ERICSSON REVIEW

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SELECTION AND TESTING OF ELECTRONIC COMPONENTS CROSS STRANDING OF TELEPHONE CABLE NEW TELEPHONE SET DIGITAL LINE EQUIPMENTS OPERATION AND MAINTENANCE CHARACTERISTICS OF AKE MAGNETO SWITCHBOARD 1977

ERICSSON REVIEW N U M B E R S 1 9 7 7 - V O L U M E 5 4

Copyright Telefonaktiebolaget LM Ericsson

Printed in Sweden, Stockholm 1977

R E S P O N S I B L E P U B L I S H E R D R . T E C H N C H R I S T I A N J A C O B / E U S

E D I T O R G U S T A F O . D O U G L A S

E D I T O R I A L S T A F F F O L K E B E R G

E D I T O R ' S O F F I C E S - 1 2 6 2 5 S T O C K H O L M

S U B S C R I P T I O N O N E Y E A R $ 6 . 0 0 O N E C O P Y $ 1 . 7 0

Contents 94 • Selection and Testing of Electronic Components for LM Ericsson's

Telephone Exchanges 105 • Cross Stranding of Telephone Cable 112 • New Telephone Set 114 Digital Line Equipments for 8 Mbit /s and 2 Mbit /s 125 • Operation and Maintenance Characteristics of AKE 13 136 • ABJ 1 0 1 - t h e Modern Public Magneto Switchboard

COVER Part of a memory board with electronic com­ponents. In the foreground a capacitor manu­factured by AB Rifa — an Ericsson company.

Selection and Testing of Electronic Components for LM Ericsson's Telephone Exchanges Berndt Agneus and Ivan Borgstrom

Electronic components have formed part of automatic telephone exchanges for a rather long time. The rapid development that these components, primarily micro circuits, have experienced during recent years and which can be expected to con­tinue for a long time is of great importance for the design of new exchange systems. In fact, the new systems are in the main based on electronic components. In the design work it is essential to select components that not only have the desired function but which are also stable during the estimated life of the exchange. The article deals with various activities, which together are aimed at ensuring the cor­rect range of components for telephone exchange equipments. The illustrations with captions provide information regarding various aids that are used in this con­nection. The article also gives a summary of electronic components of current interest and their probable development trends.

UDC 621.3.049.7 Component categories Electronic components for LM Erics­son's telephone exchange equipments are to a certain extent manufactured within the group. The remainder are bought externally and usually belong to the suppliers' standard ranges. In cer­tain cases, however, components are re­quired with characteristics that necessi­tate either special selection from the standard range or the introduction of special "customer adapted" compo­nents. The component quality cor­responds to the category "professional components", which meet higher relia­bility and long-term stability require­ments than so-called entertainment components.

Electronic components are usually di­vided into three main categories as re­gards their function, namely passive components (resistors, capacitors etc.), discrete semiconductor components (such as diodes, transistors and thyristors) and various types of micro circuits, see fig. 1.

The importance of the components in the system design The design and characteristics of an ex­change system are to a great extent de­pendent on the design of the compo­nents. There is in fact mutual effect since the system design influences the design of the components.

LM Ericsson's "great" automatic systems, the 500-line selector system and the crossbar system, were both

based on electromechanical compo­nents. However, electronic components were also included in these systems at an early stage. They were then mainly used for secondary functions such as series and parallel resistors, delay capacitors, CR units for contact protec­tion etc. These components were often soldered on to tags on the relays.

The electromechanical systems have been modernised extensively as and when the need has arisen. In connection with this, discrete components (first discharge valves, later on diodes and transistors) were brought into use at quite an early stage. They were then in­cluded in such function units as test circuits, MFC signalling systems and charging units.

SPC technique (Stored Program Con­trol) was first used in LM Ericsson in the transit exchange system AKE 13, and was then based on diodes, transistors and ferrite memories. Fast micro circuits, including semiconductor memories, became important for the further development of the SPC techni­que.

Micro circuits are now primary elements in modern exchange systems. They re­quire very little space in relation to the large number of logic functions they are able to perform. They are usually mounted on printed circuit boards, which makes for a compact construc­tion throughout. Reed switches, minia­ture relays and certain other compo­nents have also been adapted for mounting on printed boards.

The development of components and systems continues in close collabora­tion. The need for such development collaboration will increase as complex "components" containing very large numbers of functions are introduced. An example of such "components" are microcomputers, which in one or a few micro circuit packages accommodate the primary functions of a computer.

Principles for the selection of electronic components Components in telephone exchange eauiartiants musLhe_abJe to perform the

BERNDTAGNEUS Älvsjö Electronics Factory IVAN BORGSTRÖM Telephone Exchange Division Telefonaktiebolaget LM Ericsson

Fig. 1 Column 1 (lett) Some types of resistors and capacitors (passive components) (From top to bottom) Plastic foil capacitor with epoxy cover Tantalum capacitor, dry type Attenuator In the form of thick-film resistors on a ceramic substrate Varnished film resistor

Column 2 Discrete semiconductor components Display of the 7-segment type Light-emitting diode for visible light Transistor In TO 18 metal case Rectifier diode in a glass envelope for 1 A Fast logic diode In a D 35 envelope

Column 3 Micro circuits Programmable memory in a 24-pin DIL ceramic and metal package Digital micro circuit In a 16-pln ceramic Dual-in-Llne (DIL) package Linear micro circuit in an 8-lead metal envelope

intended task with high reliability dur­ing the whole life of the equipment. In addition to this basic requirement a number of other important factors must be taken into consideration when selecting electronic components, in or­der to obtain a suitable range of com­ponents. Among these may be men­tioned:

Technical status and trend. Is the com­ponent based on a new technique or new materials? If so, how well is the new technique developed and how well are the characteristics defined? Is the de­velopment likely to proceed towards the type represented by the component, or in other words has it got a future?

Supply. Since electronic components are to a great extent purchased from dif­ferent manufacturers it is important to know which manufacturers are able or will be able to supply a particular com­ponent. For reasons such as supply re­liability it is essential that there will be several approved suppliers for each type of component.

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Need. The current and future need for the component is investigated in col­laboration with circuit and system de­signers. The quantities used affect sup­ply and price.

Price situation and tendency. A com­parison with alternative components or circuit designs is made in order to as­sess the financial side of the component selection.

Standardization and coordination. It is in the interests of both LM Ericsson and the customers that the range of compo­nents used in equipments is limited, so that the number of different items is not greater than is absolutely necessary with regard to the function and reliabili­ty requirements. Consequently the in­troduction of new components requires that the proposals by component specialists are submitted to special standardization committees for deci­sion. Component selection and policy questions of particular importance are referred to a component council.

Fig. 2 Curve tracer The special type of oscilloscope, which is called a curve tracer, Is a universal Instrument tor testing semiconductor components. A family of curves are dis­played on the screen. These give an overall picture of the electrical characteristics of the component. The curves can provide Important data, such as amplification factor, breakdown voltage and reverse current

Fig. 3 Light m ic roscope Microscopes are valuable aids in construction and fault analyses of electric components. By means of direct observation or photography using light or electronic microscopes It Is possible to study and assess the design of component details or determine the causes of faults. Mlcroscoplng is indispensable when studying intricate conductive patterns or wire bondings on semiconductor crystals

Coordination between different divi­sions and companies in the Ericsson Group in questions relating to choice of electronic components takes place, for example, through a special component and circuit committee, within specialist groups and through centrally distri­buted component information.

Rules for use. The conditions that are to apply for the use of the new component in circuit designs are considered in connection with the component selec­tion. In order to attain the desired relia­bility and life it is often necessary to re­duce, to a greater or lesser extent, the values given in the manufacturer's component data for permissible electri­cal loads and operating temperatures. Permissible design data are given in special documents which also contain additional information for the de­signers.

Production engineering aspects. When manufacturing equipments it must be possible to check, assemble and con­nect the components using rational methods and production aids. This means, for example, that questions concerning automatic assembly, sol­dering and cleaning of components must be considered.

Current component range Equipments belonging to different ex­change systems, which have been de­signed at different times, are manufac­tured continuously and often in parallel. The components have been selected during different development epochs. In order to prevent this from having any negative effects the component range is continously standardized and mod­ernized.

The most important types of passive components, discrete semiconductor components and micro circuits used in the LM Ericsson exchange equipments are described below.

PASSIVE COMPONENTS

Passive components comprise various types of resistors, capacitors and trans­formers.

Resistors Carbon film resistors constitute the most common type. The resistive ele­ment consists of a carbon film on a ceramic rod. Operational experience has shown that the carbon film resistor is the most reliable type of component in telecommunication equipment.

In addition to carbon film resistors, metal film resistors are used where low temperature dependence is required. This type of resistor is being used to an increasing extent in electronic equip­ment.

A third type of film resistor that has re­cently been introduced is the metallic oxide resistor, where the resistive ele­ment consists of tin oxide with antimony oxide added.

Thick-film resistors, manufactured by means of screen printing and firing a re­sistive paste on to a ceramic substrate, are used as attenuators and fuse re­sistors.

Capacitors This category of component includes many different types, whose charac­teristics and thus fields of use are main­ly determined by their dielectric.

Aluminium electrolytic capacitors of the long-life type are used for regulating the operating times of relays. In this case a large capacitance per unit of volume is an essential characteristic.

Tantalum electrolytic capacitors are usually chosen for electronic circuits when the requirements are small size and moderate capacitance values at low voltages.

Polyester film capacitors are being used to a great extent nowadays in transmis­sion, time and contact protection circuits instead of the traditional paper capacitors

Polystyrene capacitors are used when close tolerances and good stability are required, for example in tuned circuits. Both polystyrene and polyester film capacitors are manufactured with a moulded epoxy cover and are con­structed for mounting on printed hoarriffc nnfi l i o n irt r a l n w QjatQ

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Fig. 4 X-ray camera The Inner construction and manufacture of components can be of great importance to function characteristics and life. One way of Investigating the Inside of the component Is to open the case and cut through the component. How­ever, this method Is usually destructive, so that the com­ponent properties are changed entirely. It Is often possible to obtain valuable Information regard­ing the structure of the component by means of an X-ray photograph. Inner mechanical faults can also be detected on such pictures. In addition the possibility remains of car­rying out supplementary electrical measurements on the unaffected component after the X-ray Investigation

Transformers and inductors Transformers with a core of plate frames or tape are used in transmission circuits that transmit alternating current superposed on a direct current.

Ferrite transformers with different types of cores, in certain cases adjustable, are used as current transformers and for filtering in power equipment, in tuned circuits and oscillators, for impedance matching and pulse transmission.

DISCRETE SEMICONDUCTOR COMPONENTS These components contain individual semiconductor components, each of which performs just one single func­tion. They are still used in modern ex­change systems along with micro circuits, and consist mainly of trans­istors and diodes, but thyristors and optical semiconductor components are also included in this category.

Transistors All modern transistors are made of sili­con and usually manufactured by means of so-called planar technology. For quality reasons a metal can with glass lead-throughs for the conductors is used.

Diodes The diodes used in exchange systems

can be divided into three main categories: rectifier diodes, switch diodes and voltage regulation diodes. The switch diodes are used in logic circuits and must therefore have a short recovery time in the reverse direction. The voltage regulation diodes give a de­fined voltage level in circuits where a stable reference voltage is required.

Special semiconductor components Among semiconductor components that are used to a limited extent for spe­cial functions may be mentioned the thyristor, which closes a circuit when a pulse is applied to its gate electrode, and the unijunction transistor, which is used for starting time circuits and thyristors.

Opto-electronic components have re­cently been introduced that use visible or infra-red light for their operation. Among them are light-emitting diodes, displays and opfo couplers.

Red, yellow and green light-emitting diodes are used for indicating different states in equipments, and displays show a figure or a letter depending on the applied electrical signals.

The opto coupler, on the other hand,

Fig. 5 Desk calculator Agreatnumber of measurements.provldlng large amounts of measured values, are often carried out In connection with component Investigations. These values must be pro­cessed In order to obtain essential data in a clear form. A desk calculator is used for this purpose which can be prog­rammed to process the test material In a suitable way and give the result as a printout or on a diagram. The desk calculator Is also used for certain component data calcula­tions, for example when dimensioning transformer wind-

Fig. 6 Humidity test High air humidity Is one of the most serious adverse en­vironmental conditions to which electrical components can be exposed. The dampness can affect the outside of the components by corroding metal surfaces and reduce the Insulation between the leads. It can also seep Into the components and In so doing impair their caracterlstlcs or cause total breakdown. Humidity testing Is carried out in an climatic chamber, where the air humidity and temperature are either held constant or varied cyclically with time. The compo­nents can either just be stored In the chamber or they can also be connected to an electrical voltage source during the humidity tests

uses infra-red lights as the transfer medium between a light-emitting diode and, for example, a photo transistor.

MICRO CIRCUITS Micro circuits are built up of a number of interworking semiconductor ele­ments and can integrate a number of analogue functions, digital functions or memory functions.

Analogue micro circuits In analogue circuits the voltages on the inputs and outputs can vary continu­ously over certain ranges and are thus not limited to fixed levels. In modern ex­change systems these circuits are used as — voltage regulators for power units — sensors of voltage levels — interface circuits between different

subsystems — operational amplifiers in MFC filters

etc.

Digital micro circuits These circuits are predominant among the micro circuits. Digital circuits carry out logic operations by means of digital signals, i.e. voltages on the inputs and outputs that take up values close to fixed levels. Many of these circuits be­long to specific so-called circuit families with a certain type of logic ele­ment and in other respects designed so that they can interwork in systems.

In addition to these circuit families a number of digital circuits are used that do not belong to any particular family. These can be divided into a number of groups according to function, such as registers, adders, arithmetic circuits, counters, data switches, coders and de­coders.

DTL circuits (Diode-Transistor-Logic) constituted one of the first families in micro circuit technique. They are no longer used when designing new equipments.

TTL circuits (Transistor-Transistor-Logic) are faster than DTL circuits and have gradually become the predomin­ant type.

A large number of circuits are available on the market in several different var­iants. Certain series (e.g. 74S and 74LS) have integrated so-called Schottky diodes, whereby the circuits have been made faster.

CMOS (Complementary-Metal-Oxide-Silicon) circuits operate within wide vol­tage limits and have low power con­sumption, but they are not particularly fast.

Memories constitute an important group. They can be in the form of ran­dom access memories, where the con-

Fig. 7 Programming equipment In a certain type of semiconductor memories, designated PROM (Programmable Read Only Memories), the contents of the memory cells are fed In with the aid of special prog­ramming equipment A metallic connection is thereby burnt off electrically in the cells that are to be program­med. The memory program can be fed in in different ways: manually via a push-button set, from a punched tape or with the aid of a previously programmed memory (master). The equipment also checks that the programming Is cor­rect

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Fig. 8 Load test Electrical load tests over a relatively long period (several thousand hours) constitute an Important part of the type testing ot components. For these tests the components are mounted on printed wiring boards placed In racks, which when necessary are equipped with supervisory equipment that records component faults. The electrical load Is often Intermittent, i.e. It Is switched on and off at certain Intervals In order to Imitate the stresses that can occur In some operational cases. At certain times the components are removed In order to measure their electrical characteristics. The lead tests provide Information regarding changes In the component data during operation (ageing), and In cer­tain cases regarding probable failure rate and life

tent can be changed, or read only me­mories with fixed content. Both the fast bipolar semiconductor technique and the less power-demanding MOS tech­nique are used for memory compo­nents.

Special circuits The above-mentioned micro circuits are of standard design and can be bought from different suppliers. However, for certain purposes it may be appropriate to introduce special circuits that satisfy particular function requirements.

Packaging Micro circuits are usually packaged in ceramic cases with the external pins arranged in two rows, so-called DIL packages, although certain types of analogue micro circuits are packaged in cylindrical metal cans.

Type testing The purpose of type testing is to de­termine whether a certain type of com­ponent from a manufacturer satisfies the given data and requirements. The type testing comprises measurements of data, function checks, environmental tests and load tests.

The actual type testing is usually pre­ceded by a preliminary investigation, which comprises the study of available

information concerning the type of component and a construction analysis of a small number of test items. Such methods as X-ray photography, dissec­tion, microscoping and material analy­sis are used to investigate and assess the packaging and sealing, internal connections, metal and oxide layers, diffusion pattern, cooling and mounting and connection facilities.

Type testing is carried out in ac­cordance with programs that indicate which tests and measurements are to be carried out. As far as possible the type testing programs are based on the re­commendations issued by the Interna­tional Electrotechnical Commission (IEC). When necessary, additions and modifications are made in order to adapt the type testing to the special op­erational conditions of telephone ex­change equipments. Thus particular importance is attached to the verifica­tion of the reliability and stability of the components during long periods on load.

The type testing programs normally comprise cold tests, heat tests, temper­ature cycling and humidity tests in a constant environment and also with fluctuating air humidity and tempera­ture. Moreover, the programs generally include vibration tests, soldering tests,

Fig. 9 Cold test Exchange equipments do not normally need to work at temperatures below the freezing point. However, during transport in cold areas they can be exposed to low temp­eratures, which they must be able to withstand without damage. The electrical components are therefore tested at temperatures down to at least — 40°C. In a corresponding way the effect of high temperatures Is

• • . . . . . . - . . - - • • • •

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Type of check

Type of component Me- En- El. chan- viron- param.

ical mental

Resistors, resistor net­

works and potentio­

meters S S

Capacitors S S

Diodes, transistors and thyristors S A

Micro circuits S A A

Table 1 Testing of components A Check of the whole consignment S Sampling test in accordance with MIL-STD-105

hermeticity tests, tension, bending and torsion tests on the leads and flamma­bil ity tests. Electrical tests may com prise voltage tests, current pulse tests and power loading tests. Such tests can continue for periods of 1 000 hours up to more than 10000 hours depending on the type of test and the "acceleration factor", i.e. the size of the load in rela­tion to specified component data.

Significant component data are mea­sured before, during and after the course of the type testing.

Type testing programs for electronic components also contain instructions for assessing the test results. However, the final decision as to whether a certain type of component should be accepted is always based on the expert know­ledge of the component specialist.

Quality follow-up Components delivered by approved suppliers are inspected on arrival in the way described in the next section. In addition a so-called reliability evalua­tion is carried out in accordance with a yearly plan, primarily of recently intro­duced components and components purchased in great quantities.

The reliability evaluation provides a continuous verification that the com­

ponent quality originally accepted after type testing is maintained in later com­ponent deliveries.

In this evaluation, which is carried out on samples taken from the deliveries, the inner construction of the compo­nent is studied and compared with re­ference examples from the type testing. It can then be ascertained whether the manufacturer has for example intro­duced a new type of silicon chip in a transistor or changed the connections to a capacitor foil.

The reliability evaluation also includes a limited type testing for the purpose of finding any quality defects in a com­ponent consignment within a few weeks. It is then possible to prevent the use of unsatisfactory components in the production of exchange equipments.

When a component fault is reported in equipment being manufactured, in the system testing stage or in operation, a fault analysis is carried out in order to determine the cause of the fault and when necessary to improve the compo­nent quality.

Inspection of components on arrival Inspection of purchased components

Fig. 10 The solderability tester STE 74 works in accor­dance with the solder globule method and is in­tended for measuring the solderability of compo­nents and metallized holes in printed boards etc. The test Item whose solderability is to be measured is low­ered into a molten solder globule that is placed on top of a heated iron cylinder so that the globule Is divided into two equal parts. When the solder wets the test item the latter is completely enclosed by the solder. The wetting time is measured and Is a measure of the solderability of the test item. The lowering speed, solder temperature and quantity of solder are carefully specified. The solder and test item are treated with flux and the solder must wet the iron cylinder

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on arrival takes place in accordance with test instructions based on IEC and MIL standards. The purpose of the in­spection on arrival is to ensure that the components in the system meet LM Ericsson's high demands for reliability and long life.

SCOPE The inspection on arrival is carried out on passive components, discrete semiconductors and micro circuits. It comprises checks of the mechanical properties of the components, their ability to withstand adverse environ­ments and their electrical parameters. The checks are carried out either on all components or on a sample in ac­cordance with table 1.

INSPECTION PROCEDURE All components are tested mechanically in the following way:

Mechanical dimensions are measured with vernier callipers and the solder-ability checked by means of the solder globule method or the solder bath meth­od at 230 ± 10°C (requirement of IEC 68-2-20). Fig. 10 shows a solderability tes­ter. Resistors, resistor networks and potentiometers These are tested as follows:

The resistance value is determined with the aid of a Wheatstone bridge.

The harmonic distortion factor is mea­sured with a distortion factor meter.

Fig. 11

Fig. 12 Computer-controlled test system for memory circuits, Macro-Data MD 104 M/MC Here equipped with a handler, which makes possible test­ing at Increased temperature

Capacitors (excl. electrolytic capacitors) The capacitance value is measured with a comparison bridge, where the value is set up and the deviation is read off in percentage units.

The dissipation factor is measured with a distortion factor meter.

Voltage tests are carried out using a special voltage tester, which is set up for a certain voltage and which records the insulation resistance and any breakdowns.

Electrolytic capacitors Capacitance, dissipation factor and leakage current are tested with a com­parison bridge, where the capacitance and dissipation factor values are bal­anced out, after which the leakage cur­rent is read off on a special scale.

Diodes and transistors The electrical parameters of these are measured using a go-no go tester with

automatic input. SMall consignments are checked using a curve tracer.

Micro circuits Micro circuits are subjected to rigorous checks comprising mechanical, en­vironmental and electrical tests. The testing procedure is shown in fig. 11.

The electrical testing of micro circuits deserves to be described more in detail. From the point of view of testing, the micro circuits can be divided into four groups, namely analogue, simple digi­tal, complex digital and memory cir­cuits, which require different types of advanced test equipment.

Analogue circuits The parameters concerned are checked manually in test equipment type Gener­al Radio 1730.

Simple digital circuits These are tested statically in test equipments type Teradyne J133 and Fairchild Q 901 "Qualifier".

Fig. 13 Test system Tektronix S-3260 Evaluation and checking of complex micro circuits both require such extensive measurements that special test systems are necessary. A minicomputer is used tor ex­ecuting the test programs, which apply Incoming signals on certain of the component connections. At the same time measurements are made on the output connections. The test results are processed and are then shown on a display or as a printout

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Fig. 14 Diagram of the fault ratios for micro circuits distri­buted on a manufacturer basis

Leakage faults c h e c k position 1 Electrical faults

Leakage faults C n e c k ,,,„„ 2 Electrical faults

Leakage faults c h e c k p o s | t l o n , + 2 Electrical faults Different manufacturers

Complex digital circuits The circuits are tested from a functional point of view and also statically and dynamically. For this purpose the pro­duction side uses system Tektronix S-3260, which is shown in fig. 13.

Memory circuits These are also tested as regards func­tion and statically and dynamically. The test equipment used is Macro-Data M104 M/MC, shown in fig. 12.

THE RESULTS OF INSPECTION ON ARRIVAL Inspection reports are kept for each in­dividual inspection occasion. The re­ports are compiled once a month and the statistical data are processed with the aid of the LM Ericsson computer system MAKON (Material Control Purchase). Some results are shown in the histogram for micro circuits, fig. 14. The histogram contains statistics from two different inspection positions, where the inspection differs as regards the hermeticity test. In one place the trace gas used is krypton 85 and in the other helium.

Fig. 15 shows the fault ratios for passive components and discrete semicon­ductor components.

FAULT TRACING COSTS-A COMPARISON Fig. 16 shows a comparison of fault trac­ing costs at different check levels. The figures are based on experience of actual costs. The diagram shows that it can be profitable to invest in more effec­tive fault elimination methods in the in­spection on arrival, forexample burn-in, in order to eliminate defective compo­nents that whould otherwise cause op­erational disturbances in the systems.

FUTURE PROSPECTS Development of an effective and cheap method that makes possible a one hundred per cent check for gross leak­ages is desirable. Fine leakages in the encapsulation are then checked by means of sampling. Heat storage of components is replaced by burn-in with voltage applied and increased tempera­ture. The increase in the complexity and speed of micro circuits requires large investments in systems for testing the functions and the DC and AC paramet­ers during the inspection on arrival. The test system is equipped with a main computer that controls several check stations. The check stations for the in­spection on arrival can work as inde­pendent units and utilize centrally pre­pared programs. Electrical function testing will to an increasing extent be carried out at an elevated temperature.

Fig. 15, right Fault ratios for passive components and discrete semiconductors, obtained from computer system

"Makon"

Mechanical faults Electrical faults

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Development tendencies The component development is at pre­sent progressing very rapidly, particu­larly in the field of micro circuits. There are already so many types of circuits for different purposes that existing de­mands for speed, low power consump­tion, insensivity to disturbances or volt­age variations can usually be satisfied.

However, because of the rapid de­velopment of micro circuits there has often been time for improved circuits to appear on the market during the period between the selection of components and the putting into service of the first example of a new system. It is desirable that it should be possible to exploit new achievements in the field of compo­nents in previously completed designs, for example by changing over to a new circuit family.

The development of micro circuits leads to an increasing degree of complexity and flexibility. This means that the boundaries between components, units and subsystems are being wiped out.

Programmable component types, such as micro processors and memories, will have a decisive influence on the design

and performance of the systems. This also means that a greater part of the "knowledge" and flexibility of the ex­change systems will be transferred from fixed hardware to changeable software.

As regards memory components the development is towards greater capaci­ty and speed. It is likely that the memory types that retain the information even in the case of voltage failures will become very important in future.

It is also likely that selectors with mechanical contacts will to an increas­ing extent be replaced by electronic switching elements. Opto-electronic and purely optical components will also be very important for the transmission of information.

Development of passive components follows in the wake of the applicable material and production engineering development in the semiconductor field. The resistors will be able to with­stand higher voltages and will have smaller dimensions. There are already resistor networks that are mounted in micro circuit packages. It is also likely that plastic foil, oxide and ceramic capacitors will be improved.

Fig. 16 Comparison of fault tracing costs at various check levels. (Micro circuits)

Volume checked: 2 000 000 Volume checked: 10000000

Cross Stranding of Telephone Cable

Sigurd Nordblad

LM Ericsson have developed a new manufacturing process and constructed new machines for the manufacture of pair cables. The process, which is called cross stranding, combines two methods: twinning and stranding of groups in one opera­tion and repeated changing of the relative positions of the pairs during the strand­ing. The changing can either be carried out systematically in accordance with a set plan or at random, so-called randomized cross stranding. The main purpose of the cross stranding is to reduce the extreme values of the crosstalk and thus improve the quality of the cable.

In the article the cross stranding technique is described with the emphasis on the manufacture of pair cables with randomized changing, but cross stranding can also be used with advantage for stranding single conductors, triples, quads, quintuples etc. The cross stranding technique has now been introduced at most of the telecom­munication cable factories owned by the Ericsson Group. At the Piteå plant, which wasstartedin 1972, the entire production is based on this technique. Manufacturers outside the Group also use the technique.

UDC 621.315.2 621.391.31

In addition to line attenuation, charac­teristic impedance and line resistance, the crosstalk characteristics of a cable have a very great influence on its field of use. This applies particularly in the case of trunk cables but also for subscriber cables.

One reason why the subscriber cables of today should have a low level of crosstalk is that modern telephone sets can then be utilized more efficiently. The usefulness of a telephone set is li­mited by such factors as the crosstalk level in the cable network. A reduction of the crosstalk means that greater dis­tances can be spanned or that the con­ductor size can be reduced.

High frequency systems, which are used nowadays to an ever increasing extent,

also require cables with improved electrical characteristics.

Previously the pairs, single conductors, quads etc. of a cable have usually been assembled in concentric layers. The pairs were then parallel in each layer and were adjacent to the same pairs along the whole length of the cable. Subsequently the unit cable was intro­duced, but the units were still built up of concentric layers. Efforts to improve the cable characteristics have been con­centrated on improving the precision of the wire drawing, insulation etc. and on suitable selection of lay lengths, i.e. im­provements within the pairs, and very little attention has been paid to the ef­fect of the cabling method on the electrical characteristics.

Crosstalk occurs mainly between adja­cent pairs and it is obvious that the crosstalk increases when the pairs are adjacent over a long distance. Using the conventional layer stranding technique the pairs are placed adjacently and as close as possible along the whole length of the cable. The crosstalk level between pairs varies in a cable; high level between adjacent pairs and very low level between separated pairs. However, in a telephone system the worst values often constitute a techni­cal limit and a number of very good val­ues does not alter this fact.

Fig. 1 Cross stranded cables Top, 150-pair cable with 25-pair groups Bottom, 50-pair cable with 10-pair groups, jelly-

SIGURD NORDBLAD Sieverts Kabelverk AB

Fig. 2 Cable groups with the conventional lay-up

Top, 10-palr group Bottom, 25-palr group

Fig. 3 Cross stranding lines

Top, line lor manufacturing 10-palr groups Bottom, 25-pair line with drum twister take-up

The principle of the cross stranding technique Cross stranding differs from the other stranding methods inasmuch as the pairs or other elements in question are assembled to form a group with the pairs continuously changing their rela­tive positions during the assembly. This can be carried out in different ways. The elements can be assembled in groups either in accordance with a set pattern, systematic cross stranding, or at ran­dom, randomized cross stranding. Systematic cross stranding has the dis­advantage that two elements meet at fixed intervals. In high frequency systems the intervals can correspond to wavelengths in the frequency range concerned, which can give rise to a re­sonance phenomenon that is difficult to eliminate. With randomized cross stranding the elements are crossed at random, which eliminates this reso­nance phenomenon.

Randomized cross stranding Capacitance unbalance has a predo­minant effect on the value of the cross­talk, particularly at low frequencies. If we consider a conventional, concentric 10 or 25-pair group, fig. 2, it is well known that unbalances arise mainly be­tween adjacent pairs, 1—2, 2 — 3, 3 — 4 etc. Unbalances also occur to some ex­tent between the center pairs and the pairs in the first layer, sometimes also between pairs in neighbouring layers. Unbalances between any other com­

binations are almost non-existent. It has also been established that at least the highest unbalances increase approxi­mately in direct proportion to the length of the cable.

In cross-stranded cables the random mixing ensures that two pairs are adja­cent only for a limited part of the cable length and thus the high capacitance unbalance values are reduced.

The ten pairs in a 10-pair group (fig. 2) occupy ten different positions. If, for example, we consider pair no. 1, wefind that two other pairs can be considered as adjacent. Two other positions (in the centre of the group) are slightly further away but can still be considered as adjacent. If the positions of the pairs in the group are changed at random along the length of cable, we find that two arbitrarily chosen pairs will be adjacent for only about 4/9 of the cable length. The capacitance unbalances contribute to the crosstalk mainly during this minor part of the cable length.

Cross stranding of groups that contain more than ten pairs gives an even grea­ter reduction of the unbalances. In a cross-stranded 25-pair group two pairs are adjacent for only 3/24 to 4/24 of the total cable length, which gives a cor­responding reduction of the unbal­ances. This calculated reduction is approximate, but the tendency is that a random mixing of an increasing num­ber of pairs gives a corresponding reduction of the capacity unbalances

107

between the pairs. For example, in a 100-pair group the unbalances would hardly reach measurable values.

On the other hand the 10-pair group must be considered as the smallest unit for which cross stranding gives a reasonable reduction of the unbal­ances. For practical reasons, such as colour coding, the cross stranding technique is considered suitable for groups with between 10 and 30 pairs.

Cross stranding, both randomized and systematic, gives the group a certain mechanical flexibility. Thus in this re­spect it can be compared with such pro­cesses as the braiding operation used when making flexible cables.

The most suitable mixing ratio for cross stranded 10-pair groups is approxi­mately two crossings per metre, and these crossings have proved to make the cable core looser. This results in greater separation of the pairs and thus a lower mutual capacitance compared with the conditions prevailing in a layer stranded cable. When changing over to cross-stranded cable the insulation thickness can therefore be reduced, for the same value of capacitance, which means a reduction in cost. This will be illustrated later on in the article.

Process and machines The most common pair cable specifica­tions prescribe groups containing be­tween 10 and 25 pairs. Small fixed groups are used when the cross strand­ing technique is applied. It is then pos­sible to carry out pair twinning and stranding of groups in one and the same operation.

A cross stranding line, fig. 3, consists of the following main components:

1. group twinner 2. random pulse generator 3. mixer, the cross stranding device 4. binding head 5. length measuring device 6. take-up stand

Group twinner The group twinner is basically a number of twinning machines assembled to form a unit, fig. 6. The design of the twinning machines has intentionally been kept uncomplicated. The reason for this is that the process is duplicated 10, 12, 13 or 25 times in each machine. There is greater risk of faults in sophist­icated machines and the efficiency is reduced because of the greater number of repairs.

Fig. 4 Cross stranding device for a 10-pair line

Fig. 5 Random pulse generator

Fig. 6

108

Fig. 9 Take-up stand lor connection wire

Fig. 7 Binding head and length measuring device

Fig. 8, right Take-up stands for 10-pair groups

Random pulse generator The random pulse generator, fig. 5, utilizes the white noise in a transistor to generate randomly distributed pulses for the mixer.

Mixer, cross stranding device Fig. 4 shows the cross stranding of a 10-pair group. The pairs are taken through dies which move sideways in the cross stranding device. The move­ments of the dies —the mixing of the pairs —are controlled by a motor that is started and stopped by pulses from the random pulse generator. The pairs are fanned out over rollers, after which they are assembled and bunched together. There is no systematic order between the pairs because their positions on the rollers are changed at random.

Binding head The purpose of the binding head is to fix the pairs in the same order that they have when leaving the mixer, fig. 7. The binding yarn can be used for identifica­tion purposes. A binding head usually has an electromechanical binder yarn break detector. The mechanical part of the detector has a sensing finger which is easily broken, thereby causing the machine to stop. The break detector in the cross stranding line binding head has therefore been redesigned and is fully electronic. The binder yarn tension can be adjusted during operation.

Length measuring device Fig. 7 also shows the length measuring device, including the tachometer which synchronizes all drive motors of the whole line.

Take-up stand Any type of take-up stand can be used for 10-pair groups. Fig. 8 shows a type of take-up stand where all drive equipment is placed on a frame above the drum which leaves the floor free for the trans­portation of drums. A drum twist take-up is recommended for 25-pair groups.

Final assembling A conventional stranding machine with a drum twist take-up can be used for the final assembly. It need only be equipped with a few stands for pay-off reels and can thus be simple. No back-twist of the individual groups is required.

Supplementary equipment Fig. 3 shows an ordinary cross strand­ing line. Accessories for various purposes can be included in the line, such as taping heads for different tap­ing materials.

If the group twinner is supplemented with a specially designed pay-off and take-up device it will be suitable for simultaneous twinning and coiling of connecting wire on small bobbins. Ten

109

Fig. 10 Distribution curves, showing the capacitance un­balance of 10-pair groups

Insulation: Solid polyethylene Conductor diameter: 0.5 mm Cable length: 500 m Curve A represents approximately 1000 capacitance un­balance values within 10-pair groups in cross stranded cables manufactured In the Plteä plant during 1976. Curve B represents approximately 1000 capacitance un­balance values within concentric (2 + 8) 10-pair groups. The shaded area shows the reduction of high capacitance unbalance values obtained by introducing the cross stranding technique. The expected reduction for other curves B (due to the techniques used tor the wire drawing, twinning etc.) can be calculated approximately by shifting the shaded area

Fig. 11 , right Distribution curves, showing capacitance unbal­ance between and within 10-pair groups

Insulation: Solid polyethylene Conductor diameter: 0.5 mm Cable length: 500 m Curve A represents the capacitance unbalance between 10-pair groups In cross stranded cables Curve B represents the capacitance unbalance within 10-pair groups in cross stranded cables

bobbins with pairs and triples or five bobbins with quadruples and quintu­ples can be manufactured in one opera­tion. The take-up device is shown in fig. 9.

Cross stranding line It has already been stated that since the group twinner consists of several indi­vidual twinning machines, special care was devoted to making the design reli­able and simple. Thus the 10-pair group twinner has twenty pay-off shafts, each with its own brake. The risk of a breakdown because of a brake fault is then multiplied by twenty and hence the group twinner is equipped with reliable, simple rope-brakes.

Size of reels It is generally considered that large pay-off reels give high efficiency, but experience shows that there is an optimum size. Too large reels give rise to such disadvantages as conductor elongation, long acceleration and re­tardation times etc.

Most factories in which the introduction of cross stranding lines is contemplated are already provided with pay-off and take-up drums. It must therefore be possible to adapt the cross stranding equipment for use with a wide range of such drums.

Space requirements A cross stranding line requires less floor space than conventional equipments,

owing to the fact that the pair twinning and group stranding is carried out in a single operation. The group twinner alone requires much less space than the corresponding number of single twin-ners even if these are of the high-speed type.

Operation In the group twinner all twinning heads are idle during reloading. This factor has a negative effect on the efficiency compared with production using the corresponding number of single twin­ning machines. A loading table for pay-off reels has therefore been in­cluded in the cross stranding line in or­der to reduce the loading time. A 10-pair line has to be reloaded every third to fourth hour and the reloading time is only ten minutes. Owing to the com­pactness of the line and the smooth op­eration one operator is sufficient for the supervision of three 10-pair lines. How­ever, it is desirable that two operators work together when reloading.

Electrical characteristics of cross stranded cables Capacitance unbalance The cross stranding technique reduces the high unbalance values between the pairs and the crosstalk characteristics are improved to a corresponding de­gree since no pair combinations are permitted to be systematically adjacent during any large part of the cable length. This is shown in fig. 10.

Insulation: Solid polyethylene Conductor diameter: 0.5 mm Cable length: 500 m

Insulation: Foamed polyethylene Conductor diameter: 0.5 mm Cable length: 500 m

Insulation: Solid polyethylene Conductor diameter: 0.7 mm Cable length: 500 m

Fig. 13 RMS va lues of capac i tance unba lance d is t r ibu­t ion for c ross s t randed cab le

Each cross represents the RMS value of the 45 capaci­tance unbalance values within a 10-pair group

Fig. 12 D is t r ibu t ion cu rves , show ing the capac i t ance un­

ba lance for 25 and 10-pair g roups

Insulation: Solid polyethylene Conductor diameter: 0.5 mm Cable length: 500 m Curve A represents the capacitance unbalance values within 25-pair groups In cross stranded cable Curve B represents capacitance unbalance values within 10-pair groups in cross stranded cables

The characteristics of completed cables are naturally also dependent on the quality of the individual pairs as regards the uniformity of conductors and insula­tion, lay lengths etc.

As can be seen from fig. 11 the unbal­ances between groups is much less than the unbalances within groups.

Fig. 12 shows that the unbalances in 25-pair groups are lower than the cor­responding values in 10-pair groups.

The quality of a cable as regards capaci­ty unbalance is given as the RMS (root mean square) value. The distribution diagrams in fig. 13 represent the RMS values obtained for different types of cables.

As can be seen from the diagrams, the spread is relatively large and thus a reasonably large number of measured values will be required in order to be able to establish differences in the qual­ity of cables that have been man­ufactured in different ways.

Mutual capacitance In cross stranded cables there is no systematic difference in mutual capaci­tance between pairs, caused by their positions in different layers. There are, however, some small differences in mutual capacitance because of the dif­ferent lay lengths and manufacturing

tolerances of the pairs. This is shown in table 1.

Cross stranded, PE insulated 10-pair group cables without jelly filling have a lower mutual capacitance than the cor­responding 10-group layer cables (2 + 8). The reason for this is that the cross stranded cables contain more air because of the stranding method. A re­duction in mutual capacitance of about 3 % has been noted.

The mutual capacitance relationships are different for cables with other types of insulation material.

The cables are generally specified for a fixed mutual capacitance and hence the conductor insulation in cross stranded cables can be reduced with a conse­quent reduction in material consump­tion.

High frequency characteristics The high frequency characteristics of symmetrical cables are becoming in­creasingly important. This applies wherever the cables are situated in the network and particularly when they are to be used for PCM systems. Typical crosstalk values for cross stranded ca­bles, given as the mean value m and the standard deviations, are shown in table 2.

The standard deviation, o, for near-end crosstalk is of particular interest. The

111

PE insulated, solid cable

10-pair 10-pair groups. groups.

Standard | a i d u p i n c r o s s deviation. | a y e r s stranded .1, of the ( 2 + 8 ) mutual capac­itance. percentage 1.4 0.75

Table 1

Near-end crosstalk 25-pair 10-pair at 1 MHz groups groups

m o m o

dB dB dB dB Within groups 63 6 58 6 Between adjacent groups 80 6 68 6 Between groups se­parated by one group 97 6.6 78 6

dB dB Far-end crosstalk at 150 kHz RMS 78 70

Table 2 Typical crosstalk values for cross stranded ca­bles

m Mean value a Standard deviation RMS Root mean square

Fig. 14 The number of permissible 30-channel PCM systems, N, can be read off from the diagram for

cr-value for conventional cables with 10 or 25-pair groups laid up in concentric layers is 8 - 10 dB and, as shown in table 2, the corresponding value for cross stranded groups in approximately 6 dB.

The suitability of a particular cable for PCM transmission is dependent on the following four parameters:

m Mean value, crosstalk a Standard deviation, crosstalk L Attenuation over a repeater section N Possible number of PCM systems

The possible number of 30-channel PCM systems can be determined with the aid of fig. 14 with a certain degree of statistical reliability at given m, L and a values. The diagram is based on single-frequency measurements.

Example Two cables with two 10-pair groups each, one cross stranded and the other with concentric layers, have the follow­ing typical near-end crosstalk values.

m = 68 dB a = 6 dB for the cross stranded cable a = 9 dB for the cable with concentric

layers L = 29dB(m - L =39 dB)

Fig. 14 shows that the cable with o = 6 dB can be filled (10 systems) whereas the second cable permits only one system although the mean value of the crosstalk attenuation is the same (68 dB) for both. As can be seen the lower

spread of the cross stranded cable is of great importance.

The crosstalk level between 10-pair groups in cross stranded cable is 10 dB better than the level within groups and this ratio is very constant. For groups separated by one group there is an addi­tional improvement of 10 dB. In the case of 25-pair groups the corresponding dif­ference is 17 dB.

These good and well defined values and the well organized cable lay-up make cross stranded cables very suitable for PCM systems.

Summary The method and machines for tele­communication manufacture which have been described above have re­sulted in

— improved cable quality Owing to the cross stranding of the groups the number of high capaci­tance unbalance values between pairs is reduced. The capacitance level and standard deviation of the cable are also reduced.

- reduced production costs Since twinning and group stranding are carried out in one single opera­tion, the amount of space required is reduced and also the investment, operation and maintenance costs. In addition the planning and supervi­sion of the production are simplified.

New Telephone Set

Arne Boeryd and Gunnar Wiklund

New standard telephone sets have been introduced on the market at intervals of 15-20 years. DIALOG was introduced in 1963 and soon attracted attention and appreciation because of its excellent transmission characteristics and high overall quality. During the second half of 1974 work on developing a new telephone set was started in order to meet the demands of the future as regards for example push-button dialling and more stable long-distance characteristics. As a result of this work LM Ericsson will start production of a new table set, designated DBA 100, during the autumn of 1978.

UDC 621.395.721

Fig. 1 Telephone set DBA 100

Telephone set DBA 100 has been devel­oped with the aim of providing a tele­phone set — that gives the best possible overall

economy and which remains up-to-date for 10-15 years after its intro­duction

— which is suitable for both office and domestic environments and which will meet the demands of the 1980s as regards appearance and quality

— with a simple and reliable construc­tion and which lends itself to rational production

— with an entirely modular structure that facilitates servicing

— that is suitable as the basic product for a family of telephone sets.

Work on developing and designing a new standard telephone set was started jointly by LM Ericsson and the Swedish Telecommunications Administration during the autumn of 1974, and a devel­opment assignment was placed with ELLEMTEL The work was based on jointly prepared specifications and with active participation by specialists from the Administration and LM Ericsson.

LM Ericsson's new telephone set, DBA 100, will be put on the market during the autumn of 1978 and will gradually replace DIALOG.

Design The requirements that are of prime im­portance for a telephone set relate to — the appearance — the design of the handset with regard

to handling and transmission perfor­mance

— the design of the impulsing device and its location.

A number of industrial designers were given the task of making suggestions for the external design of the set. The resultant design models were examined from an aesthetical point of view, and at the same time the possibilities of ra­tional construction were assessed.

In the design selected, fig. 1, the exte­rior of the set is built up of four units, namely the base, rear and front covers and the handset.

Mechanical construction When developing the set the possibil­ities provided by the exterior design have been exploited in order to limit the number of coloured details. This is advantageous from the point of view of manufacture, stocking sparesand main­tenance, and at the same time there is considerable scope for varying the colour of the front cover.

The base, rear cover and handset are manufactured in one colour, preferably black.

The handset has been designed so that it rests easily in the hand, reaardless of

113

ARNE BOERYD GUNNAR WIKLUND Division for Subscriber Equipments Telefonaktiebolaget LM Ericsson

Fig. 2 Exploded view of telephone set DBA 100 1. Rear cover 2. Base 3. Printed board assembly 4. Front cover

Fig. 3 Transmission properties of DBA 100 when using

whether the user grasps it in the middle or at the microphone end. The handset also rests on the body of the set in such a position that it can easily be picked up from any side of the set.

The set is designed primarily for push­button dialling.

One of the basic design aims was to create sufficient space in the set for printed boards. This means that the nec­essary mechanical components, pri­marily the push-button set and the crad­le switch, have been designed for mounting on the printed board. The basic design of the set is shown in fig. 2.

A standard set holds one printed board assembly that contains the push-button set, cradle switch, electronic compo­nents for impulsing and current feed­ing, and terminals for the handset and telephone instrument cords.

The ringing device used is a conven­tional bell. The base has been equipped with the resonators that are required for amplifying the sound from the gongs within the frequency range 1000-2000 Hz.

The construction described here has the following advantages from the points of view of manufacture, installa­tion and maintenance: — the front cover is a simple detail with­

out any fitting problems or special tolerance requirements

— the set can be tested, packed and transported without the front cover. The front cover can easily be fitted on site, when the customer has cho­sen a colour

— the weight distribution of the set is such that it is easy to carry

— the number frame is placed on top of the rear cover in front of the handset. This makes the subscriber number easy to read

— the push-button set is placed on the right-hand side of the set, with is both convenient and attractive. It is placed in an indentation in the cover. In this way the set and the printed board assembly are protected against shocks if the set should fall >n to the floor.

Circuits and components All the components are mounted on a single printed circuit board. It is there­fore possible to take full advantage of component development and market requirements. This can be especially worthwhile with regard to pushbutton dialling. The design implementation of these functional elements meets appli­cable CCITT recommendations, CEPT specifications and any additional re­quirements imposed by the telecom­munications administrations.

The set will be available either with a linear microphone and electronic speech circuit or with a carbon micro­phone and a traditional hybrid circuit.

Identical components will be used as receiver and microphone elements in the version with an electromagnetic microphone. Identical electroacoustic components for transmitting and re­ceiving are an advantage from the point of view of maintenance and stocking spares.

The version with an electret micro­phone provides the maximum quality as regards sound reproduction of the speech signal.

The reference attenuation of the set relative NOSFER will be the same ir­respective of whether an electromag­netic or electret microphone is used. Fig. 3 shows an example of these trans­mission properties when using an elec­tronic speech circuit.

Summary LM Ericsson's new telephone set DBA 100 will be the basic set in a range that will cover such applications as:

— loudspeaking telephone — executive-secretary system — office telephone systems with a var­

ious number of exchange lines being available to the set.

This range of telephone sets will be introduced successively during 1979.

Digital Line Equipments for 8 Mbit/s and 2 Mbit/s

Juho Arras and Örjan Mattsson

This article presents the digital line equipments included in LM Ericsson's new family of PCM systems in the M5 construction practice. PCM multiplex and signalling conversion equipment has been described earlier\ The two line equip­ments are intended for transmitting 8.448 Mbit/s and 2.048 Mbit/s over pair and quad cables. A unique strapping network has been introduced in the repeater equalizers which makes it possible to use existing paper-insulated cables for transmitting 8.448 Mbit/s. When designing the equipments the latest CCITT and CEPT recommendations have been taken into consideration and also the experience gained from the earlier generation of 2 Mbit/s line equipment. The equipments are characterized by high reliability with generously dimensioned lightning protection, good trans­mission characteristics and a high degree of flexibility in combination with a design that makes installation and maintenance easy. The two systems are closely related as regards their design.

UDC 621.395 343 621.3152: 621.391.31

Fig. 1 Block diagram and interfaces for digital line equipment MUX PCM multiplex equipment tor 30 circuits LTE Line terminating equipment lor a PCM system

In the terminal station Two-way intermediate repeater in a dependent repeater station

D Digital line interface 75 Q S Line Interface RS Repeater section PFS Power feeding section FLS Fault location section for repeaters CS Cable section 1) Interface cable (coax. 75 Q) 2) Line (pair or quad cable) ~ ~ Signal path

System aspects The new family of digital line equip­ments in the M5 construction practice for transmission over pair and quad cables consists of

- ZAD 8 - 2 , with a bit rate of 8.448 Mbit/s corresponding to PCM trans­mission of 120 telephony channels

- ZAD 2 - 3 , with a bit rate of 2.048 Mbit/s corresponding to PCM trans­mission of 30 telephony channels.

There are great similarities between the two systems as regards design and equipment. The article will deal mainly with ZAD 8 - 2 . The description of ZAD 2 — 3 is restricted to the parts that differ from the previous generation2 or are common for ZAD 2 - 3 and ZAD 8 - 2 .

Fig. 1 shows the structure of a digital

line equipment. The basic principles of its function have been described in de­tail earlier2. The connection to the line is via the internationally standardized coaxial D interface. The matching be­tween the interface and the cable takes place in the bay-mounted line termi­nating equipment. The bipolar line signal is regenerated in dependent two-way regenerative repeaters placed in re peater housings along the cable. Two line terminating equipments and the in­termediate line repeaters form a digital line section, which is designed as an independent functional block with its own power and alarm systems. Fault location equipment, which is usually common for several digital line sec­tions, is normally included so that any faulty repeaters can be located. The transmission takes place over symmetri­cal pairs, one pair for each direction of transmission. The two pairs can be in the same cable — single-cable operation — or in separate cables —two-cable operation. Single-cable operation means simpler system design. Two-cable operation has transmission ad­vantages since near-end crosstalk, which is often the predominant source of interference, is eliminated.

8 Mbit/s on existing cables First-order PCM systems were intro­duced in the telecommunication net­work mainly because of the increase in the capacity of existing cables that was thereby obtained. Experience has shown that these systems have provid­ed good technical performance and good economy. In the case of second-

JUHO ARRAS ÖRJAN MATTSSON Transmiss ion Divis ion Telefonakt iebolaget LM Ericsson

Fig. 2

Compar i son of the a t tenua t i on charac te r i s t i cs of pape r - i nsu la ted star quad cab le (1.2 m m , 25 nF /km) and po l y t hene - i nsu la ted cab le based on the assumpt ion that the a t tenua t ion of bo th cab les at 4.224 MHz is 60 d B . The f r equency d e p e n d e n c e of the pape r - i nsu la ted cab le co r re ­sponds to \ 7+0 .6 f and that of the po l y thene -insu la ted cab le to \ T.

^ " " Paper-insulated ^ ™ Polythene-insulated

Fig. 3 a) Overvoltage p ro tec t i on for the ZAD 2 — 3 l ine repeaters Longitudinal overvoltages build up a voltage across R1 and T so that the gas tube G strikes. During this process the repeater equipment and feeding diode D are protected by an effective current division between the power diode T and R2. b) A test pulse used for overvoltage testing. The short-circuit current of the surge voltage generator has been varied over the range 200 A to 1300 A.

115

order PCM systems it has been as­sumed that special cables with polythe­ne insulation will be used. These often contain screened groups in order to keep the near-end crosstalk at an ac­ceptable level with single-cable trans­mission. The 8 Mbit/s system makes much greater demands than the 2 Mbit/s system as regards the separation between the transmission directions. Considerable financial advantages would be attained if existing paper-in­sulated cables could also be used for second-order systems. In many cases two-cable operation can be arranged or a large cable can be utilized so that it is possible to achieve the necessary sepa­ration.

As has been mentioned in a previous article3, at the frequencies of interest when transmitting at 8 Mbit/s, paper-insulated cables have an attenuation curve that deviates from that of poly­thene-insulated cables. There are also considerable differences between dif­ferent types of paper-insulated cables. The article mentioned above gives the theoretical background for the method whereby the 8 Mbit/s line repeater can be adjusted optimally to suit different types of cables, by means of straps in the equalizer. ZAD 8 —2 therefore offers the attractive possibility of working with existing paper-insulated cables and also polythene-insulated ones. The use of only one type of repeater is advan­tageous from the point of view of main­tenance and the stocking of spares.

On the island of Funen in Denmark some fifty two-way repeaters have success­fully been installed and put into opera­tion on paper-insulated cable, conduc­tor diameter 1.2 mm, capacitance 25 nF/km. The repeaters work with two-cable operation, with a repeater spa­cing corresponding to 60 dB attenua­tion at 4.2 MHz. In order to give an idea of the difference in the attenuation characteristics, the attenuation curves of paper and polythene-insulated cables are compared in fig. 2. The difference in attenuation is as much as ±10 dB within the frequency range of the re­peater.

Reliability, overvoltage protection, maintenance

-High reliability is a prerequisite for a

line equipment. This applies particularly for the cascade-coupled line repeaters, which are often located in inaccessible places. In the design stage great efforts have therefore been made to choose the most suitable components, and to dimension the circuits with ample safety margins. In addition the burn-in method is used during manufacture in order to eliminate unreliable components.

In rural areas PCM transmission is of­ten used in cables where the repeaters can be subjected to very severe stress in connection with lightning and short circuit to earth in nearby power lines. Extensive work has been devoted to equipping repeaters and other equip­ment with efficient overvoltage protec­tion. Fig. 3 shows how such protection is arranged in the ZAD 2 — 3 line repeat­ers. The dimensioning, which can cope with pulses far in excess of the CCITT requirements, has been tested in field trials in Norway with both aerial and buried cable.

The repeater protection occupies about 20% of the area of the printed wiring board. The volume of the components concerned must be large, among other reasons because they have to be able to withstand the high powers that arise with induced currents of some tens of amperes and of long duration. The overvoltages are as far as possible leaked away via overvoltage tubes be­fore they reach the printed board. Ex­posed conductors are made wide, with a large distance to adjacent conduc­tors.

Efficient methods for locating repeat­er and cable faults are essential from the point of view of maintenance. A new fault locating system for repeaters has been developed for ZAD 8 — 2. It is based on remotely controlled fault detectors in each repeater housing and permits measurements during traffic. The line terminating equipment includes alarm circuits for supervising transmitted and received signals and the remote power feeding, so that the type of fault can easily be determined. The equipment at the terminal can be provided with a unit that carries out automatic change­over to a standby system if a fault occurs in the working system.

Fig. 5 ALBO network for automatic equalization of 0 - 2 5 dB (4.224 MHz) pair cable. The variable resistance R is controlled by the peak amplitude of the signal after the equalization

Vf+0.6f characteristic

vT characteristic

Fig. 6 Strappable correction network and its frequency

characteristic a) network configuration b) attenuation The network can be strapped to compensate for all cable attenuations of the form A d = \T+a f where (K-a^-0.6 I.e. the cable parameters3 can be selected independent of each other

Digital line repeaters for Z A D 8 - 2 a n d Z A D 2 - 3 The funct ion and block diagram for the repeaters in ZAD 8 - 2 and ZAD 2 - 3 are similar and are shown in f ig. 4. However, in the case of the 8 Mbi t /s repeater the detailed design is more comprehensive and requires more space because of the more compl icated equalization condit ions. Common tasks for the two line repeaters are equaliza­t ion, t iming recovery and pulse regene­ration.

The purpose of the equalization is to compensate for the f requency-depen­dent attenuation introduced by the cable. An opt imum selection of attenua­t ion/ f requency characteristic for the equalizer means that the effect of dis­turbance is minimized. The line repeat­ers in ZAD 8 - 2 and ZAD 2 - 3 were d i ­mensioned with the aid of a computer, and the goodness cri terion was the smallest possible signal/noise ratio at the repeater input, i.e. maximum num­ber of disturbing systems for a given bit error rate. For the purpose of d imen­sioning it was assumed that the line signal code was one of the internat ion­ally standardized transmission codes, HDB-3orAMI , and that the disturbances had crosstalk characteristics. The effect

of thermal noise has been investigated and has been found to have no signif i­cance for the cable attenuations en­countered in ZAD 8 - 2 . The maximum cable attenuation that can be equalized in the repeaters is 65 dB at 4.224 MHz for ZAD 8 - 2 and 35 dB at 1.024 MHz for ZAD 2 - 3 .

The repeaters in the ZAD 8 - 2 system each contain an equalizer consist ing of the fo l lowing parts:

— a fixed part that equalizes the maxi­mum attenuation 65 dB of paper-insulated pair cable with the attenua­t ion/ f requency characteristic Vf~+ 0.6 f.

— a fixed line bui ld ing-out (LBO) net­work which simulates a 15 dB cable (at 4.224 MHz) and which can be con­nected in for short repeater sections.

— a variable equalizer (ALBO) network for 0 - 2 5 dB cable (at 4.224 MHz).

— a strappable network which can be used to adapt the equalization to dif­ferent types of paper and polythene-insulated cable.

The conf igurat ion of the last two of these networks are shown in figs. 5 and 6. This equalizer design means that line equipment ZAD 8 - 2 can be adapted for use with all pair and quad cables en-

Fig. 4 Block diagram of the 8 Mbit/s digital repeater and the associated signal diagram

117

countered in practice that have an at­tenuation of between 25 dB and 65 dB at 4.224 MHz.

The digital line repeater for ZAD 2 - 3 contains an equalizer of the same gene­ral design as that described above, but simpler. In this case there is no need to adapt the equalizer for different types of cables, since the attenuation/fre­quency characteristic of pair cables is affected only slightly by material para­meters at frequencies less than 1 MHz.

The timing recovery is carried out by filtering the line signal in a resonant circuit (Q~80) followed by extraction of the zero transitions of the sinusoidal signal. The circuit design is such that codes with very low pulse density can be transmitted, at the same time that considerable deviations from the nomi­nal bit rate can be tolerated on the lines.

The reconstruction of the received pulse train takes place in two decision circuits, which then control the trans­mission of a new regenerated pulse. The sequence is illustrated in fig. 4.

In both systems the power feeding of the repeaters takes place with constant current over the signal transmitting wires. The feeding voltage for the ZAD

2 - 3 repeaters has been reduced in re­lation to that used for the repeaters of the previous generation. However, at­tention has been paid to the require­ment that the repeaters must be reli­able over a large range of temperatures ( -40"C to + 70°C).

The mechanical construction of the line repeaters is shown in fig. 7. As can be seen, a two-way repeater in system ZAD 2 — 3 is fitted in the same size of cassette as a one-way line repeater in ZAD 8 — 2. The latter repeater makes great de­mands as regards internal crosstalk be­cause of the great differences in levels and the mixing of analog and digital functions on the same printed board assembly. Possible crosstalk paths have been eliminated by means of a carefully designed layout and the use of screening.

Line terminating equipment The main tasks of the line terminating equipment are - to adapt the signal in the send and

receive direction between the digital link interface D and the symmetrical line interface S

- to power feed the dependent repeat­ers via the cable

- to detect and indicate alarm condi­tions.

Fig. 7 Line repeaters in system ZAD 2 - 3 (left) and ZAD 8 - 2 (right). The cassettes, with the dimen­sions 245X100X45 mm including connectors, accommodate one two-way 2 Mbit/s repeater and

- - - L ! » / *~*nAfinnntiunhi

Fig. 8 Block diagram of the line terminating equipment

i n Z A D 8 - 2

Absence of pulses, send direction

Remote power feeding fault

Absence of pulses, receive direction

High error rate, receive direction

Strappable combinatory logic

Fig. 9 Alarm functions in the line terminating equipment

Transmission path Fig. 8 shows the block diagram for the line terminating equipment in system ZAD 8 - 2 . In the send direction the bi­polar signal is converted from unbal­anced to balanced form in the send transformer of the transformer unit. In the receive direction the signal is re­generated in the terminal repeater, which is identical to a one-way de­pendent repeater. If the repeater sec­tion is short, LBO networks with an at­tenuation of 30 dB at 4.224 MHz are connected in.

The description above also applies for ZAD 2 - 3 . In this case no LBO network is required, but a 6 dB flat attenuator can be strapped in the send direction, for example to reduce the cable cross­talk. Leaving out the LBO network has made it possible to incorporate the alarm circuit in the transformer unit.

Remote power feeding The dependent repeaters along the line are power fed in series over the phan­tom circuit from the remote power feed­ing unit in the line terminating equip­ment. The same type of unit is used in both ZAD 8 - 2 and ZAD 2 - 3 . The power feeding takes place at a constant direct current of 48 mA and a voltage of up to ±106 V balanced to earth. The

distance between power feeding sta­tions varies from about 20 to 40 km in the case of 8 Mbit/s and from about 40 to 60 km for 2 Mbit/s, depending on the repeater spacing and the voltage drop in the cable. This distance can almost be doubled by using extended feeding via separate pairs or by series connec­tion of power feeding units. In order to ensure personal safety it is possible, by strapping, to limit the maximum output voltage to approximately 10 V if a break should occur in the powerfeeding loop, for example a cable break.

Alarm, automatic changeover The line terminating equipments in ZAD 8 - 2 and ZAD 2 - 3 have the same alarm functions, fig. 9. Plug-in U-links are used to set up the desired connections between primary and derived alarms.

The alarm state of the system is indica­ted by light-emitting diodes, which can be seen through the front plate of the unit.

The line terminating equipment can be provided with an auxiliary unit which, in the case of an alarm in the receive di­rection, provides automatic changeover to another, predetermined system that serves as a standby, fig. 10. The auto­matic changeover, which is tied to single-cable working, is particularly at­tractive if the standby system goes via another cable or a radio relay link.

Electronic switch

Resonant circuit

Pulse regenerating circuit

Strapping field

Transformer

Line bullding-out network

DC-DC converter

Signal path

Fig. 10 Automatic changeover between two systems that operate as the working and standby system respectively. The changeover is initiated by the absence of a signal in the receive direction or too high an error rate on the received signal

Line terminating equipment, system 1

Two-way dependent repeater

Control logic

Mechanical construction The line terminating equipments are placed in M5 single shelves", fig. 11. A shelf holds four systems in ZAD 2 - 3 and two systems in ZAD 8 - 2 . The same type of shelf is used for both single and two-cable operation and for different power feeding alternatives. In the 8 Mbit/s system, with its stringent de­mands as regards crosstalk between different parts, a one-way dependent re­peater is used as the terminal repeater, which has been made possible by adapt­ing the shelf.

The interface connections are assem­bled at the left end of the shelf. Eight easily accessible coaxial contacts, the D interface, are mounted on the inside and outside of the left side member. The first contact position at the left end of the shelf is reserved for the L interface. This is used for connecting the line ter­minating shelf to the fault location equipment in the bay and for connect­

ing in the system alarm. The interface cables are placed in the left bay side member. The station cable is brought in to the following contact positions in the shelf via connection units.

As regards the bay construction refe­rence should be made to the descrip­tion of the 30 channel PCM terminal equipment1 and the M5 construction practice 4.

The flexible bay design permits mixed equipping of 2 and 8 Mbit/s line equip­ments, PCM multiplex, digital multi­plexors and signalling equipments. When the bay is equipped with only line equipment it holds, apart from fault lo­cation equipment, 64 line terminat­ing equipments type ZAD 2 — 3 or 36 line terminating equipments type ZAD 8 - 2 . The line equipment is fed direct from the station battery - 2 4 to - 6 0 V. The alarm circuits are fed from —12 V.

Fig. 11 Line terminating shelves for ZAD 8 - 2 (top) and ZAD 2 - 3 . The top shelf is equipped with two 8 Mbit/s line terminating equipments, the bottom shelf with four 2 Mbit/s line terminating equip-

Fig. 13 Small repeater housing of the loading coil box type for one 2 Mbit/s system plus fault location and speaker circuit equipment. Alternatively the housing can be equipped with two 2 Mbit/s two-way repeaters

Housing for intermediate repeaters The development of ZAD 8 - 2 and ZAD 2 — 3 has also meant a new genera­tion of repeater housings. This work has taken place in close collaboration between the Nordic Telecommunica­tions Administrations and installation staff from LM Ericsson. The following types of housings are available.

— Two rectangular housings with ca­pacities of 23 and 8 2 Mbit/s two-way repeaters or 23 and 8 8 Mbit/s one­way repeaters in addition to equip­ment for fault location and a speaker circuit. The housings are made of steel and silumin respectively and are identical to those of the previous system generation. The compact external dimensions make these housings particularly suitable for in­stallation in manholes and on poles, but they can also be buried. Fig. 12 shows the smaller of the two hous­ings.

— A cylindrical steel housing with the same fittings and capacity as the small rectangular housing. The hous­ing is identical with the one used in FDM line equipment and is particular­ly suitable for direct burial in the ground.

- A small cylindrical housing made of stainless steel. The housing is in prin­ciple a loading coil box which can be opened, fig. 13. It is intended only for 2 Mbit/s and has a capacity of two systems or alternately one system plus fault location and speaker circuit equipment. The housing is intended mainly for easily accessible places, for example on poles or in manholes, and constitutes a finan­cially attractive solution in, for ex­ample, sparsely populated areas.

As has already been mentioned, the first three types of housings can be used for both ZAD 8 - 2 and ZAD 2 -3 . The accommodation not used can be equipped with loading and phantom coil units. In general the housings have great flexibility as regards equipping Movable dividing walls permit varying unit dimensions and the connection to the stub cable is via plug-in unit con­nection cables, fig. 14. Strapping for the power feeding alternative is done in these cables in orderto simplify installa­tion and change of repeaters. The stub cables are made up of screened cable units in order to obtain the required se­paration between the transmission di­rections for 8 Mbit/s.

The repeater housings, which are pres­sure-tight towards the cable and the en-

Fig. 12 A repeater housing equipped with eight 2 Mbit/s two-way repeaters and equipment for fault loca­tion and speaker circuit. Alternatively the hous­ing can be equipped with eight 8 Mbit/s one-way repeaters. The housing can also be used to accommodate combinations of these repeaters

121

Fig. 14 Unit connection cables are used for connecting the units to the stub cable. This arrangement gives great flexibility and simple conversion between different equipment alternatives

vironment, can be pressurized via the stub cable by means of a pneumatic resistance or an external valve. The lid is sealed with a toroidal ring seal, which has proved to be very efficient.

Location of repeater and cable faults Fault location equipment ZAD 8 — 2 A new fault location system for re­peaters has been devised in connection with the development of ZAD 8 - 2 . It has a number of advantages compared with other systems, such as — well defined fault criterion in the

form of error rate — the fact that measurements can be

carried out during operation, i.e. that preventive maintenance is permitted

— fault location from one supervising station

— identical fault location equipment in each housing.

A characteristic feature of the fault lo­cation system, fig. 15, is that each inter­

mediate repeater station contains a bi­polar error detector, which can be used to measure the error rate at the output of an arbitrary repeater in the housing. The supervising terminal station can indicate the repeater to be tested via a loaded pair, the fault location pair, which is common for all intermediate repeater stations. The indication is car­ried out by sending a pulse train that contains housing and repeater addres­ses. The same fault location pair is used for sending the error detector result back to the supervising station. The communication over the fault loca­tion pair takes place via data modems of the FM type. A transmission speed of 750 error pulses/s has been chosen as giving a suitable compromise between information speed, modem complexity and demands on the transmission me­dium. It corresponds to a maximum transmitted error rate of approximately 10"4. Addressing takes place at the low speed of 100 bauds in order to ensure that the addressing is reliable. The range is limited by the power fed out

Fig. 15 Block diagram of the fault location system with remotely controlled bipolar error detectors

Transformer unit

One-way repeater

Electronic switch (built Into the one-way repeater)

Address generator Error analyzer Address counter

Unit connection cable

Connec­tion strip

2 Mb i t / s two-way repeater

8 Mb i t / s one-way repeater 8 Mb i t / s one-way repeater Filter and service unit Fault detector and service unit Side c i rcu i t loading coi l unit Phantom c i rcu i t loading coi l unit Phantom c i rcu i t load ing coi l unit Measur ing box Th rough-connec ­t ion b lock

Loop connec t ion adapter

122

and the attenuation of the fault location pair. A maximum of 32 housings can be connected, on condition that the atten­uation is less than 40 dB. The power feeding is carried out from the supervis­ing station using parallel feeding. This permits branching of the fault location pair.

The fault location equipment in the re­peater housing consists of the fault de­tector and service unit, which also con­tains the speaker circuit equipment. All housings are equipped with identical units and the address identity is determ­ined by means of straps.

At the terminal the fault location equip­ment is assembled in a fault detector shelf of the M5 single shelf type, fig. 16. The shelf can accommodate the above-mentioned unit for supervising the ter­minal repeaters in the bay. In the super­vising terminal the shelf is also equip­ped with the instrument units, including power equipment, required for the fault location. The received error rate is indi­cated by a light-emitting diode strip.

The error pulses are also available on a counter and a recorder output. The housing and repeater addresses are set up with thumb-wheel switches. Each fault detector shelf can terminate and

monitor six fault location pairs.

The unit is prepared for control from an external computer which makes poss­ible automatic supervision.

Fault location equipment ZAD 2 — 3 The fault location method with filter2

used in the previous system generation has been retained in the 2 Mbit/s equip­ment. A mechanical adaptation to the new system has been carried out. In the intermediate repeater stations the fault location filter has been combined with the speaker circuit unit, and this unit can also be placed in the terminal in a fault location shelf. The fault location shelf, a M5 single shelf, can accom­modate two fault localisation filters, and terminate up to six fault location pairs. The shelf can also accommodate one line terminating equipment, a facil­ity that has been provided to cater for small stations where there is only one PCM system.

The reasons for retaining the filter method were that it is simple and that many administrations have access to the required measuring instruments. It is, however, possible to adapt ZAD 2 - 3 to the fault location system of ZAD 8 - 2 .

Fig. 16 Fault detector shelf and fault detector and ser­vice unit. The instrument units are placed to the left in the shelf

123

Locating of cable faults in Z A D 8 - 2 a n d Z A D 2 - 3 In the case of cable breaks it is possible to locate the fault in the cable with the aid of the power feeding. The power feeding unit is then switched over to voltage feeding with reversed polarity. A current contribution is obtained from each repeater before the break point and the faulty repeater section can be singled out by measuring the sum cur­rent at the terminal. If the cable fault consists of a short circuit between the two pairs in the power feeding loop the fault can be located by measuring the output voltage.

Line test sets LM Ericsson have developed special instruments, line test sets, for 8 Mbit/s and 2 Mbit/s digital line systems, in order to simplify planning, installa­tion and fault tracing. The two instru­ments have the same general structure and each consists of a transmitter, re­ceiver and accumulator with a charging device. Fig. 17 shows the 2 Mbit/s line test set, which is now in pro­duction. The various parts are com­bined into one robust mechanical unit. The transmitter can generate bipolar pulse trains, one of which is crystal controlled. The receiver consists of a modified one-way repeater, supple­

mented by a bipolar error detector and a counter. The modification makes it possible to control the equalizer and the position of the decision thresholds manually. The transmission quality of the cable can be checked with the in­struments by measuring the error rate, cable attenuation and eye opening. The last test provides a measure of the effi­ciency of the equalization and of any reflections. With the 8 Mbit/s instru­ment it is also possible to check the strapping chosen in the equalizer with regard to the type of cable. The instru­ments can also be used for crosstalk measurements or for checking the ef­fect of external disturbances such as signalling disturbances. During these measurements it is often only the end points of the cable that are accessible, i.e. no working signal can be applied to the receiver. Through a unique property of the instruments the result can be obtained in the form of an equivalent error rate. Alternatively the noise power at the decision point can be measured. The interpretation then assumes a knowledge of its amplitude distribut ion.

Summary When developing the digital pair cable systems ZAD 8 - 2 and ZAD 2 - 3 experi­ence from the previous generation of 2 Mbit/s line systems has been utilized

Fig. 17 The instrument "2 Mbit/s line test set" lor in­vestigating whether cables are suitable tor PCM transmission. Practical operating conditions can be simulated and evaluated with the aid of this instrument

124

and the possibi l i t ies offered by con­struct ion practice M5, component de­velopment etc. The two systems have much in common. The use of the same housing admits common equipping and facilitates future conversion. ZAD 8 - 2 has several unique characterist ics, for example the possibil ity of using both polythene and exist ing paper- in­sulated cables, and a fault location

system that permits measurements dur­ing operat ion. Compared with the pre­vious generat ion of 2 Mbi t /s line sys­tems ZAD 2 - 3 has lower power con­sumpt ion per repeater, i.e. larger power feeding distance, better l ightning pro­tect ion and a more advanced alarm sys­tem. Furthermore the volume of the ter­minal equipment is only half that of the previous generat ion.

Technical data Electrical data

Line signal Bit rate/symbol rate Code Impedance Pulse amplitude

Intermediate repeater Equalization range

Power consumption per one­way repeater, max. Temperature range

Power supply Primary current source

Feeding of intermediate repeaters Nominal regulated current Output voltage, max

Z A D 2 - 3

D1 interface 2.048 Mb/s

S1 interface 2.048 Mbaud

Bipolar HDB-3 or AMI 75Q unbal

±2.37 V 120L> bal.

+ 3.0 V

5 -35 dBat 1 MHz

8 2 V/48 mA -40°C to + 70"C

ZAD 8 - 2

D2 interface 8.448 Mb/s

S2 interface 8.448 Mbaud

Bipolar HDB-3 or AMI 75Q unbal.

±2.37 V 150Q bal

+ 3.3 V

40-65 dB strappable 25-50 dBat 4.2 MHz

15.5 V/48 mA - 4 0 ' C t o + 70"C

Battery 24. 36, 48, 60 V Rectifier tor 110, 127, 220 V (45-65 Hz)

Series feeding via the phantom circuit 48 mA DC ±106 V bal.

Mechanical data

References 1. Lindquist, S. and Widl, W.: 30-

Channel PCM Terminal Equip­ment in the M5 Construction Prac­tice. Ericsson Rev. 53 (1976):1, pp. 38-49.

2. Arras, J. and Tarle, H.: PCM Line Equipment ZAD 2. Ericsson Rev. 49(1972):2, pp. 47-55.

3. Fredricsson, S.: Transmission Pro­perties of Paper-Insulated Twin Cables at High Frequencies. Erics­son Rev. 54 (1977):1, pp. 2 8 - 3 1 .

4. Axelson, K., Harris, P.-O. and Store-sund, E.: M5 Construction Practice for Transmission Equipment. Ericsson Rev. 52 (1975):3/4, pp. 94-105.

Terminal repeater station Shelf dimensions Capacity per line terminating shelf

Bay height Capacity per bay

Intermediate repeater station Dimensions h x w x l or hx® Weight Number of two-way 2 Mbit/s repeaters or one-way 8 Mbit/s repeaters (incl. fault loca­tion and speaker circuit equipment)

ZDD 532 310*280x430 mm

40 kg

8

122x225-473 mm

ZAD 2 - 3 : 4 systems ZAD 8 - 2 : 2 systems

Max. 2743 mm ZAD 2 - 3 : 64 systems ZAD 8 - 2 : 36 systems

ZDD 533 700x510 mm

110 kg

8

ZDD 534 320x480x610 mm

80 kg

23

ZDD 535 280x195 mm

10 kg

1 (2 Mbit/s)

Operation and Maintenance Characteristics of AKE 13

Lars G. Ericsson and Åke Persson

This article is devoted primarily to a description of the operation and maintenance characteristics of AKE 13. The article also gives some examples of operational experience, but this will be described in more detail in a later issue of Ericsson Review. Certain of the facilities offered by AKE 13 as regards international and intercontinental traffic are also touched upon.

UDC621 395 343 AKE 13 is an SPC system intended for medium-sized to very large transit ex­changes for national as well as interna­tional and intercontinental traffic. The first version of AKE 13, AKE 131 with control system APZ 130, was taken into service in Rotterdam in 1971 and was then the first SPC transit ex­change in the world and also the first

multi-processor exchange1. However, Rotterdam was not LM Ericsson's first SPC exchange. AKE 13 was based on experience gained from the combined local and transit exchange system AKE 12, which was put into operation in Tumba, outside Stockholm, as early as 19682.

The latest version of the system, which is designated AKE 132 and which con­tains the new control system APZ 150, has been described previously3,5.

As can be seen from table 1, 18 AKE 13 exchanges have been put into operation in eight countries in three continents

Country

Austral ia

Czechoslovakia

Denmark

Fin land

Italy

Mex ico

Hol land

Norway

Sweden

England

Total:

Exchange

Sydney (1974) - Broadway Sydney - Paddington

Prag

Å lborg (1974) Arhus (1977) Alber ts lund Copenhagen (1974) Hi l lerod Odense (1976) Slagelse V i rum

Helsinki , PLH (1974) Hels inki , PLH Helsinki , HT(1976) Turku (1974)

Bari (ASST)

Napol i (SIP) Palermo (SIP) (1975) Padova(SIP) Verona (SIP) Salerno (SIP)

Guadalaiara (1975) Mexico D F (1973)

Monterrey (1975)

Dordrecht (1976) Rotterdam DC (1971) Rotterdam INT (1976)

Bergen Drammen Oslo (1976) Skien Stavanger

Go thenburg , Vrr Stockho lm, FRE (1974) S tockho lm, HY(1976)

London-Thames

Natio­nal

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

x X

X

X

x

x X

X

X

X

X

X

X

Inter­nat ional

* X

X

X

X

X

X

•

X

X

X

X

X

Mul t ip le-capaci ty

in operat ion

2 400

9600 12 000

20 400

9 600

15 000

4 000 8 000

3 600

8 400 18 600

6 000

3 600 9600 3 600

9600

2 400 12 000

158 400

Oil order

3 600 7 200

18 000

9600

6 000

7 200 6 000

29 400 8 000 3 200

6 000 6 600 3 600 3 600 4 800 4 800

7 200

1 200

3 600

3 600 2 400

3 600 3600

4 800

16800

174 400 332 800

Table 1 AKE 13 exchanges in operation or on order on o • k ~ 1cl 1077

LARS G ERICSSON AKE PERSSON

Telephone Exchange Division Telefonaktiebolaget LM Ericsson

and a further 18 exchanges are on or­der. Thus the further development of the system that is continuously being car­ried out is founded on long and exten­sive experience.

System characteristics The telecommunications administra­tions' need of transit exchanges with high capacity increases with the expan­sion of the long-distance traffic. Previ­ously it has not always been possible to satisfy the need for such large ex­changes, and hitherto it has been a fairly common practice to combine a number of exchange units, each with a relatively low maximum capacity, to form what is, from the point of view of the network, one common switching point.

A way of solving these problems that is often more economical for the tele­communications administrations and also technically more suitable, not least from the point of view of operation and maintenance, is to install a single ex­change having the required capacity.

AKE 13, with the following inherent properties, is able to satisfy all reasona­ble demands as regards lines and traf­fic:

— maximum number of incoming lines, 30000

— maximum number of outgoing lines, 30 000

— maximum number of switched calls per hour for the control system APZ 150, 750 000

— maximum traffic capacity of the switching network with an internal congestion of 0.2 %, 25 000 erlangs

The switching capacity is among the largest in the world. The system also meets very stringent demands as re­gards reliability and operating characteristics.

Synchronously duplicated multi-processor system The AKE 13 control system is built up of a suitable number of synchronously du­plicated data processing blocks (DPB), with a maximum of eight blocks. The

Fig 1 Block diagram of the hardware. An AKE 13 ex­change can be equipped with up to 8 data proces­sing blocks (DPB)

Each DPB is synchronously duplicated, i.e. consists of two sides, each with a complete processor with stores and a transfer unit. Both sides normally work In synchro­nism, which Is continuously monitored. However, only one side Is executive. When there Is a discrepancy between the sides the faulty unit Is disconnected. The duplication can be utilized, for example, when changing programs

Table 2 Extended AKE 13 exchanges Exchange

Helsinki Copenhagen Mexico City Mexico City Rotterdam DC Rotterdam DC Rotterdam INT Turku Turku Ålborg Ålborg

Time

Jan -77 Nov -75 Sept -75 Sept -76 Mar -73 July-74 May -76 Feb -75 April -77 Aug -74 Feb -77

Size of the extension

4 processors 9 600 multiple pos. 2 „ 7 200 2 „ 7 200 1 „ 3 600

1 200 1 ,, 3 600 1 „ 1 200

800 1 600 3 000 1 200

Size of the exchange after the extension

6 processors 15 000 multiple pos. 6 ,, 20 400 4 „ 15 000 5 .. 18 600 2 „ 6 000 3 „ 9 600 2 ,, 3 600 2 „ 6 400 2 „ 8 000 4 „ 8 400 4 „ 9 600

data processing capacity of the system can thus be extended in step with the increase in the number of lines, fig. 1. This means that the central, most com­plex and, for the operation most im­portant part of the system need never be larger than what is required for handling the traffic on the lines connected at the time.

The factor that is most important for the reliability is the synchronous duplica­tion of the control system. It provides

— the fastest possible fault detection through continuous comparison of the function of the duplicated units

— the possibility of easily separating software faults and hardware faults

— simple and reliable fault localization — the minimum loss of traffic handling

capacity when a fault occurs — the possibility of introducing new

functions without disturbing the traffic

Thanks to the duplication it is possible to reload the whole system without dis­turbing the traffic. The two sides in the system, A and B, which normally work in synchronism, are separated by means of a command, fig. 2. Side A is then loaded with exchange programs and data from a tape unit. Side B continues the traffic handling without interrup­tion. When the loading is completed, the newly loaded side A takes over the traf­

fic handling and side B is put in the standby state. If any operational dis­turbance should then occur because of faults in the newly loaded software, there will be an automatic changeover to side B and the traffic handling will again be carried out by the original software. When it has been proved that the newly loaded side A works satisfactorily, updating of side B is ordered and also a return to synchron­ously duplicated operation. The updat­ing is carried out by copying the con­tents of the A-side stores.

This facility, which through separation of the system sides enables programsto be loaded and verified without interrup­tion of the traffic handling, can also be used when making major changes in the software.

The possibilities offered by the syn­chronous duplication and multi-pro­cessor arrangement have been utilized in the methods for installation and test­ing of extensions that have been used successfully in AKE exchanges already in operation, table 2.

The system can be extended by the addition of new data processing blocks without it being necessary to change the contents of the program stores already in service. If the extension does not include any new functions it is suffi­cient to make certain minor adjust-

Fig.2

128

Fig. 3 Standardization of program packages

merits in the data stores of the data pro­cessing blocks in service, in order to show the changed configuration of the control system.

When extending an exchange, the addi­tional data processing block and switching equipment are tested by means of special installation test pro­grams, which are run in the new proces­sor before the new units are connected to the data processing blocks that are already in service.

Division into function blocks Several of the basic characteristics of AKE 13 have been obtained by dividing the system into function blocks. The blocks contain program sequences with functionally associated data and also hardware units. The interfaces between the various blocks are clearly defined and the interworking between blocks is carried out with the aid of special sig­nals.

Experience has shown that this struc­ture is essential for the design, mainte­nance and production of such a large and complex SPC system as AKE 13. To develop clear, lucid and well adapted documentation for a large program system is generally considered as one of the most difficult and resource-de­manding tasks in the production of software. However, thanks to the divi­sion into blocks it has been possible to apply the same product handling and documentation rules for the AKE 13

software as have long been used suc­cessfully for LM Ericsson's earlier ex­change systems. This has been advantageous both for the operation and the maintenance of the AKE 13 systems.

Standardized software When designing software a natural aim is to create software that can be used for as many exchanges as possible. Standardization means simplified doc­ument handling and improved program quality because of the wider field of application. This has a favourable effect on both exchange and product maintenance.

Thanks to the division into function blocks it has been possible to build up an extensive program library over the years. As regards the switching system this library comprises approximately 200 blocks with standard functions and a further 200 blocks containing mar­ket-dependent functions. Altogether this means a total program volume of approximately 700 000 words for the switching system. The standard blocks cover all signalling systems that have been specified by CCITT, including No. 6, and a comprehensive operation and maintenance system for the exchange switching system. The operating system in APZ 150 is general for all exchanges, and it has therefore been possible to create a standardized system file forthis system that applies for all exchanges, both as regards the revision status of the blocks and their placing (allocation) in the program store. The operating sys­tem is handled as a superior product and its status is marked with a revision state indication for the whole operating system. The revision state is changed when functions are added to the operat­ing system. The operating system in service can be exchanged for a more modern one without the environment, i.e. the software in the switching sys­tem, being affected.

It is also possible to standardize the switching system software in the same way. This has been done for several administrations. The aim has been to create, for each administration, as large a share of standard allocated program volume as possible. The blocks that are uniaue for a particular exchanqe are

129

Software f o r / h e swi tching system (APT) Standard products 36 %

Software for the swi tch ing system New product ion 4 %

Fig. 4 Distribution of the software volume between standard and new production

then added, fig. 3. Thanks to the block structure of the software it has thus been possible to create standard alloca­tions, despite functional differences be­tween different exchanges, a fact which has been a considerable help in reduc­ing an administration's costs.

The far-reaching standardization has meant that nowadays when LM Erics­son deliver an exchange in a new mar­ket it is only necessary to design on av­erage 4 % of the total amount of soft­ware to be included in the exchange. The remainder can be collected as ver­ified standard products from a library, fig. 4, without any changes being necessary.

AKE 13 in the international network When AKE 13 was designed, one of the prerequisites was that the system should be suitable for the handling of international and intercontinental traf­fic with particularly complex demands. The large traffic handling capacity, adaptability to different signalling sys­

tems and traffic routing requirements and the comprehensive maintenance functions are examples of characteris­tics that are important for the interna­tional traffic and which it has been pos­sible to realize through the SPC techni­que. Of the 36 AKE 13 exchanges which have hitherto been put into operation or ordered, no less than 14 are interna­tional exchanges (table 1).

System AKE 13 is able to provide all traf­fic facilities that have been agreed in­ternationally, and the system is suitable for all levels in the international hierarchy. Today practically all the in­ternational signalling systems are in operation in AKE 13 exchanges, namely R1, R2, CCITT 4, CCITT 5 and CCITT 6. Facilities for connecting echo suppres­sors, individual ones or from a common group, and for automatically connect­ing in and disconnecting attenuators are built into the system. All current forms of international accounting are catered for. Charging can be carried out by means of repeated metering pulses or toll ticketing. For example, pulse charging can be used for the national traffic and toll ticketing for the interna­tional traffic.

Fig. 5

130

International maintenance centre, IMC In an international exchange it is natur­ally particularly important that the maintenance of the exchange and lines can be carried out efficiently and that suitable aids are available. CCITT re­commend that the supervision meas­urement and testing of international lines and the associated telephone ex­change equipment should be carried out at an international maintenance centre, IMC, in the international ex­change. IMC comprises the following parts:

ISMC Maintenance centre for the inter­national exchange

ITMC Maintenance centre for the inter­national lines

ISCC Administrative centre for coordi­nation of the maintenance of ex­change and lines. ISCC does not require any special equipment and will therefore not be discus­sed here.

ISMC ISMC has access to functions for

— supervising the operation

— testing devices and localizing faults — carrying out traffic recording

In AKE 13 these functions are built into the system and are reached via typewriters. The ISMC activities are therefore usually carried out in the con­trol room of the AKE exchange, fig. 6. The AKE system provides ISMC with some fifty operation and maintenance functions for the switching equipment Some of these are:

— supervision of fuses and control circuits

— supervision of traffic disturbances, congestion and blocking. An alarm and printout are obtained when a cer­tain threshold value is exceeded. The system also contains functions for a more detailed study of each dis­turbance

— automatic and semi-automatic supervision of the quality of connec­tions and calls through observation of randomly selected connections

— automatic checking that each line has at least one call every 24 hours

— automatic signalling check on out­going lines

Fig. 6 The control room in an AKE 132 exchange with maintenance panel, magnetic tape units and other I/O devices

131

- tracing of the connection path through the exchange for a certain connection

- recording of changes in the state of devices in a certain connection

- circuit tester for code receivers and code senders. The testing is initiated by a command or automatically from the disturbance supervision

— periodic testing of the speech paths through the switching network

— blocking of lines, devices and links by means of commands

— traffic recording and the collection of statistics on line routes, device groups and link routes. There is a wide range of measurement types available in addition to those recom­mended by CCITT.

ITMC Extensive equipment is placed at the disposal of ITMC for the maintenance of international lines:

Measuring equipment for making fully automatic transmission measurements in accordance with CCITT recommen­dations. The measuring equipment is of two types. One type is designated ATME (Automatic Transmission Measuring Equipment) and carries out level and noise measurements on international

lines in accordance with CCITT recom­mendation No. 2. The other type con­sists of automatic test equipments; CCITT 12 which uses CCITT measuring methods Nos. 1 and 2 for routine checks of the transmission quality on interna­tional lines with signalling in ac­cordance with CCITT system No. 4, and STC (Simplified Transmission Check) for routine checks on international lines with signalling in accordance with signalling system R2. The measure­ments are controlled by a previously stored program that indicates when the measurements are to be made and on which lines.

A test desk with instrument for making manual or semi-automatic transmission measurements, fig. 7. As can be seen from fig. 8, these desks can be con­nected both to the station side of the junction line relay sets, via the selector network, and to the line side, via U-link racks (jack racks that give access out towards the line or in towards the sta­tion).

U-link racks for connecting lines to the test desk. (Certain telecommunications administrations, however, consider that the U-link racks can be omitted, since a junction line relay set in an SPC ex-

WilH UiUdl/r IP ,1

Fig. 7 Test desk for ITMC

The desk contains, among other things, — a level meter — a variable oscillator — a frequency meter — a psophometer The desk can be connected to the lines either automati­cally via the switching stages In response to a command from the typewriter, or manually, via the U-link racks In

Table 3 Number of component faults per rack and operat­ing year in the Rotterdam DC

Type of equ ipment

Central data process ing equ ipment Test and cont ro l equ ipment I/O equ ipment Code swi tches Other sw i tch ing devices

1972 No. of racks

30 17

2 64 90

No. of faults per rack

0 4 1 82 2.0 0.047 0.28

1975 No. of racks

47 32

4 128 158

No. of faults per rack

0.28 1.53 0.25 0 0 5 6 0082

change is small compared with one in a convent ional exchange because the logic funct ions are carried out by the software.)

Route supervision panel wi th lamps for indicat ing route congest ion and various types of b locking.

A typewriter that provides access to all supervisory funct ions in the system.

The design of ITMC can vary depending on the size of the exchange and the number of international lines. The equipment is normally placed in a sepa­rate room. It is then often most conve­nient to put the U-link racks in the same room. For small international ex­changes, the ITMC equipment can be placed in the maintenance desk in the control room.

Operational experience The SPC technique has made it possible to rationalize the operat ion and mainte­nance work to a very great extent. The administrat ions have pointed out many advantages, for example:

- that changes of traffic routing data, route information and also charging and account ing data require consid­erably less resources and can be car­ried out in very much shorter t ime in AKE 13 than in conventional systems. A comparison is made on the last page of this article

— that it is easier to introduce funct ion­al changes since the majority of the

funct ions are realized in software — that the efficient supervision and

fault localization funct ions have meant that in AKE 13, more often than in previous systems, faults are de­tected and cleared before the sub­scribers complain

— that the connect ion and testing of lines is greatly faci l i tated by the maintenance panel and typewriter provided.

Extension of exchanges in operation It has already been mentioned that sev­eral AKE 13 exchanges in operation have been extended, table 2. The great f lexibi l i ty of the system offers an almost unl imited variety of extension config­urations, but experience from the ex­tensions that have been carried out shows that it is possible to l imit and standardize the number of extension conf igurat ions and stil l meet all re­quirements that the administrations may have. This standardization has the fo l lowing advantages:

— it admits the development of stan­dardized methods and aids

— it gives short installation and testing times

— extensions can be carried out with­out specialist aid

— it reduces the amount of resources required in all handl ing stages

— it gives high reliability

Component reliability In a previous article on the first AKE 13 exchange, Rotterdam DC1, the number

Fig. 8 Connection of the ITMC test desk to the switching network

133

Fig. 9 Sys tem res ta r t f r equency in the AKE 13 exchange

in the Rotterdam DC with 3 DPBs and 9 600

multiple pos i t i ons

1. The switching stages and data store were extended at the beginning of 1973

2 The exchange was extended by the inclusion ot a new data processing block and new switching stages at the beginning ot 1974

3. An inverter fault occurred in June 1975 — Mean value tor 4 months

of component faults per rack was given for the various parts of the system dur­ing the first year of operation, 1972. These figures are given in table 3, sup­plemented by the corresponding fig­ures for 1975. It should be noted that the exchange has been extended in the meantime. The table verifies the high component reliability, which has proved to be very stable and has even improved during the time the exchange has been in operation.

System restart The system restart is such a basic func­tion in the fault clearing operations that it deserves a special description.

A software or handling fault can man­ifest itself in several ways, for example through

— the normal program handling ceas­ing wholly or partly

— an unauthorized attempt to write data in a specially protected area

— a jump in the software being addres­sed to an unequipped part of the store.

Number of au tomat ic res tar ts per mon th

Restart takes place when any of the above fault situations occur.

A hardware fault in a processor side is detected by the system maintenance unit and can usually be located and iso­lated without affecting the operation. In the exceptional cases when this is un­successful, for example if a double fault occurs, restart also takes place. Such a restart means that temporary data are cleared, whereby any faulty data are re­moved. The system is then set to an in­itial state where it can start handling traffic again.

In the first place a normal system restart takes place, in which calls in the register state (i.e. connections in the process of being set up) are disconnected. This takes approximately 30 seconds and during this time no new calls can be ac­cepted. Calls that have already been established are not affected.

If repeated normal restarts do not have the desired effect, or if the number of restarts during a short period of time exceeds a preset value, a major system restart takes place. This means that established calls are also disconnected. Such restarts are relatively few and are carried out in approximately 15 sec­onds.

The exchange staff may sometimes want to "clean up" the exchange and they then carry out a manual restart, normal or major. A manual system re­start is also carried out in connection with function changes and when a data processing block is put into operation or taken out. These manual restarts are usually carried out during periods of low traffic.

Subscribers who happen to call the ex­change during a system restart experi-

ence this as congestion or a "silent connection". They then make a new and normally successful call attempt, and thus the traffic handling disturbances must be considered as negligible.

Fig. 9 shows the system restart rate for the very first AKE 13 exchange, Rot­terdam DC, from when it was first put into operation until the end of 1976. The great reduction in the number of re­starts is a result of the successive im­provement and stabilization of the system and also the clearance of faults in the exchange itself. A number of faults were revealed during the period immediately after the system had been put into operation, when new programs were taken into use and more lines con­nected in. Hence the number of system restarts per month increased until March 1972, and then started to de­crease.

When a system restart is carried out a printout is obtained, stating the type of fault. Atthesametime data are recorded on magnetic tape that define the system statewhen thefaultoccurred. Thisgives valuable information for further fault tracing.

In conclusion it can be said that the system restart function is an excellent aid for maintaining the operational re­liability of the system, since it limits the effects of a fault and also provides the basic data for clearing program and handling faults. Furthermore the possi­bility of initating a system restart man­ually means that the operating staff have been given a tool that enables them to clear up a complex fault situa­tion.

Successive improvements The AKE 13 system has been delivered to some of the most technically advanced administrations in the world. In meeting their different requirements AKE 13 was successively sup­plemented, so that it now constitutes a system that meets the most divergent demands, especially as regards opera­tion and maintenance. The administra­tions' experience of AKE 13 has also provided views that have resulted in new facilities and improved handling methods.

As new techniques become available the possibility arises of adding new sys­tem components, which increase the capacity of the system, enlarge its field of use, improve its operational reliability and simplify its handling. Processor APZ 150 is one example of this de­velopment. This modern processor was used for the first time in Stockholm (Hammarby exchange) in June 1976. Three more exchanges with APZ 150 were taken into service in 1976, in Odense (Denmark), Oslo (Norway) and Helsinki (Finland). By using integrated circuit engineering and semiconductor memories it has been possible, for example, to reduce the space require­ments and increase the traffic handling capacity. A list of the most important improvements that APZ 150 has brought the AKE 13 system is given on the oppo­site page.

Another example of the continuous de­velopment of the AKE system is the in­troduction of regional computers. Thus the LM Ericsson type APN 163 minicomputers are used in AKE 13 for several applications, for example forthe signalling terminals in the latest AKE generation of the CCITT signalling sys­tem No. 6, and for the display-based operator system ANE 403. Minicomput­er APN 163, which is also used in several applications outside the telecommuni­cations field, has been designed with special regard paid to the stringent de­mands on reliability that a telephone exchange makes. The list of instruc­tions for APN 163 has also been de­signed to meet the requirements of SPC exchanges. Since the minicomputer has been designed by LM Ericsson it has been possible to integrate its opera­tion and maintenance with that of the remainder of the exchange.

The use of regional computers provides valuable flexibility at the interface be­tween the central functions in the con­trol part of the AKE system and the func­tions of the peripheral units, which are often affected by external conditions. In certain cases it has also been possible to relieve the central control system of routine but capacity demanding func­tions, which would otherwise have re­duced the overall traffic handling r a n a r i t u n f t h o QvQtpm

134

Comparison of the amount of work required for installing a new route in a conventional exchange and in an AKE 13 exchange

Assume that a new outgoing line route of 30 lines is to be connected in. The exchange has 3 000 incoming and 3 000 outgoing lines.

In a conventional transit exchange the work includes the following work opera­tions: I.New number analysis wirings, route

type markings and wires for idle mark­ing must be included in the route mark­ing devices.

2. In the line selection devices the line test wires must be strapped to connection relays.

3. Straps must be made in the exchange intermediate distribution frames (IDF) for connecting the idle marking, test, control and statistics wires of the new junction line relay sets.

4. The speech and control wires of the junction line relay sets must be con­nected in the IDF to the correct position in a selector stage multiple (in AKE 13 this is normally done by means of fixed exchange wiring).

The work is carried out in different parts of the exchange rack room and is con­cluded by deblocking the lines with the aid of the deblocking button on each junction relay set in the relay set rack in question. The exchange is assumed to comprise 22 central route marking devices and 25 line selecting devices. A total of approximately 740 straps or IDF wires must be connected. Assuming that the work is carried out by experienced staff, a total of approximately 31 work hours is required (8 hours of which are required for connecting the lines to the switching stages).

In an AKE 13 exchange all operations are carried out from the control room with the aid of commands, which are either typed on a typewriter or punched and read in via a punched tape reader

1. A command that creates the new route and gives it the correct characteristics

5 -10 minutes

2. One command per line to include it in the new route

approximately 30 minutes

3. Commands for dimensioning supervi­sion of congestion, disturbance and blocking for the route 10-15 minutes

4. Changing the digit analysis tables approximately 30 minutes

Consists of the following operations: - punching the analysis command - mounting the magnetic tape for the

change in the analysis table and writ­ing the command

- read-in of the control commands - read-in of the analysis commands - read-in of the loading tape

5. One deblocking command per line approximately 30 minutes

A total of 11/2-2 hours is required, in­cluding the punching of the command tapes.

APZ 150 compared with APZ 130 - The capacity approx. 3 times larger - The amount of space required for

the control part is reduced to 1/3 for the same capacity

- The power required for the control part is reduced to 1/3 for the same traffic handling capacity

- The use of integrated circuits in­stead of discrete components in­creases the reliability

- More efficient fault localization system

- Automatic restart with reloading and safeguarding of certain data when the system restart function cannot restore the exchange to traffic handling

- Improved aids in the system for functional changes and changes in size

- Improved facilities for tracing program faults

- Advanced system for safe introduc­tion of program changes during operation

References 1. Hamstad.O. and Norén, L.-O.AKE

131 Rotterdam Exchange and Ex­perience from First Year of Opera­tion. Ericsson Rev. 50 (1973):2, pp. 58-64.

2. Sundblad, A.: Operating Experi­ence from AKE 120, Tumba. Erics­son Rev. 47 (1970):2, pp. 42-49.

3. Meurling, J., Norén, L.-O. and Svedberg, B.: Transit Exchange System AKE 132. Ericsson Rev. 50 (1973):2. pp. 34-57.

4. Norén, L.-O. and Sundström, S.: Software System for AKE 13. Erics­son Rev. 5? (1974):2, pp. 34-47.

5. Nilsson, R. and Norén, L.-O.: In-Plant System Testing. Ericsson Rev. 53 (1976):1, pp. 19-27.

6. Norén, L.-O. and Sundström, S.: Development, Production and Maintenance of Software for AKE 13. Ericsson Rev. 53 (1976):3, pp. 152-160.

135

ABJ 101-the Modern Public Magneto Switchboard

Lennart Aldestam

The demand for manual magneto switchboards has increased considerably during recent years. There are large areas where conditions are such that magneto switch­boards offer a satisfactory and economical solution to the telecommunication problems as also long lines of not very high quality can be connected. In order to meet this growing demand LM Ericsson, in close cooperation with many of their customers, have developed a modern, single-position magneto switchboard designated ABJ 101. ABJ 101 can operate either as an independent exchange or in combination with an automatic exchange (PAX), AKD 860.

UDC 621.395 343 ABJ 101 - a compact and flexible switchboard ABJ 101 is a compact and modern magneto switchboard with all equip­ment built into the chassis. The switchboard has a modular structure, fig. 1, and the units are connected via plugs and jacks, which means simple and fast installation.

This construction gives a low initial cost and permits extension in stages. No special tools are required for assembl­ing and installing the switchboard.

Owing to the fact that the switchboard is built up of such components as minia­ture relays and cord winders it has very small dimensions and thus requires only a third of the volume of space re­quired for traditional floor switchboard.

ABJ 101 can be placed on a desk, coun­ter etc. and merges well with different surroundings. The small dimensions mean that it is usually very easy to find a suitable place for the switchboard.

Can be extended from 10 to 240 lines The switchboard can be extended in un­its of ten magneto lines, four CB junc­tion lines and one cord pair.

The final capacity is 18 cord pairs and either 240 magneto lines or 220 magneto lines and 8 CB junction lines, fig. 2. Thus the system covers a very wide capacity range. The switchboard is already wired for full capacity when de­livered. All apparatus is inserted from the front.

Administrations with several switch­boards may find it advantageous to stock extension equipment themselves. The cost of this will be small.

High operator efficiency A summary of the most important characteristics of ABJ 101 is given at the end of the article. Many of these characteristics are such that they en­able the operator to handle a larger amount of traffic and provide better service for the subscribers.

Fig. 1 ABJ 101 equ ipped for 40 magne to l ines, 4 CB junction l ines and 10 cord pairs

1. Base unit 2. Line shelf 3. Line unit for magneto lines 4. Line unit for CB junction lines 5. Supervision unit 6. Cord pair unit 7. Position unit

137

LENNARTALDESTAM Telephone Exchange Division Telefonaktiebolaget LM Ericsson

Some examples of such aids are au­tomatically generated ringing (trans­istorized), ring-back tone to the calling subscriber and supervision of a junction line when other calls are being handled. The switchboard can also be equipped with a push-button set for decadic im­pulsing when required. Furthermore control devices have been positioned and the colour scheme selected with the aim of making the work of the operator as easy and pleasant as possible.

Independent exchange ABJ 101 can be included in a network as an independent exchange and then in-terwork with all types of manual and au­tomatic public systems.

Mechanical design From the points of view of construction and function the switchboard consists of two main parts, namely the basic unit and line boxes. The basic unit contains a central wiring unit to which all wiring between different equipment has been concentrated. Position and cord pair equipment is mounted in the basic unit. The line boxes contain magneto line and CB junction line units. The first line box also contains a supervision unit.

Incoming lines are connected to the

equipment in the line boxeseitherdirect or via a wall-mounted connection box. (Magneto lines are connected via 10-pair cables and CB junction lines via 4-pair cables.)

The sides of the switchboard are made of teak in natural colour with a plastic finish. The plates for mounting the line units and the top and rear plates of the exchange are finished in green. The cord pair and position units are framed by anoidized light metal sections. Vac­ant positions are covered by green plas­tic strips.

All components meet the requirements for good insulation, shape permanence etc. even during extreme climatic condi­tions.

Cord pair equipment The components for a cord pair have been made up into a cord pair unit, fig. 4.

The unit, the front of which is covered with green plastic, contains two cord winders with 3-pole cords and plugs. The plugs have covers and protective spirals made of grey plastic, which to­gether with the design of the cord wind­er considerably reduce the mechanical stress on the cord.

Fig. 2 ABJ 101 with two line boxes equipped for 130 magneto lines and 14 cord pairs (maximum 160 magneto lines with two line boxes). The capacity is extended to the final number of 240 magneto

Fig. 3 Line unit for 10 magneto lines

LM Ericsson have long operational ex­perience of cord winders obtained from the portable switchboards ABM 10, which have withstood severe trials un­der field conditions. The cord winder is a corner stone in the ABJ construction and has greatly contributed to the re­duction of the switchboard volume.

The cord pair unit also includes a 3-position switch and a clearing drop in­dicator. The switch, which has small dimensions, has a frame and lever arm made of grey plastic.

Line equipment The line units are available in two vari­ants, one for magneto lines, fig. 3, and one for CB junction lines. Among other things the line units contain a drop indi­cator jack strip. The CB junction line units also contain a printed board as­sembly with line components.

Position equipment The dial and other control devices that

are common for the whole of the ex­change have been assembled in a posi­tion unit. The front of the unit is covered with a plate of green plastic.

Supervision equipment A flag indicator, battery indicator and jacks for checking cords etc. are as­sembled in a supervision unit.

ABJ 101 in combination with automatic exchange AKD 860 For certain telecommunication re­quirements in rural areas LM Ericsson can offer a new, economical system consisting of the magneto switchboard ABJ 101 in combination with an automa­tic exchange (PAX) AKD 860, fig. 5.

This system is intended for very small communities and can be used while waiting for the demand to increase suf­ficiently to justify a changeover to a fully automatic rural exchange system. The combination of these two standard

Fig. 4 Cord pair unit

AP Answering cord RP Calling cord SK-RK 3-posltlon switch with speech position SK and

ringing position RK SL Clearing drop indicator

139

Fig. 5 ABJ 101 in combination with automatic exchange AOK 860 gives dialled calls in a the densely popu­lated area and manually extended calls in the surrounding rural area. Both subscriber cate­gories have the possibility of national and inter­national calls via the operator in the manual switchboard

products gives subscribers in densely populated areas many of the advantages of automatic calls and at the same time offers remote subscribers in the surrounding areas telephone service via the modern manual switchboard.

All subscribers have the possibility of national and international calls via the operator at the manual switchboard. A subscriber in AKD 860 calls the operator by dialling a single-digit number. Calls within AKD 860 are set up automatically and within ABJ 101 manually. Calls be­tween subscribers in ABJ 101 and AKD 860 are set up by the operator.

Exchange AKD 860 is built up of plug-in units, which means simple and fast in­stallation and low maintenance costs. This construction permits extension in stages and facilitiates any future move of the equipment to another location. The small dimensions enable the system to be installed in suitable exist­ing premises, and thus no special build­ing is required.

The operator at the manual switchboard handles the charging for trunk calls and in certain cases this is done in coopera­tion with the trunk operator in the superior exchange.

Accessories for ABJ 101 Power equipment For power supply four dry-cell batteries BKA 1002 for 1.5 V and 50 Ah are re­commended. These are placed in bat­tery box BKY 1012.

Main distribution frame Main distribution frame NBA is recom­mended for switchboards with more than 40 lines. A main distribution frame, but without line fuses, can also be obtained through using double the number of connection boxes.

Telephone sets Ordinary magneto telephone sets are connected to the exchange, for exam­ple LM Ericsson's model DAG 11102/8.

External bell When an acoustic signal is needed out­side the switchboard, a bell KLD 1303, can be connected. Various outdoor bells can also be used.

Wooden sides ABJ 101 can be adapted to the environ­ment at the installation site. The switch­board is delivered with teak sides. Sets of side pieces made of jacaranda, natural pine, walnut or other types of wood can be supplied on special re­quest.

" The maximum number of lines that can be con­nected to a certain switchboard is dependent on a number of factors, such as the amount of traffic, type of traffic, the efficiency of the operator, calling habits, local tradition etc. Under normal circum­stances an operator in a single-position magneto switchboard can handle a maximum of 100-140 magneto lines and 8 CB lines. However, if the amount of traffic is very small it is possible in some cases to handle more than 200 lines

Technical data

Maximum number* of magneto lines CB junction lines Cord pairs

Dimensions in cm Width Height Depth

Approx. weight, kg

Base unit + 1 line box

80 70 60 - 4 8 18 18 18

62 41 53

40

Base unit + 2 line boxes

160 150 140 - 4 8 18 18 18

62 59 53

55

Base unit + 3 line boxes

240 230 220 4 8

18 18 18

62 77 53

70

Operating voltage 6VD.C.

Line resistance The maximum value of the line resis­tance for magneto lines is dependent on the line attenuation, which may amount to 15 dB. The leakage resistance must not be less than 10000 ohms. The indi­cators drop at 9 mA. The resistance and leakage values for CB junction lines are primarily dependent on the limit values of the main exchange. Thus the max­imum values for these lines must be calculated on the basis of data from the main exchange.

ABJ 101 Advantages The exchange has been given all the characteristics that can be demanded from a modern manual magneto switchboard which means that — CB junction lines from manual or au­

tomatic exchanges can be connected — magneto junction lines can be con­

nected to the ordinary magneto units — supervision of a junction line can be

carried out by the operator while handling other calls

— line splitting gives the operator the possibility of talking to one party without the other being able to over­hear the conversation

— the ringing is generated automatic­ally (transistorized)

— a flag indicator indicates that a signal is being sent out

— a ring-back tone is sent to the caller — backward ringing can be carried out

via the answering cord — acoustic signals can be obtained

concurrently with incoming seizure and clear-forward signals

— a fixed acoustic signal can be con­nected in by means of a switch

— dropped indicators are automatically reset during the handling of the calls

— a battery indicator indicates when it is time to check the condition of the battery

— testing of the cords can be performed — an automatic fuse eliminates fuse

changes — anextrajack is provided for connect­

ing in a handset for an assistance operator

— space is provided forthe memoranda that the operator needs to have readi­ly accessible in order to be able to work rapidly and efficiently.

The Ericsson Group With associated companies and representatives

EUROPE SWEDEN Stockholm 1. Teletonaktiebolaget LM Ericsson 2. LM Ericsson Telematenel AB 1. ABRrfa 1. Sieverts Kabelverk AB 5. ELLEMTEL Utvecklings AB 1. AB Transvertex 4. Svenska Elgrossist AB SELGA 1. Kabmatik AB 4. Holm & Ericsons Elektriska A8 4. Mellansvenska Elektriska AB 4. SELGA Mellansverige AB Alingsås 3. Kabeldon AB

Gävle 2. Vanadts Entreprenad AB Gothenburg 4. SELGA Västsverige AB Kungsbacka 3. Bota Kabel AB Malmö 3. Bjurhagens Fabrikers AB 4. SELGA Sydsverige AB Norrköping 3. AB Norrköpings Kabelfabrik 4. SELGA östsverige AB Nyköping 1. Thorsman & Co AB Spånga 1. Svenska Radio AB Sundsvall 4. SELGA Norrland AB Växjö 1. Widells Metallprodukter AB

EUROPE (excluding Sweden) DENMARK Copenhagen 2. LM Ericsson A/S 1. Dansk Signal Industri A/S 3. GNT AUTOMATIC A/S 1. I. Bager&Co A/S Tfistrup 2. Thorsman & Co Aps 2. LM Ericsson Radio Aps FINLAND Helsinki 2. Oy Thorsman & Co Ab Jorvas 1. Oy LM Ericsson Ab FRANCE Colombes 3. Société Francaise des

Telephones Ericsson Paris 2. ThorsmansS.a.r.l. Bologne sur Mer 1. RIFAS.A. Marseille 4. Etablissements Ferrer-Auran S.A. IRELAND Athlone 1. LM Ericsson Ltd. Drogheda 2. Thorsman Ireland Ltd. ITALY Rome 1. FATMESoc. per Az 5. SETEMERSoc per Az. 2. SIELTESoc. perAz

The NETHERLANDS Rijen 1. Ericsson Telefoonmaatschappij B.V.

NORWAY Nesbru

Oslo 2. SRA Radio A/S 4. A/S Telesystemer 4. A/S Installatör Drammen 3. A/S Norsk Kabelfabrik

POLAND Warszaw 7. Telefonaktiebolaget LM Ericsson

PORTUGAL Lisbon 2. Sociedade Ericsson de Portugal Lda

SPAIN Madrid 1 . Irtdustrias de Telecommunicact6n S.A.

(Intelsa) 1. LM Ericsson S.A.

SWITZERLAND Zurich 2. Ericsson AG

UNITED KINGDOM Horsham 4. Thorn-Ericsson Telecommunications

(Sales) Ltd 2. Swedish Ericsson Rentals Ltd. 5. Swedish Encsson Company Ltd. 3. Thorn-Ericsson Telecommunications

(Mfg) Ltd. London 6. Thorn-Encsson Telecommunications

Ltd 4. United Marine Leasing Ltd. 4. United Marine Electronics {UK) Ltd

WEST GERMANY Frankfurt-am-Maln 2. Rifa GmbH Hamburg 4. UME Marine Nachnchtentechnik, GmbH Hanover 2. Ericsson Centrum GmbH Ludenschetd'Ptepersloh 2. Thorsman & Co GmbH

Representatives In: Austria, Belgium, Greece, Iceland, Luxem­bourg. Yugoslavia.

LATIN AMERICA ARGENTINA Buenos Aires 1. Cia Ericsson S.A C.I. 1. Industrias Eléctricas de Quilmes S.A. 5. Cia Argentina de Teléfonos S.A 5. Cia Entrernana de Telétonos S.A.

BRAZIL Säo Paulo 1. Ericsson do Brasil Comércio e

Industria S.A. 4. Sielte SA. Instalacöes Elétricas e

Telefönicas 4. TELEPLAN, Projetos e Planejamentos

de Telecommunicates S A Rio de Janeiro 3. Fios e Cabos Plåsticos do

Brasil S.A Säo José dos Campos 1. Telecomponentes Comércio e

Industria S.A.

CHILE Santiago 2. Cla Ericsson de Chile S.A.

COLOMBIA Bogota 1. Ericsson de Colombia S.A. Call 1. Fåbricas Colombianas de Materiales

Eléctricos Facomec S.A

COSTA RICA San José 7 Tp l f f nnak t i phn l f l no t I M EricSSOn

ECUADOR Quito 2. Teléfonos Ericsson C A

GUATEMALA Guatemala City 7. Teletonaktiebolaget LM Ericsson

HAITI

Port-au-Prince 7. LM Encsson

MEXICO Mexico D.F. 1. Teleindustna Ericsson, S.A 1. Latinoamericana de Cables S A

de C V 2. Teléfonos Ericsson S.A. 2. Telemontaje, S.A. de C.V.

PANAMA Panama City 2. Telequipos S A

PERU Lima 2. Cia Ericsson S A

EL SALVADOR San Salvador 7. Teletonaktiebolaget LM Encsson

URUGUAY Montevideo 2. Cia Ericsson S.A.

VENEZUELA Caracas 1. Cla Anonima Ericsson

Representatives in: Bolivia. Costa Rica, Dominican Republic, Guadeloupe, Guatemala. Guyana, Haiti, Honduras. Martinique, Netherlands Antilles, Nicaragua, Panama, Paraguay, El Salvador, Surinam, Trinidad. Tobago

AFRICA ALGERIA Algiers 7. Telefonaktiebolaget LM Ericsson

EGYPT Cairo 7. Telefonaktiebolaget LM Ericsson

MOROCCO Casablanca 4. Société Marocaine des Telephones et

Telecommunications "SOTELEC"

TUNISIA Tunis 7. Teletonaktiebolaget LM Ericsson

Zambia Lusaka 2. Ericsson (Zambia Limited) 2. Telefonaktiebolaget LM Ericsson

Installation Branch

Representatives In: Angola, Cameroon, Central African Repub­lic, Chad, People's Republic of the Congo, Dahomey, Ethiopia, Gabon, Ivory Coast, Kenya, Liberia, Libya, Malagasy, Malawi, Mali, Malta, Mauretania, Mozambique, Niger, Nigeria, Republic of South Africa, Reunion, Senegal, Sudan, Tanzania. Togo, Tunisia, Uganda, Upper Volta, Zaire.

ASIA INDIA Calcutta 2. Ericsson India Limited

INDONESIA Jakarta 2. Ericsson Telephone Sales

Corporation AB

IRAQ Baghdad 7. Teletonaktiebolaget LM Ericsson

IRAN Teheran 3. Simco Ericsson Ltd. 4. Aktiebolaget Erifon

KUWAIT Kuwait 7. Telefonaktiebolaget LM Ericsson

LEBANON Belrouth 2. Société Libanaise des Telephones

Ericsson

MALAYSIA Shah Alam 1. Telecommunication Manufacturers

(Malaysia) SDN BHD

OMAN Muscat 7. Teletonaktiebolaget LM Encsson

SAUDIARABIA Riyadh 7. LM Ericsson

THAILAND Bangkok 2. Ericsson Telephone Corporation

Far East AB

TURKEY Ankara 2. Ericsson Turk Ticaret Ltd Sirketi

Representatives in: Bahrein, Bangladesh. Burma, Cyprus, Hong Kong, Iran, Iraq. Jordan, Kuwait, Lebanon, Macao, Nepal, Oman. Pakistan, Phillip-pines, Saudiarabia, Singapore, Sri Lanka, Syria, United Arab Emirates

UNITED STATES and CANADA UNITED STATES Woodbury NY. 2. LM Ericsson Telecommunications Inc New York, NY. 5. The Ericsson Corporation CANADA Montreal 2. LM Ericsson Limitée/Limited

AUSTRALIA and OCEANIA Melbourne 1. LM Ericsson Pty Ltd 1. Rifa Pty. Ltd. 5. Telenc Pty Ltd Sydney 3. Conqueror Cables Ltd.

Representatives In: New Caledonia, New Zealand, Tahiti.

1. Sales company with manufacturing 2. Sales and installation company 3. Associated sales company with manu­

facturing 4. Associated company with sales and

installation 5. Other company 6. Other associated company 7. Technical office

TELEFONAKTIEBOLAGET LM ERICSSON

ISSN 0014-0171 fltinlnri i n C u i f l r i a n I u i n n l n r a ^ n o n O r e b f O l 9 7 !