Analog Trunk Interface

EXERCISE OBJECTIVE

When you have completed this exercise, you will know the role of the analog trunk interface in a PABX. You will be able to explain the various functions of the analog trunk interface. You will be familiar with the block diagram and operation of the analog trunk interface in the Lab-Volt PABX.

DISCUSSION

Role of the Analog Trunk Interface

A PABX performs basic enterprise switching, that is, it makes the connections required to establish all calls between telephone sets located in an enterprise. A PABX also concentrates all telephone sets in the enterprise to a limited number of telephone lines that home in on the local CO. In many installations, conventional (analog) telephone lines, referred to as PABX analog trunks, are used to connect a PABX to the local CO.

Today's PABX's are all-digital integrated systems that switch digitized voice signals and use digital signaling protocols. Because of the intrinsic nature of modern PABX's, some kind of hybrid circuitry is required to interface each analog trunk (telephone line) with the digital circuitry in a PABX. This is the role of the analog trunk interfaces as shown in Figure 1-4. On one side, each analog trunk interface exchanges digital signals with the PABX circuitry. On the other side, it transmits and receives analog signals via a PABX analog trunk. In other words, the role of the analog trunk interface is to make the PABX, which is essentially a digital machine, appear as a conventional analog telephone set to the local CO. Note that one analog trunk interface is required for each PABX analog trunk. Also note that the analog trunk interfaces in a PABX are usually grouped in banks as are the analog line interfaces in central offices.

Figure 1-4. Analog trunk interfaces make a link between the digital circuitry in a modern PABX and analog trunks.

Functions of the Analog Trunk Interface

To correctly link the digital circuitry in a PABX to the analog trunks and make the PABX appear as a conventional analog telephone set to the local CO, the analog trunk interface performs the following functions:

1. Detects the ringing voltage applied across the analog trunk when there is an incoming call to the PABX;

2. Provides trunk status information (analog trunk loop open or closed) to allow the PABX call processor to perform trunk supervision;

3. Closes the analog trunk loop to answer an incoming call to the PABX or initiate an external call;

4. Converts time-division multiplexed serial digital signals (voice, DTMF dialing tones) from the PABX circuitry into analog signals that can be transmitted via

Analog Trunk Interface

5. Converts analog signals (voice, call progress tones, caller identification data) received from the analog trunk into time-division multiplexed serial digital signals before they are routed to the PABX circuitry;

6. Performs wire to four-wire conversion because the analog trunk is a two-wire circuit whereas the PABX circuitry uses 4-two-wire circuits;

7. Opens the analog trunk loop to terminate an external call.

The remaining subsections of this discussion relate the above functions to the block diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX.

Block Diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX Figure 1-5 is a block diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. The tip (T) and ring (R) terminals of the ANALOG TRUNK INTERFACE connect to the PABX analog trunk (local loop going to the Lab-Volt CO). Analog signals (voice signals, call progress tones, and DTMF dialing tones) are exchanged via these terminals. On the other side of the ANALOG TRUNK INTERFACE, the receive (RX2), transmit (TX2), sampling clock, bit clock, trunk status (TKS2), and data terminals connect to the call processor and the switching and signaling circuits of the Lab-Volt PABX. These terminals carry digital signals (digitized voice signals, DTMF dialing tones, call progress tones, etc).

Operation of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX This subsection explains the operation of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. Many of the explanations that follow are based on the block diagram shown in Figure 1-5. Furthermore, the subsection highlights the fact that the ANALOG TRUNK INTERFACE is somewhat a mixture of analog line interface circuitry (CODEC, TSAC, 2W/4W conversion circuit and loop current detector in a subscriber loop interface circuit) and analog telephone set circuitry (switchhook and ringing voltage detector in an electronic ringer circuit).

Analog Trunk Interface

Figure 1-5. Block diagram of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX.

ANSWER RELAY

The ANSWER RELAY is a double-pole single-throw contact relay as shown in Figure 1-6. It plays a role similar to that of the switchhook in an analog telephone set. When the PABX analog trunk is not in use, the relay contacts are open as shown in Figure 1-6 (a). During a call, the relay contacts are closed to connect the

Analog Trunk Interface

analog trunk loop as shown in Figure 1-6 (b). This is because the battery feed circuit of the analog line interface in the CO permanently applies DC voltage across the tip and ring wires of the analog trunk.

Figure 1-6. The ANSWER RELAY plays a role similar to that of the switchhook in an analog telephone set.

Analog Trunk Interface

The ANSWER RELAY is controlled by the PABX call processor via TIME-SLOT ASSIGNMENT CIRCUIT 1 (TSAC 1) of the ANALOG TRUNK INTERFACE. When the PABX call processor sends data to TSAC 1 that forces its ANS output to go to logic state 1, the ANSWER RELAY closes. In brief, the ANSWER RELAY, in conjunction with TSAC 1, performs functions 3 and 7 mentioned in the previous subsection (i.e. closing the analog trunk loop to answer or initiate an external call and opening the analog trunk loop to terminate an external call).

RING AND TRUNK STATUS DETECTOR

The RING AND TRUNK STATUS DETECTOR performs the first two functions of an analog trunk interface mentioned in the previous subsection: ringing voltage detection and trunk status indication. When the ringing voltage is applied across the PABX analog trunk by the CO, it is detected by the RING AND TRUNK STATUS DETECTOR which sets its output to logic state 1. Similarly, when the ANSWER RELAY closes, DC current starts to flow in the analog trunk loop. This current is detected by the RING AND TRUNK STATUS DETECTOR which sets its output to logic state 1. Note that all analog signals exchanged via the PABX analog trunk pass through the RING AND TRUNK STATUS DETECTOR without being modified.

Also note that the functions performed by the RING AND TRUNK STATUS DETECTOR are basically the same as the AC ringing voltage detection in the electronic ringer circuit of an analog telephone set and the loop current detection in the subscriber loop interface circuit (SLIC) of an analog line interface.

DUPLEXER

The DUPLEXER performs function 6 mentioned in the previous subsection: two-wire to four-wire conversion. This function is equivalent to the hybrid function performed by the SLIC of each analog line interface in a CO. Figure 1-7 illustrates the operation of the DUPLEXER.

The DUPLEXER converts the balanced signal received from the PABX analog trunk (via the ANSWER RELAY and the RING AND TRUNK STATUS DETECTOR) into a single-ended analog signal, and routes this signal to the ENCODER/

DECODER (CODEC) analog input. Conversely, the DUPLEXER receives a single-ended analog signal from the CODEC, and converts this signal into a balanced signal to be transmitted via the PABX analog trunk. Furthermore, the DUPLEXER prevents the balanced signal to be transmitted from being returned to the CODEC analog input as a single-ended signal. This prevents the signal to be transmitted via the PABX analog trunk from being echoed in the PABX.

Note: A single-ended signal is available on a single wire. However, the voltage related to such a signal is measured (or sensed) by connecting an instrument (or any other electronic device) between this wire and a wire connected to the circuit's common terminal.

Analog Trunk Interface

Figure 1-7. Operation of the DUPLEXER in the ANALOG TRUNK INTERFACE.

ENCODER/DECODER and TIME-SLOT ASSIGNMENT CIRCUIT 1

The ENCODER/DECODER (CODEC) and TIME-SLOT ASSIGNMENT CIRCUIT 1 (TSAC 1) perform functions 4 and 5 mentioned in the previous subsection. In fact these two functions are very similar to the coding function performed by the CODEC and TSAC of each analog line interface in a CO.

In brief, the CODEC converts the analog signal received from the PABX analog trunk, via the RING AND TRUNK STATUS DETECTOR, ANSWER RELAY, and DUPLEXER, into a digital signal. This signal consists of serial 8-bit PCM codes occurring at a rate of 8000 codes/s. Each PCM code is then time-division multiplexed to line TX2 which connects to the switching circuit of the PABX. TSAC 1 receives data from the call processor that determines the time slot assigned to the CODEC to multiplex each PCM code to line TX2. TSAC 1 produces a rectangular pulse signal at its TXE output that is aligned with the assigned transmit (TX) time slot. Figure 1-8 shows an example of the TSAC-1 TXE output signal when TX time slot 5 is assigned to the CODEC.

Analog Trunk Interface

Figure 1-8. Multiplexing and demultiplexing signals at the TXE and RXE outputs of TSAC 1.

In the opposite direction, the CODEC receives a digital signal, to be transmitted via the PABX analog trunk, from line RX2 of the PABX switching circuit. This digital signal consists of time-division multiplexed serial 8-bit PCM codes occurring at a rate of 8000 codes/s. The CODEC demultiplexes the digital signal received via line RX2 and converts it into an analog signal that is transmitted via the PABX analog trunk. TSAC 1 receives data from the call processor that determines the time slot assigned to the CODEC to demultiplex a digital signal from line RX2. TSAC 1 produces a rectangular pulse signal at its RXE output that is aligned with the assigned receive (RX) time slot. Figure 1-8 shows an example of the TSAC-1 RXE output signal when RX time slot 7 is assigned to the CODEC.

Note that both the TX and RX time slots of the CODEC in the ANALOG TRUNK INTERFACE are dynamically assigned by the PABX call processor according to the connections to be established.

TIME-SLOT ASSIGNMENT CIRCUIT 2

The analog trunk interfaces in a PABX are usually grouped in banks as mentioned earlier in this discussion. The status of the analog trunk associated with each interface in a bank is generally time-division multiplexed so that complete status information about all trunks in the bank can be routed to the signaling circuit of the

Analog Trunk Interface

This method is used in the ANALOG TRUNK INTERFACE of the Lab-Volt PABX, that is, one line (labeled TKS2) is used to transmit time-division multiplexed trunk status information. The PABX analog trunk status, available at the RING AND TRUNK STATUS DETECTOR output, is routed to a tri-state buffer controlled by TIME-SLOT ASSIGNMENT CIRCUIT 2 (TSAC 2). TSAC 2 produces a rectangular pulse signal at its TSTXE output that is aligned with the time slot assigned to the ANALOG TRUNK INTERFACE for the transmission of the trunk status. The assigned time slot corresponds to the TSAC-2 address. For example, when the TSAC-2 address is 5, the PABX analog trunk status is multiplexed to time slot 5, that is, the tri-state buffer is enabled during the corresponding time interval. This is illustrated in Figure 1-9. Note that a dialog box in the Lab-Volt PABX allows the TSAC-2 address to be set.

Figure 1-9. The status of the PABX analog trunk is time-division multiplexed to line TKS2.

CALLER IDENTIFICATION ACCESS PATH

When a central office directs a call toward an analog telephone set, it sends data about the calling party after the first burst of ringing. This feature is referred to as caller identification (caller ID). The caller ID data is transmitted to the analog telephone set via the local loop as a frequency-shift keying (FSK) signal.

Similarly, when a call is directed toward the Lab-Volt PABX, it receives the caller ID data via the analog trunk after the first burst of ringing. However, since the ANSWER RELAY remains open as long as the call is not answered, some alternate path must be provided in the ANALOG TRUNK INTERFACE so that caller ID data can reach the DUPLEXER. This is the role of the CALLER IDENTIFICATION ACCESS PATH in the ANALOG TRUNK INTERFACE. This path allows the FSK signal bearing the caller ID data to reach the DUPLEXER. On the other hand, a voltage limiter in the CALLER IDENTIFICATION ACCESS PATH clips the high-level AC ringing voltage to avoid damaging the DUPLEXER.

Analog Trunk Interface

Even though a voltage limiter in the CALLER IDENTIFICATION ACCESS PATH clips the high-level AC ringing voltage when the ANSWER RELAY is open, the DUPLEXER is provided with its own voltage limiter. This is because, when a call is answered while the AC ringing voltage is applied across the analog trunk, the ANSWER RELAY closes and the AC ringing voltage is applied directly to the DUPLEXER until the CO detects that the call is answered. This generally takes about 100 ms for a CO to detect that a call is answered, and this should never exceed 200 ms.

Procedure Summary

In the first part of the exercise, you will set up an analog trunk between a Lab-Volt PABX and a Lab-Volt Central Office.

In the second part of the exercise, you will study the operation of the ANSWER RELAY and RING AND TRUNK STATUS DETECTOR of the ANALOG TRUNK INTERFACE in the Lab-Volt PABX. To do so, you will observe the signals involved in the operation of these components when receiving or making an external call.

In the third part of the exercise, you will study the operation of the DUPLEXER, CODEC, and TSAC 1 of the ANALOG TRUNK INTERFACE. To do so, you will observe the input and output signals of these components during a call between a digital telephone set connected to the Lab-Volt PABX and an analog telephone set connected to the Lab-Volt CO.

In the last part of the exercise, you will observe how the status of the analog trunk associated with the ANALOG TRUNK INTERFACE is digitized and time-division multiplexed to a trunk status line.

EQUIPMENT REQUIRED

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

PROCEDURE

Setting Up an Analog Trunk Between a Lab-Volt PABX and a Lab-Volt Central Office

Note: In this exercise, it is assumed that a single host computer is used to download the PABX and CO programs to two Reconfigurable Training Modules, Model 9431. This host computer is used to monitor the Lab-Volt PABX.

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

Analog Trunk Interface

Install the 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 a central office (CO). Connect an analog telephone set to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on this telephone set.

CAUTION!

Do not connect or disconnect the analog telephone set when the Reconfigurable Training Module is turned on. High voltages are present on the standard telephone connectors of the Dual Analog Line Interface.

Connect the AC/DC power converter supplied with the analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of the AC/DC power converter to the DC power input connector on the analog telephone set.

* 2. Install the Digital Telephone Interface, Model 9476, into the digital (D) slot or one of the two analog/digital (A/D) slots of the other Reconfigurable Training Module. This module will be used as a PABX. 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.

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.

Finally, install the PABX Analog Trunk Interface, Model 9477, into one of the two analog/digital (A/D) slots of the Reconfigurable Training Module used as a PABX. Using the analog trunk line provided with the PABX Analog Trunk Interface (two-wire cable terminated with standard [RJ-11]

male telephone connectors), connect the RJ-11 female connector on the PABX Analog Trunk Interface installed in the Reconfigurable Training Module used as a PABX to the remaining RJ-11 female connector on the Dual Analog Line Interface installed in the Reconfigurable Training Module used as a CO.

* 3. Turn on the host computer.

Turn on the TTS Power Supply, then turn on the Reconfigurable Training Modules.

Analog Trunk Interface

* 4. On the host computer, start the Telephony Training System software, then download the PABX program to the Reconfigurable Training Module used as a PABX.

Note: If the host computer is unable to download the PABX program to the Reconfigurable Training Module used as a PABX, make sure that the proper IP address is used to communicate with this Reconfigurable Training Module.

* 5. 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.

* 6. On the host computer, designate digital telephone set AL of the Lab-Volt PABX as the attendant's telephone set. Make sure that no external call restriction is applied to digital telephone sets AL and AR. Set the DTMF dialing tone duration to 1 s.

* 7. On the host computer, download the CO program to the Reconfigurable

In document Analog Access to the Telephone Network (Page 73-103)