Maintenance and Test in in Digital Telephone Switching System


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This article describes the operations and maintenance functions required for the efficient and reliable operation of the CO digital exchanges. Local operations and maintenance capabilities shall be provided in each exchange. The exchanges shall provide all required equipment and means necessary to interface/interact these functions with the centralized operation & maintenance facilities.

Maintenance and Test in in Digital Telephone Switching System

5.39.2 Scope of Maintenance Functions

The following automatic and semiautomatic capabilities shall be accessible from system consoles, both locally at the exchanges and centrally.
a) Administration of the exchange(s).
b) Exchange supervision and maintenance.
c) Outside plant supervision and tests functions.
d) Transmission tests and signaling system tests.
Testing and diagnostic capabilities shall be provided to systematically monitor the performance of the exchanges, to obtain operating status information, detect fault conditions, activate alarms, and identify faulty hardware and software modules. A description of standard on-line and off-line test facilities built into the exchanges shall be provided.

Diagnostic capabilities shall be provided to permit the identification of faulty units at the lowest level of readily replaceable hardware modules, such as individual circuit boards. After a part or parts suspected of failure have been replaced, the running of diagnostic programs shall determine, display, and print whether the trouble has been cleared or the unit is still faulty.
The output of the diagnostic tests of a faulty unit shall yield direct information as to the location of the failed circuit boards. For the infrequent case where a trouble or multiple troubles cannot be quickly located, the system shall provide the actual results of each diagnostic test or specific subsets of tests.

The diagnostic programs shall not interfere with the normal call processing programs or system operations. The testing and diagnostic functions shall include, but not be limited to, those normally associated with “test desk” functions, such as the following:
a) Extension of the testing circuits toward both the subscribers’ lines and the exchange equipment.
b) Identification and location of reported faults.
c) Dial-speed testing and DTMF testing.
d) Ringing testing.

e) Verification that the line is connected to station equipment.
f) Testing subscriber line loop resistance and insulation resistance.
g) Transmission testing.
h) Testing for extraneous potential.
5.39.3 Administration Functions
The administrative functions required at each exchange are those needed to retrieve status information and control the operation of the exchange. Such functions involve database manipulations, including entry of data, day-to-day modifications (i.e., provisioning, etc.), entry of commands, as well as the retrieval of information to verify loads, to make modifications, and to validate changes.
a) Entry of Data Fields
Entry of data fields shall be made via system consoles and other local and remote data processing peripherals, such as magnetic tape units. The kinds of data to be entered involve the allocation or assignment of facilities, and instructions to the system regarding its internal operations, its interaction with other exchanges. All input and output messages used for maintenance and test facilities shall conform to ITU-T Recommendations on man-machine language.
Data assignments shall include, but not be limited to, the following:
1. Directory numbers to subscribers’ lines.
2. Calling classes of service for lines and incoming trunks.
3. Called classes of service for lines and outgoing trunks.
4. Lines to hunting groups (PABX).
5. Type-of-service categories to incoming lines and trunks, i.e., maintenance, coin boxes, etc.
6. Addresses to trunk groups using the same route treatment.
7. Type-of-service to outgoing trunks, i.e., PABX or DID.
8. Digit deletion or addition to incoming or outgoing trunk groups.
9. Service circuits.
b) Day-to-Day Data Modification

Operations personnel shall be able to modify or add new data, either from consoles. This standard feature will be used for day-to-day operation of the system and will allow for:
1. Adding new subscribers.
2. Removing subscribers.
3. Call interception.
4. Number changes.
5. Adding or deleting trunks or trunk groups.
6. Modifying call routing.
7. Service class change.
The system shall also have the capability of providing all relevant data associated with line records as part of the database.
c) Initialization or Replacement of Database
The means employed for initializing the database during installation or re-initializing it a fault shall be described in detail. This information shall describe the procedure used, the storage methods, and the duration of procedure.
d) Integrity of Database
The methods employed to ensure that the required data are, in fact, entered into the memory areas of the equipment and are protected once entered, shall be described in detail.
e) Report Generation and Data Retrieval
Upon request from keyboard, the system shall provide system status reports and retrieve specified data files on a VDU and/or printed copy.
f) Traffic Measurement
The system shall provide for the control and retrieval of traffic measurement information. At the option of the console operator, output data shall be made available in the form of a display, hard copy and/or stored on tape or disc.
5.39.4 Exchange Maintenance
a) Traffic Performance Supervision

performance supervision shall include the following:
1. Supervision of quality of originating traffic sources. This is required to verify that at least one call is made through each originating traffic source (i.e., line unit or trunk group) in the exchange during a predetermined period.
2. Blocking supervision. This is required to establish that the number of blocked services on certain routes does not exceed the predetermined limit value for the grade of service.
3. Supervision of faults. This is required to detect and indicate which traffic-carrying units, functional units, and routes through an exchange are not operating properly. This will indicate the number of faults, the time and status of the call when the fault occurred and whether there was a failure after a second attempt to complete a call. If the number of faults exceeds a predetermined value, an alarm must be actuated.
4. Supervision of the digital switching network is required to determine that the number of blocked connections does not exceed the predetermined limit.
5. Supervision of processor loading. This is required to prevent interruption of service due to overloading the processor. Hence, it must be possible to control the intensity of the traffic in the processor by reducing overhead programs, reducing time-out intervals, and increasing line-scanning intervals.
6. Verification of the transmission path through the digital switching network. This is required to establish calls from specific inlets toward specific outlets through the switching network and, also, to control specific switching points of the switching network.
7. Tracing the transmission path of a call. This is required to identify the elements involved in a specific connection. Tracing can be initiated either by an instruction or automatically by means of a diagnostic program.
8. Inhibition of disconnection. This is required to stop release time-out on demand, on a specific connection to allow the continuation of a connection, i.e., in which a signaling failure or release time failure has occurred.

b) Testing with Traffic Equipment
Test calls shall be set up and checks shall be made at different phases of the test calls. When a fault occurs, the connection shall be held and the location of the fault indicated. It shall be possible to make test connections between two groups of test numbers for checking and recording transmission levels, metering, ringing signals, insertion loss, crosstalk, etc. It is not required to provide tests on the external line plant or subscriber errors; the success or failure of call made with this capability will be entirely dependent on the condition of the exchange equipment.
c) Maintenance Actions
When a malfunction has been detected and an equipment fault isolated, the system software diagnostics shall provide sufficient information, available at the local console and at the Saudi Aramco network operation control center facility’s console, to direct a craftsperson to the faulty circuit boards.
Following replacement of the faulty unit, system diagnostics shall validate the successful completion of the maintenance action.
The system shall provide information by the rank order, indicating the percentage of faulty events in which faulty equipment can be localized to only one circuit board; to two circuit boards; to three circuit boards; and so forth.
d) Other Equipment
Maintenance methods and routines shall be described, and test equipment shall be included for all other equipment such as batteries, rectifiers, diesel generators, and HVAC. Actions to be taken by maintenance persons shall be described in response to the operation of the alarm indicators and sensors from such equipment.
5.39.5 Outside Plant Supervision and Test
a) Checking of Line from Subscribers’ Premises
In addition to making tests with the cooperation of the wire chief, the repair person shall be able to check the condition of the line from the subscribers’ premises by dialing special access codes, via a telephone set. These codes shall provide, as a minimum, the following tests:
1. Measurement of voltage on the line.
2. Insulation resistance of wire “a” to wire “b” and of wire “a” or “b” to ground.
3. Measurement of loop resistance. (The limit value of loop resistance is 2000 ohms (maximum), including the telephone instrument, 1900 ohms is acceptable).
4. Dial speed test.
5. Telephone instrument ringer test.
6. DTMF test.
7. Verification of correct directory number. A repair person(s) at subscriber stations shall be able to report to the exchange maintenance center or network operation control center facility by dialing a code.
b) Tests Made Before Completing a Call
Automatic subscriber line tests shall be made to verify the absence of extraneous voltages on the line. These tests shall be made automatically on the lines of subscribers A and B during the establishment of the call. If a line condition precludes the extension of the call to or from one of the subscribers, the relevant lines shall be “busied-out,” and a record of the fault shall be made. Such lines shall be automatically checked for correction of the condition.
c) Local Test Positions
Each exchange shall be equipped with more than one test position to provide all the functions listed below in this sub-section. Each position shall be able to test both the subscriber’s lines and the exchange equipment.
It shall be possible to exercise, at a remote centralized test center, all functions performed at the local test positions. All necessary equipment capable of performing these tests in a remote place, shall be furnished. The functions provided shall include the following.
1. Measuring resistance: Measure the two-terminal DC resistance a-to-b, a-to-ground, or b-to-ground.

2. Measuring capacitance: Measure the two-terminal capacitance a-to-b, a-to-ground, or b-to-ground.
3. Distance to an open circuit: Measure the line capacitance to determine the distance to an opening on a line.
4. Line circuit test: In order to test the line battery and verify that dial tone can be received.
5. Rotary dial analysis: In order to measure the pulse count, dial speed, and the percent of break.
6. Speaking and ringing circuits: Apply a ringing voltage to a line with the receiver off-hook to verify that the ringer works.
7. An interrupted earth key for connection of interrupted earth to ground.
8. Remote test facilities.
9. Balance test: In order to determine if an attached line is balanced or unbalanced.
10. A howler circuit, or “receiver off-hook” tone injection, that allows the attendant to manually request that howler or equivalent tone be applied to the line.
11. Automatic and/or manual test call generation, to check dial tone, DTMF/DP reception ringing, ring trip, and two-way transmission.
12. Cable fault alarm.
13. Measuring foreign potentials: Measuring the potential of an AC or DC source present at the a-to-b, a-to-ground, or b-to-ground terminals.
14. Other standard test facilities, including new subscribers tests, tone demonstration, outgoing lines to both DP and DTMF telephone sets, and night alarm and trunk circuits.
15. Coin line test facilities (if required), for application of coin return, coin collect voltages, and reverse battery.
16. Possibility of connecting two headsets.
5.39.6 Blocking of Equipment
It shall be possible to block traffic-carrying units or functional units of the exchange(s), either manually or automatically, by means of instructions entered at the console, in order to isolate them from normal call processing, so that they can be part of a desired test connection.
5.39.7 Alarms
The switching equipment shall include audible and visual alarm systems to alert maintenance personnel of trouble conditions, or of imminent equipment failures. The alarms shall give maintenance personnel information on the nature of the trouble, its severity, and the location of the malfunctioning component. It shall be possible to extend or transfer alarm signals to the network operation control center facility. Audible alarm signals shall be suppressible when a reported fault is being addressed by maintenance personnel. However, visual alarm signals shall be cancelable only when the fault has been cleared. Audible alarms shall have a cut-off key.
The visual and audible alarms signals shall indicate the type of fault, whether it is prompt or deferred and its location, and shall be classified according to the following levels:
a) Critical Alarms
These alarms indicate faults that effect adversely the operation of the exchange (service interruption) and need immediate action, such as:
1. Fire in switching room.
2. Blown major fuses or tripping of main circuit breaker.
3. Processor failure.
b) Major Alarms
These alarms indicate faults that partially impair service and may interrupt service to a group of subscribers and need immediate response, such as:
1. Ringing current failure.
2. Failure of battery charger.
3. High or low battery voltage.
4. Failure in alarm sender.
5. Failure in peripheral devices.
c) Minor Alarms

These alarms indicate faults that do not result in interruption of service, but that should be corrected as soon as possible, such as:
1. Service circuit time out.
2. Permanent loop.
3. Failure in HVAC.
Extension of alarms: The extension of alarms to a remote center may be through the interconnection link with each exchange, as well as independently. In either case the utilization of a dedicated digital link per exchange is required, through which a minimum of twenty (20) types of alarms will be transmitted.
Alarm details: Alarm reports shall include the name of the exchange from which the alarm originated, the type of alarm, the time of occurrence, and the time the fault was corrected.
5.39.8 Tools
Sets of tools and instruments necessary for the operations and maintenance of the digital exchanges shall be provided in adequate quantities, per detailed design, for each exchange.
5.39.9 System Recovery
In case of hardware failure, the system shall automatically reconfigure itself into a working system without intervention of maintenance personnel. The exchange shall have the capability of initiating an on-line stand-by recovery program to recover the system on line without interrupting the service. The exchange shall have manual configuration capabilities to override the automatic recovery capabilities. After recovery, the exchange shall provide a record of all recovery actions (both automatic and manual). In the case of automatic recovery, the recorded data obtained shall allow analysis of the fault condition. A history of these recovery actions and the associated data shall be maintained in the exchange. A manual control shall also be included to initialize all call memory in the system.
5.39.10 Call Monitoring
The exchanges shall be designed in such a way that will allow the monitoring of all calls in process of connection. On demand it shall be possible to identify problems with any call that should become ineffective. Upon identifying the call in the fault state, the system shall automatically print out the appropriate indications of the call’s state.

It shall also indicate the circuit involved (trunk, register, line, etc.) to facilitate dispatching of maintenance personnel.
5.39.11 Information Requested
The following information, on maintenance and test facilities, shall be provided by the manufacturer.
a) Means used for initializing the database during installation or its re-initialization after a fault. The information required shall include procedures, methods of storage, and duration of the procedures.
b) Method used to ensure and to verify entry of data into memory areas.
c) Method used to protect data entered into memory areas.
d) Specific categories of data fields and types of reports generated by the maintenance system.
e) The method of processing alarm information; the type and number of alarms, and the triggering causes of each alarm.
f) Estimated cost maintenance of system, expressed in man-hours, per line, per annual.

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