DISCUSSION Outgoing Inter-Exchange Call … Inter-Exchange Call Routing Sequence 2-14 CO A CALLING...

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EXERCISE OBJECTIVE

When you have completed this exercise, you will be familiar with the sequence ofevents that occurs in a Lab-Volt CO when an outgoing inter-exchange call isestablished. You will also be familiar with the two different sequences of events thatcan occur in a Lab-Volt CO when an inter-exchange call is terminated. Gettingfamiliar with these sequences of events will allow you to understand themechanisms by which outgoing inter-exchange calls are established and terminatedin Lab-Volt CO's.

DISCUSSION

Outgoing Inter-Exchange Call Establishment

Figure 2-3 shows the typical sequence of events that occurs in a Lab-Volt CO whenan outgoing inter-exchange call is established, that is, when an inter-exchange callis initiated by this CO (CO A in Figure 2-3). This sequence consists of the call-control messages exchanged with the CO of the called party (CO B in Figure 2-3)plus the actions performed by the CALL PROCESSOR in the CO of the calling party(CO A in Figure 2-3).

Outgoing Inter-Exchange Call Routing Sequence

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CO A

CALLING PARTY CALLED PARTY

SETUP

CALPRC

IE's : 04H, 18H, 6CH, 70H

IE : 18H

ALERT

D CHANNEL OF DIGITAL TRUNK

TELEPHONE CONVERSATION

CONN

CO B

CALL PROGRESS TONE TRANSMISSION

CALL PROGRESS TONE REMOVAL

INTER-EXCHANGE CALL SETUP

CALL PROGRESS TONE TRANSMIT

PATH RELEASE

CONACK

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Figure 2-3. Sequence of events that occurs in a Lab-Volt CO when an outgoing inter-exchange call

is established.

The sequence begins once CO A has processed a telephone number received fromthe calling party's telephone set and determined that it is a valid number. Thiscauses CO A to send a SETUP message to CO B via the D channel of the digitaltrunk. This message includes a Bearer Capability (04H) IE that indicates therequested bearer service, a Channel Identification (18H) IE that identifies theB channel (time slot) of the digital trunk which will be used to convey voice data, aCalling Party Number (6CH) IE that provides the telephone number and name of thecalling party, and a Called Party Number (70H) IE that contains the telephonenumber of the called party.

Note: In a Lab-Volt CO, a telephone number received from a calling party'stelephone set is considered valid when it exactly corresponds to the completetelephone number associated with the other analog line interface in this CO(intra-exchange call), or when the first digits in the number received from thecalling party's telephone set correspond to the number of the CO at the otherend of the digital trunk (inter-exchange call).

On reception of the SETUP message from CO A, CO B analyzes the contents ofthe IE's in this message and determines that it has all the information necessary toestablish the inter-exchange call, and consequently, replies to CO A with a CALLPROCEEDING (CALPRC) message. This message includes a Channel


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Identification (18H) IE that confirms the B channel (time slot) of the digital trunkwhich will be used to convey voice data. CO B then verifies whether or not thecalled party's telephone set is busy. If not, CO B sends an ALERTING (ALERT)message to CO A and makes this telephone set ring.

On reception of the ALERT message from CO B, CO A starts transmitting the ringback tone to the calling party's telephone set at the ringing cadence. This isillustrated in Figure 2-3 by the CALL PROCESSOR actions Call Progress ToneTransmission and Call Progress Tone Removal. Note that at this point, aconnection has already been setup in CO A for the transmission of call progresstones to the calling party's telephone set.

When the called party answers the call, this is detected by CO B which immediatelyconnects the analog line interface associated with this telephone set to the digitaltrunk via the digital trunk interface. CO B also sends a CONNECT (CONN)message to CO A to indicate that the called party answered the call and is nowconnected to the digital trunk.

On reception of the CONN message from CO B, CO A immediately stopstransmitting the ring back tone to the calling party's telephone set and releases theconnection allowing transmission of call progress tones to this telephone set (CALLPROCESSOR action Call Progress Tone Transmit Path Release in Figure 2-3).Once this is done, CO A connects the analog line interface associated with thecalling party's telephone set to the digital trunk via the digital trunk interface (CALLPROCESSOR action Inter-Exchange Call Setup in Figure 2-3). It also sends aCONNECT ACKNOWLEDGE (CONACK) message to CO B to confirm thereception of a CONN message and indicate that the connection is established at itsend of the digital trunk. The two parties involved in the call can now have atelephone conversation through the connection established between CO A andCO B via the digital trunk, their voice signals being conveyed through the B channel(time slot) indicated by the Channel Identification IE in the call-control messagesexchanged to establish the call.

Inter-Exchange Call Termination

Figure 2-4 shows the typical sequence of events that occurs in a Lab-Volt CO whenthe local party, that is, the party connected to this CO (CO A in Figure 2-4) hangsup first at the end of a telephone conversation to initiate termination of an inter-exchange call. This sequence consists of the call-control messages exchanged withthe CO of the other party involved in the call (CO B in Figure 2-4) plus the actionsperformed by the CALL PROCESSOR in the CO of the local party (CO A inFigure 2-4).


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CO A

THE CALL

DISC

RLSE

IE : 08H

RLCOM

D CHANNEL OF DIGITAL TRUNKCO B

CLEAR SIGNAL RECEPTION

INTER-EXCHANGE CALL TERMINATION

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TO TERMINATE

HANGS UP FIRST

LOCAL PARTY

Figure 2-4. Sequence of events that occurs in a Lab-Volt CO when the local party hangs up first to

initiate termination of an inter-exchange call.

The sequence begins when the local user hangs up. This is detected by CO A(CALL PROCESSOR action Clear Signal Reception in Figure 2-4) whichimmediately disconnects the analog line interface (ALI) associated with thetelephone set of the local party from the digital trunk and marks this ALI as available(CALL PROCESSOR action Inter-Exchange Call Termination in Figure 2-4).Once this is done, CO A sends a DISCONNECT message to CO B. This messagecontains a Cause (08H) IE that indicates normal call clearing.

On reception of the DISC message from CO A, CO B immediately disconnects theanalog line interface associated with the telephone set of the other party involvedin the call from the digital trunk. It then replies to CO A with a RELEASE (RLSE)message to confirm the reception of a DISC message and indicate that theconnection has been released at its end of the digital trunk.

On reception of the RLSE message, CO A sends a RELEASECOMPLETE (RLCOM) message to CO B in order to confirm the reception of theRLSE message and that the connection has been released at its end of the digitaltrunk.

Figure 2-5 is another sequence of events that can occur in a Lab-Volt CO when aninter-exchange call is terminated. In this sequence, it is the remote party, that is, theparty connected to the other CO involved in the call (CO B in Figure 2-5) that hangsup first at the end of a telephone conversation to initiate termination of the inter-exchange call. The sequence consists of the call-control messages exchanged withthe CO of the remote party plus the actions performed by the CALL PROCESSORin the CO of the local party (CO A in Figure 2-5).


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CO A

THE CALL

DISC

RLSE

IE : 08H

RLCOM


INTER-EXCHANGE CALL TERMINATION


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TO TERMINATE

HANGS UP FIRST

REMOTE PARTY

Figure 2-5. Sequence of events that occurs in a Lab-Volt CO when the remote party hangs up first

to initiate termination of an inter-exchange call.

The sequence begins when the remote party hangs up. This is detected by CO Bwhich immediately disconnects the analog line interface associated with thetelephone set of the remote party from the digital trunk. Once this is done, CO Bsends a DISCONNECT message to CO A. This message contains aCause (08H) IE that indicates normal call clearing.

On reception of the DISC message from CO B, CO A immediately disconnects theanalog line interface associated with the telephone set of the local party from thedigital trunk (CALL PROCESSOR action Inter-Exchange Call Termination inFigure 2-5). It then replies to CO B with a RELEASE (RLSE) message to confirmthe reception of a DISC message and indicate that the connection has beenreleased at its end of the digital trunk.

On reception of the RLSE message, CO B sends a RELEASECOMPLETE (RLCOM) message to CO A in order to confirm the reception of theRLSE message and that the connection has been released at its end of the digitaltrunk.

When the local party hangs up, this is detected by CO A (CALL PROCESSORaction Clear Signal Reception in Figure 2-5) which marks the analog line interfaceassociated with the telephone set of the local party as available.

Comparing the sequences of events in Figures 2-4 and 2-5 reveals that they containthe same call-control messages and CALL PROCESSOR actions. However, thesemessages and actions occur in a different order in each sequence. Furthermore, themessages transmitted by a CO in one sequence are received by this CO in theother sequence. Similarly, the messages received by a CO in one sequence aretransmitted by this CO in the other sequence.


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CO A

CALLING PARTY CALLED PARTY

SETUP

CALPRC

IE's : 04H, 18H, 6CH, 70H

IE : 18H

PROG


CALL PROGRESS TONE TRANSMISSION


CALL PROGRESS TONE REMOVAL

CALL PROGRESS TONE TRANSMIT

PATH RELEASE

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IE's : 08H, 1EH

IE : 08H

RLCOM

RLSE

DISC

Failed Establishment of an Outgoing Inter-Exchange Call

The sequence of events in Figure 2-3 showed successful establishment of anoutgoing inter-exchange call. Figure 2-6 is another sequence of events that occursin a Lab-Volt CO when establishment of an outgoing inter-exchange call failsbecause the called party is busy or the dialed number is not a valid number (i.e.unallocated number).

Figure 2-6. Sequence of events that occurs in a Lab-Volt CO when establishment of an outgoing

inter-exchange call fails.

The sequence of events in Figure 2-6 begins in the same way as the sequence inFigure 2-3, that is, CO A sends a SETUP message to CO B which then replies witha CALL PROCEEDING (CALPRC) message. However, when CO B analyzes thecontents of the Called Party Number (70H) IE included in the SETUP messagereceived from CO A, it determines that the call cannot be established because it isdirected toward a telephone set that is busy, or because the requested telephonenumber is not a valid number. This causes CO B to send a PROGRESS (PROG)message to CO A instead of an ALERTING (ALERT) message. This PROGmessage includes a Cause (08H) IE indicating the reason why the call cannot beestablished and a Progress Indicator (1EH) IE indicating that in-band information(appropriate call progress tone) is to be provided to the calling party.


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On reception of the PROG message from CO B, CO A starts transmitting theappropriate call progress tone (busy tone or special information tone) to the callingparty's telephone set (CALL PROCESSOR action Call Progress ToneTransmission in Figure 2-6).

When hearing the call progress tone, the calling party understands that the callcannot be established and hangs up. This is detected by CO A (CALLPROCESSOR action Clear Signal Reception in Figure 2-6) which immediatelyceases transmission of the call progress tone toward the calling party's telephoneset and releases the connection used to transmit this call progress tone. This isillustrated in Figure 2-6 by the CALL PROCESSOR actions Call Progress ToneRemoval and Call Progress Tone Transmit Path Release. CO A then terminatesthis unsuccessful attempt to establish an inter-exchange call by exchanging thenormal sequence of call-control messages (DISC, RLSE, and RLCOM) with CO B.

CALL PROCESSOR Log Function in Lab-Volt CO's

A function of the CALL PROCESSOR in Lab-Volt CO's allows recording of all thecall-control messages (ISDN layer-3 messages) that are exchanged with anotherCO via the digital trunk during inter-exchange calls as well as all actions performedby the CALL PROCESSOR that result from the exchange of these messages. Therecorded call-control messages include all IE's as well as their contents(parameters). The recorded CALL PROCESSOR actions include all phases of eachaction. This recording function is referred to as the Call Processor Log.

The operation of the Call Processor Log is similar to that of a recorder: the recordingof the call-control messages and CALL PROCESSOR actions can be started andstopped when required. A recorded sequence of call-control messages and CALLPROCESSOR actions can then be displayed, or printed, to perform step-by-stepanalysis.

The Call Processor Log is included in Lab-Volt CO's for educational purposes only.It is used extensively in the remaining exercises of this manual to study thesequences of events that occur in Lab-Volt CO's during inter-exchange calls.

Procedure Summary

In the first part of the exercise, you will set up a digital trunk between two Lab-VoltCentral Offices (CO's).

In the second part of the exercise, you will establish an outgoing inter-exchange calland then terminate the call by hanging up the handset of the calling party's (localparty's) telephone set first. While doing this, you will record the call-controlmessages exchanged between the two CO's, as well as the CALL PROCESSORactions performed in the CO of the calling party, using the Call Processor Log. Youwill then analyze the recorded messages and their IE contents, as well as therecorded actions and their different phases.

In the third part of the exercise, you will establish an outgoing inter-exchange calland then terminate the call by hanging up the handset of the called party's (remote


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party's) telephone set first. While doing this, you will record the call-controlmessages exchanged between the two CO's, as well as the CALL PROCESSORactions performed in the CO of the calling party. This will allow you to comparethese messages and actions to those recorded when the call is terminated when thecalling party hangs up first.

In the last part of the exercise, you will try establishing an outgoing inter-exchangecall by dialing an invalid number on the calling party's telephone set. While doingthis, you will record the call-control messages exchanged between the two CO's, aswell as the CALL PROCESSOR actions performed in the CO of the calling party.You will then analyze the recorded messages and actions.

Note: This exercise is quite long. However, it can be divided in two parts andcarried out in two laboratory periods.

In the first laboratory period, carry out the PROCEDURE sub-sections entitled:– Setting Up a Digital Trunk Between two Lab-Volt Central Offices– Establishment and Termination of an Outgoing Inter-Exchange Call

In the second laboratory period, carry out the PROCEDURE sub-sectionsentitled:– Setting Up a Digital Trunk Between two Lab-Volt Central Offices– Termination of an Outgoing Inter-Exchange Call by the Called (Remote)

Party– Failed Establishment of an Outgoing Inter-Exchange Call

EQUIPMENT REQUIRED

Refer to Appendix A of this manual to obtain the list of equipment required toperform this exercise.

PROCEDURE

Setting Up a Digital Trunk Between two Lab-Volt Central Offices

Note: In this exercise, it is assumed that a single host computer is usedto download the CO program to two Reconfigurable Training Modules,Model 9431. This host computer is used to monitor one of the twoLab-Volt CO's (the one designated as CO A throughout the exercise).

* 1. Make sure that two Reconfigurable Training Modules, Model 9431, areconnected to the Power Supply, Model 9408.

Make sure that there is a network connection between each ReconfigurableTraining Module and the host computer.

* 2. Install a Dual Analog Line Interface, Model 9475, into one of the twoanalog/digital (A/D) slots of a Reconfigurable Training Module. This module


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will be used as CO A. Connect two analog telephone sets to this DualAnalog Line Interface. Make sure that the tone dialing mode is selected oneach telephone set.

CAUTION!

Do not connect or disconnect the analog telephone sets

when the Reconfigurable Training Module is turned on. High

voltages are present on the standard telephone connectors

of the Dual Analog Line Interface.

Install a Digital Trunk Interface, Model 9478, into the digital (D) slot or theremaining analog/digital (A/D) slot of the Reconfigurable Training Moduleused as CO A.

* 3. Install a Dual Analog Line Interface, Model 9475, into one of the twoanalog/digital (A/D) slots of the other Reconfigurable Training Module. Thismodule will be used as CO B. Connect two analog telephone sets to thisDual Analog Line Interface. Make sure that the tone dialing mode isselected on each telephone set.

Install a Digital Trunk Interface, Model 9478, into the digital (D) slot or theremaining analog/digital (A/D) slot of the Reconfigurable Training Moduleused as CO B.

* 4. Using the digital trunk line provided with one of the Digital Trunk Interfaces(multi-wire cable terminated with RJ-45 male telephone connectors),connect the RJ-45 female connector on the Digital Trunk Interface installedin the Reconfigurable Training Module used as CO A to the RJ-45 femaleconnector on the Digital Trunk Interface installed in the ReconfigurableTraining Module used as CO B.

* 5. Connect two of the AC/DC power converters supplied with the analogtelephone sets to the AC power outlets on the Power Supply. Then,connect the DC power output jack of each AC/DC power converter to theDC power input connector on either of the analog telephone sets A ofCO's A and B.

Note: If another Power Supply, Model 9408, is available,connect the two other AC/DC power converters supplied with theanalog telephone sets to the AC power outlets on this PowerSupply. Then, connect the DC power output jack of eachAC/DC power converter to the DC power input connector oneither of the analog telephone sets B of CO's A and B.

* 6. Turn on the host computer.


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Turn on the Power Supply, then turn on the Reconfigurable TrainingModules.

* 7. On the host computer, start the Telephony Training System software, thendownload the CO program to the Reconfigurable Training Module used asCO A.

Note: If the host computer is unable to download theCO program to the Reconfigurable Training Module used asCO A, make sure that the proper IP address is used tocommunicate with this Reconfigurable Training Module.

* 8. On the host computer, download the CO program to the ReconfigurableTraining Module used as CO B, using the function in the Tools menu thatallows a functionality to be downloaded to another RTM.

Note: If the host computer is unable to download theCO program to the Reconfigurable Training Module used asCO B, make sure that the proper IP address is used tocommunicate with this Reconfigurable Training Module.

Furthermore, if you downloaded the CO program to theReconfigurable Training Module used as CO B from a secondhost computer, make sure that the digital trunk TDM format(DS1 or E1) selected in CO B is the same as that selected inCO A. Also make sure that the number of CO B differs from thenumber of CO A. These parameters, which are accessed via theLVTTS Options dialog box, must be set as mentioned above toenable establishment of inter-exchange calls via the digital trunk.These parameters are set automatically when a single hostcomputer is used to download the CO program to the twoReconfigurable Training Modules.

Once the CO program has been downloaded, a box representing CO B willappear connected to CO A, via a digital trunk, in the diagram of CO Adisplayed on the host computer screen.

Record below the telephone numbers associated with ANALOG LINEINTERFACEs A and B of CO B.

ANALOG LINE INTERFACE A (ALI A) of CO B:

ANALOG LINE INTERFACE B (ALI B) of CO B:

* 9. On the Digital Trunk Interfaces installed in the Reconfigurable TrainingModules used as CO's A and B, make sure that the alarm indicators (redand yellow LED's) are not lit, thereby confirming that the digital trunk is nowready to carry inter-exchange calls.


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* 10. On the host computer, make sure that the addresses of the TSAC's inALI's A and B are set to 01 and 02, respectively.

Also, make sure that the addresses of the TSAC's of SERVICE CIRCUITs1 and 2 in the SIGNALING CIRCUIT are set to 01 and 02, respectively.

Establishment and Termination of an Outgoing Inter-Exchange Call

* 11. Make the following settings on the host computer:

– Display the control registers of TIME SLOT INTERCHANGERs 1 and2 (TSI-1 and TSI-2 Control Registers) of the DIGITAL TRUNKINTERFACE.

– Display the contents of the HOOK STATUS BUFFER MEMORY in theSIGNALING CIRCUIT.

– Display the control register of the SPACE-DIVISION SWITCH (SDSControl Register) in the SWITCHING CIRCUIT.

– Display the Call Processor Log window.

– Adjust the view in order to be able to simultaneously observe the TSI-1and TSI-2 Control Registers, the HOOK STATUS BUFFER MEMORY,the SDS Control Register, and the CO-A circuitry in its entirety.

* 12. Prepare yourself to observe the sequences of events that occur in CO Awhen an outgoing inter-exchange call from CO A to CO B is established,and then terminated when the calling party (local party at CO A) hangs upfirst. To do so, read the next step, then proceed to this step while observingall the events described in it.

* 13. Start recording the call-control messages and CALL PROCESSOR actionsin CO A.

Lift off the handset of telephone set A of CO A and observe the sequenceof events described below.

– In the HOOK STATUS BUFFER MEMORY, bit D0 at memoryaddress 00 (with TDM over 24 time slots) or 01 (with TDM over 32 timeslots) goes from logic state 0 to logic state 1.

– The CALL PROCESSOR connects ALI A to SERVICE CIRCUIT 1(SC1) for transmission of digitized call progress tones produced by SC1to ALI A.

The CALL PROCESSOR also connects SC1 to ALI A for transmissionto SC1 of the digitized DTMF dialing tones that will eventually beproduced by ALI A.


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– The dial tone produced by the TONE GENERATOR in SC1 can beheard in the handset earpiece of telephone set A of CO A.

On telephone set A of CO A, slowly dial each digit in the number oftelephone set A of CO B, and observe the sequence of events describedbelow.

– When the first digit is dialed, it is detected by the DTMF DETECTORof SC1 and indicated by the DETECTED DIGIT display of SC1. Thiscauses dial tone generation to cease in SC1.

– When each of the other digits is dialed, it is detected by the DTMFDETECTOR of SC1 and indicated by the DETECTED DIGIT display ofSC1.

– As soon as the last digit is dialed, the CALL PROCESSOR releases theconnection for the transmission of the digitized DTMF dialing tonesproduced by ALI A to SC1.

– CO A sends a SETUP message to CO B.

– CO B replies with a CALL PROCEEDING (CALPRC) message toCO A.

– Telephone set A of CO B starts to ring.

– CO B sends an ALERTING (ALERT) message to CO A.

– The ring back tone produced by the TONE GENERATOR of SC1 canbe heard in the handset earpiece of telephone set A of CO A.

Lift off the handset of telephone set A of CO B to answer the call andobserve the sequence of events described below.

– CO B sends a CONNECT (CONN) message to CO A.

– Ring back tone generation ceases in SC1 and the CALL PROCESSORreleases the connection for transmission of digitized call progress tonesproduced by SC1 to ALI A.

– The SPACE-DIVISION SWITCH connects line TX0 to line RX2 duringtime slot 1, and line TX2 to line RX0 during time slot 3, while the ALI-ARX time slot is set to 03. This connects ALI A to the DTI, in bothtransmission and reception.

– TIME SLOT INTERCHANGER 2 in the DTI performs time slotinterchange from time slot 1 to time slot 6. TIME SLOTINTERCHANGER 1 in the DTI performs time slot interchange fromtime slot 6 to time slot 3.

– CO A sends a CONNECT ACKNOWLEDGE (CONACK) message toCO B.


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– A normal conversation can occur between telephone sets A of CO's Aand B.

Hang up the handset of telephone set A of CO A to terminate the call andobserve the sequence of events described below.


– Time slot interchange from time slot 1 to time slot 6 is released in TIMESLOT INTERCHANGER 2 of the DTI, while time slot interchange fromtime slot 6 to time slot 3 is released in TIME SLOT INTERCHANGER 1of the DTI.

– The SPACE-DIVISION SWITCH releases the TX0-to-RX2 connectionmade during time slot 1 and the TX2-to-RX0 connection made duringtime slot 3, while the ALI-A RX time slot is set to none. Thisdisconnects ALI A from the DTI.

– CO A sends a DISCONNECT (DISC) message to CO B.

– CO B sends a RELEASE (RLSE) message to CO A.

– CO A replies with a RELEASE COMPLETE (RLCOM) message.

Hang up the handset of telephone set A of CO B. Stop recording the call-control messages and the CALL PROCESSOR actions in CO A.

* 14. Display the contents of each call-control message (IE's) recorded in theCALL PROCESSOR Log, as well as the contents of each IE(IE components). Also, display the additional information about each CALLPROCESSOR action recorded in the log.

Make sure that the circuitry of the ALI's, SWITCHING CIRCUIT,SIGNALING CIRCUIT, CALL PROCESSOR, and DIGITAL TRUNKINTERFACE of CO A can be observed at the same time.

* 15. Print the contents of the CALL PROCESSOR Log.

Note: If a printer is not available, save the CALL PROCESSORLog to an LVTTS file. This will allow this log to be compared toother logs you will record in this exercise and in the next one.


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Outgoing Inter-Exchange Call Establishment

* 16. In the CALL PROCESSOR Log, examine the actions performed by theCALL PROCESSOR in CO A to initiate the outgoing inter-exchange call:

– the actions performed when the calling party at telephone set A ofCO A lifts off the telephone handset (Service Request, Call ProgressTone Transmit Path Setup, DTMF Dialing Tone Receive PathSetup, and Call Progress Tone Transmission actions);

– the actions used to receive the dialed digits, stop dial tone generation,and release the connection used for reception of DTMF dialing tones(Dialed Digit Reception, Call Progress Tone Removal, and DTMFDialing Tone Receive Path Release actions);

– the action used to recover the complete telephone number of the calledparty (Dialed Telephone Number Processing action).

From your observations, which ALI is associated with telephone set A ofCO A? To which SERVICE CIRCUIT of CO A is this interface connectedto for reception of digitized call progress tones and transmission of digitizedDTMF dialing tones? Explain.

What does the CALL PROCESSOR in CO A determine once it hasrecovered the complete telephone number of the called party? Explain.

* 17. Locate the SETUP message sent by CO A to CO B after the CALLPROCESSOR action Dialed Telephone Number Processing. What is thepurpose of this message?


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* 18. In the SETUP message, examine the contents of the Bearer Capability IE.What are the B-channel characteristics required within the digital trunkinterface of CO B in order to establish an inter-exchange connection (call)?Explain.

* 19. In the SETUP message, what is the function of the Channel IdentificationIE?

According to the Interface Type component included in the ChannelIdentification IE, of what type must the digital trunk interface of CO B be?Explain.

According to the other components included in the Channel IdentificationIE, which B channel (time slot) of the digital trunk will be used to conveyvoice data? Is CO B allowed to request that another channel be used?Explain.

* 20. In the SETUP message, examine the contents of the Calling PartyNumber IE. What is the function of this IE? Explain.


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* 21. In the SETUP message, examine the contents of the Called Party NumberIE. What is the function of this IE? Explain.

* 22. Using the Playback function of the Call Processor Log, play back theSETUP message. From your observations, explain how the CALLPROCESSOR of CO A sends this message and the information includedin it to CO B via the D channel of the digital trunk.

* 23. In the CALL PROCESSOR Log, observe that, upon reception of theSETUP message from CO A, CO B returns a CALL PROCEEDING(CALPRC) message to CO A. Explain why.

* 24. In the CALPRC message, examine the contents of the ChannelIdentification IE. According to the Interface Type component included in thisIE, of what type must the digital trunk interface of CO A be? Explain.

According to the Channel Number component included in the ChannelIdentification IE, does CO B confirm that channel 6 (time slot 6) of thedigital trunk will be used to convey voice data? Explain.


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* 25. Play back the CALPRC message sent by CO B to CO A. From yourobservations, explain how the signaling data corresponding to thismessage, received from CO B via the D channel of the digital trunk, isrouted to the CALL PROCESSOR of CO A.

* 26. What does the ALERTING (ALERT) message sent by CO B to CO Aindicate? Explain.

* 27. Observe that, upon reception of the ALERTING (ALERT) message fromCO B, the CALL PROCESSOR in CO A performs the actions CallProgress Tone Transmission and Call Progress Tone Removal.

Play back these actions. Why and how does the CALL PROCESSOR inCO A perform these actions?

* 28. What does the CONNECT (CONN) message sent by CO B to CO Aindicate?


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* 29. What two actions are performed by the CALL PROCESSOR in CO A uponreception of the CONNECT (CONN) message from CO B?

* 30. What is the purpose of the action Call Progress Tone Transmit PathRelease performed by the CALL PROCESSOR in CO A?

* 31. What is the purpose of the action Inter-Exchange Call Setup performedby the CALL PROCESSOR in CO A?

Play back the action Inter-Exchange Call Setup. How does the CALLPROCESSOR in CO A perform this action? Explain.

* 32. Why does CO A send a CONNECT ACKNOWLEDGE (CONACK)message to CO B?


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Outgoing Inter-Exchange Call Termination Initiated by the Calling (Local) Party

* 33. In the CALL PROCESSOR Log, locate the call-control messages and CALLPROCESSOR actions used to terminate the outgoing inter-exchange callfrom CO A to CO B when the calling party (local party at CO A) hangs upfirst.

Notice that the signaling procedure for call termination begins with theCALL PROCESSOR action Clear Signal Reception. What is the meaningof the following information about this action: "clear forward signal receivedfrom ALI HS0, 01"? (Answer by playing back this information).

* 34. Locate the CALL PROCESSOR action Inter-Exchange Call Termination.What is the purpose of this action?

Play back the action Inter-Exchange Call Termination. How does theCALL PROCESSOR in CO A perform this action? Explain.

* 35. Why does CO A send a DISCONNECT (DISC) message to CO B?


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Examine the contents of the Cause IE included in the DISC message.According to the Location component, where does this message originatefrom?

According to the Cause Value component, why must the connection bereleased?

* 36. Why does CO B send a RELEASE (RLSE) message to CO A? Why doesCO A reply with a RELEASE COMPLETE (RLCOM) message?

* 37. As a review of what you have learned in this part of the exercise, use thePlayback function of the Call Processor Log to display the completesequence of events illustrating what happens in CO A when an outgoinginter-exchange call from CO A to CO B is established, and then terminatedwhen the calling party (local party at CO A) hangs up first.

* 38. Clear the contents of the CALL PROCESSOR Log.

Termination of an Outgoing Inter-Exchange Call Initiated by the Called(Remote) Party

* 39. On the host computer, make sure that the view is adjusted so that thefollowing elements can be simultaneously observed:

– The TSI-1 and TSI-2 Control Registers of the DTI;

– The contents of the HOOK STATUS BUFFER MEMORY in theSIGNALING CIRCUIT;

– The SDS Control Register in the SWITCHING CIRCUIT;

– The CO-A circuitry in its entirety.


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* 40. Prepare yourself to observe the sequence of events that occurs in CO Awhen an outgoing inter-exchange call established from CO A to CO B isterminated when the called party (remote party at CO B) hangs up first. Todo so, read the next step, then proceed to this step while observing all theevents described in it.


Lift off the handset of telephone set A and dial the number of telephoneset A of CO B. Lift off the handset of telephone set A of CO B to answerthe call and have a normal telephone conversation.

Hang up the handset of telephone set A of CO B to terminate the call andobserve the sequence of events described below.

– CO B sends a DISCONNECT (DISC) message to CO A.

– Time slot interchange from time slot 1 to time slot 6 is released in TIMESLOT INTERCHANGER 2 of the DTI, while time slot interchange fromtime slot 6 to time slot 3 is released in TIME SLOT INTERCHANGER1 of the DTI. The ALI-A RX time slot is set to none.

– The SPACE-DIVISION SWITCH releases the TX0-to-RX2 connectionmade during time slot 1 and the TX2-to-RX0 connection made duringtime slot 3. This disconnects ALI A from the DTI.

– CO A sends a RELEASE (RLSE) message to CO B, which replies witha RELEASE COMPLETE (RLCOM) message.

Hang up the handset of telephone set A of CO A:


Stop recording the call-control messages and the CALL PROCESSORactions in CO A.




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* 43. In the CALL PROCESSOR Log, locate the call-control messages and CALLPROCESSOR actions used to establish the outgoing inter-exchange callfrom CO A to CO B. These messages and actions are identical to thoserecorded in the CALL PROCESSOR Log previously saved in this exercise.

Now locate the call-control messages and CALL PROCESSOR actionsused to terminate the inter-exchange call from CO A to CO B when thecalled party (remote party at CO B) hangs up first.

What causes CO B to send a DISCONNECT (DISC) message to CO A?What is the purpose of this message?

Examine the contents of the Cause IE included in the DISC message.According to the Location component, where does this message originatefrom?

According to the Cause Value component, why must the connection bereleased?

* 44. Play back the action Inter-Exchange Call Termination performed by theCALL PROCESSOR of CO A upon reception of the DISCONNECTmessage from CO B. How does the CALL PROCESSOR perform thisaction?


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* 45. Why does CO A send a RELEASE (RLSE) message to CO B? Why doesCO B reply with a RELEASE COMPLETE (RLCOM) message?

* 46. In the CALL PROCESSOR action Clear Signal Reception, what is themeaning of the following information: "clear forward signal received fromALI HS0, 01"? (Answer by playing back this information).

What is the meaning of the following information: mark ALI HS0, 01 asavailable? Explain.


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Comparison of the Logs (Call Termination by Calling Party Versus Call Terminationby Called Party)

* 47. According to the two logs recorded in this exercise, does the sequence ofcall-control messages and CALL PROCESSOR actions used to terminatean outgoing inter-exchange call from CO A to CO B differ depending onwhether the calling party (local party at CO A) or the called party (remoteparty at CO B) hangs up first? Explain.


Failed Establishment of an Outgoing Inter-Exchange Call

* 49. On the host computer, make sure that the view is adjusted so that thefollowing elements can be observed simultaneously:

– The contents of the HOOK STATUS BUFFER MEMORY in theSIGNALING CIRCUIT;

– The SDS Control Register in the SWITCHING CIRCUIT;

– The CO-A circuitry in its entirety.

* 50. Prepare yourself to observe the sequence of events that occurs in CO Awhen an outgoing inter-exchange call initiated from CO A to CO B cannotbe established because the telephone number dialed by the calling party(party at CO A) is invalid. To do so, read the next step, then proceed to thisstep while observing all the events described in it.


Lift off the handset of telephone set A of CO A to initiate an inter-exchangecall. On this telephone set, dial a number whose first digits correspond to


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the number of CO B and that has the length of the telephone numbersassociated with the ALI's in this CO, but which is not one of thesetelephone numbers. Upon dialing the last digit in this number, observe thesequence of events described below.

– The SPACE-DIVISION SWITCH releases the TX0-to-RX1 connectionmade during time slot 1 for the transmission of the digitized DTMFdialing tones produced by ALI A to SERVICE CIRCUIT 1 (SC1).

– CO A sends a SETUP message to CO B.

– CO B replies with a CALL PROCEEDING (CALPRC) message, andthen a PROGRESS (PROG) message to CO A.

– The special information tone (SIT) can be heard in the handsetearpiece of telephone set A of CO A.

Hang up the handset of telephone set A of CO A and observe thesequence of events described below.


– The SPACE-DIVISION SWITCH releases the TX1-to-RX0 connectionmade during time slot 1 for transmission of the digitized call progresstones produced by SC1 to ALI A.

– CO A sends a DISCONNECT (DISC) message to CO B.

– CO B sends a RELEASE (RLSE) message to CO A, which replies witha RELEASE COMPLETE (RLCOM) message.

Stop recording the call-control messages and the CALL PROCESSORactions in CO A.




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* 53. Observe that the CALL PROCESSOR actions and call-control messagesrecorded in the Call Processor Log correspond to those normally used toinitiate establishment of an outgoing inter-exchange call, until when CO Bsends a PROGRESS (PROG) message to CO A. What is the purpose ofthis message?

* 54. In the PROGRESS (PROG) message sent by CO B to CO A, examine thecontents of the IE's.

According to the contents of the Cause IE, explain why the inter-exchangecall cannot be established.

In the Progress Indicator IE, what does the Progress Descriptioncomponent indicate? Explain.

* 55. What action is performed by the CALL PROCESSOR in CO A uponreception of the PROGRESS (PROG) message from CO B? Why does itperform this action?

* 56. What causes the CALL PROCESSOR in CO A to perform the action ClearSignal Reception? Explain.


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* 57. Explain why the CALL PROCESSOR in CO A does not mark ALI HS0, 01(ALI A) as available as soon as it receives a clear signal from this lineinterface.

* 58. What is the purpose of the action Call Progress Tone Transmit PathRelease performed by the CALL PROCESSOR in CO A?

* 59. Why does CO A send a DISCONNECT (DISC) message to CO B?

* 60. What message does CO B return to CO A to confirm the reception of aDISC message and indicate that the connection has been released at itsend of the digital trunk?

* 61. What message does CO A return to CO B to indicate that the connectionhas been released at its end of the digital trunk?

* 62. As a review of what you have learned in this part of the exercise, you canuse the Playback function of the Call Processor Log to display the completesequence of events illustrating what happens in CO A when an outgoinginter-change call from CO A to CO B cannot be established because thecalled number is invalid (unallocated).


* 64. In the previous manipulations, if you had dialed a number that is valid(allocated) while the called party was busy, what indications would havebeen given by the Cause and Progress Indicator IE's included in the


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PROGRESS (PROG) message sent by CO B to CO A, to indicateunsuccessful inter-exchange call establishment? Explain.

Note: You can verify your answer by using the Call ProcessorLog to record the call-control messages and CALLPROCESSOR actions in CO A when the called party is busy. Todo so, start recording the call-control messages and the CALLPROCESSOR actions in CO A, then perform the following steps:

– Lift off the handset of telephone set A of CO B.

– Using telephone set A of CO A, place a call to telephoneset A of CO B. This will cause a busy tone to be transmittedto telephone set A of CO A.

– Hang up the handset of telephone set A of CO A, then hangup the handset of telephone set A of CO B.

Stop recording the call-control messages and the CALLPROCESSOR actions in CO A. Display the contents of thePROG message recorded in the Call Processor Log, as well asthe contents of the IE's included in this message (IE compo-nents).


* 66. On the host computer, close the Telephony Training System software.

Turn off the TTS Power Supply, as well as the host computer (if it is nolonger required).

Disconnect the AC/DC power converters from the TTS Power Supply andthe telephone sets.

Remove the digital trunk line (multi-wire cable terminated with RJ-45 maleconnectors) connecting the Digital Trunk Interface installed in theReconfigurable Training Module used as CO A to the Digital TrunkInterface installed in the Reconfigurable Training Module used as CO B.

Disconnect the telephone sets from the Dual Analog Line Interface installedin the Reconfigurable Training Module used as CO A. Remove the DualAnalog Line Interface and the Digital Trunk Interface from this module.


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Disconnect the telephone sets from the Dual Analog Line Interface installedin the Reconfigurable Training Module used as CO B. Remove the DualAnalog Line Interface and the Digital Trunk Interface from this module.

CONCLUSION

In this exercise, you became familiar with the signaling procedure used by the CALLPROCESSOR in a Lab-Volt CO to establish, maintain, and terminate a connectionwith another Lab-Volt CO (inter-exchange call), via the digital trunk. You learnedthat this procedure, which is defined in layer 3 of ISDN, makes use of a series ofcall-control messages, also defined in ISDN layer 3, that are exchanged in apredetermined logical order. You learned that these call-control messages cancontain information elements (IE's) conveying one or more parameters related to thecontrol of the call.

Using the Call Processor Log of a Lab-Volt CO, you recorded the sequence ofevents that occurs in this CO when an inter-exchange call initiated by this CO(outgoing call) is established and terminated. This allowed you to make theobservations below.

– During the establishment of the outgoing inter-exchange call, the following call-control messages are exchanged between the CO of the calling party and thatof the called party: SETUP, CALL PROCEEDING, ALERTING, CONNECT, andCONNECT ACKNOWLEDGE messages. Meanwhile, the call processor in theCO of the calling party performs the same actions as those used to establishan intra-exchange (local) call, except that when the called party answers thecall, it performs an inter-exchange call setup (that is, it connects the ALIassociated with the calling party's telephone set to the digital trunk) instead ofan intra-exchange call setup.

– Termination of the outgoing inter-exchange call results in the exchange of thefollowing call-control messages between the two CO's: DISCONNECT,RELEASE, and RELEASE COMPLETE messages. Meanwhile, the callprocessor in the CO of the calling party performs an inter-exchange calltermination and it marks the ALI associated with the calling party's telephoneset as available. The order in which these messages and call processor actionsoccur, as well as the direction of transmission of these messages, depend onwhether the local party (calling party) or remote party (called party) hangs upfirst.

Finally, you recorded the sequence of events that occurs in a CO when an inter-exchange call initiated by this CO cannot be established because the called numberis invalid (unallocated). You saw that, once it has determined that the number isinvalid, the CO of the called party sends a PROGRESS message to the CO of thecalling party indicating the reason why the call cannot be established. This causesthe CO of the calling party to transmit a special information tone to the callingparty's telephone set and, when this party hangs up, to exchange the usual call-control messages for terminating a call with the other CO.


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REVIEW QUESTIONS

1. Briefly explain the main difference between subscriber signaling in the PSTNand signaling between exchanges of the PSTN.

2. What does layer 3 of ISDN, or network layer, define?

3. List below, in the order in which they occur, the call-control messagesexchanged between two Lab-Volt CO's interconnected via a digital trunk toestablish and terminate an inter-exchange call, assuming that the called partyhangs up first at the end of the conversation.


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4. Explain what the Lab-Volt CO of a calling party does to establish an inter-exchange call when, after recovery of the complete called party number, itdetermines that it is an inter-exchange call request.

5. Describe the sequence of events that occurs in a Lab-Volt CO (labeled CO A)when it receives a DISCONNECT message from a remote CO (labeled CO B)requesting that the inter-exchange connection be released.

DISCUSSION Outgoing Inter-Exchange Call … Inter-Exchange Call Routing Sequence 2-14 CO A CALLING...

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Transcript of DISCUSSION Outgoing Inter-Exchange Call … Inter-Exchange Call Routing Sequence 2-14 CO A CALLING...