Message from discussion Telephone subs. line carriers

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From: "Ian Bartholomew" <news...@iandb.org.uk>
Newsgroups: uk.telecom
References: <00nC7.10640$o56.956811@news2-win.server.ntlworld.com>
Subject: Re: Telephone subs. line carriers
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Date: Sat, 27 Oct 2001 09:50:59 +0100
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"squilsh" <squi...@ntlworld.com> wrote in message
news:00nC7.10640$o56.956811@news2-win.server.ntlworld.com...
> while from 'Local Telecommunications 2', Griffiths ed, 1988, chapter 14
> describes the '1+1 Carrier'  system, also known as WB600. This was
> introduced around 1972, to allow two subscribers to use one line pair
> simultaneously. One uses the line as normal, the other is connected
> through the WB600 unit, with the frequencies:
>
> 40 kHz    Subscriber to exchange
> 64 kHz    Exchange to subscriber
>
> This seems to be still in use, in 1997 I came across one when doing some
> DIY on a friend's house, which was divided into three flats, she wanted
> two old BT boxes removing as she had changed to cable TV phone, I
> opened them to see what was inside, and there was a PCB marked
> '1+1 carrier' and a battery. The two other flats still used BT phones
> so it was a good job I didn't just pull them out...I had thought they
> were an old external bell.

I've never heard of WB600, maybe that was the number given to a prototype
version?. The one you describe above is almost certainly what was known as a
"Carrier System 1+1" or WB900 and was very common - the exchange I worked in
had around 200 when I was last involved with them (~1993). They came in
during the late 70's early 80's and (again when I was last involved) had
evolved into about 9 slightly different, and often incompatible, versions.
There is a removable exchange unit for each line (provided in "shelves" of
10) and the customer's end has the box you describe along with a little
black filter that sits in the Audio customers line.

WB900 was brought it to replace Shared Service lines but was itself replaced
by DACS around 1995. I have no idea if a full replacement program, swapping
WB900 over to DACS, was put in place though - I'd gone by then.

They worked reasonably well but could be prone to some common problems.
- If the exchange unit version was different to the customers unit version
then it would sometimes work - but sometimes it wouldn't.
- The battery you mention above are rechargeable and the line current,
supplied from the Audio line circuit, was used to keep the charge up. If the
Audio customer was a busy line then the battery never got enough charge and
the Carrier customer eventually stopped working - usually after a period of
degraded service and the battery slowly died.
- Any line faults, that would normally be unnoticeable, could affect this
charging, with the same results.
- The Carrier was unusable for anything other than simple voice.

Happy memories!!

Ian (TXK, TXE, ND, UAX, SAX and bar)