Telephone exchanges (from UK Telecom FAQ)

Q: What types of exchange are there, and which can be digital ? {*}

The types are:

- Strowger - rotary switches, etc (UAXs, SAX, TXS)
- Electromechanical crossbar (TXKs)
- Electronic-control reed-relay switches (TXE2)
- SPC non-digital (TXE4*)
- Digital (Systems X and Y - AXE10: may be further categorised as to whether
  they're ISDN-capable, I suppose; 5ESS; DMS100; UXD5.)

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.


Q: What is the difference between a System X and System Y exchange ? {*}

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.


Q: What is the number that will dial my 'phone back to test the ringing?

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...


Q: When I called 175 from home there was a message telling me my phone
number, then when I hung up I was called back and offered a mysterious
menu.  Anyone know how to work it?

- 1.  Dial 175
- 2.  Exchange answers and says "You are connected to <number>.  Start test."
- 3.  Hang up
- 4.  Exchange calls back and says "line testing ok" (assuming it is ok, of
      course).  Then it enters a loop, inviting you to "dial next test" and
      performing a test based on the single-digit code you give it.  From
      memory, test 1 is the keypad test (dial 123456789*0#) and 3 is the
      coin pulse test.

                              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


Q: What do the various messages from the 175 tester on System X really
mean, eg. "Earth A, Battery B"?

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 ...


Q: Is there a way I can pulse-dial a digital exchange number and then tone
dial from there? {*}

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).