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 //                     4TEL in TXK1 Local Exchanges                      //
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 This article decsribes experience in Severnside District of using the 4Tel
 Automated Subscriber Loop Test System, supplied by Teradyne Ltd., to replace
 the Automatic Line Insulation Tester (ALIT) in TXK1 crossbar Local Exchanges.
 The article decsribes some of the problesm encountered, and how the
 redundant ALIT was modified to make the 4TEL equipment more efficient and
 cheaper to purchase.


 INTRODUCTION
 ============
 Early in 1983, the then Bristol Telephone Area began trials of a new
 American-designed system for testing customers lines, supplied by Teradyne
 Ltd. The system, known as the 4TEL Automated Subscriber Loop Test System,
 comprised a central main computer installed at the Bristol Repair Service
 Centre, and remote dial-and-test units fitted at distant satellite telephone
 exchanges.

        The remote units, known as Central Office Line Testers (COLTs), seize
 and test customers lines via the normal test access circuits. Tests can be
 carried out overnight by using a routine mode in which lines are
 automatically tested in the sequence 0000-9999. The results are stored in
 the central computer and can be printed out when required. Alternatively,
 individual lines can be tested on demand, and the results displayed on a
 Visual Display Unit.

        One of the exchanges chosen for the trail was the TXK1 crossbar
 exchange at Henbury, which had a multiple of 10,500 lines. Investigations
 showed that a COLT with one dial circuit could test only about 4000 circuits
 each night. Another Dial Circuit was added at extra cost, but still only
 about 8000 lines per night could be tested. The problem was caused by the
 method used to access line circuits in a TXK1 exchange.

        Each COLT dial circuit was connected to a test transmission relay
 group (TTRG), and dialled a four-digit code to seize each line circuit,
 including all spare lines. The two dial circuits tended to operate
 simultaneously and dial consecutive numbers. At the Linemarker stage of a
 call set-up, only one terminating call at a time can be processed. The
 co-incident calls sometimes led to an apparent system failure and,
 consequently, a fault print-out on the exchange tele-printer. Fig. 1 shows
 original equipment configuration.

 Fig. 1 - Original Method of interfacing the COLT to a TXK1 Exchange
 -------------------------------------------------------------------

                        ROUTER SWITCHES  DISTRIBUTOR SWITCHES
  Ŀ   Ŀ   Ŀ Ŀ Ŀ Ŀ   Ŀ
   COLT Ĵ TTRG - RSA - RSB - DSA - DSB -Ĵ  LINE   -
               CIRCUIT 
                                                        
                                                              
           Ŀ                                         
            INCOMING                                          
            REGISTER                                          
            DETECTOR                                          
                                                    
                                                              
                                                              
           Ŀ     Ŀ                    Ŀ
            INCOMING --Ĵ ROUTER  --Ĵ  LINE  
            REGISTER       CONTROL                      MARKER 
                                    

 Incoming Register Connector               Operations Doubled
 Router Controls                           Operations Doubled
 'Preference Choice' Incoming Registers    Operations Ten Times Doubled

 The massive increase in the use of registers caused concern because the
 wear, and subsequent failure, of pulsing relays is major service hazard
 in TXK1 exchanges. It was therefore suggested that the automatic line
 Insulation Tester (ALIT), made redundant by the 4TEL system, could be
 used to set up the 4TEL calls. The ALIT is, in essence, a register
 that has the ability to test lines, and the great advantage of being able
 to increment calls sequentially without re-dialling. Using the ALIT as
 an interface between the the 4TEL and the router control (see Fig.2) would
 eliminate the extra wear on the registers.

 Fig. 2 - New method of Interfacing
 ----------------------------------
                        ROUTER SWITCHES  DISTRIBUTOR SWITCHES
  Ŀ   Ŀ   Ŀ Ŀ Ŀ Ŀ   Ŀ
   COLT Ĵ ALIT -Ĵ RSA - RSB - DSA - DSB -Ĵ  LINE   -
               CIRCUIT 
                                                         
                                                               
                                                               
                                                               
                                                               
                                                               
                                                               
                                                               
                                                               
                           Ŀ                    Ŀ
                Ĵ ROUTER  --Ĵ  LINE  
                             CONTROL                      MARKER 
                                                

 Preliminary investigations also indicated that time could be saved by
 changing the COLT control program. In Strowger Exchanges, the 4TEL program
 includes some call incrementing. The COLT dials via the test selector access
 to the first outlet of the test final selector bank. After the test has been
 completed, the wipers are stepped on to the second outlet. This continues
 up to digit 9, when the COLT clears and redials the number ending in '0'.
 It was thought that this increment feature of 4TEL could be used in a TXK1
 exchange. Calculations showed that, by using only one dial circuit, more
 than 10,000 tests per night should be possible.


 DEVELOPMENT
 ===========

        The ALIT is normally set up for a test programme by code digits
 dialled from a remote access point into relay stores, as follows :

 First Digit                            Type of start; for example, immediate
                                        or delayed

 Second Digit                           Insulation Test Limits

 Third Digit                            10,000-line group to be tested

 Fourth Digit                           1000-line start group to be tested
                                        (For example, 2000)

 Fifth Digit                            100-line start group to be tested
                                        (For example, 2300)

 Sixth Digit                            1000-line finish group
                                        (For example, 3000)

 Seventh Digit                          100-line finish group
                                        (For example, 3599)

 The tens and units counting stores are automatically set to zero, ready
 to increment from there. In the example given, the ALIT would have tested
 from 2300-3599. The first stage of the development was to list the COLT
 requirements, check the COLT signalling arrangements, and then modify the
 ALIT accordingly. The COLT had to be able to

 A) Set up a call to any number, and increment if neccesary for the routine
    mode;
 B) Control the increment;
 C) Receive Information on Line status, that is, FREE, BUSY, Number
    UnObtainable, etc, etc...
 D) Test the Line if FREE;
 E) Monitor the Line if BUSY; and
 F) Clear on completion of the test programme.

 The first two digits of the original ALIT programming were not neccesary
 as the type of start was always going to be immediate and the COLT would do
 the testing. The first two ALIT  information stores were therfore strapped
 out, so that the digits dialled by the COLT became:

 First Digit                    10,000-line group to be tested
 Second Digit                   1000-line group to be tested
 Third Digit                    100-line group to be tested
 Fourth Digit                   10-line group to be tested
 Fifth Digit                    Unit number to be tested

 The rewiring work that allowed the COLT to operate the tens and units stores
 for digits four and five was a major task. The OPERATE leads that originally
 went to the finish stores of the ALIT had to be diverted to the tens and
 units stores. This enabled the COLT to pre-set the tens and units stores
 to any required number. As spare relay contacts were used, only four extra
 diodes were required to carry out these changes. The increment feature of
 the ALIT stayed much the same as before, but, instead of it being automatic,
 the COLT controlled it. Thus the COLT initiates a one-step increment, checks
 that one step has occurred, and then proceeds with testing.

        Once the fifth set-up number has been received, the ALIT reverses
 the potential of the dialling wires. The COLT recognises this signal, and
 sends an earth to the ALIT on the START leads. The ALIT then acts as a
 register and seizes a free router control. When the router control has
 interrogated the line circuit required, the relevant information on line
 status is returned to the ALIT. If the line is FREE, then the call is
 completed and the ALIT signals to the COLT. If the Line is BUSY, then the
 call is completed and the ALIT signals the two facts to the COLT. Where
 there a is Number UnObtainable (NU), Change Number Interception (CNI), or
 Equipment engaged tone (EET), this information is passed from the ALIT
 to the COLT on seperate leads.

        When a call is completed from the ALIT to a line circuit, a test is
 made by the COLT. To facilitate this, the test wires of the ALIT were
 diverted from the original testing element to the COLT test Circuit. The
 Line test takes about two seconds, and then the COLT can either increment
 or Clear Down.

        If the line under test is BUSY, then the call is still completed.
 This is not a normal feature of the ALIT, but was achieved by giving the
 ALIT a Class-Of-Service (COS) isentical to the TTRG. Thus the router
 control overrides the BUSY signal, and the call is set up as if the line
 is FREE. If the COLT monitoring facility is in use, then the line being
 tested is checked. If voice frequency Modulation is present (KpT : ie,
 If yer gabbing to yer bird, or yer modem is whistling dixie) the COLT
 recognises this as a succesful call in progress. This information can
 then be shoen in the results. If monitoring is not required, the COLT
 can increment immediately.

        A call is not set up to a line circuit where there is NU tone, CNI,
 or EET. In these cases, the information on line conditions is stored, and
 the COLT increments or Clears Down. When the COLT has finished its tests,
 an earth signal is sent to operate the ALIT Clear-Down relay.

 OPERATIONAL EXPERIENCE
 ======================

        The modified ALIT has been in use since mid-July 1984, and has proved
 to be very reliable. Fig. 3 shows a comparison between the old and the new
 system for testing ten lines, and shows an approximate saving of 22-24
 seconds per ten tests. This saving means that, under the new arrangement,
 only one dial circuit in the COLT needs to be used to test 10,000 lines
 each night. Assuming that all lines are working, and no re-tests are
 required, then the time required to test 10,000 lines is approximately
 10 Hours.

 Fig. 3 - Comparison of test timings between the Original and new methods
 ------------------------------------------------------------------------
               OLD                                      NEW

       COLT          REGISTER    ELAPSED TIME    COLT          ALIT

COLT Seizes -                 0 COLT Seizes ALIT -
Register                            1                   
                                    2                   
Dials Number                        3  Dials Number     
xxx0 (Four   - Call            4  xxxx0 (Five      
 Digits)            Set Up          5   Digits)         
     COLT Tests                     6                - Call
  COLT Clears-                7        COLT Tests      Set Up
COLT Seizes -                 8       COLT Clears-   ALIT Releases
Register                            9 COLT Seizes ALIT -
                                    10                  
Dials Number                        11                  
xxx1 (Four   - Call            12 Dials Number     
 Digits)            Set Up          13 xxxx1 (Five      
     COLT Tests                     14  Digits)         
  COLT Clears-                15               - Call
COLT Seizes -                 16      COLT Tests       Set Up
Register                            17  COLT Increment -
                                    18        -ĿALIT Increments - xxx2
Dials Number                        19      COLT Tests  Call
xxx2 (Four   - Call            20  COLT Increment -         Set Up
 Digits)            Set Up          21      -Ŀ      ALIT Increments - xxx3
     COLT Tests                     22 COLT Tests Call Set Up
  COLT Clears-                23               -
COLT Seizes -                 24                  
Register                            25                  
                                    26                  
                                    27                  
                                    28                  
                                    29                  
                                    30                  
                                    31                  
                                    32                  
                                    33                  
                                    34                  
                                    35         End of Test on 10 Circuits
                                    36 -
                                    37                  
                                    38                  
                                    39                  
                                    40                  

           End of Tests
        On Ten Circuits Approx.
               60 s.

 As previously explained, the COLT can increment 1-9, but then clears and
 re-dials to 0. Changes to the software could make the COLT increment from
 0-9, and possibly 0-99. These modifications would enable the time required
 to test 10,000 lines to be reduced to approximately 8.8 and 8.0 hours
 respectively.

 CONCLUSION
 ==========

        The New ALIT-COLT interface has proved a sucess. The wear and tear
 on registers is back to normal, and the nightly 4TEL routine can now test
 10,000 lines per night.

        Financial as well as time savings have been achived as only one
 dialler instead of two is required. The modifications take about three days
 to complete, and the extra equipment needed is one standard relay, six
 diodes, and the cabling.

        The new system is being adopted in other large TXK1 exchanges in
 Severnside District, and interest has been shown from other Areas/Districts
 suffering from the same problems with the 4TEL system.