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Old 7th Feb 2023, 5:11 pm   #14
Pellseinydd
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Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 707
Default Re: NTS (Alcatel) London 8 PABX.

Quote:
Originally Posted by rambo1152 View Post
We maintained loads of London systems during my time with the ill fated Nationwide Telephone Company based on Bury.
I can't really recall (no pun intended) the trick with extentions TBR that you described, but I do remember some of those systems doing earth leg recall on the trunks as they were earth-start lines.

I never really understood what the advantage was over normal loop-start.
It certainly confused us techs the first time we encountered them!

Did some versions support meter pulses? I cant remember.
The first of the London range only supported LD telephones and were designed to use old telephones that the customer might have - most of which probably didn't have an Earth Recall Button. Hence designing the system to use the initial break of the pulse train as 'TBR'

Also as all the BT systems of the day only used Earth Recall, with the introduction of small PABXs with liberalisation in August 1984, it was rulled by OFCOM that only Earth Recall could be used on PABXs connected to the PSTN. (BT convincing OFCOM?) It was annoying as the Plessey ISDX had the facility to provide both ER/TBR but TBR wasn't initially allowed by the PXML which had to be adhered to. Thus ISDXs had to use Earth Recall with MF phones from 1984 onwards initially

As time went by OFCOM relaxed things and TBR was allowed. Hence the reason that the London guys used their crafty idea = not ony saved money on the phones but could use 2 wires to a phone not two pairs!

The London 8/12 upwards could support Meter Pulse Detection by changing the line cards. The London 8 I still have has that facility on the exchange lines.

Prior to Liberalisation, many exchange lines provide for older PABXs were 'Earth Calling' from the days of earlier PABXs. BT loved providing them when you ordered new exchange lines despite being asked for loop calling rather the earth start.
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