Exercise 1-2. Digital Trunk Interface EXERCISE OBJECTIVE

Exercise 1-2 Digital Trunk Interface EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain the role of the digital trunk interface in a central office. You will be familiar with the DS1 and E1 TDM formats that can be used in the Lab-Volt digital trunk. You will be able to describe the operation of the digital trunk interface used in Lab-Volt CO's using the simplified block diagram of this interface. DISCUSSION Role of the Digital Trunk Interface As mentioned before in this unit, a digital trunk is a link between two switching offices of the PSTN that can carry many digitized voice signals at a same time and in both directions. This is achieved by multiplexing in time digitized voice signals according to one of the various TDM formats available in the older North American and European hierarchies of digital transmission systems (DS1 to DS4 and E1 to E5 digital signals) and the SONET/SDH hierarchy of digital transmission systems (STS-1 to STS-192 and STM-1 to STM-64 electrical signals). The role of the digital trunk interface is to provide a link between the digital circuitry of the CO (call processor, signaling circuit, and switching circuit) and a digital trunk. This is illustrated in Figure 1-4. 1-25

DIGITAL TRUNK INTERFACE DIGITIZED VOICE SIGNALS FROM SWITCHING CIRCUIT SIGNALING DATA FROM CALL PROCESSOR (VIA SIGNALING CIRCUIT) TRANSMITTED SERIAL DIGITAL SIGNAL PAIR OF COPPER WIRES TO OTHER CO DIGITAL TRUNK (FOUR WIRES) DIGITIZED VOICE SIGNALS TO SWITCHING CIRCUIT SIGNALING DATA TO CALL PROCESSOR (VIA SIGNALING CIRCUIT) RECEIVED SERIAL DIGITAL SIGNAL PAIR OF COPPER WIRES FROM OTHER CO Figure 1-4. The digital trunk interface provides a link between the digital circuitry of the CO and a digital trunk. In brief, the digital trunk interface receives many digitized voice signals (64-kb/s PCM signals) from the switching circuit and signaling data from the call processor (via the signaling circuit). It time multiplexes these signals and data, according to a certain TDM format (DS1, E4, STS-1, STM-1, etc), to form a serial digital signal that is transmitted via the digital trunk line. Conversely, the digital trunk interface receives a serial digital signal corresponding to a certain TDM format (DS1, E4, STS-1, STM-1, etc) from the digital trunk line. It extracts (demultiplexes) the various digitized voice signals (64-kb/s PCM signals) and the signaling data it contains, and sends these signals and data to the switching circuit and the call processor (via the signaling circuit) of the CO, respectively. Note that depending on the TDM format used, the transmission media can be pairs of copper wires (as shown in Figure 1-4), coaxial cables, or optical fibers. TDM Formats Used in the Lab-Volt Digital Trunk Two TDM formats are available for the digital trunk used to interconnect Lab-Volt CO's. When the digitized voice signals are multiplexed in time over 24 time slots in Lab-Volt CO's, the DS1 format from the North American hierarchy of digital transmission systems is used. When time-division multiplexing is made over 32 time slots in Lab-Volt CO's, the E1 format from the European hierarchy of digital transmission systems is used. Figure 1-5 illustrates the DS1 TDM format. This format divides time into intervals of equal duration that are referred to as frames. The duration of each frame is 125 µs, which exactly corresponds to the reciprocal of the sampling frequency (8 khz) used in CO's to digitize voice signals. Each frame is subdivided into 24 time intervals that are called time slots. These time slots are numbered 1 to 24. Each time slot can carry a digitized voice signal (8-bit PCM code). An extra bit (F bit) is added at the 1-26

beginning of each frame to convey framing information (the use of this bit is covered later in this discussion). The F bit and the digitized voice signals in the 24 time slots form a serial digital signal which consists of 193 bits that repeat every frame, thereby leading to a bit rate of 1.544 Mb/s. DIGITIZED VOICE SIGNAL (8-BIT SERIAL PCM CODE) SIGN BIT SIGNAL MAGNITUDE b7 b6 b5 b4 b3 b2 b1 b0 MSB LSB F 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 24 TIME SLOTS CONTAINING 24 EIGHT-BIT SERIAL PCM CODES OR 23 EIGHT-BIT SERIAL PCM CODES PLUS 1 EIGHT-BIT WORD OF SIGNALING DATA (192 BITS) FRAMING BIT (F BIT) FRAME = FRAMING BIT + 24 EIGHT-BIT TIME SLOTS (193 BITS) 125 µs Figure 1-5. DS1 time-division multiplexing (TDM ) format. Note that one of the 24 time slots in each DS1 frame can be used to convey signaling data (instead of a digitized voice signal) related to the digitized voice signals transmitted. This is the case in the Lab-Volt digital trunk where time slot 24 conveys signaling data and the remaining 23 time slots can carry digitized voice signals. Digital signaling between CO's is covered extensively in the next unit of this manual. Figure 1-6 illustrates the E1 TDM format. This format also divides time into frames having a duration of 125 µs. Each frame is divided into 32 time slots that are numbered 0 to 31. Each of time slots 1 to 15 and 17 to 31 can carry a digitized voice signal (8-bit PCM code), for a maximum of 30 digitized voice signals. Time slot 0 is used to convey framing information (the use of this information is covered later in this discussion). Time slot 16 is used to convey signaling data related to the digitized voice signals transmitted. The 32 time slots form a serial digital signal which consists of 256 bits (32 time slots x 8 bits) that repeat every frame, thereby leading to a bit rate of 2.048 Mb/s. 1-27

DIGITIZED VOICE SIGNAL (8-BIT SERIAL PCM CODE) SIGN BIT SIGNAL MAGNITUDE b7 b6 b5 b4 b3 b2 b1 b0 MSB LSB 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 15 TIME SLOTS CONTAINING 15 EIGHT-BIT SERIAL PCM CODES 15 TIME SLOTS CONTAINING 15 EIGHT-BIT SERIAL PCM CODES TIME SLOT CONTAINING FRAMING INFORMATION TIME SLOT CONTAINING SIGNALING INFORMATION FRAME = 32 EIGHT-BIT TIME SLOTS (256 BITS) 125 µs Figure 1-6. E1 time-division multiplexing (TDM) format. Simplified Block Diagram of the DIGITAL TRUNK INTERFACE Figure 1-7 is a simplified block diagram of the DIGITAL TRUNK INTERFACE used in Lab-Volt CO's. This diagram is divided in two major sections that are referred to as the RECEIVER and the TRANSMITTER. The RECEIVER receives a serial digital signal in DS1 or E1 format from the digital trunk line, extracts (demultiplexes) the various digitized voice signals and the signaling data it contains, and sends these signals and data to the switching circuit and the call processor (via the signaling circuit) of the CO, respectively. Conversely, the TRANSMITTER receives many digitized voice signals from the switching circuit and signaling data from the call processor (via the signaling circuit), time multiplexes these signals and data to form a DS1 or E1 digital signal that is transmitted via the digital trunk line. The remaining two subsections in this discussion describe the operation of the circuitry in the RECEIVER and TRANSMITTER of the DIGITAL TRUNK INTERFACE. 1-28

DIGITAL TRUNK INTERFACE RECEIVER TO SWITCHING CIRCUIT OF CO TO CALL PROCESSOR OF CO VIA SIGNALING CIRCUIT TX2 DORX TIME SLOT INTERCHANGER 1 RECOVERED DATA RECOVERED FRAME SYNC. RECOV. MF SYNC. RECOV. TS CLOCK LFA RECOVERED BIT CLOCK FRAMING CIRCUIT AND ALARM DETECTOR LINE DECODER LOS DATA/ CLOCK RECOVERY CIRCUIT IRX RECEIVE LINE FROM OTHER CO BIT CLOCK AIS RAI FRAME SYNC. DIGITAL TRUNK TRANSMITTER ITX TRANSMIT LINE FROM SWITCHING CIRCUIT OF CO RX2 TIME SLOT INTERCHANGER 2 FRAMING AND SIGNALING CIRCUIT LINE CODER TO OTHER CO BIT CLOCK FRAME SYNC. FROM CALL PROCESSOR OF CO VIA SIGNALING CIRCUIT DOTX TX BIT CLOCK TX FRAME SYNC. TX MF SYNC. Figure 1-7. Simplified block diagram of the DIGITAL TRUNK INTERFACE used in Lab-Volt CO's. Operation of the RECEIVER The RECEIVER consists of a DATA/CLOCK RECOVERY CIRCUIT, a LINE DECODER, a FRAMING CIRCUIT AND ALARM DETECTOR, and TIME SLOT INTERCHANGER 1. The operation of each of these circuits is explained in this subsection. The explanations sometimes refer to the simplified block diagram in Figure 1-7. DATA/CLOCK RECOVERY CIRCUIT The DATA/CLOCK RECOVERY CIRCUIT receives a serial digital signal from the digital trunk line. This signal usually exhibits some distortion caused by the transmission over the digital trunk line. The DATA/CLOCK RECOVERY CIRCUIT 1-29

recovers the bit clock from the received serial digital signal. The bit clock is a square-wave signal having a frequency that corresponds to the bit rate (1.544 or 2.048 Mb/s) of the received digital signal. The recovered bit clock is used to synchronize the operation of the FRAMING CIRCUIT AND ALARM DETECTOR and TIME SLOT INTERCHANGER 1 with the incoming serial digital signal. The DATA/CLOCK RECOVERY CIRCUIT uses the recovered bit clock to regenerate a "clean" serial digital signal from the distorted signal received from the digital trunk line. The regenerated serial digital signal is sent to the LINE DECODER. Figure 1-8 shows a slightly distorted serial digital signal received from a digital trunk line and the RECOVERED BIT CLOCK signal and regenerated serial digital signal at the outputs of the DATA/CLOCK RECOVERY CIRCUIT. Notice that the rising edges in the RECOVERED BIT CLOCK signal are aligned with the middle of the pulses in the distorted serial digital signal to make the regeneration of the serial digital signal easier. BINARY SEQUENCE 1 1 0 1 0 1 0 0 1 SERIAL DIGITAL SIGNAL RECEIVED RECOVERED BIT CLOCK SIGNAL REGENERATED SERIAL DIGITAL SIGNAL Figure 1-8. Input and output signals of the DATA/CLOCK RECOVERY CIRCUIT. Note that the DATA/CLOCK RECOVERY CIRCUIT detects whether or not a signal is received from the digital trunk line. When a serial digital signal of sufficient magnitude is received from the digital trunk, the signal at the loss-of-signal (LOS) output of the DATA/CLOCK RECOVERY CIRCUIT is at logic level zero. Conversely, the signal at the LOS output goes to logic level 1 when no signal is received from the digital trunk line. 1-30

LINE DECODER The serial digital signal received from the digital trunk line is coded to ensure a minimal amount of transitions in this signal, and thereby, facilitates clock recovery. When the DS1 TDM format is used, a line code referred to as bipolar with eight-zero substitution (B8ZS) is applied to the serial digital signal. When the E1 TDM format is used, another line code called high-density bipolar of order 3 (HDB3) is applied to the serial digital signal. In brief, the B8ZS and HDB3 line codes are modified versions of another line code called alternate mark inversion (AMI). When the AMI line code is used, each binary zero is transmitted as no signal on the line and each binary one is transmitted as a rectangular pulse whose polarity alternates from one binary one to the next. When the B8ZS line code is used, the serial digital signal is still AMI coded but each sequence of 8 successive binary zeroes in the digital signal is replaced with a sequence of pulses containing two bipolar violations, that is, a sequence where the alternation in pulse polarity is not respected. Similarly, when the HDB3 line code is used, the serial digital signal is also AMI coded but each sequence of 4 successive binary zeroes is replaced with a sequence of pulses containing one bipolar violation. Figure 1-9 shows a sequence of binary ones and zeros in non-return-to-zero (NRZ) format and the digital signals that result from AMI, B8ZS, and HDB3 line coding. Notice that the return-to-zero (RZ) format is used in each coded digital signal, that is, the duration of each pulse representing a binary one is equal to one half the bit interval. BINARY SEQUENCE (NRZ FORMAT) 1 0 0 0 0 0 0 0 0 1 1 AMI CODED DIGITAL SIGNAL B8ZS CODED DIGITAL SIGNAL BIPOLAR VIOLATION HDB3 CODED DIGITAL SIGNAL BIPOLAR VIOLATION Figure 1-9. Serial digital signals coded using the AMI, B8ZS, and HDB3 line codes. 1-31

The LINE DECODER in the RECEIVER of the DIGITAL TRUNK INTERFACE receives the serial digital signal regenerated by the DATA/CLOCK RECOVERY CIRCUIT, and converts this signal to the NRZ format. The resulting serial digital signal (recovered data) is sent to the FRAMING CIRCUIT AND ALARM DETECTOR and TIME SLOT INTERCHANGER 1. Figure 1-10 is an example of signals at the input and output of the LINE DECODER. BINARY SEQUENCE 0 1 1 0 0 0 1 0 1 0 LINE DECODER INPUT SIGNAL LINE DECODER OUTPUT SIGNAL Figure 1-10. Signals at the input and output of the LINE DECODER. FRAMING CIRCUIT AND ALARM DETECTOR The FRAMING CIRCUIT analyzes the framing information contained in the serial digital signal coming from the LINE DECODER (recovered data) to find the beginning of each frame. This process is referred to as frame alignment, that is, the RECEIVER aligns itself with the serial digital signal received. Frame alignment is absolutely essential to correctly recover the digitized voice signals and the signaling data contained in the various time slots of each frame. When the FRAMING CIRCUIT is able to achieve frame alignment, a rectangular pulse signal appears at its RECOVERED FRAME SYNC. output, each pulse in this signal being aligned with the first time slot of each frame as shown in Figure 1-11. Furthermore, the signal at its loss-of-frame-alignment (LFA) output is at logic level zero. Conversely, the signal at the LFA output goes to logic level 1 when frame alignment is lost. 1-32

BEGINNING OF A FRAME FRAME INTERVAL (125 µs) TIME SLOT INTERVALS RECOVERED FRAME SYNC. SIGNAL RECOVERED TIME-SLOT CLOCK SIGNAL RECOVERED MULTIFRAME SYNC. SIGNAL Figure 1-11. Output signals of the FRAMING CIRCUIT. The FRAMING CIRCUIT also produces a recovered time-slot clock (RECOVERED TS CLOCK) signal and a recovered multiframe synchronization (RECOVERED MF SYNC.) signal. The RECOVERED TS CLOCK signal is a square-wave signal aligned with the time slot intervals. The RECOVERED MF SYNC. signal consists of a rectangular pulse that occurs at the beginning of each multiframe, the duration of this pulse being equal to one frame (125 µs). Note: Appendix C of this manual provides information related to the multiframe structures of the DS1 and E1 digital signals. The ALARM DETECTOR analyzes the serial digital signal coming from the LINE DECODER (recovered data) to detect the presence of remote alarm signals. When an alarm signal is detected, either the AIS or RAI output of the ALARM DETECTOR goes to logic level 1. Alarms are discussed extensively in the next exercise of this unit. TIME SLOT INTERCHANGER 1 TIME SLOT INTERCHANGER 1 (TSI 1) receives a serial digital signal in NRZ format (recovered data) from the LINE DECODER. It performs time slot interchange, that is, it transfers digitized voice signals contained in certain time slots of the RECOVERED DATA signal to other time slots in the serial digital signal at its output (line TX2 of the DIGITAL TRUNK INTERFACE). For example, a digitized voice signal received in time slot 4 of the RECOVERED DATA signal can be made available in time slot 1 of the serial digital signal on line TX2, as shown in 1-33

Figure 1-12. The use of time slot interchange in the DIGITAL TRUNK INTERFACE allows a digitized voice signal to be received from the digital trunk in a certain time slot while being space-division switched in another time slot in the switching circuit (space-division switch) of the CO. This flexibility greatly reduces the chances of having an inter-exchange call blocked because no communication path can be established. Data coming from the call processor of the CO determines what time slot interchanges are to be performed by TSI 1. DATA TO SWITCHING CIRCUIT OF CO TIME SLOT INTERCHANGER 1 MEMORY RECOVERED DATA FROM LINE DECODER TIME SLOTS TIME SLOTS TX2 1 2 3 4 5 6 1 2 3 4 5 6 DATA FROM CALL PROCESSOR OF CO RECOVERED FRAME SYNC. SIGNALING DATA TO CALL PROCESSOR OF CO DORX RECOVERED BIT CLOCK BIT CLOCK FROM CO FRAME SYNC. FROM CO Figure 1-12. TIME SLOT INTERCHANGER 1 (TSI 1) performs time slot interchange and extracts signaling data contained in the RECOVERED DATA signal. TSI 1 also extracts the signaling data contained in either time slot 16 or 24 of the RECOVERED DATA signal. The extracted signaling data, available at the DORX output of TSI 1, is sent to the call processor of the CO (via the digital signaling processor in the signaling circuit of the CO) where it is analyzed. Notice that two bit clock signals and two frame synchronization signals are provided to TSI 1 shown in Figure 1-12. The RECOVERED BIT CLOCK and RECOVERED FRAME SYNC. signals, which are synchronized with the RECOVERED DATA signal, are used to write the contents of the RECOVERED DATA signal into the memory of TSI 1. Conversely, the BIT CLOCK and FRAME SYNC. signals from the CO are used to read the contents of the memory in TSI 1. Operation of the TRANSMITTER The TRANSMITTER consists of TIME SLOT INTERCHANGER 2, a FRAMING AND SIGNALING CIRCUIT, and a LINE CODER. The operation of each of these circuits is explained in this subsection. The explanations sometimes refer to the simplified block diagram in Figure 1-7. 1-34

TIME SLOT INTERCHANGER 2 TIME SLOT INTERCHANGER 2 (TSI 2) receives digitized voice signals from the switching circuit (space-division switch) of the CO via line RX2. These digitized voice signals are to be transmitted to another CO via the digital trunk. TSI 2 performs time slot interchange to place the digitized voice signals to be transmitted in the proper time slots of the serial digital signal sent to the remote CO via the digital trunk. For example, if a digitized voice signal is received in time slot 3 and is to be transmitted in time slot 5 on the digital trunk, TSI 2 makes the digitized voice signal received from line RX2 during time slot 3 available during time slot 5 of the serial digital signal at its output. Data coming from the call processor of the CO determines the time slot interchanges that must be carried out by TSI 2 for the digitized voice signals to be transmitted via the digital trunk during the proper time slots. The BIT CLOCK and FRAME SYNC. signals from the CO are used to to read the contents of the memory in TSI 2. Notice that two bit clock signals and two frame synchronization signals are provided to TSI 2 shown in Figure 1-7. The BIT CLOCK and FRAME SYNC. signals from the CO are used to write the contents of the serial digital signal on line RX2 into the memory of TSI 2. Conversely, the TX BIT CLOCK and TX FRAME SYNC. signals, which are synchronized with the TDM format used in the digital trunk, are used to read the contents of the memory of TSI 2. The serial digital signal at the output of TSI 2 is sent to the FRAMING AND SIGNALING CIRCUIT. Note that when a time slot is not used to carry a digitized voice signal, the output signal of TSI 2 is held at logic level one. This adds a sequence of binary ones in the serial digital signal to be transmitted, and thereby, increases the number of transitions in the coded signal transmitted via the digital trunk. This feature of TSI 2 and the use of B8ZS or HDB3 line coding ensure easy clock recovery in the receiver at the other end of the digital trunk. FRAMING AND SIGNALING CIRCUIT The FRAMING AND SIGNALING CIRCUIT (FSC) adds framing information and signaling data to the serial digital signal coming from TSI 2. The framing information is generated locally by the FSC. When the DS1 TDM format is used, the FSC inserts the framing information in the F bit position of each frame (bit located just before time slot 1). When the E1 TDM format is used, the FSC inserts the framing information in time slot 0 of each frame. The FSC receives signaling data from the call processor of the CO, via the digital signaling processor in the signaling circuit of the CO and the DOTX input of the DIGITAL TRUNK INTERFACE. The signaling data is information exchanged between two CO's that allows control of inter-exchange calls. When the DS1 TDM format is used, the FSC inserts the signaling data in time slot 24 of each frame, one octet at a time. When the E1 TDM format is used, the FSC inserts the signaling data in time slot 16 of each frame. Note that the FSC inserts binary ones in time slot 24 or 16 whenever there is no signaling data to be transmitted. This adds a sequence of binary ones in the serial digital signal to be transmitted, and thereby, increases the number of transitions in the coded signal transmitted via the digital 1-35

trunk. This helps in ensuring easy clock recovery in the receiver at the other end of the digital trunk. Note that the FSC requires the TX BIT CLOCK and TX FRAME SYNC. signals from the CO in order to be able to insert the framing information and the signaling data into the correct time intervals of each frame. LINE CODER The LINE CODER applies a line code to the serial digital signal coming from the FRAMING AND SIGNALING CIRCUIT, which is in NRZ format, before transmission via the digital trunk. When the DS1 TDM format is used, the LINE CODER applies the B8ZS line code to the serial digital signal. When the E1 TDM format is used, the LINE CODER applies the HDB3 line code to the serial digital signal. The use of these line codes ensures a minimum amount of transitions in the serial digital signal transmitted via the digital trunk, and thus, enables easy clock recovery by the receiver at the other end of the trunk. Procedure Summary In the first part of the exercise, you will set up a digital trunk between two Lab-Volt Central Offices (CO's). In the second part of the exercise, you will observe how framing and signaling are performed in the DIGITAL TRUNK INTERFACE (DTI) used in Lab-Volt CO's. To do so, you will observe the signals involved in the operation of the FRAMING AND SIGNALING CIRCUITs in the DTI TRANSMITTER and RECEIVER. You will then observe the serial digital signals exchanged via the digital trunk when an interexchange call is initiated. In the last part of the exercise, you will determine how time slot interchange is performed in the DTI. To do so, you will observe the signals at the input and output of the TIME SLOT INTERCHANGERs in the DTI TRANSMITTER and RECEIVER when an inter-exchange call is established. EQUIPMENT REQUIRED Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise. 1-36

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 used to download the CO program to two Reconfigurable Training Modules, Model 9431. This host computer is used to monitor one of the two Lab-Volt CO's (the one designated as CO A throughout the exercise). * 1. Make sure that two Reconfigurable Training Modules, Model 9431, are connected to the Power Supply, Model 9408. Make sure that there is a network connection between each Reconfigurable Training Module and the host computer. * 2. Install a Dual Analog Line Interface, Model 9475, into one of the two analog/digital (A/D) slots of a Reconfigurable Training Module. This module will be used as CO A. Connect two analog telephone sets to this Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each 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 the remaining analog/digital (A/D) slot of the Reconfigurable Training Module used as CO A. * 3. Install a Dual Analog Line Interface, Model 9475, into one of the two analog/digital (A/D) slots of the other Reconfigurable Training Module. This module will be used as CO B. Connect two analog telephone sets to this Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each telephone set. Install a Digital Trunk Interface, Model 9478, into the digital (D) slot or the remaining analog/digital (A/D) slot of the Reconfigurable Training Module used 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 installed in the Reconfigurable Training Module used as CO A to the RJ-45 female connector on the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO B. 1-37

* 5. Connect two of the AC/DC power converters supplied with the analog telephone sets to the AC power outlets on the Power Supply. Then, 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 A of CO's A and B. Note: If another Power Supply, Model 9408, is available, connect the two other AC/DC power converters supplied with the analog telephone sets to the AC power outlets on this Power Supply. Then, 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 B of CO's A and B. * 6. Turn on the host computer. Turn on the Power Supply, then turn on the Reconfigurable Training Modules. * 7. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module used as CO A. Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module used as CO A, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module. * 8. On the host computer, download the CO program to the Reconfigurable Training Module used as CO B, using the function in the Tools menu that allows a functionality to be downloaded to another RTM. Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module used as CO B, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module. Furthermore, if you downloaded the CO program to the Reconfigurable Training Module used as CO B from a second host computer, make sure that the digital trunk TDM format (DS1 or E1) selected in CO B is the same as that selected in CO A. Also make sure that the number of CO B differs from the number of CO A. These parameters, which are accessed via the LVTTS Options dialog box, must be set as mentioned above to enable establishment of inter-exchange calls via the digital trunk. These parameters are set automatically when a single host computer is used to download the CO program to the two Reconfigurable Training Modules. 1-38

Once the CO program has been downloaded, a box representing CO B will appear connected to CO A, via a digital trunk, in the diagram of CO A displayed on the host computer screen. Record below the telephone numbers associated with ANALOG LINE INTERFACEs 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 Training Modules used as CO's A and B, make sure that the alarm indicators (red and yellow LED's) are not lit, thereby confirming that the digital trunk is now ready to carry inter-exchange calls. * 10. On the host computer, make sure that the addresses of the TSAC's in ALI's A and B are set to 01 and 02, respectively. Also, make sure that the addresses of the TSAC's of SERVICE CIRCUITs 1 and 2 in the SIGNALING CIRCUIT are set to 01 and 02, respectively. Framing and Signaling in the DIGITAL TRUNK INTERFACE * 11. On the host computer, zoom in on the DIGITAL TRUNK INTERFACE (DTI) of CO A, then zoom in on the TRANSMITTER of this interface. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP8 (TX FRAME SYNC. signal), TP10 (TX TS CLOCK signal), TP7 (TX BIT CLOCK signal), and TP11 (LINE CODER input) of the DTI, respectively. * 12. 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 1-39

Channel 4 Mode...................................... Normal Sensitivity.................................... 5 V/div Input Coupling................................... DC Time Base.................................... 20 µs/div Trigger Source....................................... Ch 1 Level......................................... 1 V Slope................................... Positive (+) Display Mode................................... Square Display Refresh.............................. Continuous Position the horizontal trigger point two divisions from the left-hand side of the Oscilloscope screen. * 13. On the Oscilloscope screen, observe the TX FRAME SYNC. signal at TP8 of the DTI. This signal consists of a rectangular pulse that occurs every frame. The TX FRAME SYNC. signal is used to control the read operations in the memory of TIME SLOT INTERCHANGER 2 (TSI 2). It also allows the FRAMING AND SIGNALING CIRCUIT (FSC) to insert framing information and signaling data into the serial digital signal coming from TSI 2. Measure the period of the TX FRAME SYNC. signal. This period corresponds to the duration of one frame. From your measurement, do frames occur at the same rate as voice signals are sampled in the Lab-Volt CO's, i.e., 8000 times per second? Explain. * 14. Observe the TX TS CLOCK signal at TP10 of the DTI. Each cycle in the TX TS CLOCK signal is equal to the duration of one time slot, that is, 5.2 µs when the DS1 TDM format is used, or 3.9 µs when the E1 TDM format is used. Determine the number of cycles that occur in the TX TS CLOCK signal within one complete frame. This corresponds to the number of time slots that occur within one complete frame. 1-40

* 15. Set the Oscilloscope time base to 5 µs/div. Observe that the width of the pulse in the TX FRAME SYNC. signal at TP8 is equal to the duration of one time slot. What is the purpose of this pulse? Explain. * 16. Set the Oscilloscope time base to 1 µs/div. Observe the TX BIT CLOCK signal at TP7 of the DTI. In conjunction with the TX FRAME SYNC. signal, the TX BIT CLOCK signal is used to control the read operations in the memory of TSI 2. It also allows the FSC to insert framing information and signaling data into the serial digital signal coming from TSI 2. Determine the number of cycles that occur in the TX BIT CLOCK signal within one time slot. This corresponds to the number of bits that can be conveyed in each time slot. From your observation, is each time slot perfectly suited to carry a digitized voice signal (8-bit PCM code)? * Yes * No * 17. Measure the period of the TX BIT CLOCK signal at TP7 of the DTI. This corresponds to the time interval between transmission of two successive bits via the digital trunk. Determine the bit rate of the digital trunk (i.e., the number of bits per second which can be transmitted via the digital trunk). To do so, divide 1 by the period of the TX BIT CLOCK signal. 1-41

* 18. (Skip this step if the E1 TDM format is used.) Position the horizontal trigger point seven divisions from the left-hand side of the Oscilloscope screen. Observe that, with the DS1 TDM format, the cycle of the TX TS CLOCK signal that occurs just before the beginning of the pulse in the TX FRAME SYNC. signal lasts a little longer than the other cycles (9 bits instead of 8 bits). Explain why. Position the horizontal trigger point two divisions from the left-hand side of the Oscilloscope screen. * 19. Set the Oscilloscope time base to 20 µs/div. Observe the serial digital signal at TP11 (LINE CODER input) of the DTI. Notice that this signal seems to be at logic level 1 most of the time. This occurs because all time slots that are not used to carry a digitized voice signal are filled with binary ones by TSI 2, while the time slot used to carry signaling data is filled with binary ones by the FSC when there is no signaling data to be transmitted. * 20. Decrease the Oscilloscope time base to 5 µs/div. Observe that the serial digital signal at TP11 sometimes goes to logic level 0 during brief time intervals, since framing information appears during a short segment of this signal in the form of continuous bit transitions between logic states 0 and 1: When the DS1 TDM format is used, the framing information appears just before time slot 1 (F bit position) of each frame. When the E1 TDM format is used, the framing information appears in time slot 0 of each frame. Is this your observation? * Yes * No * 21. Set the Oscilloscope time base to 10 µs/div. Using telephone set A of CO A, place a call to telephone set A of CO B and let it ring. While doing this, observe that signaling data momentarily appears in one time slot of the serial digital signal at TP11 (LINE CODER input) of the DTI. 1-42

Note: Since the signaling data appears for a very short period of time in the serial digital signal at TP11, you might have to hang up and repeat the above step several times in order to be able to observe this data on the Oscilloscope screen. Using the TX FRAME SYNC. signal and the TX TS CLOCK signal, determine in which time slot of the serial digital signal at TP11 the signaling data appears. Note: If necessary, hang up and repeat this step as many times as required to be able to answer the above question. Hang up the handset of telephone set A of CO A. * 22. On the Oscilloscope, select the manual display refresh mode. Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP3 (RECOVERED FRAME SYNC. signal), TP5 (RECOVERED TS CLOCK), TP6 (RECOVERED BIT CLOCK signal), and TP2 (LINE DECODER output) of the DTI RECEIVER, respectively. On the Oscilloscope, set the time base to 20 µs/div and select the continuous display refresh mode. * 23. On the Oscilloscope screen, observe the signals produced by the FRAMING CIRCUIT in the DTI RECEIVER, as a result of successful frame alignment: Observe the RECOVERED FRAME SYNC. signal at TP3 of the DTI. This signal consists of a rectangular pulse that occurs every frame, each pulse being aligned with the first time slot of each frame. Set the Oscilloscope time base to 5 µs/div. Observe the RECOVERED TS CLOCK signal at TP5 of the DTI. Each cycle in this signal is equal to the duration of one time slot. Set the Oscilloscope time base to 1 µs/div. Observe the RECOVERED BIT CLOCK signal at TP6 of the DTI. This signal consists of a square wave whose frequency is 8 times higher than that of the RECOVERED TS CLOCK signal (8 cycles per time slot). The RECOVERED FRAME SYNC., RECOVERED TS CLOCK, and RECOVERED BIT CLOCK signals are all synchronized with the RECOVERED DATA signal at TP2 (LINE DECODER output) of the DTI. * 24. Set the Oscilloscope time base to 20 µs/div. 1-43

Observe the RECOVERED DATA signal at TP2 of the DTI. Notice that this signal seems to be at logic level 1 most of the time, since neither a digitized voice signal nor any signaling data is currently received from the digital trunk line. * 25. Decrease the Oscilloscope time base to 5 µs/div. Observe that the RECOVERED DATA signal at TP2 sometimes goes to logic level 0 during brief time intervals, since framing information appears during a short segment of this signal in the form of continuous bit transitions between logic states 0 and 1: When the DS1 TDM format is used, the framing information appears just before time slot 1 (F bit position) of each frame. When the E1 TDM format is used, the framing information appears in time slot 0 of each frame. Is this your observation? * Yes * No * 26. Set the Oscilloscope time base to 10 µs/div. Using telephone set A of CO A, place a call to telephone set A of CO B and let it ring. While doing this, observe that signaling data momentarily appears in one time slot of the RECOVERED DATA signal at TP2 of the DTI. Note: Since the signaling data appears for a very short period of time in the serial digital signal at TP2, you might have to hang up and repeat the above step several times in order to be able to observe this data on the Oscilloscope screen. Using the RECOVERED FRAME SYNC. signal and the RECOVERED TS CLOCK signal, determine in which time slot of the RECOVERED DATA signal at TP2 the signaling data appears. Hang up the handset of telephone set A of CO A. Time Slot Interchange in the DIGITAL TRUNK INTERFACE * 27. On the Oscilloscope, select the manual display refresh mode. Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP16 (RX2 line), TP17 (TSI-2 output), TP10 (TX TS CLOCK signal), and TP8 (TX FRAME SYNC. signal) of the DTI TRANSMITTER, respectively. 1-44

Make the following settings on the Oscilloscope: Trigger Source..................................... Ch 4 Display Refresh............................... Continuous * 28. Using telephone set A of CO A, place a call to telephone set A of CO B. Answer the call and have a normal telephone conversation. While talking into the handset of telephone set A of CO A, observe the PCM codes representing the voice signal that originates from this telephone set at TP16 (RX2 line) and TP17 (TSI-2 output) of the DTI. Notice that these codes appear in different time slots of the signals at TP16 and TP17. Using the TX FRAME SYNC. signal and the TX TS CLOCK signal, find in which time slot of the signal at TP16 the PCM codes appear. Explain why the PCM codes appear in this time slot. Using the TX FRAME SYNC. signal and the TX TS CLOCK signal, find in which time slot of the signal at TP17 the PCM codes appear. Explain why the PCM codes appear in this time slot. * 29. From your observations, what is the function of TIME SLOT INTERCHANGER 2 (TSI 2) in the TRANSMITTER of the DTI? What signals are required for TSI 2 to achieve this function? Explain. 1-45

* 30. Display the control register of TIME SLOT INTERCHANGER 2 (TSI-2 Control Register) of the DIGITAL TRUNK INTERFACE. Observe that this register indicates 6 in the cell at the intersection of the row labeled INCOMING TIME SLOT (LINE RX2) 1 and the column labeled OUTGOING TIME SLOT (TP17). This indicates that TSI 2 performs time slot interchange from time slot 1 to time slot 6, as observed in the previous steps. Close the TSI-2 Control Register. * 31. Do not hang up. On the Oscilloscope, select the manual display refresh mode. Disconnect all the Oscilloscope Probes. Connect Oscilloscope Probes 1, 2, 3, and 4 to TP2 (LINE DECODER output), TP15 (TX2 line), TP5 (RECOVERED TS CLOCK signal), and TP3 (RECOVERED FRAME SYNC. signal) of the DTI RECEIVER, respectively. On the Oscilloscope, select the continuous display refresh mode. * 32. While talking into the handset of telephone set A of CO B, observe the PCM codes representing the voice signal that originates from this telephone set in one time slot of the RECOVERED DATA signal at TP2. Using the RECOVERED FRAME SYNC. signal and the RECOVERED TS CLOCK signal, determine in which time slot of the signal at TP2 the PCM codes appear. Is this time slot the same as that used for transmission, via the digital trunk, of the PCM codes representing the voice signal that originates from telephone set A of CO A? Explain. * 33. While talking into the handset of telephone set A of CO B, observe that PCM codes representing the voice signal that originates from this telephone set also appear in one time slot of the signal at TP15 (TX2 line). * 34. On the Oscilloscope, select the manual display refresh mode. Disconnect Oscilloscopes Probes 3 and 4 and connect them to TP29 (TS CLOCK signal) and TP30 (FRAME SYNC. signal) of the SIGNALING CIRCUIT, respectively. 1-46

On the Oscilloscope, select the continuous display refresh mode. The FRAME SYNC. signal of CO A is now used to trigger the Oscilloscope sweep. * 35. While talking into the handset of telephone set A of CO B, observe the PCM codes representing the voice signal that originates from this telephone set in one time slot of the signal at TP15 (TX2 line). Using the FRAME SYNC. signal and the TS CLOCK signal, determine in which time slot of the signal at TP15 the PCM codes appear. Is this time slot the same as that in which the PCM codes appear in the RECOVERED DATA signal at TP2 of the DTI? Explain. * 36. From your observations, what is the function of TIME SLOT INTERCHANGER 1 (TSI 1) in the RECEIVER of the DTI? What signals are required for TSI 1 to achieve this function? Explain. * 37. Display the control register of TIME SLOT INTERCHANGER 1 (TSI-1 Control Register) of the DIGITAL TRUNK INTERFACE. Observe that this register indicates 3 in the cell at the intersection of the row labeled INCOMING TIME SLOT (TP2) 6 and the column labeled OUTGOING TIME SLOT (LINE RX2). This indicates that TSI 1 performs time slot interchange from time slot 6 to time slot 3, as observed in the previous steps. 1-47

Close the TSI-1 Control Register. Hang up the handset of telephone sets A of CO's A and B. * 38. 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 telephone sets. Remove the digital trunk line (multi-wire cable terminated with RJ-45 male connectors) connecting the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO A to the Digital Trunk Interface installed in the Reconfigurable Training Module used as CO B. Disconnect the telephone sets from the Dual Analog Line Interface installed in the Reconfigurable Training Module used as CO A. Remove the Dual Analog Line Interface and the Digital Trunk Interface from this module. Disconnect the telephone sets from the Dual Analog Line Interface installed in the Reconfigurable Training Module used as CO B. Remove the Dual Analog Line Interface and the Digital Trunk Interface from this module. CONCLUSION In this exercise, you learned that the digitized voice signals and signaling data transmitted via a digital trunk can be multiplexed according to various TDM formats. Thus, you saw that the digital trunk used to interconnect two Lab-Volt CO's supports two TDM formats: the North American DS1 format and the European E1 TDM format. You learned that both formats divide time into intervals of equal duration called frames, each frame being subdivided into time intervals called time slots. You saw that the DS1 frame uses 24 time slots, while the E1 frame uses 32 time slots. In both cases, one of the time slots is used exclusively to convey signaling data, and framing information is enclosed in a specific bit position (in DS1 TDM format) or time slot (in E1 TDM format) of the frame. The remainder of the time slots in the frame are used to convey digitized voice signals (8-bit PCM codes). You became familiar with the role of the digital trunk interface in a CO. You learned that this interface provides a link between the digital circuitry of the CO and a digital trunk. You saw that the digital trunk interface consists of two major sections: a transmitter and a receiver. In the transmitter, the digitized voice signals and signaling data coming from the digital circuitry of the CO are time multiplexed to form a serial digital signal that is transmitted on the digital trunk line. In the receiver, the digitized voice signals and signaling data contained in the serial digital signal received from the digital trunk line are demultiplexed (extracted) from this signal and sent to the digital circuitry of the CO. 1-48

You familiarized yourself with the block diagram and operation of the DIGITAL TRUNK INTERFACE (DTI) used in Lab-Volt CO's. You saw that the DTI TRANSMITTER contains: a time slot interchanger that places the CO digitized voice signals in the proper time slots of the serial digital signal to be transmitted on the digital trunk line; a framing and signaling circuit that adds framing information and signaling data to the serial digital signal coming from the time slot interchanger; a line coder that applies the B8ZS or HDB3 line code to the serial digital signal coming from the framing and signaling circuit, before transmission on the digital trunk line. On the other hand, you saw that the DTI RECEIVER contains: a data/clock recovery circuit that recovers the bit clock from the serial digital signal received from the digital trunk line and regenerates this signal so as to eliminate the effect of distortion; a line decoder that converts the regenerated serial digital signal from the B8ZSor HDB3-format to the NRZ format (recovered data); a framing circuit that finds the beginning of each frame and an alarm detector that detects the presence of remote alarm signals; a time slot interchanger that transfers digitized voice signals contained in certain time slots of the recovered data signal to other time slots in the signal at its output. REVIEW QUESTIONS 1. What is the role of the digital trunk interface in a CO? How does it achieve this role? Explain. 1-49

2. What are the two TDM formats available for the digital trunk used to interconnect Lab-Volt CO's? How do these formats resemble each other? How do they differ? 3. What is the use of time slot interchange in the DIGITAL TRUNK INTERFACE of a Lab-Volt CO? What device determines what time slot interchanges are to be performed in the DIGITAL TRUNK INTERFACE? 4. What circuit in the DIGITAL TRUNK INTERFACE of a Lab-Volt CO recovers the bit clock from the serial digital signal received from the digital trunk line? What are two uses of the recovered bit clock? 1-50

5. What is the function of the FRAMING AND SIGNALING CIRCUIT in the TRANSMITTER of the DIGITAL TRUNK INTERFACE of a Lab-Volt CO? How does this circuit achieve this function and what two signals does it require to do this? Explain. 1-51