More on the SxS switching:
On the old exchanges it wasn't at all unusual to hear a little crosstalk from other lines, and the step-pulsing sounds of other calls being set up could often be heard in the background. Here's a recording of Step-pulse crosstalk
on an otherwise quiet line.
When you were calling "digital - digital" the number usually just rang out immediately but when calling digital to ARF / ARK crossbar or to a mobile or abroad (e.g. to the UK to an old switch) the bebebebe tone was played down the line rather than letting you hear the actual signaling.
The pre-digital exchanges here didn't let the caller hear the signaling, but there was no "call progressing" tone applied. However, the AC9 signaling system -- which was very widely used for many years in the U.K. -- used 2280Hz for supervision, and you would often hear the chirp of 2280 on a trunk call when the distant party answered.
One thing that was
done on British crossbar exchanges was the application of a long first burst of ring-tone when a call switched through before dropping into the normal ring cycle.
Here's an example: TXK Ring
The idea was to prevent extra delay if the call happened to switch through during the silent period between rings, although considering that the break between rings on the British/Irish cadence is only 2 seconds, it probably wasn't that big a deal. SxS switches never adopted this approach.
In other countries if a switch is swamped it just won't give a dial tone at all. Here the switch will take the call, let you dial and then make you wait until it's ready to process the call.
Digital switching has resulted in a vastly different network compared to the older switching systems.
In SxS without directors, the dial pulses control the selectors directly, so the only way to ensure an instant dial-tone to all subsscribers without fail would have been to provide one first selector per line.
In director areas (which in Britain were just the big cities: London, Birmingham, Edinburgh, etc.) it was the director which provided dial tone and accepted the dialed digits, storing them until enough were received to pulse out the correct routing codes into the selectors. Directors were complex and expensive pieces of equipment, so optimizing their use was essential, and providing one per line was totally out of the question.
In TXK (crossbar) exchanges, a similar situation existed with the register-translator. As with the director, this stayed on the line just long enough to accept the dialed digits and set up the call path before dropping out to be used by another caller.
On some small SxS offices you could sit listening to dial-tone all day, but on director SxS or crossbar if you hadn't dialed after at most 30 to 60 seconds, you'd be dumped off the director or register-translator to free it for other calls.
Where circuits were all busy at the moment a call was dialed, there was no question of waiting for a line to become free. The equipment just connected Equipment Engaged Tone
to tell the caller to try again.
If the congestion was on routes out of one of the big tandem centers, then the caller would be given the luxury of a recorded message, such as this
The recordings I've linked to here are on Rob Grant's Telephone Website
. Lots more interesting stuff there.
[This message has been edited by pauluk (edited 09-15-2003).]