T1 Tutorial

Chapter 1 - Introduction

Modern telecommunications user companies often purchase one product from vendor A, another product from vendor B, and so forth. No single vendor necessarily integrates the entire end-to-end transmission project. The telecommunications user company must design its own facilities, purchase the correct products from the many vendors, install and test the products properly, and document what it has for future reference. Unfortunately, the first step (engineering design) sometimes is loosely considered until the installation fails to test correctly. This document addresses engineering planning and design of a T1 repeatered line so service can be delivered on a more predictable schedule.

Signals

The 1.544 Mbps bipolar Pulse Code Modulation (PCM) line signals of T1-type terminal equipment (such as channel banks, data terminals, and repeatered lines) are designated DS-1, meaning "digital signal first level." At the standard cross-connect point for DS-1 signals, DSX-1, the voltage level of pulses is about +3 volts and -3 volts. DS-1 means that the signal meets the interface specification for DS-1 signals (1.544 Mbps, bipolar +3 and -3 volt pulses, 50% duty cycle, etc.). The data pattern means the 192 bits of payload data contains live traffic (that's 24 time slots and 8 bits per time slot) or some test pattern. After each 192 bits, there is a framing bit for #193. That bit is either one or zero depending on whether it must follow the special pattern for Superframe Format (SF, also referred to as D4) or Extended Superframe Format (at this point, the use of SF or ESF is purely at customer discretion based on the types of channel banks and other network elements present in the network). See Figure 1. This 193-bit frame is repeated 8000 times per second. This is also called a frame rate of 8 kHz. Each of 24 channels is sampled and transported 8000 times per second. If a terminal receiver cannot determine the start and finish of a frame, then receivers will always be "out of frame sync" with respect to transmitters and very poor performance is noted. There is one more qualifier on the signal, and that is Line Code. The two line codes are Alternate Mark Inversion (the old standard) and B8ZS (bipolar eight zero substitution). For network elements that carry live traffic, this is very important.

 

Figure 1

What does Alternate Mark Inversion (AMI) mean? In the data communications world, all digital data is either One or Zero, nicknamed either Mark or Space. If the data bore a pattern of All Ones, then the first One is transmitted on the AMI transmission link as +3 volt pulse, then the next One as -3 volt pulse, then the next One as +3 volt pulse. In a live traffic signal with mixed Ones and Zeros, there will be Zeros between the positive and negative pulses as depicted in the following figure. See Figure 2. Internally to the electronics of equipment, the 1.544 Mbps signal might be unipolar, but everywhere accessible to the user, the signal is bipolar. DS-1 signals may fall into one of three categories: SF, ESF, and possibly Unframed. Sometimes test equipment will use an unframed DS-1 test signal to check a transmission facility, but most terminal equipment must be selected for SF or ESF only. If terminal equipment expects to see SF and it receives ESF instead, it will probably not be able to lock its framer circuit onto the input signal and everything will stay in alarm.

 

Chapter 2 - Description of a Repeatered Line

A T1 repeatered line provides a physical 4-wire transmission path for cable carrier systems that transmit bipolar pulse streams at bit rates of 1.544 Mbps. At each central office, cable pairs connect to an office repeater. Between offices, line repeaters are located at nominal spacing of 32 dB at 772 kHz. Automatic line build out (ALBO) equalizers in each repeater can compensate for a range of losses in the preceding cable section. Note that the following overview diagram (Figure 3) is simplified by the use of one line in each direction to symbolize one twisted pair of wires. Other diagrams use one line to indicate both directions (two twisted pairs), and still other diagrams use four lines to indicate both pairs. The nature of the diagram dictates which symbol convention is used. The triangular symbol indicates a digital regenerator, which is somewhat related to an amplifier. Note that office repeaters use a regenerator in only one direction, and line repeaters use regenerators in two directions. With this view of DS-1 signals, let's examine other network elements that may connect.

 

Figure 3

A T1 Channel Service Unit (CSU) interfaces a typical piece of customer equipment (such as a channel bank) with a public T1 transmission facility. There are several specific types of CSUs and the most common type has a DS-1 signal interface on each side of the unit (the Data Terminal side and the Network side). In some respects, this CSU resembles an office repeater; however, additional diagnostic features are present on most CSU products. One of the most common functions is that of Ones Density Enforcement (also referred to as Ones Stuffing). Customer DS-1 signals are not allowed onto the public network containing very long strings of zeros (since there may be network elements that require occasional ones for timing purposes). As a result, most CSU devices change a zero to one to suppress the 16th consecutive zero, depending on specific requirements for a public facility. If the traffic has simply voice circuits, an occasional forced error of this sort is a negligible problem. If, on the other hand, the traffic is high-priority unrestricted data, then an occasional error is not acceptable. If this is the case, then the Line Code must be selected as B8ZS and not standard AMI. B8ZS uses selected bipolar violations (BPVs) as a means of signaling the far end that strings of zeros are in the customer data. Therefore, no errors are introduced and the system works fine. If B8ZS Line Code is used, then each and every network element in the path must be provisioned for B8ZS instead of AMI (or else the element must completely ignore Line Codes). Note that some T1 office repeaters are aware of B8ZS and others are not aware. Note that these repeaters will not be disrupted by one line code or the other, but many have BPV monitors that will detect the unintentional BPVs but ignore the intentional BPVs that are part of B8ZS coding.

 

DS-1 Signal Pre-compensation and Signal Levels

In most central offices, there is a DSX-1 cross-connect jackfield located between the channel bank and the office repeater, or between any two dissimilar elements in a whole end-to-end system. The jackfield serves as a maintenance test point for craft people and as a wire wrap interconnection point between the two elements. See Figure 4. If a transmit port must send a DS-1 signal only 50 to 100 feet to the DSX-1 panel (typical in small equipment rooms), then it can send a "normal" DS-1 signal waveform. That waveform won't be affected much by only 50 to 100 feet of cable capacitance if the cable is good, so the waveform looks perfect at the DSX-1 panel when viewed on an oscilloscope. That is the objective, to get a perfect signal waveform presented to the DSX-1 panel. This standard signal is referred to as having a level of 0 dBdsx. A standard signal that has been attenuated by several hundred feet of cable might have a level of -3 dBdsx.

 

Figure 4

Looking at the outbound DS-1 signal going from the terminal equipment toward the span line, if it is perfect at the DSX-1 panel, then it gets minimally affected by the 50 to 100 feet of cable capacitance, so the signal still looks fairly normal at the receiver. Often the assumption is made that the transmit cable distance to the DSX-1 is the same as the receive cable distance from the DSX-1 (on the same side of the DSX-1). If the cable distance to the DSX-1 is much longer, say 655 feet, and then a normal signal is going to be attenuated and "rounded off" due to cable capacitance. In many pieces of equipment, a pre-equalizer is used in the DS-1 interface on the transmit port. This pre-equalizer "sharpens up" the pulse edges to exaggerated amplitudes. This is launched down through the 655 feet of cable and the sharp edges become smoothed down ("rolled off") due to cable capacitance. When it arrives at the DSX-1 panel, it should be exactly a perfect waveform (see Figure 5). When the signal comes into an office repeater port after passing 655 feet of cable from the DSX-1, it can be successfully received without errors. It might be possible to attenuate the signal even more without degradation, say from 700 or 800 feet, but errors in transmission might also appear (which would be unacceptable). It depends on the specific equipment and cable in use. Some types of Inside Plant equipment are only capable of transmitting and receiving a signal through 0 to 133 feet to the DSX-1 panel. It is relatively important to have each and every network element meeting the "perfect waveform" mask, which is an industry standard.

 

Figure 5

The idea is to consider each direction of signal flow. Know what type of cable is in use and roughly what its attenuation characteristics are (22 gauge ABAM cable has a nominal capacitance of 14 to 16 pF/foot @ 772 kHz). Also consider that a 24 gauge cable has approximately 25% higher attenuation than standard 22 gauge. Know what signal levels should be present at each point and what the receive sensitivity is for each device. Many problems are traced to the use of cable that is not intended for T1 use. Office repeaters are somewhat more complex, since they have one set of capabilities on the Equipment side (inside) and one set on the Facility side (outside). A typical office repeater has a digital regenerator only on the receive side (receive from facility), and has enough sensitivity to regenerate a signal from a line repeater that is about 3000 to 4000 feet away. It has a typical inside cable specification for 0 to 655 feet, depending on the exact pre-equalization. See Figure 6.

 

Chapter 3 - Span Engineering

Pulse Transmission

Pulses generated by the terminal equipment (e.g. channel bank) and repeaters are subject to distortion by attenuation and phase characteristics of the cable. In the line and office repeater units, just preceding the actual regenerator is an ALBO equalizer which restores adequate pulse shape for detection and regeneration. Pulses generated in the terminal equipment must reach the office repeater in a predictable fashion even if it is in the same room. In many office repeaters, the line build out (LBO) setting tells it through how many feet of cable the signal has traveled since the DSX-1 cross-connect. The office repeater adapts to that attenuated signal. Most LBO settings assume the use of 22 gauge twisted-pair cable (with around 14 to 16 pF per foot of capacitance). Note that a 24 gauge cable has approximately 25% more loss than 22 gauge. By default, many office repeaters are shipped set for 0 to 133 feet of cable (assuming 22 gauge). For transmission calculations, the cable attenuation at 772 kHz is used. This is half of the 1.544 Mbps clock rate, but this is valid because the power spectrum of the pulse stream is maximum at approximately 772 kHz.

Error Rates

Pulses sent along the repeatered line are regenerated at each repeater point. The repeater looks at each time slot and decides whether or not a pulse is present. If the logic circuit determines that there is a pulse, the repeater outputs a new pulse that is free of noise, distortion, or interference. As a result of many possible factors, a few pulses may be incorrectly regenerated. A Zero is sent instead of a One, or vice versa. The ratio of error pulses to the total number of time slots is called the error rate or the bit error rate (BER). One error in 1000 is referred to as BER 10-3. One error in a million is BER 10-6. For strictly voice circuit application to T1, a BER of 10-6 or 10-7 might be considered acceptable performance, although a BER of 10-8 or 10-9 might be required for some data purposes. In the most common systems, once live traffic errors have crept into the transmission stream, they cannot be sorted out

or corrected. Error rates tend to accumulate through an end-to-end system, although the rates tend to be low for digital systems compared to analog systems. The exception to this is found in a few sophisticated microwave radio systems where forward error correction is used. However, its expense limits application.

Overall System Length

Each repeater in the series adds a small amount of jitter to every pulse of the bit stream. A conservative limit of 200 tandem repeaters in a system ensures that the accumulated jitter won't exceed the synchronization capability of terminal equipment or higher-order multiplexers. Based on the accumulation of error rates in tandem repeater sections, and particularly in end sections (the repeater section next to the central office), the system should not include more than ten tandem span lines (nine intermediate offices).

Design Criteria

Long-range design of the span line is necessary to plan the expected cross section of the span. The selection of one-cable or two-cable operation, locations for the line repeaters, and repeater section length will depend on the future requirements of the route. Also, the cable plant must be carefully studied in terms of the number, type, age of cables; freedom from bridge taps and branches; splicing integrity; suitability for line repeater locations; and minimum exposure to electrical and mechanical hazards. Major factors that control the design of the span include:

1. Ultimate number of systems within the cable

2. Cable pair attenuation at 772 kHz

3. Crosstalk coupling loss between cable pairs

4. Central office noise

5. Ambient temperature range

Single-Cable or Dual-Cable Operation

In normal one-cable operation, low-level repeater inputs and high-level repeater outputs appear at the same point of the cable. As a result, near-end crosstalk (NEXT) is the limiting factor in repeatered line design. The number of systems that can be installed in a single cable is mainly controlled by the physical separation of the pairs in the two directions of transmission. Greater separation increases the coupling loss, resulting in decreased interference. A general rule is that if transmit and receive pairs are in the same cable binder group, the maximum section loss should be reduced to 15 dB to prevent crosstalk.

In two-cable operation, NEXT does not limit the number of systems for one cable. The choice of one-cable or two-cable operation is based on cable route, circuit requirements, availability of suitable cables, and economics.

Chapter 4 - Repeater Spacing

Maximum Section Loss

A span line includes one or more repeater sections. Typical maximum section loss is 32 dB (normal section) measured at 772 kHz. In the end section next to the central office, maximum loss is limited to 50 to 70% of the normal loss limit, or about 23 dB. In some early line repeaters, the ALBO equalizer has a range of 6 dB to 31 dB. See Figure 7. In many newer line repeaters, the ALBO has a range of 0 dB to 35 dB. This means that the signal can be attenuated through up to 35 dB of cable loss and still become regenerated properly. In practice, repeater sections are designed with a safety factor of several dB, so spacings of 28 to 32 dB are quite common.

 

Figure 7

Minimum Section Loss

The minimum section loss is frequently set at 9 dB because of repeater design and to attenuate reflections. In other words, loss is generally added via a XMT span pad or via a 7.5 dB equalizer in the office repeater for the end section. Equalizers help control near end crosstalk (NEXT) by simulating cable attenuation characteristics whereas the pad displays flat loss.

Cable Loss Measurement

If a maximum length section is engineered, cable loss should be measured before the final repeater location is established. This allows for changes and small errors. Note that aerial cable can be exposed to higher ambient temperatures compared to buried cable and ducted cable. Higher temperature equates to higher copper attenuation and DC resistance. For design purposes, the losses for aerial cable are estimated at about 5% higher than for buried cable.

Cable TypeNominal Capacitance uf/mile

Buried Cable Loss Estimate dB/1000 feet

Aerial Cable Loss Estimate dB/1000 feet

22 AWG PIC, D-shield, filled

0.083 4.19 4.37

22 AWG PIC, D-shield, filled

0.083 4.58 4.75

24 AWG PIC, D-shield, filled

0.083 5.17 5.39

24 AWG PIC, D-shield, filled

0.083 5.73 5.87

Example: One section of 6250 feet of 22 AWG PIC, D-shield, filled, buried has a loss of approximately 26.2 dB. That leaves a nice safety margin.

Example: One section of 6000 feet of 24 AWG PIC, D-shield, unfilled, aerial has a loss of approximately 35.2 dB. This exceeds the line repeater capability and could be a problem performer. This exceeds the 35 dB limit for typical line repeaters.

Example: One end section of 2200 feet of 24 AWG PIC, D-shield, unfilled, buried has a loss of approximately 12.6 dB. This is far less than the 50% guideline, so it is somewhat wasteful. This should be stretched out.

Example: One end section of 5200 feet of 22 AWG PIC, D-shield, unfilled, aerial has a loss of approximately 24.7 dB. This exceeds the 70% guideline and should be avoided.

T1 Repeatered Lines

In the case of a CO to CO T1 repeatered line, there is normally one Originating Office Repeater and one Terminating Office Repeater. The convention is that the Originating end powers the line with simplex readings of +V and -V, where the -V is typically 10 volts more in magnitude with respect to the +V. If both ends originate power, then there may be a simplex power loop strap set at one of the line repeaters out in the middle of the span. Note that the voltage will appear at the span line interface even if that is open loop. Current will not flow into an open loop, however, so the voltage sensed across the 10 ohm current sensing resistor is the best indication of the validity of the simplex current. The newer ASPR Originating Office Repeaters may be "universal" which means that a mid-span loop strap is not required and powering is shared by each end.

Chapter 5 - Near End Crosstalk (NEXT)

In one-cable operation, errors can be caused by NEXT between cable pairs in opposite directions of transmission. Physically separating the different groups of pairs as much as possible is preferred. Refer to Figure 8. Shortening the repeater spacings will reduce the level differential and the sensitivity to NEXT. One single T-1 system in a cable has only itself with which to interfere (transmit signal with respect to receive signal). Twenty systems in a cable makes many more cases for NEXT. Not only does one single system have itself for interference, but nineteen other systems are also producing NEXT. If a particular cable use is expected to grow over time to its maximum capacity, then the correct safety margin must be calculated into the design. This cannot be corrected later.

 

Figure 8

Chapter 6 - Center Office Noise and the End Section of a Repeatered Line

As stated earlier, in the end section next to the central office, maximum loss is limited to 50 to 70% of the normal loss limit (32 dB), or about 23 dB. The 23 dB limit includes the loss in the tip cable and in the office wiring to the line terminating shelf for office repeaters. If exchange service is mixed in the same cable with T1, then an extra safety factor of 8 dB should be allowed to tolerate impulse noise. This brings down the end section to a maximum of 15 dB. An end section of 15 dB represents approximately 3700 feet of cable, depending on the exact type. If this cable distance is too short to be economically tolerable, then consider leaving out the exchange service pairs to gain back the 8 dB safety margin.

 

Chapter 7 - Center Office Cabling

Office wiring typically goes from the cable vault directly to the office repeater shelf, from the office repeater shelf to the Automatic Protection Switch (if equipped), and from there to the channel bank, radio mux, or other terminal equipment. Along the way, there may be one or more DSX-1 cross connect panels. Western Electric ABAM cable is a traditional selection to connect among all of these elements. Use a loss figure of approximately 0.4 dB per 100 feet of office cabling, assuming 22 AWG. Remember that 14 to 16 pF per foot is the normal cable capacitance. Cable with higher capacitance will give you problems on the longer cable runs. CAT 5 LAN cable was not intended for T1, but it has low capacitance, so it makes a good substitute.

In some rural service areas, the incidence of lightning strikes on aerial cable is so frequent that one extra measure is applied at the cable vault. The outside cable might be 22 gauge and the tip wiring to the line terminating shelf might be 22 gauge, but one short section of higher gauge (smaller diameter) cable is added at the cable vault. This 25 to 50 foot length of 24 or 26 gauge cable is called fuse cable. As its name implies, it acts as a fuse element that will open when huge currents from lightning appear. This technique is effective in keeping lightning surges out of the central office, but it has side effects. The 24 or 26 gauge insert may be short, but it interjects one extra attenuation factor in cable loss estimation. Further, when the fuse cable opens up, that pair must be abandoned. That is a lot of work to correct.

Chapter 8 - Repeater Housings on a Repeatered Line

If route junctions are present along the cable, then it is advisable to adjust repeater section spacings to place repeater housings at the junctions. Otherwise, there is greater possibility of signal level differences from adjoining cable branches, which would contribute to crosstalk. If a "repeaterless junction" is necessary, T1 span pads may be used to equalize levels between branches. See Figure 9. Manufacturers of repeater housings typically offer several configurations depending on number of line repeater units in the "nest," whether there are attached stub cables, the length of such stubs, presence or absence of lightning protectors, etc. It often seems as though each repeater housing vendor has chosen to describe T1 pairs by different methods. Just be aware that in a single direction, the transmit pair from one repeater becomes the receive pair 6000 feet away. The east-west transmit is separate from the west-east transmit.

 

Figure 9

Chapter 9 - Cable Details

Gauge, D-Shield or T-Screen Type, and Other Factors

In most areas, 22 AWG is the preferred cable for most types of T1. In some metropolitan areas, limited 24 AWG and 26 AWG are used. Keep in mind the different attenuation characteristics of each. Larger sizes, such as 19 AWG, were never very popular for T1 because of the economics of buying copper wire versus buying electronic systems. D-shield is a type of cable with one half of the pairs (transmit in one direction) organized into one D-shaped half of the cable, as viewed in cross-section. This shielding effectively makes one-cable operation almost as good as two-cable operation in terms of crosstalk reduction. Consult the cable manufacturers for complete details. Filled cable contains a gel that prevents ground water from easily leaking into the cable. Although this cable has higher initial cost, it tends to produce longer cable life in rainy regions. Note that filled cable has slightly different cable attenuation figure compared to unfilled cable.

Selection of Conductors

T1-class cables are commonly manufactured with multiples of 25-pair bundles. For single-cable operation, the standard choice is the east-west direction of transmission to be in one bundle and the west-east direction to be in a separate bundle. Furthermore, 104-pair cables are organized with two bundles for each direction and four pairs for the various voice-frequency circuits associated with T1 lines (order wire, fault locate, etc.). If the cable does not have this bundle organization, then it becomes necessary to test for potential crosstalk (distortion factor, or D-factor) from every cable pair to every other cable pair to determine which pairs have the most coupling loss.

Chapter 10 - Spare Lines and Automatic Spare Line Switching

Murphy's Law states "Anything that can go wrong will go wrong."

Although modern line repeaters are very reliable, they are used in a hostile environment (temperature extremes, vibration, lightning, automobile damage, rodent damage, etc.). As a result, line repeaters can fail any time. Poor cable splicing techniques might lead to intermittent noise after installation. When channel banks or customer services have gone into alarm at the central office, how long the service disruption will remain is a function of the restoration technique. Most user companies have implemented spare span lines within each T1 trunk cable. When a working span begins to fail, the traffic at the ends is rerouted to the spare span line. Some choose to do this manually (via patch cords) at each attended office. Others who place more value on availability of the span line for live traffic have chosen to implement automatic transfer equipment.

The Automatic Protective Switch (APS) is located at each end of a span line facility and takes care of signal failure detection and automatic transfer. APS products generally fall into two categories: one-for-one switching (1:1) or one-for-N switching (1:N). The term 1:1 means that there is one protection line for each normal working line. Another way of stating this is one standby line and one normal line. This is most commonly used in ring networks and in applications involving mixed transmission media (one fiber facility and one copper facility, for example). In contrast, 1:N means that there is one protection line for a number (N) of normal working lines. In practice, the integer N could be 1, and the upper limit on N may be dozens. The number N is commonly 2 to 8. This type is most commonly used on "straight T1 cable carrier" applications, especially for subscriber loop carrier.

Chapter 11 - Fault Locating

Description

If a copper T1 span line has failed, it can be very time-consuming to isolate the fault. Traditional fault location is a technique for determining precisely which line repeater or cable section has failed within one span line. Fault location utilizes one loaded VF pair within the T1 cable (T1 pairs are always unloaded).

When central office alarms indicate that a span line has failed, normal traffic is blocked or removed from it at the office repeater. A special T1 test set injects so-called fault locating codes into the span line. These fault codes contain bipolar violations that are repeated at a discrete audio frequency. This is most easily thought of as sending T1 frequency modulated with an audio tone. At the first repeater housing an audio tone filter with frequency "A" is present. There is a special output from each regenerator that is fed to the audio tone filter. If that regenerator has a good signal containing "A" tone, the tone will pass down through the filter to the loaded pair named Fault Locate. If tone "A" successfully passes back to the central office and can be read on a meter, then assume that regenerator #1 is OK.

If tone "B" is sent from the central office, it may be intended for the regenerator in the second repeater housing. If a cable fault has stopped the signal, then the "B" tone will not get as far as the second repeater (in fact, nothing will if the actual fault is between #1 and #2). The "B" tone will not pass through the filter and the "B" tone will not be measured back at the central office. The technician can make the assumption that some type of cable problem has occurred between repeaters #1 and #2, possibly including the line repeater plug-in at #2.

System Layout

Fault locating schemes can be developed with a dozen or more audio frequencies to cover very long span lines of many repeaters. Filter frequencies are usually assigned in alphabetical order along the span line. The newest fault locate test sets are automated to

send all of the frequencies in sequence and monitor the tone receive level for each one. It decides which repeater is likely to have the fault.

Newer Techniques

After traditional fault locating was established, some newer types of T1 line repeaters were invented for the purpose of eliminating the fault locating filters and the associated cable pair. These non-traditional repeaters had a programmable looping feature which works fine in principle. However, one big headache occurs after a lightning hit to the cable. Each of these looping repeaters takes a period to reset itself after the preceding repeater in the series string. For a long repeatered line, this can be a sizable time delay (minutes).

Chapter 12: Order Wire

The order wire is simply a loaded voice frequency pair that runs through the T1 cable and appears inside each line repeater housing. When one technician is troubleshooting at the remote housing and one technician is at the central office, it is very convenient to have a place to plug in a technician's telephone ("butt-in phone"). The order wire is typically terminated to an automatic ring down line at the office repeater area of the central office. If the copper order wire is not implemented, then the technician at the remote housing must have a two-way radio or cellular telephone with plenty of battery and talk time.

Chapter 13: Lightning Protection

Lightning is an electrical discharge pulse in the atmosphere, which averages 20 kA or more in current. Commonly, these discharges are viewed as a flash from cloud to ground, although it can flash from the ground to the cloud. Once ionization of the atmosphere occurs, this becomes a luminescent, conductive plasma (a lightning "bolt") sometimes reaching 60,000 degrees F. Lightning can deliver a tremendous discharge of energy at any grounded object. It can easily explode a tree with a direct strike.

Lightning strikes are somewhat predictable over a geographical region. The isokeraunic level is the number of thunderstorm days per year. This isokeraunic number varies from over 100 along the Gulf coast of Florida to less than 5 in the Pacific Northwest. Nevertheless, virtually all areas in North America are subject to lightning strikes to some degree. Exterior equipment, rooftop equipment, and equipment connected to aerial and buried copper cables are subject to possible damage. In low lightning areas, protected-type line repeaters are used along aerial cable and unprotected-type line repeaters are used along buried cable. In contrast, in areas of heavy lightning activity, it is quite common to use protected-type line repeaters regardless of whether the cable is aerial or buried.

Aerial cables are especially susceptible to lightning strikes. The huge energy pulse from lightning momentarily raises the potential of ground at that strike point and then travels along the copper pairs to the central office where it finds a lower ground potential. At the central office, primary protection consists of three-element gas tube protectors, typically installed at the well-grounded protection frame or near the cable vault. In some central offices, gas tubes are installed at the top of the relay rack with office repeaters.

Primary protectors must be present and grounded properly. Many items of transmission equipment, such as office repeaters, have secondary protection in the form of solid state surge limiters, but they are not effective if primary protection is bad. When the lightning voltage causes the primary gas tube to conduct, it represents a short circuit to the span powering regulator.

In a typical T1 transmission line, span power is fed only from the Central Office. In some cases, additional power comes from the far end unit. During normal operation, the 60 mA simplex current flows normally, the gas tubes sit idly, waiting for a lightning strike, and the DS1 traffic moves along. In the instant of a lightning strike (somewhere mid-span), the lightning acts as a huge current pulse that raises the ground potential at that strike location. If the cable is not properly grounded everywhere, the lightning can enter the copper pairs and flow toward wherever the best ground point might be, which might be toward the nearest Central Office or, in the other direction, toward the remote end. The lightning might be in the form of a metallic voltage appearing from tip to ring on a pair. It might be in the form of a metallic voltage appearing from tip to ground or from ring to ground. Or, it might be in the form of a longitudinal current surge.

If a metallic voltage appears at a three-element gas tube protector (called the primary protector), the tube will fire either tip to ground, ring to ground, or tip to ring. This assumes that the gas tube is both working and grounded correctly. Some companies, however, fail to periodically test their gas tubes with a gas tube checker. If measured with a simple meter, gas tubes appear to be an open circuit whether they are working or not working, making it unreliable. If the gas tube has an improper voltage rating, it will not work correctly. If the voltage rating is too high, lightning voltage can seep in before it fires (thereby stressing equipment). If the voltage rating is too low, the normal DC voltage applied, at the simplex power feed end of the span, and is enough to set it off

prematurely, or at least, to hold the gas tube in "glow" mode after the strike (thereby forcing a failure situation after the lightning strike).

When the gas tube fires on schedule, it is effectively producing a short to ground. If there is some span power feed repeater nearby, this acts as a dead short on its current loop, which causes a big current surge. Various T1 products have built-in secondary surge protectors to withstand this secondary surge, called the current surge. But if the primary surge protector is not doing its job, it will most likely burn out the equipment or anything around it. In some cases, this surge protector is in the form of extra series resistance to limit current peaks. In other cases, this protector is in the form of a fuse that will open up at a high current point. In yet others, the protector is a combination resistor and fuse.

As a general rule, however, two-element protectors are not recommended for T1 circuits. Two-element protectors are suitable for plain old telephone service (POTS) on a two-wire circuit. Due to the nature of T1 and its simplex current loop, equipment may be damaged through the use of anything other than three-element gas tubes.

Some telephone operating companies have the policy that T1 is only placed on new cables, dedicated for T1. This is because the headaches of rehabilitating old exchange cable can be severe. The job of eliminating every last bridge tap is difficult on some older cables. Impulse noise from ordinary analog subscriber loops can become a problem when mixed side-by-side with T1.

Chapter 14: Repeater Power Feed

Simplex Power Design

Office repeaters are located at the central office where noise-free -48 VDC power is plentiful. Line repeaters seldom have any local source of AC or DC power. Instead, the power for line repeater electronics is fed down the copper pairs from the office repeater. Inside a line repeater, there are two digital regenerators designated side 1 and side 2. Line repeaters are powered by DC current flow through a loop formed from the simplexes of the two cable pairs associated with side 1 and side. The line repeater must have current flowing though it in the right polarity for proper operation. In this case, the repeater represents an equivalent resistance of 100 to 120 ohms on a common 60 mA simplex loop. Note that in most cases the loop must be completed at the office repeaters for this to be a valid loop, and office repeaters must have some type of switch or jumper specifically for this purpose. In a few cases, the simplex loop is not made this way. Sometimes the power is fed all the way into some CPE terminal equipment and the loop is made there, but this is not the most common case.

After this simplex power loop is engineered and installed correctly, a voltage drop can be measured across side 1 of a line repeater (about 7 VDC). Similarly measure from the span receive side of the office repeater to the span transmit side (about 7 to 12 VDC). In many far end office repeaters, T1 Channel service units, "Smart Jacks" and Network Interface Units (NIUs) that receive power from the span line, this simplex arrangement presents a voltage drop of 11 to 12 VDC. This is one method of verifying that the current has the correct polarity. If it is wrong, then the voltage measurement is only 0 to 1 VDC. Here is the way to remember it: The positive current flows in the same direction as the PCM signal direction. Note that there are alternative schemes of powering each element in a T1 transmission facility, but the most common one is illustrated here. See Figure 10.

 

Figure 10

In the classic T1 repeatered line, the simplex power planning must account for the equivalent DC resistances of office repeaters, line repeaters, attenuation pads and equalizers, and the copper conductors themselves. The longer the span line is, the more repeaters must be in series, hence, the more voltage must be applied at one end to feed the current loop. In a short length repeatered line, this voltage might be only 20 to 30 VDC, but as the length is stretched out to 10 to 15 miles, this might become ±130 VDC. As it gets extremely long, simplex current might be fed from both ends with the simplex looping back both ways in the middle. If these DC calculations must be made and there are no further engineering guidelines, assume that 22 gauge cable has an equivalent DC resistance of about 18 ohms per 1000 feet. Of course, 24 gauge and 26 gauge have much higher resistance. Assume that each line repeater is 120 ohms and one unpowered office repeater is 170 ohms.

There are small additional resistances for LBO networks and other pads in the circuit, and these must be added into the calculation. If you know this total equivalent resistance in the loop and you know the current is 60 mA, then apply Ohm's Law to solve for the minimum necessary voltage. Note that many of the most modern Automatic Span Powering Repeaters simply need local -48 VDC and they will develop the necessary voltage to regulate 60 mA into the simplex loop up to a maximum loop of 4000 to 4200 ohms.

However, in many high-rise building installations, the T1 span line is rather short, perhaps from the basement equipment room to the tenth floor. So short, in fact, that no line repeaters are needed. We can name it the Un-repeatered T1 Span Line. In these, we might see an office repeater at the near end and a "Smart Jack" or Network Interface Unit (NIU) at the far end, only 3000 cable feet away. In this short span line case, the voltage necessary to drive the simplex current loop is only 20 VDC, but the current loop must still be connected at the ends for the current flow to be correct. One small aggravation is that experienced T1 transmission people will refer to simplex power, loop current, simplex voltage, plus a few other terms, and they use these terms interchangeably to mean about the same thing. Obviously, the voltage applied to a loop resistance yields a loop current. Just keep in mind that equivalent resistance through cable is based on one twisted pair acting as parallel resistance (resistance of one wire ÷2). However, the simplex loop current must pass out the cable length then pass back the same length (×2). This, effectively, makes equivalent resistance the same as one wire for the one-way length.

Chapter 15: DS-1 Signal Details

Data Error Rates

Pulses sent along the repeatered line are regenerated at each repeater point. The repeater looks at each time slot and decides whether or not a pulse is present. If the repeater logic determines that there is a pulse, the repeater puts out a new pulse free of noise, distortion, or interference incurred in the preceding repeater section. Owing to degradation factors, a small number of pulses may be incorrectly regenerated; that is, a pulse will be transmitted where it was not present or vice versa. The ratio of transmitted time periods (pulse or no pulse) that are received incorrectly at the end point to the total number of time periods is called the "error rate."

The total error rate is the arithmetic sum of the error rates of the individual repeater sections. Because of the effect of impulse noise from central office equipment, end sections (the section nearest the central office) are the principal source of errors and, therefore, are shortened to increase the signal-to-noise ratio at the office repeater. Between the terminal ends, a maximum error rate of 1 in 106 results in good voice communications. Pulse errors cause transients in the individual voice channels, but at this rate they are not noticeable to the average listener. On the other hand, a higher error rate of 1 in 105 (BER=10-5) results in acceptable voice transmission although audible clicks are noticeable. Many data systems are less tolerant of bit errors, and facilities are engineered to meet error rates of 1 in 108, 109, or 1010 wherever possible. Any bit error on the span line results in a Bipolar Violation (BPV).

There is one specific type of error that could show up in a T1 span line. A BPV occurs as a result of bad wiring connections anywhere. A normal DS-1 signal uses Alternate Mark Inversion as the line code. In other words, if the terminal data stream is 11111111, then the line code is transmitted as marks (pulses) of alternating polarity, so we would see +1, -1, +1, -1, +1, -1, +1, -1. If a one BPV has been created, we would see two consecutive pulses of the same polarity. In this case, we might see +1, -1, +1, +1, -1, +1, -1, +1. B8ZS is a different line code that intentionally sends and receives BPVs in a specific pattern to carry a special meaning related to 64 kbps Clear Channel unrestricted data. This is often used for Primary Rate ISDN.

Most basic transmission line elements such as line repeaters are completely transparent to any type of framing (SF, ESF, or otherwise). In contrast, most pieces of DS-1 terminal equipment, such as channel banks, are very sensitive to proper framing format. Between these two types of elements, there might be intermediate elements, such as higher order multiplexers and automatic protection switches. These elements may or may not be sensitive to framing format. Some elements even convert from one framing format to another. A few also have the ability to auto-configure depending on the signal format that is first received.

Voltage and Temperature Factors

Normal simplex loop current is 60 mA for modern line repeaters. A few older repeaters use more current (100, 120, or 150 mA). Occasionally, problems with AC power induction can be overcome by increasing the loop current within the tolerance of the repeaters. However, in the absence of 60 Hertz induction, most simplex loops are set between 55 mA and 65 mA. Copper cable tends to have more resistance at higher ambient temperatures (it will take more voltage to drive the constant current through the loop), so knowing the expected cable temperature extremes for a locality helps set a strategy for optimizing loop current for best performance. Aerial cable is exposed to much hotter temperatures than buried cable, and this must be calculated into the design.

Chapter 16: Network Interface Units (NIU)

NIUs are sometimes referred to as "Smart Jacks" in that they have intelligent functions that are necessary for the demarcation point between the telco and the customer premises. NIUs bear a strong resemblance to terminating office repeaters that do not feed span power to the facility. They also bear a resemblance to T1 Channel Service Units (CSUs). Different NIUs have different feature sets, but diagnostic loopbacks are most commonly found.

The interesting situation is when there are no line repeaters. If the total length of the facility is less than 3000 to 4000 feet, then probably no mid-span T1 line repeaters are necessary. In that case, the DC polarity might accidentally be applied backwards at the CO and there is nothing on the copper line to fail until it gets to the NIU. So, if the NIU does not work on Day 1, first establish that DC is going correctly (this takes a DC voltmeter across the span side, and test for a 6 to 8 VDC voltage drop). Then move to tracing the high frequency signal. A DS-1 signal at 1.544 MHz is pretty unique and can be traced from point to point with the right kind of full-featured DS-1 test set, not a "receive only" DS-1 monitor. Remember that simplex current is only seen from the office repeater or NIU to the outside. Once DS-1 signals are inside the CO, inside from the office repeater, then simplex is no longer present. Inside, the DS-1 signals are +3 volt and -3 volt pulses with no simplex.

Chapter 17: Planning for Installation

Study all associated vendor information and compare to transmission engineering standards for your organization. Prepare a complete facility sketch, showing all outside plant involved with this project (include cable pairs, repeater housing details, fault locate scheme, etc.). Prepare a similar sketch of all inside plant involved (include floor plan, bay layout, signal wiring, power wiring, alarms, etc.). Show transmission calculations that justify repeater spacing for the particular cable. After engineering approval, supply copies of all sketches to the installation crew, the local central office files, and headquarters files.

Once the installation has been completed, the installation crew must supply "as-built" drawings for files. These may very well be the original untouched drawings or they may have notations from the crew for the changes that they had to make during installation. Examples include setting option switches differently to achieve better performance, using different cable pairs from the plan, and installing plug-in cards into specific slots.

In some operating companies, this is all handled on paper drawings. In others, this is handled exclusively with electronic files.

Chapter 18: Test Equipment Requirements

Every installation team and maintenance technician must have a good T1 bit error rate test (BERT) instrument. In many cases, a single BERT is adequate, but in other cases having one BERT at each end of a facility is an advantage. The primary BERT must have transmit and receive capability. Often, receive-only T1 monitor instruments are useful in conjunction with the primary set. The best BERTs feature two receive ports instead of one. The second port becomes handy for a synchronization troubleshooting situation.

For 98% of troubleshooting, all that is needed is a good BERT. One of the most common test instruments is generically called a T-BERD and is manufactured by TTC. In a really complex case of "finger-pointing" over the exact DS-1 waveform, the only way to solve it is to use a good high-frequency dual trace oscilloscope. With an oscilloscope, use two calibrated high-impedance probes set for A-B, or differential mode. Tie the two probe grounds together at the logic ground of the device under test. That way you can see if there is a DC offset voltage present.

Test Set Details

There are many brands of DS-1 test sets. A typical example is the TTC FireBERD 209. It has buttons and indicators on the front panel. When the user is trying to test from a CO out to a Larus NIU, he might insert the 209 signal at the office repeater and send the so-called Loop-Up Code. Keep in mind that there are several varieties of these codes for loopback purposes. Different products of different vintages use different loop codes to accomplish this function. There are four-bit, five-bit, and six-bit codes. Just because the technician pushes the button for Send Loop Up, he may not know exactly what code he is sending. If it happens to be the wrong code for the Larus unit, it won't be recognized, and that will cause confusion. Some test sets, like the 209, will allow you to walk down through detailed menus until you can see exactly what code bits are sent from that particular set. Just make sure you know.

Chapter 19 - Troubleshooting and Problem Isolation

General

Is this a new installation problem or troubleshooting existing equipment? What are all of the basic symptoms? If you can't get the symptoms clear enough to write them down in black and white, you will have trouble explaining it to any other troubleshooter. Has the product practice or user manual been read and understood? Was this caused by a human error, such as accidentally bumping a working pair of wires? Wires can break. What was the last thing that happened before the symptoms were seen? Segment the problem and solve the segments before you solve the big picture.

Keep in mind that there is a standard terminology problem that always exists around the telephone industry. In some places T & R indicates Tip and Ring, which identifies one wire from the other wire in a two-wire pair (don't confuse the term ‘ring’ with the term ‘ringing signal’). In other places T & R indicates Transmit and Receive. Frequently these are not single wires. In DS-1, these are Transmit pair and Receive pair. Of the Transmit pair, there is a ‘tip’ wire and a ‘ring’ wire. We could just as easily call them Transmit pair, wire A and wire B. But by some tradition, they were named tip and ring. Sometimes on a schematic they are named TT and TR (for Transmit Tip and Transmit Ring) and then RT and RR (for Receive Tip and Receive Ring). On still other equipment these are named T and R on the transmit pair and T1 and R1 on the receive pair. Again, remind yourself that the

Transmit signal at the near end becomes the Receive signal at the far end. In some places Transmit is abbreviated as XMT or Tx and Receive is abbreviated as RCV or Rx. There is no consistency, so be aware.

Another set of terms is East and West. In the early days of long-distance circuits, AT&T Long Lines developed standard terminology to explain what long-haul equipment at one town was connected to what equipment at the next town, etc. So they arbitrarily called all transmission directions either East-West or West-East. So it is quite common to be tracking a signal from one Office Repeater Shelf marked "East," meaning this is facing the span to the east. It might then go to another Office Repeater Shelf marked "West," meaning this is facing the span to the west. It is often more foolproof to simply draw lots of arrows on a circuit diagram, indicating the direction of all signals.

Switch Options, Framing and Line Code

With DS-1 signals inside the Central Office, you have to verify where the framing format is SF (also erroneously called D4) or ESF. On a few products, it doesn't matter too much (they automatically detect and set SF/ESF), but it is good to know (i.e. line repeaters and office repeaters generally don't care). You have to verify whether the Line Coding is AMI or B8ZS. On most products, this matters. It is good to sketch in these attributes onto the overall facility sketch as you go along. Verify all option settings.

Problem Classification

Exact symptoms are extremely important. An intermittent problem is much harder to fix than a constant problem. If it is intermittent, then its pattern of occurrence must be made reproducible. In other words, if it fails once per day every day, then that means something. If it fails only on the hottest day of the year, then that means something different. If it is an intermittent noise problem on a DS-1 circuit, then this is a little tougher. In general with an intermittent signal, if you can keep testing it until you can get it to fail consistently, you are half-way to solving the problem. To a large extent on DS-1 signals, either it works, or it doesn't work at all. Rarely is there a marginal noisy problem except with grounding problems. If we have a problem with a poor logic bit error rate, then that can be tracked down. "The test signal measures perfect at A, then again at B, but it is poor at C." We can then concentrate on the B to C connection. If there is a problem with Bipolar Violations (BPVs), then this means something.

If you see a "clean" BER with one legal DS-1 test pattern, then you see a "dirty" BER with another legal pattern, often this is a clue to a bridge tap on the copper pair.

As a general rule, BPVs are generated from two sources. One is a poorly performing T1 line repeater or bad copper T1 line pairs. The other source is the last ten feet of line coming into the failing product. Very commonly, there is a problem with the last few feet of DS-1 jumper wires, like a single broken wire (not a broken pair) at a wire-wrap post. Or maybe a "shiner" (a wire where the plastic insulation has been sliced off to reveal the shiny, tinned, copper conductor). Shiners can easily make an intermittent short to a grounded shelf or anything else. Thermal intermittents are hard to reproduce without a lab temperature chamber. A can of freeze spray can help once in a while, but this is unusual. Fortunately, thermals are not that common anymore with modern low-power components. Another easy test is to tap an intermittent shelf with the handle of a screwdriver to see if some intermittent component will produce a bit error. If you can get an intermittent problem to the point where you can consistently make it come or go, then you are halfway to solving the problem.

Chapter 20: Acceptance Testing

Many systems and products include complete documentation, covering acceptance testing. Individual circuit packs may have their own segmented "card-level" tests, but after everything is hooked up end-to-end, a real-world test must be made to verify that the entire system is fully ready to support live traffic. For many T1 transmission systems, the bit error rate tests use QRSS (Quasi Random Signal Sequence pattern). There may be a different test strategy for each different type of T1 product, but a few common strategies are suggested in the following figure, Figure 11. The exact error-free performance requirements vary from one location to another, but if there are no other guidelines to use, start with BER of 10-8 or better if the live traffic contains only voice channels. If data channels are present, try to start with BER of 10-10 or better. Over some facilities, still better performance can be measured, but it takes a tremendous amount of time. For normal telco transmission facilities, acceptance tests are commonly run for periods of a few hours at most. It is most important to record all information regarding the acceptance tests. It is good form to include a copy of the product practice that lists the test procedure along with the actual results and a sketch of how the tests were run. Acceptance tests should be kept on file in every central office or equipment room.

 

Figure 11

Chapter 21: Glossary and Acronyms

A/D Analog to digital conversionADPCM Adaptive differential pulse code modulation

AISAlarm indication signal; a signal transmitted in lieu of the normal signal to maintain transmission continuity, and indicates to the receiving terminal that there is a transmission fault located upstream or at the transmitting terminal

ALBO Automatic line build outAMI Alternate mark inversionANSI American National Standards InstituteAPS Automatic protection switchASPR Automatic span powering repeaterAWG American wire gauge

B8ZSBipolar eight zero substitution; a line coding scheme to substitute 000 + - 0 - + for 00000000 if the preceding pulse was +, and 000 - + 0 + - for 00000000 if the preceding pulse was -. Done to maintain ones density

BER Bit error rate

BERT Bit error rate testBOC Bell Operating Companybps Bits per second1BPV Bipolar violation; the detection of any isolated error, whether it deletes a pulse or adds a pulse

CCC1Clear channel capability; allows unrestricted data bearing any number of consecutive zeros to pass through a transmission facility. This usually requires B8ZS line coding on all elements

CO Central office; of a telephone company; the location of the switching centerCODEC Coder-decoder; converts analog voice to digital and vice-versaCPE Customer premises equipment

CRCCyclic Redundancy Check; a method of detecting the existence of errors in the transmission of a digital signal using polynomial division

CSU Channel Service Unit; the interface from CPE to the public T-1 lineD4 Fourth generation digital channel bank

DACSDigital access and cross-connect system; a digital switching device for routing T-1 lines and possibly DS-0 portions of lines, among multiple T-1 ports

dB DecibeldBdsx Decibels with respect to the standard level at the DSX-1 cross-connectDC Direct current; battery powerDLC Digital loop carrierDS-0 Digital signal, level 0; 64 kilobits per second, the international standard rate for digitized voice channelsDS-1 Digital signal, level 1; 1.544 megabits per second, the North American standardDS-3 Digital signal, level 3; 44.736 megabits per second, the North American standardDSU Digital service unit; converts RS232, RS422, or V.35 terminal interface data to DSX-1 interfaceDSX-1 Digital service cross connect, level 1; part of the DS-1 specificationDTMF Dual tone multi-frequency; TOUCHTONE dialing, as opposed to rotary dialingE&M Signaling leads on a voice tie line; known as Ear and Mouth

ESFExtended Super Frame, a DS-1 framing format of 24 frames. In this format, 2 Kbps are used for framing pattern sequence, 4 Kbps are used for the Facility Data Link, and the remaining 2 Kbps are used for CRC

FB Framing bitFCC Federal Communcations CommissionFDL Facility data link; an embedded overhead channel within the ESF formatFEC Forward error correctionHz Hertz, cycles per secondISDN Integrated Services Digital NetworkIXC Interexchange carrier; a long distance telephone company (not a LEC)kA KiloampereskHz Kilohertz, thousands of cycles per secondLATA Local access and transport areaLEC Local exchange carrierM24 Multiplexer that converts one DS-1 line to 24 voice channels for a C.O.M44 Multiplexer that converts one T-1 of 44 ADPCM channels into two T-1s with PCM encodingmA MilliamperesMHz MegahertzMux MultiplexerNI Network interface; demarcation point between the public network and CPENIU Network interface unit; the unit installed at the demarcation which provides loopback test capabilitiesOOF Out of frameOOS Out of synchronization or else out of servicePAM Pulse amplitude modulationPBX Private branch exchange; a private telephone switching systemPCM Pulse code modulationpF Picofarads, a measure of capacitance PIC Polyethylene insulated conductorPL Private line; a leased line, not switchedPOP Point of presence; termination of long distance line from a long distance carrier at a central officePOTS Plain old telephone servicePSTN Public switched telecommunications networkQRSS Quasi random signal sequenceRBOC Regional Bell Operating CompanyRZ Return to zero; DS-1 signals pause at zero voltage between each pulse when making zero crossingsSLC Subscriber loop carrierSF Superframe Format; a DS-1 framing format of 12 frames

T1 A 1.544 Mbps transmission standardT-BERD A trade name for T1 bit error rate tester made by TTCTDM Time division multiplexer

TSITime slot interchange; a method of temporarily storing data bytes so they can be sent in a different order than received; a way to switch

VF Voice frequencyVG Voice grade; the common analog telephone line

VMRViolation monitor removal; a function which removes all violations which are detected so that they do not propagate along the span line

ZBTSIZero Byte Time Slot Interchange; a method of providing Clear Channel Capability without using B8ZS. This was an interim solution advocated by Verilink