In brief, as I understand it as a signalling engineer, the technology has changed quite a bit over the years, so the answer varies- and being a railway there is probably somewhere that is still using the technology of 100 years ago and every generations since up to the latest stuff installed in this year.
Nowadays it really is not a lot different to a normal telephone system other than the interface to the signaller- the incoming calls are "mapped" to a specific "button" on screen to reflect the origin of the call- however we are now all used to seeing on our mobile phones the calling number before we take the call so not now that different.
The drivers' phones start ringing the relevant control centre as soon as they are picked up; no need to dial a number from the SPTs. The phones all have their own lines, if not all the way back to the control centre, to a remote "branch exchange" that then routes the selected call back to the control centre over a shared circuit. However really just like the standard international public telephone system but ust on a smaller scale.
Historically however there tended just to be one physical pair of wires for a long length of phones alongside the railway and it was in this scenario that there was the real problem of preventing two of them being simultaneously connected to the line and the wrng driver hearing the signallers instructions intended for another person. There were various ways of achieving; I remember the units that had two Stowger uniselectors (solenoid driven stepper motors moving a multiway rotary switch) that sent out a series of pulses as a code to identify which phone was calling the centre (in those days couldn't ring out from the centre!)- I seem to recall that the "address" of each phne was such that the sum of the two numbers always came to 25. If a driver picked up a phone when the line was busy then the phone would not connect to it and they needed to wit to try later; nowadays the call tends to get through to the control centre and the signaller can see it flashing even if unavble to answer since engaged on another call (and hence can themselves ring back once free). Some concentrators have two separate handsets and so a second call can be taken by another person, or the signaller leave one on hold whilst taking a more urgent call. Typically the phones are auto muted unless the signaller holds a button on the handset, so if left off hook then the phoe does not pick up conversation intended for someone else being spoken to locally or using another phone.
In the old world, telecomms always used a 50V battery with the positive leg being earthed (to do with avoiding corrosion of the earth rods I believe)- I wouldn't be surprised that this voltage is still used for ringing current. However nowadays in the UK the amount of copper in the circuit is generally limited to the final few km to the phone from the nearest "access node" at which the call is multiplexed into the optical signal carried on fibre optics and hence derived locally rather than fed all the way from the control centre.
That is my simplistic signal engineer view; it would be good to be corrected / answers refined by someone who really knows the detail!
(29-06-2013, 01:29 PM)Chall1986 Wrote: Hello there, I am from a signalling background but don't have much knowledge of the telecoms side.
I am wondering if anyone could share their knowledge with me.
I am wanting to find out:
The process by which the lineside telephones communicate with the central control?
how are the telephones prevented from overhearing the conversation between driver and signaller?
voltage and current requirements for a standard lineside phone?
Any information would be much appreciated.
Cheers
