EXERCISE OBJECTIVE
When you have completed this exercise, you will be familiar with the control functions performed by the call processor during the processing of a call. You will learn how to record and observe the control actions performed by the call processor of the Telephony Training System during the processing of a call.
DISCUSSION Introduction
As stated in the discussion of fundamentals of this unit, all interconnections made in the switching circuit of today's central offices are under stored program control (SPC), i.e., under the control of a central computer (call processor).
Figure 3-1 shows a simplified diagram of a central office using stored program control. Each analog line interface (ALI), trunk interface, and service circuit (the service circuits are integrated to the SIGNALING CIRCUIT in Figure 3-1) is connected to both sides of the switching circuit (these connections are not shown to keep the diagram clear) to allow each of theses devices to transmit and receive digitized signals. The figure also shows that the call processor exchanges data with the analog line interfaces, the signaling and switching circuits, and the trunk interfaces to perform four control functions: system supervision, signaling, dialed telephone number reception and processing, and connection control (switching circuit control).
System supervision is performed by reading circuit status information (telephone set hook status, trunk interface idle/busy status).
Signaling mainly consists of transmitting commands to analog line interfaces to make telephone sets ring, and sending data to the signaling circuit to generate call progress tones which are routed to the proper telephone sets via the switching circuit.
Dialed telephone number reception and processing is performed by reading dialed digits from the signaling circuit one by one to recover the complete number, and analyzing this number to determine the connections to be made.
Note that when pulse dialing is used, dialed number reception is carried out by monitoring the circuit (hook status) status information.
Connection control consists in sending the proper connection setup and release commands to the switching circuit.
Call Processor Functions
Figure 3-1. Simplified diagram of a central office using stored program control.
Control Functions Performed by the CALL PROCESSOR in the CENTRAL OFFICE of the Telephony Training System
Figure 3-2 is a simplified diagram of the CENTRAL OFFICE in the Lab-Volt Telephony Training System. This subsection explains how the CALL PROCESSOR in the CENTRAL OFFICE performs the control functions.
System Supervision
The CALL PROCESSOR supervises the system by cyclically reading the contents of the HOOK STATUS BUFFER MEMORY in the SIGNALING CIRCUIT. The hook status signals indicate the CALL PROCESSOR if a telephone set requests service, remains active or becomes inactive.
Signaling
The signaling function performed by the CALL PROCESSOR consists in sending commands to the TSAC of ANALOG LINE INTERFACEs to make telephone sets ring. The CALL PROCESSOR also sends data to TONE GENERATORs in the SERVICE CIRCUITs to control the generation of call progress tones.
Call Processor Functions
DATA
PULSE DIALING
DETECTOR CALL PROCESSOR DETECTED NUMBER ANALOG LINE INTERFACE B
ANALOG LINE INTERFACE A RX0
RX1
SERVICE CIRCUIT 2 FOR ANALOG LINE INTERFACES SERVICE CIRCUIT 1 FOR ANALOG LINE INTERFACES
HS0
HOOK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT R / V
Figure 3-2. Simplified diagram of the CENTRAL OFFICE in the Telephony Training System.
Dialed Telephone Number Reception and Processing
Depending on the type of dialing used (tone or pulse), telephone numbers are received by reading the dialed digits from DTMF DETECTORs in the SERVICE CIRCUITs or by scanning the contents of the HOOK STATUS BUFFER MEMORY.
Once a complete telephone number is recovered, the CALL PROCESSOR analyzes this number to determine the connections required.
Call Processor Functions
Connection Control
Connection control is performed by writing data in the SPACE-DIVISION SWITCH Control Register, and by dynamically controlling the receive (RX) time slot assigned to each ANALOG LINE INTERFACE and each SERVICE CIRCUIT in the SIGNALING CIRCUIT according to the connections to be established.
CALL PROCESSOR Log Function in the CENTRAL OFFICE of the Telephony Training System
A special function of the CALL PROCESSOR in the CENTRAL OFFICE of the Telephony Training System allows the recording of the control actions performed by the call processor during the processing of a call. This function is referred to as the Call Processor Log function.
The operation of the Call Processor Log function is similar to that of a recorder: the recording of the control actions can be started and stopped when required. A recorded sequence can be played back, or printed, to perform step-by-step observation.
This function is included in the Telephony Training System for educational purposes only.
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 identify the paths through which the CALL PROCESSOR performs each of its control functions.
In the third part of the exercise, you will use the log function of the CALL PROCESSOR to record the control actions performed when a call is initiated without being completed. You will then relate each action recorded in the log to a specific control function of the CALL PROCESSOR. You will also relate the actions performed when you attempt to make the call to the control actions recorded in the log.
In the last part of the exercise, you will make a step-by-step observation of what happened in the Central Office for each of the actions recorded in the log.
EQUIPMENT REQUIRED
Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise.
Call Processor Functions
PROCEDURE
Setting Up the Central Office
* 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 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 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 pro-gram 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.
Call Processor Functions
Paths Through Which the CALL PROCESSOR Performs the Control Functions
* 4. Draw in Figure 3-3 the paths through which the CALL PROCESSOR performs each of the control functions listed below. Use the specified line symbols.
– System supervision (line symbol: • • • • ) – Signaling (line symbol: — — — — )
– Dialed telephone number reception (line symbol: + + + + ) – Connection control (line symbol: )
The Log Function of the CALL PROCESSOR
* 5. Make sure that the address of the TSAC in ANALOG LINE INTERFACE A is set to 01.
On the host computer, display the Call Processor Log window.
* 6. Start recording the control actions performed by the CALL PROCESSOR.
Lift off the handset of telephone set A and dial two digits on the keypad, then hang up. While doing this, observe that control actions are recorded in the Call Processor Log window as they are being performed.
Stop recording the control actions performed by the CALL PROCESSOR.
* 7. Display the detailed information about each control action recorded in the Call Processor Log window.
Call Processor Functions
DATA
PULSE DIALING
DETECTOR CALL PROCESSOR DETECTED NUMBER ANALOG LINE INTERFACE B
ANALOG LINE INTERFACE A RX0
RX1
SERVICE CIRCUIT 2 FOR ANALOG LINE INTERFACES SERVICE CIRCUIT 1 FOR ANALOG LINE INTERFACES
HS0
HOOK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT R / V
Figure 3-3. Paths through which the CALL PROCESSOR performs the control functions.
* 8. In Table 3-1, classify the control actions recorded in the Call Processor Log window according to the control function of the CALL PROCESSOR which they are related to.
Call Processor Functions
CALL PROCESSOR CONTROL FUNCTION
CALL PROCESSOR CONTROL ACTIONS
System supervision
Signaling
Dialed telephone number reception and processing
Connection control
Table 3-1. Relating the control actions recorded in the Call Processor Log window to the control functions of the CALL PROCESSOR.
* 9. In Table 3-2, relate the actions you performed when you attempted to make a call to the control actions recorded in the Call Processor Log window.
EVENT CALL PROCESSOR CONTROL ACTIONS
Handset of telephone set A is lifted
Two digits are dialed on the keypad of telephone set A
Handset of telephone set A is replaced on the cradle
Table 3-2. Relating the actions performed when attempting to make a call to the control actions recorded in the Call Processor Log window.
Call Processor Functions
* 10. In the control action named Service Request, what is the meaning of the following detailed information: received from HS0, 01 ? Explain.
* 11. In the control action named Service Request, what is the meaning of the following detailed information: mark ALI HS0, 01 as busy?
* 12. What is the purpose of the control action named Call Progress Tone Transmit Path Setup?
* 13. Explain why the control action named Call Progress Tone Removal is not immediately followed by the control action named Call Progress Tone Transmit Path Release.
Step-By-Step Observation
* 14. In the LVTTS window, adjust the view so as to be able to see the circuitry of both ANALOG LINE INTERFACEs, the SWITCHING CIRCUIT, the SIGNALING CIRCUIT, and the CALL PROCESSOR.
Using the playback function of the Call Processor Log, make a step-by-step observation of what happened in the Central Office for each of the control actions recorded in the log.
Note: The Previous View function of LVTTS allows you to reobtain the initial general view after you have zoomed on a particular section.
Call Processor Functions
From your observations, were several resources of the Central Office required to process the call, even if the call was not completed? Explain.
* 15. 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 the call processor functions: system supervision, signaling, dialed telephone number reception and processing, and connection control. You learned that, in order to perform these control functions, the call processor must communicate with the analog line interfaces, as well as the signaling and switching circuits of the central office.
You learned that the Telephony Training System has a useful function that can provide a chronological record, or log, of all the control actions performed by the CALL PROCESSOR as it processes a call. You used this function to obtain a log of the control actions performed by the CALL PROCESSOR when a call is initiated without being completed. You related each control action recorded in the log to a specific control function of the CALL PROCESSOR. Finally, you made a step-by-step observation of what happened in the Central Office for each control action recorded in the log. This allowed you to see that, whenever a call is attempted by a subscriber, several resources of the Central Office are required to process the call even when the call is not completed.
REVIEW QUESTIONS
1. How does the CALL PROCESSOR of the Telephony Training System supervise the status of the telephone circuits?
Call Processor Functions
2. How does the CALL PROCESSOR of the Telephony Training System perform telephone number detection and processing?
3. How does the CALL PROCESSOR of the Telephony Training System perform the signaling function?
4. How does the CALL PROCESSOR of the Telephony Training System perform connection control?
5. What is the purpose of the log function of the CALL PROCESSOR of the Telephony Training System?