EXERCISE OBJECTIVE DISCUSSION Introduction 2-31

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1 Exercise 2-3 Two-Dimensional Switching EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with two-dimensional switching. DISCUSSION Introduction The first two exercises of this unit showed that either a time-division switch or a space-division switch can be used to interconnect two telephone sets together, or a telephone set with a service circuit in the signaling circuit of the central office. However, practical considerations limit the number of channels (interconnections) which a time-division switch or a space-division switch can establish. With timedivision switches, the maximum practical bit rate limits the number of time slots available, and thereby, the number of channels. With space-division switches, the maximum number of pins on VLSI IC packages is the fundamental limitation to the number of channels. Today, the number of telephone lines homing in on a central office has become so large that a single switch (be it of the time-division or space-division type) is not sufficient. Combining time-division switching with space-division switching is a clever means for designing digital switches that can handle a large number of channels. This is referred to as two-dimensional switching. The benefits of combining time-division switching and space-division switching are demonstrated in the following example. Suppose that a 4096-channel digital switch is required, it could be implemented with a stage of time-division switches (time slot interchangers) followed by a space-division switch as shown in Figure 2-9. Each input of the space-division switch is fed by a time-division switch. The timedivision switch performs time slot interchange while the space-division switch provides connections in space. Since the time-division switch works with 64 time slots and the space-division switch is a non-blocking switch, a maximum number of 4096 interconnections or channels (64 time slots in the time-division switch times 64 inputs in the space-division switch) can be established. This leads to time-division multiplexed (TDM) signals with a bit rate of about 4 Mb/s (64 time slots/frame 8 bits/time slot 8000 frames/second) and an array of 4096 crosspoints in the space division switch which can easily be fitted into a VLSI IC package. This is well within the practical limit of the technology. 2-31

2 64 TIME SLOTS TIME-DIVISION SWITCH SPACE-DIVISION SWITCH TX RX1 SDS TX2 2 RX2 64 x 64 TX64 64 RX64 Figure 2-9. A time-space (TS) switch. By contrast, if a single time-division switch with 4096 time slots were used, the bit rate would be about 262 Mb/s (4096 time slots/frame 8 bits/time slot 8000 frames/second), a very high rate! Similarly, if a single-space division switch were used, it would require a switch, meaning about crosspoints and a VLSI IC package with more than 8000 pins! Note that the DIGITAL SWITCHING circuit of the Telephony Training System provides the same benefits as the TS switch described above, although it does not perform time slot interchange (real time-division switching). The equivalent of time slot interchange is achieved by dynamically assigning receive (RX) time slots to the analog line interfaces, service circuits, etc, according to the connections to be established. Digital switches used in today's central offices usually consist of multiple stages of time-division and space-division switches. Two popular digital-switch architectures are the space-time-space (STS) and time-space-time (TST) architectures. These digital switches provide maximum flexibility and efficiency. Figure 2-10 shows an example of a time-space-time switch (TST). Information arriving via a TX input line is delayed in a first time-division switch stage until an appropriate path through the space-division switch stage is available. At that time, the information is transferred through the space-division switch stage to a second time-division switch stage where it is held until the desired outgoing time slot occurs. Note that two connection paths are required for each telephone conversation. Also note that the great flexibility of the TST switch allows the use of time slots in the space-division switch stage that bear no relationship with the data in the ingoing and outgoing time slots. 2-32

3 TX0 RX0 TX1 SDS RX1 TXN RXN Figure A time-space-time (TST) switch. Procedure summary In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS). In the second part of the exercise, you will analyze two-dimensional switching. To do so, you will establish a communication between two telephone sets. You will determine which circuits of the Central Office are interconnected through observation of the SPACE-DIVISION SWITCH and the TX and RX TIME SLOT NUMBER displays. You will explain what is the purpose of these connections. In the third part of the exercise, you will manually change the connection settings in the Control Register of the SPACE-DIVISION SWITCH and determine how this affects voice transmission between two telephone sets. This will allow you to see that two connections are required in the Central Office in order to establish a normal bidirectional conversation. In the last part of the exercise, you will observe coordination of two-dimensional switching by the CALL PROCESSOR. To do so, you will set the TX time slot of a line interface to several different values and observe the subsequent changes that occur in the time and space connections made in the Central Office. Finally, you will calculate the maximum number of connections that can be made in the Central Office. EQUIPMENT REQUIRED Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise. 2-33

4 PROCEDURE Setting Up the Central Office * 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model Make sure that there is a network connection between the Reconfigurable Training Module and the host computer. Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module. Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each analog telephone set. CAUTION! High voltages are present on the standard telephone connectors of the Dual Analog Line Interface. Do not connect or disconnect the analog telephone sets when the Reconfigurable Training Module is turned on. Connect the AC/DC power converter supplied with each analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of each AC/DC power converter to the DC power input connector on either of the analog telephone sets. Note: The analog telephone set requires an auxiliary DC power source for the digital display to be operative. * 2. Turn on the host computer. Turn on the TTS Power Supply, then turn on the Reconfigurable Training Module. * 3. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module. The CO program configures the Reconfigurable Training Module so that it operates as a central office. Note: If the host computer is unable to download the CO 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. 2-34

5 * 4. On the host computer, make sure that the address of the TSAC in ANALOG LINE INTERFACE A is set to 01. Set the address of the TSAC in ANALOG LINE INTERFACE B to 07. Analysis of Two-Dimensional Switching * 5. Zoom in on the SWITCHING CIRCUIT of the Central Office. Set the TIME SLOT SELECTOR of the SPACE-DIVISION SWITCH to 01. This will enable display of the connections made by this switch during time slot 1. * 6. While observing the SPACE-DIVISION SWITCH, lift off the handset of telephone set A. You should observe that, as soon as the handset is lifted, the SPACE- DIVISION SWITCH establishes two distinct connections during time slot 1: one between line TX0 and line RX1, the other between line TX1 and line RX0. Is this your observation? * Yes * No Replace the handset of telephone set A on the cradle. * 7. Again lift off the handset of telephone set A. Based on the current reading of the TX and RX TIME SLOT NUMBER displays of the ANALOG LINE INTERFACEs and SERVICE CIRCUITs, and on the connections made by the SPACE-DIVISION SWITCH during time slot 1, which circuits of the Central Office are interconnected during time slot 1? What is the purpose of these connections? Replace the handset of telephone set A on the cradle. * 8. Lift off the handset of telephone set A and dial the number of telephone set B. Lift off the handset of telephone set B to answer the call and establish a communication. 2-35

6 Observe that the SPACE-DIVISION SWITCH now performs a single connection during time slot 1, which is from line TX0 to line RX0. Is this your observation? * Yes * No Now set the TIME SLOT SELECTOR of the SPACE-DIVISION SWITCH to 07. Observe that the switch also performs a connection during time slot 7 which, again, is from line TX0 to line RX0. Is this your observation? * Yes * No * 9. Based on the connections made by the SPACE-DIVISION SWITCH during time slots 1 and 7, and on the current reading of the TX and RX TIME SLOT NUMBER displays in the ANALOG LINE INTERFACEs, which circuits of the Central Office are connected during time slot 1? During time slot 7? Why? * 10. Do not replace the handsets of telephone sets A and B on their cradles. Bidirectional Telephone Conversation * 11. Display the Control Register of the SPACE-DIVISION SWITCH (SDS Control Register). Set the SPACE-DIVISION SWITCH Control Mode (SDS Control Mode) of the SWITCHING CIRCUIT to Manual. While doing this, have a normal telephone conversation and observe the connections performed by the SPACE-DIVISION SWITCH in time slots 1 and 7, as well as the contents of the SDS Control Register. 2-36

7 Describe what happens when the SDS Control Mode passes from Automatic to Manual. Briefly explain why. * 12. Set the TIME SLOT SELECTOR of the SPACE-DIVISION SWITCH to 01. In the SDS Control Register, enter "00" in the cell at the intersection of column TX0 and row Time Slot 1. This will force the SPACE-DIVISION SWITCH to connect line TX0 to line RX0 during time slot 1. * 13. Try having a normal telephone conversation. What do you observe? Why? * 14. Set the TIME SLOT SELECTOR of the SPACE-DIVISION SWITCH to 07. In the SDS Control Register, enter "00" in the cell at the intersection of column TX0 and row TS7. This will force the SPACE-DIVISION SWITCH to connect line TX0 to RX0 during time slot 7. * 15. Try having a normal telephone conversation. What do you observe? Why? 2-37

8 * 16. From the observations you have made up to this point, what is the number of connections required in the Central Office in order for a normal (bidirectional) conversation to occur between telephone sets A and B? Why? * 17. Do not replace the handsets of telephone sets A and B on their cradles. Coordination of Two-Dimensional Switching * 18. Set the SDS Control Mode of the SWITCHING CIRCUIT back to Automatic. * 19. Connect Oscilloscope Probes 1, 2, and 4 to TP6 (line TX0), TP9 (line RX0), and TP17 (FRAME SYNC. signal) of ANALOG LINE INTERFACE A, respectively. Connect Oscilloscope Probe 3 to TP29 (TS CLOCK signal) of the SIGNALING CIRCUIT. * 20. Start the Oscilloscope. Make the following settings on the Oscilloscope: Channel 1 Mode Normal Sensitivity V/div Input Coupling DC Channel 2 Mode Normal Sensitivity V/div Input Coupling DC Channel 3 Mode Normal Sensitivity V/div Input Coupling DC Channel 4 Mode Normal Sensitivity V/div Input Coupling DC Time Base us/div 2-38

9 Trigger Source Ch 4 Level V Slope positive (+) Display Refresh Continuous Observe that PCM codes appear in the signals on lines TX0 and RX0 during time slots 1 and 7 (especially when having a normal telephone conversation). These PCM codes correspond to the digitized voice signals from ANALOG LINE INTERFACES A and B. * 21. Set the TX time slot (TSAC address) of ANALOG LINE INTERFACE A to several different values. After each new TX time slot setting, talk into the handset of telephone set A while observing the signals on lines TX0 and RX0 on the Oscilloscope screen. Observe that the time slot during which the digitized voice signal from ANALOG LINE INTERFACE A appears on these lines has changed as a result of the change in the TX time slot setting of this interface. observe that the time slot during which the SPACE-DIVISION SWITCH performs the TX0-to-RX0 line connection used to route the digitized voice signal from ANALOG LINE INTERFACE A to ANALOG LINE INTERFACE B has changed in order to follow the change in the TX time slot setting of ANALOG LINE INTERFACE A; observe that the RX time slot assigned to ANALOG LINE INTER- FACE B for reception of the digitized voice signal from ANALOG LINE INTERFACE A has changed in order to follow the change in the TX time slot setting of ANALOG LINE INTERFACE A. Which device of the Central Office causes the SPACE-DIVISION SWITCH and ANALOG LINE INTERFACE B to "automatically adapt" to each new TX time slot setting of ANALOG LINE INTERFACE A in order to maintain a normal, bidirectional conversation between telephone sets A and B? Explain. Replace the handsets of telephone sets A and B on their cradles. * 22. From the observations you have made up to this point, does combining time-multiplexed switching with space-division switching provide several different "time-space" connections? * Yes * No 2-39

10 * 23. Calculate the maximum number of time-space connections that can be made in the Central Office of the Telephony Training System. Then, compare your result to the maximum number of connections that could be made if either time-multiplexed switching (through a single physical line) or space-division switching were used alone. What can you conclude? * 24. 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 AC/DC power converters from the TTS Power Supply and the analog telephone sets. Disconnect the analog telephone sets from the Dual Analog Line Interface. Remove the Dual Analog Line Interface from the Reconfigurable Training Module. CONCLUSION In this exercise, you became familiar with two-dimensional switching as it is implemented in the Telephony Training System. You saw that time-multiplexed switching is performed through dynamic assignment of the receive time slots to the ANALOG LINE INTERFACEs and SERVICE CIRCUITs of the Central Office. In addition, space-division switching is performed through physical connection of transmit and receive lines during the proper time slots. You saw that the dynamic assignment of the receive time slots and the control of the connections made by the SPACE-DIVISION SWITCH are both orchestrated by the CALL PROCESSOR. You learned that, in order for a normal, bidirectional conversation to occur between two telephone sets, two connections must be made by the Central Office to allow transmission of the voice signal from each telephone set. Finally, you saw that two-dimensional switching is a clever means of designing digital switches that can perform a great number of connections. 2-40

11 REVIEW QUESTIONS 1. What is the main limitation to the number of connections that a time-division switch can make? 2. What is the main limitation to the number of connections that a digital spacedivision switch can make? Why? 3. What does "two-dimensional switching" mean? 4. What is the advantage of two-dimensional switching over time-division switching and space-division switching? 5. Briefly explain how two-dimensional switching is performed in the Telephony Training System. 2-41