The last crossbar and Strowger exchanges have now been removed from service.
The UXD5, a public-exchange variant of the Monarch digital PABX, probably remains only in the Highlands of Scotland and rural Wales.
You can tell whether you have an electronic exchange by trying to dial a number with DTMF dialling. If you get unobtainable when you dial * or # then you have an electronic one, but not digital.
On TXE4(RD) exchanges (the original design), a change to the class-of-service threading is needed to enable DTMF. The TXE4A (with a newer design of processor and register) has DTMF enabled for all lines.
Dial *#001#. If you get "No services are in operation on this line" or a list of services, you're System-X. If you get "Sorry, you have dialled an invalid service code" it's AXE10.
Also, *55* will prompt you for a time on System-X; it won't on AXE10. If you get Number Unobtainable, you're not on a digital exchange. If you still have dial tone, you're on an "old technology" exchange. If you hear nothing (except crackling in your case) you're probably on an Ericsson (System-Y)
Now, type (a) accepts your dialling at the point in the switching matrix that your call has so far reached. Every switch has its own dial pulse decoding and routing intelligence. Short of generating the dial pulses at the MDF (where the lines come into the exchange), there's no way to convert them to DTMF. And what would be the point? - pulsing dial pads are _so_ cheap that pretty much everyone's telephone can do the job.
All but type (a) accept the digits you dial into a register, and make the routing decisions centrally based on the contents of the register. In principle, any such exchange can easily be converted to accept DTMF: it simply requires a different input box for the register.
(Historical note: the first electronic research exchange to hit public service in the mid-60s, the TXE1 at Leighton Buzzard (RIP) was DTMF capable. As a person involved with fiddling 'phones at the time, I arranged to go round it with some like-minded friends: we were shown a DTMF 'phone in the exchange manager's office, and were told that there was precisely one `public' user of the service.)
Pretty much any SPC exchange (types (d) and (e)) can in principle offer network services. We know that type (d) can in some cases, since we've had a post to that effect from someone at STC (was it - sorry, I've forgotten your name). We know that type (e) can do it, since they all do...
Five designs of digital exchange are being installed. System X, AXE 10, UXD 5, 5ESS PRX and DMS100. System X is an all-British family of digital exchanges manufactured by GEC Plessey Telecommunications (GPT); AXE 10 is a Swedish design and manufactured in the UK by Ericsson Telecommunications Ltd; UXD 5 is a small system specifically devleoped by BT for rural areas, and 5ESS PRX is the European version of a system developed by the American company AT&T and widely used in North America. The DMS100, designed and built by Northern Telecom (NT) is being used for Featurenet services.
The last analogue switch was removed from the trunk network in June 1990. All traffic on the trunk network is now handled by 59 fully interconnected Digital Main Switching Units (DMSUs) and four partially connected Digital Switching Units (DSUs) which help handle high call volumes from between London and the Home Counties. International traffic is handled by 4 dedicated exchanges.
At the local level, around 97% of customers are connected to digital or modern electronic exchanges.
At 30th September 1993, BT operated 7,537 local exchanges in the UK, comprising 5,532 digital, 1,301 electronic, 38 crossbar and 666 Strowger (electromechanical). By March 1996 all customers will be served by modern electronic exchanges.
System X was a co-operative development by Post Office Telephones, GEC, Plessey and STC. There was a big bust-up in which STC were removed (and given exclusive rights to supply TXE4) and GEC and Plessey were given all the rights and obligations to develop System X as a commercial system which BT would then buy by competitive tender.
It was decided that there should be at least TWO suppliers to the PO/BT, and there was a large international tender to decide on a second, competitive system. Ericsson made the AXE10 which offered practically the same flexibility in a smaller box (mainly because System X was designed by committee: it was already dated before the first release model became available). The AXE10 was chosen and was then (and still is) bought by BT in competition with System X. The AXE10 became BT's 'System Y' just to keep things 'simple'.
If anything the AXE10 is more full-featured than the 'X' but BT only took the basic software package on each, so both offer practically the same. Incidentally, Cellnet tried to use System X as their mobile switch but dumped it as it was very old tech. Vodafone tried Ericsson, and that is why Vodafone runs totally on AXE10s.
Dialling 174 on modern electronic exchanges.will make the exchange ring back when you put the phone on hook.
On a System X/Y dialling 177 will make the exchange read out your number, and dialling 175 will produce the exchange test...
A replacement for both 174 and 175 has also been mentioned, which is
17070
Ringing 17070 results in
"This circuit is defined as <number> " and then " BT line test facilities, please press 1 for ring-back test or clear down." Keying 1 then results in "BT ring-back test, please clear down." After a few seconds, phone rings, and announcement is "BT ring-back test completed."
Exchange Test Numbers
Abbreviations
SPM = Subscriber Pulse Metering FRB = Ringback Test
CPI = Cable Pair Indicator (Loud tones on line to identify cable)
LPI = Line Pair Indicator (Gives telephone number connected to)
SALT = Subscriber automatic line test
TXE4 - Electronic Equipment:
SALT Dial 175 and wait for "Start Test" message
Replace handset. Rings
Message: "Line Testing Ok" or fault if present.
Continue, wait for dial tone for dial test.
Dial 1 3 0 5. Rings. Message "Testing Ok" or fault if present
FRB Dial 174
LPI Dial 188 gives directory number & equipment, or if not,
Dial 187 gives equipment number.
CPI 176 plus full national code (i.e. 176 081 553 7104)
Tone is present on line when dialled. Replace receiver
to cancel tone.
TXD-X - System X Equipment
SALT Dial 175
FRB Await ringback and listen for message.
Continuation tests. Await interrupted dial tone.
Dial "1" for dial test. On LD phone, dial 1-0.
On MF phone, dial 1-9*0#.
Dial "4" for SPM test. 10 metering pulses are sent, then
NU tone, followed by 10 more pulses.
CPI Dial 176 plus full national code (i.e. 176 081 553 7104)
(This will only work from same processor)
Wait for short burst of tone reminder, then tone is transferred
to the specified line. To disconnect tone, replace handset.
TXD-Y - System Y Equipment
SALT Dial 175
Await ringback and listen for tone.
Dialtone=OK Engaged=Suspect Faulure=Fault
Dial next test if required, tests as follows:
2 Low A/B Insulation test |
3 A/B to earth | Insulation tests
4 A/B to battery |
5 A/B loop to earth |
9 Bell answer before 5th ring
11 SPM test
7 Loop Res test
8 Dial test. LD Dial 0. MF Dial 1-9 0
10 Rec all pre
6 Line reversal for diode check
Dialtone=OK Engaged=Suspect Faulure=Fault
FRB 174
A-wire (leg) - connected to earth at the exchange via effectively a current source.
B-wire (leg) - connected to negative 50v at the exchange likewise.
Looking from the exchange, there should be a large resistor (100k I think), across 2microfarads, and a slice of resistance in series (1k nominal at dc) - the bell. (the 100k is only on new lines)
Earth or battery A, as reported by the tester is a fault. it should be isolated totally. Earth or battery B likewise.
Low resistance A to B likewise again.
Each indicates a weak insulation, and it depends on what path the weakness has allowed. other exchange lines around it are still connected, to earth and -50v after all ...
Well, if you are not in an area served by Mercury, I would think the only way to do this would be with a BT ChargeCard - you pulse dial the "144", and can tone dial from then on.
All BT payphones switch to tone dialling as a matter of course, after receiving a metering pulse. Most dual-equipped phones, if switched to pulse dialling, will change over to tone dialling by pressing the '*' key. Operator and non-payment numbers (144, 0800) do not switch.
The main snag, (apart from the extra delay caused by entering your ID and PIN), is that you then have to pay for your call at call-box rates (8.5p/unit +VAT instead of about 4.2p +VAT).