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// 4TEL in TXK1 Local Exchanges //
// Expunged Into Digital Form by Keltic Phr0st //
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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 -�������Ŀ�ALIT 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.