Quote:
Originally Posted by Oldcodger
I'll answer #1 with another question. When did the operator direct routes come into existence? Before STD became fully into effect there was a set of routes for direct dialing solely for the use of operators with a set of differing codes. I first heard of it when some engineers at Stone ( Yarnfield college as it's known these days AFAIK) used this to phone home for free and were caught.
From memory of the heady days in a TRS, I seem to remember that there were set levels depending on the grade of the junction. I seem to remember that the worst case was -4dB overall loss( sent at 0dBm and received at other end receiver at -4dBm , approx half power/or voltage if you prefer)
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Answer to question at beginning - back in the 1800's once two exchanges were interconnected
There had been a separate network for the trunk circuits since 1896 when the trunk circuits of the National Telephone Company were 'nationalised' and the running of the trunk network became the GPO's responsibility. Thus from then on, the National Telephone Co only handled local calls until the GPO took it over on 1st Jan 1912.
The telephone network expanded but some exchanges as late as 1955 only had two trunks to their parent exchange - Glenmoriston for instance only had two junctions to Fort Augustus although there were twenty subs on the exchange at the time. A trunk call would involve the Glenmoriston operator connecting the caller to Fort Augustus operator who would connect the call to the main manual switchboard at Inverness which handled trunk calls. the Inverness 'trunk' operator would then set the call up 'down the line to each manual exchange in turn. That all took time. Hence most long distance calls were worked on the 'delayed' system were the caller was rung back once the call had been set up.
About 1951/2 saw the start of 'Trunk Mechanisation' - the conversion of the trunk telephone network to automatic working. This was a separate network accessible only by the operator from manual exchanges or the 'auto-manual' boards at that was reached by dialling 0 (or 01 on some exchanges) and from the TOL/TRUnk manual switchboards in London, Birmingham and Manchester Director areas..
The automatic trunk network consisted of 'Zone centres (each had direct routes to all other twenty odd zone centres), Each zone centre then had direct routes to all the Group Switching Centres in its area. Each GSC had direct routes to the Minor exchanges in its area. And the there were 'Dependent' exchanges off some Minor exchanges. all the way to the wanted subscriber Assuming all the GSCs, minor and dependent exchanges were automatic, an originating operator could dial all the way! The GSCs, Minor and Dependent exchanges were part of the local network. Once STD started, dependent exchanges were connected directly to the Group Switching Centre. Whilst dialling from the Trunk network was normal, dialling from the local network to the Trunk network wasn't permitted. However very occasionally, there were errors in the trunking of circuits - for instance when Mold exchange went automatic, a level '1x' route was left 'open' allowing access into the trunk network by 'ordinary/coinbox lines. In other places, 'naughty' engineers set up their own level which didn't meter the calls and gave NU tone if dialled but you just carried on dialling. I have heard of this happening in the Bristol area and it being followed by a court case - engineers involved were sent packing .
However it wasn't a case of just dialling the 'access code' and then the STD code. The codes on the trunk network were different from each location (the operators had them listed in a VIF (Visual Indication File - a flip up card setup) so knew what to dial. Once STD appeared, the STD codes were the same all over the UK to an exchange but the local 'register translator' converted it to the required trunk network code from the originating GSC as it accessed the trunk network.
The audio levels between zone centres was certainly 0db loss, between Zone centre and GSC was 3db and to Minor exchange from GSC was 3 dB also giving a maximum of 12db - I'll see if I can find the info.