Exercise 1-1. Architecture of a Digital PABX EXERCISE OBJECTIVE

Exercise 1-1 Architecture of a Digital PABX EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the architecture of a digital PABX (the Lab-Volt PABX). You will be able to highlight major resemblances and differences between the architecture of a digital PABX and that of a central office. You will also be able to demonstrate how two telephone sets are interconnected in the Lab-Volt PABX. DISCUSSION Block Diagram of the Lab-Volt PABX Today's PABX's are all-digital integrated systems as mentioned in the Unit's Discussion of Fundamentals, and the Lab-Volt PABX is no exception. It is completely digital inside, except for some analog circuitry mainly located in the trunk interface and the trunk service circuit, and uses digital telephone sets of the ISDN type. A simplified block diagram of the Lab-Volt PABX is shown in Figure 1-4. Like any other PABX, the Lab-Volt PABX consists of the same basic elements as a central office, that is, line interfaces for the telephone sets, a switching circuit, a signaling circuit, a call processor, and a trunk interface. Each of these elements has the same function as in a central office. However, the implementation of certain elements differs from that used in a central office (like the Lab-Volt Central Office for example), mainly because the telephone sets changed from analog to digital technology. For instance, in the signaling circuit the DIGITAL SIGNALING PROCESSOR and the CALL PROGRESS TONE GENERATOR replace the service circuits for analog line interfaces. The switching circuit still consists of a SPACE- DIVISION SWITCH but a device called a DIGITAL CONFERENCE BRIDGE is added to allow conference calling. And obviously, the analog line interfaces are replaced with digital line interfaces. The Lab-Volt PABX also has an additional basic element, called ANNOUNCEMENT CIRCUIT, which is not found in a central office. The ANNOUNCEMENT CIRCUIT allows intercom calls to be performed using any of the digital telephone sets connected to the Lab-Volt PABX. Intercom calling is discussed in detail in the next unit of this manual. 1-5

1 2 4 5 7 8 * 0 1 2 4 5 7 * 1 2 4 5 7 8 * 0 1 2 4 5 7 * 8 0 8 0 3 6 9 # 3 6 9 # 3 6 9 # 3 6 9 # Architecture of a Digital PABX LAB-VOLT PABX TELEPHONE SETS LINE INTERFACES TX0 SWITCHING CIRCUIT RX0 TRUNK INTERFACE DUAL DIGITAL LINE INTERFACE A TX0 RX0 D0TX D0RX TX1 TX2 TX3 TX4 SPACE- DIVISION SWITCH RX1 RX2 RX3 RX4 RX2 TX2 TS2 ANALOG TRUNK INTERFACE TO AND FROM CO TX5 RX5 DUAL DIGITAL LINE INTERFACE B TX1 RX1 D1TX D1RX D0TX TX4 TX5 DIGITAL CONFERENCE BRIDGE SIGNALING CIRCUIT RX4 RX5 TX3 D0RX D1TX D1RX DIGITAL SIGNALING PROCESSOR ANALOG TRUNK SERVICE CIRCUIT RX3 INTERCOM SPEAKERS RX3 TX3 CALL PROGRESS TONE GENERATOR TRUNK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT TS2 ANNOUNCEMENT CIRCUIT CALL PROCESSOR Figure 1-4. Simplified block diagram of the Lab-Volt PABX. The DUAL DIGITAL LINE INTERFACE Figure 1-5 shows the block diagram of the digital line interfaces used in the Lab-Volt PABX. This block diagram is totally different from that of the analog line interfaces used in the Lab-Volt Central Office. In fact, the only elements that remain are the TXn and RXn terminals that allow the digital line interface to be connected to the SPACE-DIVISION SWITCH, and the time-slot assignment circuit (TSAC) that generates the control signals used to multiplex and demultiplex the digitized voice signals (PCM signals on the TXn and RXn lines). As the analog line interfaces in central offices, the digital line interfaces in the Lab-Volt PABX are grouped in banks. 1-6

1 2 4 5 7 8 * 0 1 2 4 5 7 * 8 0 3 6 9 # 3 6 9 # Architecture of a Digital PABX DUAL DIGITAL LINE INTERFACE DIGITAL TELEPHONE SETS TX RX IRX LINE DECODER (ASI/NRZ) DEMUX B1 B2 D 8 8 2 P/S P/S 2 B1 TXE B2 TXE TXn DnTX TO A TX INPUT OF SPACE-DIVISION SWITCH TO DIGITAL SIGNALING PROCESSOR 2 E TX RX ITX LINE CODER (NRZ/ASI) MUX D B1 B2 8 8 S/P 2 B1 RXE DnRX RXn FROM DIGITAL SIGNALING PROCESSOR FROM AN RX OUTPUT OF SPACE-DIVISION SWITCH S/P B2 RXE B1 TXE B2 TXE B1 RXE TSAC FROM CALL PROCESSOR B2 RXE Figure 1-5. Block diagram of the DUAL DIGITAL LINE INTERFACE of the Lab-Volt PABX. Each digital line interface is of the ISDN-BRI type and allows connection of two digital telephone sets to the Lab-Volt PABX. This is why each digital line interface in the Lab-Volt PABX is called a DUAL DIGITAL LINE INTERFACE. Note: Those who are not familiar with the Integrated Services Digital Network (ISDN) can refer to Appendix B of this manual which provides information on ISDN that is relevant to the Lab-Volt PABX. For each direction of transmission (from the digital telephone sets to the PABX and vice versa), each DUAL DIGITAL LINE INTERFACE provides two bearer channels (labeled B1 and B2), each having a 64-kb/s transmission rate, and a data channel (labeled D) having a transmission rate of 16 kb/s. Each bearer channel, or B channel, is used to transmit a digitized voice signal (voice data) while the D channel is used to transmit signaling data. This explains why each DUAL DIGITAL LINE INTERFACE can accommodate two digital telephone sets. Up to 1-7

four digital telephone sets can be connected to the Lab-Volt PABX because two DUAL DIGITAL LINE INTERFACEs are available. All data from the two digital telephone sets connected to a DUAL DIGITAL LINE INTERFACE (referred to as the ISDN layer-1 data) is time multiplexed to the B channels and the D channel. This data, which is coded using the alternate space inversion (ASI) line code, is received via the IRX terminal of the DUAL DIGITAL LINE INTERFACE. Figure 1-6 illustrates the paths through which the voice data and signaling data received at the IRX terminal are routed to the SPACE-DIVISION SWITCH and the DIGITAL SIGNALING PROCESSOR of the PABX, respectively. DUAL DIGITAL LINE INTERFACE VOICE DATA TO SPACE-DIVISION SWITCH VOICE DATA FROM CHANNEL B1 IRX DATA RECEIVED ISDN LAYER-1 FORMAT LINE DECODER (ASI/NRZ) DEMUX B1 B2 P/S P/S VOICE DATA FROM CHANNEL B2 TXn 4 5 6 7 8 TIME SLOTS DIGITAL TELEPHONE SETS B1 B2 D SIGNALING DATA VOICE DATA VOICE DATA MUX E D SIGNALING DATA IN ISDN LAYER-2 FORMAT DIGITAL SIGNALING PROCESSOR CALL PROCESSOR D ITX LINE CODER (NRZ/ASI) SIGNALING DATA IN ISDN LAYER-3 FORMAT B1 S/P RXn B2 S/P Figure 1-6. Paths through which the voice data and signaling data are routed to the SPACE- DIVISION SWITCH and the DIGITAL SIGNALING PROCESSOR of the PABX, respectively. At the IRX terminal of the DUAL DIGITAL LINE INTERFACE, a line decoder converts the received ASI-coded data to non return-to-zero (NRZ) coded data. A demultiplexer (DEMUX) then separates (demultiplexes) the voice data and the signaling data from the B channels and the D channel. Voice data is available at the B1 and B2 outputs of the DEMUX while the signaling data is available at the D output. Two parallel-to-serial (P/S) converters and their respective tri-state buffer multiplex the recovered voice data to two different time slots (which are determined by the CALL PROCESSOR according to the connection to be established) for transmission to the SPACE-DIVISION SWITCH via the TX line associated with the DUAL DIGITAL LINE INTERFACE. The signaling data recovered at the D output of the DEMUX is sent to the DIGITAL SIGNALING PROCESSOR. Note that this data is coded according to the layer-2 format of the ISDN signaling protocol. 1-8

Note: Variable transmit (TX) time slots are used in the Lab-Volt PABX (except for the transmission of call progress tones) to minimize the chances of having a telephone call blocked because no path is available to establish the required connection. The TX time slots are dynamically assigned by the PABX call processor according to the connections to be established. This differs from the Lab-Volt CO which uses fixed TX time slots, configured through dialog boxes, in order to facilitate the study of its operation. In the other direction of transmission (from the PABX to the digital telephone sets), the PABX uses either one of the B channels of the interface to transmit voice data to a telephone set and the D channel to transmit signaling data to the two telephone sets. In other words, all data is time multiplexed to the B channels and the D channel to form an ISDN layer-1 data string that is transmitted to the two digital telephone sets via the ITX terminal of the DUAL DIGITAL LINE INTERFACE. Figure 1-7 illustrates the paths through which voice data from the SPACE-DIVISION SWITCH and signaling data from the DIGITAL SIGNALING PROCESSOR are routed to digital telephone sets. DUAL DIGITAL LINE INTERFACE DEMUX B1 P/S TXn IRX LINE DECODER (ASI/NRZ) B2 P/S SIGNALING DATA IN ISDN LAYER-3 FORMAT D DIGITAL TELEPHONE SETS VOICE DATA VOICE DATA SIGNALING DATA MUX E D SIGNALING DATA IN ISDN LAYER-2 FORMAT DIGITAL SIGNALING PROCESSOR CALL PROCESSOR B1 B2 D ISDN LAYER-1 FORMAT ITX DATA TRANSMITTED LINE CODER (NRZ/ASI) B1 B2 S/P S/P VOICE DATA TO CHANNEL B1 RXn 4 5 6 7 8 TIME SLOTS VOICE DATA TO CHANNEL B2 VOICE DATA FROM SPACE-DIVISION SWITCH Figure 1-7. Paths through which voice data from the SPACE-DIVISION SWITCH and signaling data from the DIGITAL SIGNALING PROCESSOR are routed to digital telephone sets. In brief, voice data to be transmitted to the digital telephone sets is received through the RX line associated with the DUAL DIGITAL LINE INTERFACE. Two serial-toparallel (S/P) converters and their respective tri-state buffer demultiplex voice data 1-9

from two different time slots (which are determined by the CALL PROCESSOR according to the connection to be established). The two demultiplexed digitized voice signals are sent to inputs B1 and B2 of a multiplexer (MUX). The signaling data from the DIGITAL SIGNALING PROCESSOR is available at the D input of the multiplexer. The multiplexer combines all data available on its various inputs into a single data string, that is, the voice data at inputs B1 and B2 are multiplexed to channels B1 and B2, respectively, and the signaling data at the D input is multiplexed to the D channel. A line coder converts the data string from the NRZ format to the ASI format to obtain ISDN layer-1 data that is transmitted to the digital telephone sets. Note that the signaling data received from the digital telephone sets connected to a DUAL DIGITAL LINE INTERFACE (data recovered at the D output of the DEMUX) is routed to the echo (E) input of the MUX as shown in Figure 1-8. This allows the signaling data to be echoed back to the digital telephone sets via echo (E) bits that are interleaved with the data in the B and D channels of the ISDN layer-1 data. This provides a means to resolve D channel contention. Additional explanation about D channel contention is provided in Appendix B of this manual. DUAL DIGITAL LINE INTERFACE DEMUX B1 P/S TXn IRX ISDN LAYER-1 FORMAT LINE DECODER (ASI/NRZ) B2 P/S B1 B2 D D DIGITAL TELEPHONE SETS ECHO BITS INTERLEAVED WITH DATA IN CHANNELS B1, B2, AND D MUX E SIGNALING DATA IN ISDN LAYER-2 FORMAT DIGITAL SIGNALING PROCESSOR CALL PROCESSOR D B1 B2 D ISDN LAYER-1 FORMAT ITX LINE CODER (NRZ/ASI) B1 S/P RXn B2 S/P Figure 1-8. Signaling data is echoed back to the digital telephone sets as a means of resolving D channel contention. As in the analog line interfaces of the Lab-Volt Central Office, a TSAC receives data from the call processor to generate pulse signals that are used to multiplex and demultiplex digitized voice signals (PCM signals on the TXn and RXn lines). However, in the Lab-Volt PABX, each TSAC provides two transmit enable (TXE) signals and two receive enable (RXE) signals (see Figure 1-5) because each DUAL 1-10

DIGITAL LINE INTERFACE accommodates two digital telephone sets. The B1 TXE and B2 TXE signals determine the time slots in which voice data from channels B1 and B2 (voice data at the B1 and B2 outputs of the DEMUX), respectively, is multiplexed to the TX line of the line interface. Similarly, the B1 RXE and B2 RXE signals indicate the time slots during which voice data is to be read from the RX line for redirection to channels B1 and B2 (inputs B1 and B2 of the MUX), respectively. Figure 1-9 shows an example of the TSAC output signals when voice data received from channels B1 and B2 is to be multiplexed to the TX line during time slots 5 and 8, respectively, and voice data to be transmitted to the telephone sets via channels B1 and B2 is to be read from the RX line during time slots 6 and 7, respectively. TIME SLOTS 2 3 4 5 6 7 8 9 10 11 12 13 B1 TXE SIGNAL TIME B2 TXE SIGNAL TIME B1 RXE SIGNAL TIME B2 RXE SIGNAL TIME Figure 1-9. Example of TSAC output signals used to multiplex and demultiplex digitized voice signals (PCM signals on the TXn and RXn lines). The Switching Circuit The SPACE-DIVISION SWITCH in the switching circuit operates in the same manner as the switch in the Lab-Volt Central Office. In brief, the SPACE-DIVISION SWITCH performs space connections (connections of an input [TX] line to any output [RX] line), at every time slot. The switching circuit in the Lab-Volt PABX has an additional device called DIGITAL CONFERENCE BRIDGE. This device can combine two digitized voice signals into a single digitized signal. It is used to implement conference calling, which is studied in the next unit of this manual. 1-11

The Signaling Circuit The signaling circuit in the Lab-Volt PABX consists of the DIGITAL SIGNALING PROCESSOR, the CALL PROGRESS TONE GENERATOR, the ANALOG TRUNK SERVICE CIRCUIT, and the TRUNK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT (see Figure 1-4). The DIGITAL SIGNALING PROCESSOR is somewhat the equivalent of the service circuits for analog line interfaces used in the Lab-Volt Central Office, that is, it acts as a kind of interpret circuit. The DIGITAL SIGNALING PROCESSOR converts the signaling data coming from the telephone sets (data in the D channel), which is in the ISDN layer-2 format, to ISDN layer-3 signaling data. This data is then transferred to the CALL PROCESSOR. Conversely, the DIGITAL SIGNALING PROCESSOR converts signaling data from the CALL PROCESSOR, which is in the ISDN layer-3 format, to ISDN layer-2 signaling data that is transmitted to the telephone sets via the DUAL DIGITAL LINE INTERFACE. The exchange of signaling data between the digital telephone sets and the PABX will be studied thoroughly in other exercises of this manual. Although digital signaling is used in the Lab-Volt PABX, call progress tones are still required to keep telephone users aware of call progression. The call progress tones are provided by the Lab-Volt PABX because the digital telephone sets used with this PABX do not produce these tones internally. The CALL PROGRESS TONE GENERATOR in the signaling circuit of the Lab-Volt PABX produces different digital call progress tones (dial tone, busy tone, etc.). These digital call progress tones are permanently available on line TX3, each tone being multiplexed to a different time slot. Any one of the digital call progress tones can be routed to a digital telephone set via the SPACE-DIVISION SWITCH and one of the B channels of the corresponding DUAL DIGITAL LINE INTERFACE. The operation of the CALL PROGRESS TONE GENERATOR as well as the routing of call progress tones to digital telephone sets is studied in detail in another exercise of this unit. The ANALOG TRUNK SERVICE CIRCUIT and the TRUNK STATUS DEMUL- TIPLEXING AND STORAGE CIRCUIT make the link between the ANALOG TRUNK INTERFACE and the CALL PROCESSOR. The operation of the ANALOG TRUNK SERVICE CIRCUIT and the TRUNK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT is studied in another manual of the Lab-Volt Telephony Training System. The Trunk Interface The ANALOG TRUNK INTERFACE in the Lab-Volt PABX converts voice data received from the SPACE-DIVISION SWITCH into an analog voice signal that can be transmitted on an analog trunk line. Conversely, it converts an analog voice signal received from a trunk line into voice data that is routed to the SPACE- DIVISION SWITCH. The operation of the ANALOG TRUNK INTERFACE is studied in another manual of the Lab-Volt Telephony Training System. 1-12

Procedure Summary In the first part of the exercise, you will set up a PABX with the Telephony Training System (TTS). In the second part of the exercise, you will study the architecture of the Lab-Volt PABX. This will allow you to see the resemblances and differences between the Lab-Volt PABX and the Lab-Volt Central Office. In the third part of the exercise, you will lift off the handset of a digital telephone set and determine which B channel is assigned to this telephone set for voice data transmission in the corresponding digital line interface of the Lab-Volt PABX. In the last part of the exercise, you will determine how the Lab-Volt PABX routes the digitized voice signal coming from a digital telephone set to another digital telephone set. Note: Before proceeding with the procedure below, it is strongly recommended that you go through Section 4 of the Telephony Training System User Guide (part number 32964-E0) all the way. This section, entitled "Familiarization with the Lab-Volt PABX", provides detailed information on how to set up the Lab-Volt PABX, use its essential features, and ensure that the digital telephone sets connected to it are set up to operate properly. EQUIPMENT REQUIRED Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise. PROCEDURE Setting Up the Lab-Volt PABX * 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model 9408. Make sure that there is a network connection between the Reconfigurable Training Module and the host computer. Install the Digital Telephone Interface, Model 9476, into the digital (D) slot or one of the two analog/digital (A/D) slots of the Reconfigurable Training Module. Connect two digital (ISDN) telephone sets provided with the Lab-Volt PABX to the left and right connectors of interface A of the Digital Telephone Interface. These telephone sets will be referred to as digital telephone sets A-left (AL) and A-right (AR), respectively, throughout the exercise. 1-13

CAUTION! Do not connect or disconnect the digital telephone sets when the Reconfigurable Training Module is turned on. High voltages are present on the RJ-45 connectors of the Digital Telephone Interface. * 2. Turn on the host computer. Turn on the TTS Power Supply, then the Reconfigurable Training Module. * 3. On the host computer, start the Telephony Training System software, then download the PABX program to the Reconfigurable Training Module. The PABX program configures the Reconfigurable Training Module so that it operates as a private automatic branch exchange (PABX). Note: If the host computer is unable to download the PABX program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor check if the computer is using the proper IP address to communicate with the Reconfigurable Training Module. * 4. Lift off the handset of each digital telephone set connected to the Lab-Volt PABX, while observing the Lab-Volt PABX diagram in the LVTTS software window. When the handset of a digital telephone set is lifted off, an icon representing this telephone set should appear connected to the corresponding DUAL DIGITAL LINE INTERFACE in the Lab-Volt PABX diagram. If so, this indicates that this telephone set is properly programmed to be operational with the Lab-Volt PABX. Otherwise, this means that this telephone set is not properly programmed to be operational with the Lab-Volt PABX. Note: An icon representing the digital telephone set may already be displayed in the Lab-Volt PABX diagram before you lift off the handset of this telephone set. This occurs because each digital telephone set automatically begins communication with the Lab-Volt PABX a certain time after it is powered up in order to identify itself to the Lab-Volt PABX. This indicates that this digital telephone set is properly programmed to be operational with the Lab-Volt PABX. If a digital telephone set does not seem to be properly programmed, refer to Section 4 of the Telephony Training System User Guide (part number 32964-E0), entitled "Familiarization with the Lab-Volt PABX", to know how to properly program the digital telephone sets connected to the Lab-Volt PABX. Hang up the handset of each digital telephone set. 1-14

Basic Elements of the Lab-Volt PABX * 5. On the host computer, examine the block diagram of the Lab-Volt PABX without zooming in on it. Notice that the Lab-Volt PABX contains the same basic elements as the Lab-Volt Central Office, which are: line interfaces for the telephone sets; a switching circuit; a signaling circuit; a call processor; a trunk interface. On the other hand, notice that the Lab-Volt PABX additionally contains an ANNOUNCEMENT CIRCUIT, which is not found in the Lab-Volt Central Office. * 6. On the host computer, zoom in on the icons representing digital telephone sets AL and AR. Using the Pan command, observe that each digital telephone set is connected to DUAL DIGITAL LINE INTERFACE A (DDLI A) of the Lab-Volt PABX, through two pairs of wires: one pair for transmission of the ISDN layer-1 data coming from the telephone set to the interface via one of the interface IRX terminals; one pair for reception of the ISDN layer-1 data coming from the interface via one of the interface ITX terminals. * 7. Pan across DDLI A to examine its contents. Notice that the contents of this interface differs markedly from that of the analog line interfaces used in the Lab-Volt Central Office. The only elements that remain are the TX0 and RX0 lines that connect DDLI A to the SPACE-DIVISION SWITCH of the Lab-Volt PABX; the TSAC. What are the TX0 and RX0 lines used for? What is the function of the TSAC? * 8. Observe that, in addition to line TX0, DDLI A uses another transmit line, D0TX. This line permits the routing of the signaling data coming from digital telephone sets AL and AR (data recovered at the D output of the demultiplexer of DDLI A) to the DIGITAL SIGNALING PROCESSOR of the Lab-Volt PABX. The data on line D0TX is in ISDN layer-2 data format. 1-15

Also, observe that, in addition to line RX0, DDLI A uses another receive line, D0RX. This line permits the routing of the signaling data coming from the DIGITAL SIGNALING PROCESSOR to digital telephone sets AL and AR, via the D input of the multiplexer of DDLI A. The data on line D0RX is in ISDN layer-2 data format. * 9. Go to DUAL DIGITAL LINE INTERFACE B (DDLI B) and observe that it does not use the same transmit and receive lines as DDLI A. What does this imply? Which lines are used by DDLI B to transmit and receive multiplexed digitized voice signals respectively? Which lines are used by DDLI B to transmit and receive signaling data to and from the DIGITAL SIGNALING PROCESSOR? * 10. Go to the SWITCHING CIRCUIT of the Lab-Volt PABX. Observe that this circuit contains a TIME-SLOT SELECTOR and a SPACE-DIVISION SWITCH used to perform space connections, as in the case of the SWITCHING CIRCUIT of the Lab-Volt Central Office. Does the SPACE-DIVISION SWITCH of the Lab-Volt PABX has more inputs and outputs than that of the Lab-Volt Central Office? * Yes * No Observe that an additional device called DIGITAL CONFERENCE BRIDGE is included in the SWITCHING CIRCUIT of the Lab-Volt PABX. What is the use of this bridge? 1-16

Through which transmit (TX) and receive (RX) lines are the SPACE- DIVISION SWITCH and DIGITAL CONFERENCE BRIDGE interconnected? * 11. Go to the SIGNALING CIRCUIT of the Lab-Volt PABX and examine its contents. Observe that a DIGITAL SIGNALING PROCESSOR and a CALL PROGRESS TONE GENERATOR replace the service circuits for analog line interfaces and the hook status demultiplexing and storage circuit used in the SIGNALING CIRCUIT of the Lab-Volt Central Office. Which two other elements of the SIGNALING CIRCUIT of the Lab-Volt PABX (possibly appearing faded to show that they are not currently available in your Lab-Volt PABX) are not included in the SIGNALING CIRCUIT of the Lab-Volt Central Office? * 12. Examine the DIGITAL SIGNALING PROCESSOR of the SIGNALING CIRCUIT. This device converts the signaling data coming from the digital telephone sets via the DDLI's, which is in ISDN layer-2 data format, to ISDN layer-3 data format. Conversely, it converts the signaling data coming from the CALL PROCESSOR, which is in ISDN layer-3 data format, to ISDN layer-2 data format. This data is then transmitted to the digital telephone sets via the digital line interfaces. Through which lines does the DIGITAL SIGNALING PROCESSOR receive the signaling data from the digital telephone sets? Through which lines does the DIGITAL SIGNALING PROCESSOR transmit the signaling data from the CALL PROCESSOR to the digital telephone sets? * 13. Examine the CALL PROGRESS TONE GENERATOR in the SIGNALING CIRCUIT. Observe that it consists of four tone generators associated with four CODEC's, and a TSAC that generates the control signals used to multiplex each digital call progress tone to a different time slot on line TX3. 1-17

Any one of the digital call progress tones present on line TX3 can be routed to a digital telephone set, via the SPACE-DIVISION SWITCH and one of the B channels of the corresponding digital line interface. What are the four call progress tones generated by the CALL PROGRESS TONE GENERATOR? * 14. Go to the ANNOUNCEMENT CIRCUIT of the Lab-Volt PABX. This circuit is used for implementation of intercom calling. It converts the digitized voice signal coming from the intercom user's telephone set, via the SPACE- DIVISION SWITCH and one of the B channels of the corresponding digital line interface, into an analog voice signal. This signal is amplified and then routed to intercom speakers. Through which line is the ANNOUNCEMENT CIRCUIT connected to the SPACE-DIVISION SWITCH? Assignment of a B Channel to a Digital Telephone Set * 15. Go to DDLI A and zoom in on its B-CHANNEL ASSIGNMENT display. While observing this display, lift off the handset of digital telephone set AL. Notice that, as soon as the handset is lifted off, the SPID (Service Profile IDentifier) of digital telephone set AL appears in the B1 field of the display, indicating that channel B1 of DDLI A is assigned to this telephone set for voice data transmission. Is this your observation? * Yes * No * 16. Do not hang up. While observing the B-CHANNEL ASSIGNMENT display of DDLI A, lift off the handset of digital telephone set AR. Notice that, as soon as the handset is lifted off, the SPID of digital telephone set AR appears in the B2 field of the display, indicating that channel B2 of DDLI A is assigned to this telephone set for voice data transmission. Is this your observation? * Yes * No 1-18

* 17. Hang up the handset of digital telephone set AL. Does this cause the B1 field of the B-CHANNEL ASSIGNMENT display of DDLI A to return to "----", indicating that channel B1 of this interface is now unused? * Yes * No * 18. Hang up the handset of digital telephone set AR. Does this cause the B2 field of the B-CHANNEL ASSIGNMENT display of DDLI A to return to "----", indicating that channel B2 of this interface is now unused? * Yes * No Routing of the Voice Data from a Digital Telephone Set to Another in the Lab-Volt PABX * 19. Connect Oscilloscope probes 1, 2, 3, and 4 to TP5 (channel B1 P/S Converter output), TP11 (TSAC B1 TXE output), TP7 (TX0 line), and TP16 (FRAME SYNC. signal) of DDLI A, respectively. * 20. Start the Oscilloscope. Make the following settings on the Oscilloscope: Channel 1 Mode....................................... Normal Sensitivity................................... 5 V/div Input Coupling................................... DC Channel 2 Mode....................................... Normal Sensitivity................................... 5 V/div Input Coupling................................... DC Channel 3 Mode....................................... Normal Sensitivity................................... 5 V/div Input Coupling................................... DC Channel 4 Mode....................................... Normal Sensitivity................................... 5 V/div Input Coupling................................... DC Time Base..................................... 10 µs/div Trigger Source........................................ Ch 4 Level........................................ 1.0 V Slope.................................... Positive (+) Display Refresh............................... Continuous Display Mode.................................... Square 1-19

* 21. Lift off the handset of digital telephone set AL and dial the terminal number associated with digital telephone set AR. Lift off the handset of digital telephone set AR to answer the call and establish a communication. Note: The terminal number associated with each digital telephone set can be found in the Addressing Cross-Reference Table of the Lab-Volt PABX. * 22. Observe that a PCM signal appears at TP5 (channel-b1 P/S Converter output) of DDLI A on the Oscilloscope screen, especially when talking into the handset of digital telephone set AL. Does this confirm that channel B1 of DDLI A is currently assigned to digital telephone set AL for voice data transmission? Explain why. * 23. According to the TIME SLOT NUMBER displays of DDLI A, which transmit (TX) time slot is currently assigned to channel B1 of DDLI A? Explain. Note: Time slots 1 to 4 are reserved for transmission of the call progress tones produced by the CALL PROGRESS TOME GENERATOR. This will be discussed in detail in Exercise 1-4. * 24. On the Oscilloscope screen, observe that a pulse occurs in the TSAC B1 TXE output signal (TP11) during time slot 5. Explain why. Note: The pulse in the FRAME SYNC. signal at TP16 is aligned with time slot 0 of each frame. * 25. Observe that the PCM signal at TP5 (channel B1 P/S Converter output) of DDLI A appears in the signal at TP7 (TX0 line) of this interface during time slot 5. Explain why. 1-20

* 26. From the observations you have made up to this point, explain how the voice data from a digital telephone set is routed to an input of the SPACE- DIVISION SWITCH of the SWITCHING CIRCUIT in the Lab-Volt PABX. * 27. Go to the SWITCHING CIRCUIT of the Lab-Volt PABX. Using the TIME- SLOT SELECTOR of this circuit, observe the connections made by the SPACE-DIVISION SWITCH during time slot 5. You should observe that during time slot 5, which is the TX time slot assigned to channel B1 of DDLI A, the SPACE-DIVISION SWITCH connects line TX0 (DDLI-A transmit line) to line RX0 (DDLI-A receive line). What is the purpose of this connection? * 28. Go to the B-CHANNEL ASSIGNMENT display of DDLI A. Which B channel of this interface is currently assigned to digital telephone set AR for voice data transmission? Explain. * 29. According to the TIME SLOT NUMBER displays of DDLI A, which receive time slot is currently assigned to channel B2 of this interface? 1-21

Does this time slot correspond to the transmit time slot assigned to channel B1 of DDLI A? What does this imply? * 30. Disconnect Oscilloscope probes 1 through 3. Connect Oscilloscope probes 1, 2, and 3 to TP10 (RX0 line), TP14 (TSAC B2 RXE output), and TP9 (channel-b2 S/P Converter input), respectively. * 31. Does a pulse occur in the signal at TP14 (TSAC B2 RXE output) of DDLI A during time slot 5? Why? * 32. Observe that a PCM signal appears at TP10 (RX0 line) of DDLI A, especially when talking into the handset of digital telephone set AL, due to the TX0-to-RX0 connection made by the SPACE-DIVISION SWITCH of the SWITCHING CIRCUIT during time slot 5. Is this PCM signal routed to the channel-b2 S/P Converter input (TP9) of DDLI A? Explain. 1-22

* 33. From your observations, explain how the voice data from an output of the SPACE-DIVISION SWITCH is routed to a digital telephone set. * 34. To summarize what you have learned in this exercise, explain how the CALL PROCESSOR in the Lab-Volt PABX established a connection between the B channels assigned to digital telephone sets AL and AR in DDLI A to permit a normal (bidirectional) conversation between the users at these telephone sets. Note: In the preceding steps, you studied voice data transmission in a single direction only, that is, from digital telephone set AL to digital telephone set AR. To answer the above question, you can use LVTTS as you have done so far to determine how voice data transmission occurs in the other direction, that is, from digital telephone set AR to digital telephone set AL. CALL PROCESSOR actions performed to establish a connection for transmission of the voice data from digital telephone set AL to digital telephone set AR: 1-23

CALL PROCESSOR actions performed to establish a connection for transmission of the voice data from digital telephone set AR to digital telephone set AL: * 35. Compare the technique used to interconnect two line interfaces in the Lab-Volt PABX to that used to in the Lab-Volt Central Office. * 36. Hang up the handsets of digital telephone sets AR and AL. * 37. 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 no longer required). Disconnect the digital telephone sets from the Digital Telephone Interface. Remove the Digital Telephone Interface from the Reconfigurable Training Module. 1-24 CONCLUSION In this exercise, you familiarized yourself with the architecture of the Lab-Volt PABX. You saw that, similar to the Lab-Volt Central Office, the Lab-Volt PABX is made up of line interfaces, a switching circuit, a signaling circuit, a call processor, and a trunk interface. Unlike the Lab-Volt Central Office, however, the Lab-Volt PABX uses digital line interfaces instead of analog line interfaces, its switching circuit includes a digital conference bridge to allow conference calling, its signaling circuit contains a digital signaling processor and a call progress tone generator in place of service

circuits for analog line interfaces and a hook status demultiplexing and storage circuit, and it includes an announcement circuit. You learned that the digital line interfaces of the Lab-Volt PABX are of the ISDN-BRI type: for each direction of transmission (from the digital telephone sets to the Lab-Volt PABX and vice versa), these interfaces provide two bearer channels, labeled B1 and B2, that are used to convey voice data, as well as a data channel, labeled D, that is used to convey signaling data. Because of this, each digital line interface can accommodate two digital telephone sets and is thus referred to as a dual digital line interface (DDLI). You saw that, as soon as the handset of a digital telephone set is lifted off, the Lab-Volt PABX assigns a B channel to this telephone set in the corresponding digital line interface. You learned that the voice data produced by a digital telephone set is time multiplexed to the assigned B channel, while the signaling data produced by this telephone set is time multiplexed to the D channel, all the data being sent in ISDN layer-1 data format to the corresponding digital line interface in the Lab-Volt PABX. Conversely, you learned that the Lab-Volt PABX transmits voice data to a digital telephone set by time multiplexing this data to one of the B channels of the interface, and that it transmits signaling data to this telephone set by time multiplexing this data to the D channel, all the data being sent to the telephone set in ISDN layer-1 data format. Finally, you compared the technique used to interconnect two line interfaces in the Lab-Volt PABX to that used in the Lab-Volt Central Office. You saw that, in both cases, the line interfaces are interconnected by combining time-multiplexed switching with space-division switching. The only difference lies in the way timemultiplexed switching is performed: in the Lab-Volt PABX, it is performed by dynamically assigning transmit and receive time slots to the line interfaces; in the Lab-Volt Central Office, it is performed by dynamically assigning receive time slots to the line interfaces, the transmit time slots being fixed and user-determined through the use of dialog boxes. REVIEW QUESTIONS 1. How do the architectures of the Lab-Volt PABX and Lab-Volt Central Office resemble? How do they differ? 1-25

2. Explain why each digital line interface on the Lab-Volt PABX can accommodate two digital telephone sets. 3. Describe the paths through which the voice data and the signaling data from a digital telephone set are routed to the SPACE-DIVISION SWITCH and the DIGITAL SIGNALING PROCESSOR of the Lab-Volt PABX. 4. How does the Lab-Volt PABX transmit voice data and signaling data to the two digital telephone sets associated with a digital line interface? 5. What is the use of the Bn TXE and Bn RXE signals generated by the TSAC of a digital line interface in the Lab-Volt PABX? 1-26