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uk.telecom FAQ, Part 3/3 - Technical matters

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Archive-name: uk-telecom/part3
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See reader questions & answers on this topic! - Help others by sharing your knowledge
               Frequently Asked Questions for uk.telecom

                       Part 3 - Technical matters

        Compiled/ Posted by James Grinter <>

~See <> for the question index,
and further information about this FAQ.~

These articles attempt to summarise answers to some of the more often
asked questions in the newsgroup uk.telecom.

This third part of the FAQ is about 'phones and 'phone technology, how
it works, and what you can do with it.


Subject: How do phones work?

Q: Okay, so tell me this. How do phones work ?

A very good question, that one, and for the moment the only answer I can
direct you to is the file in the comp.dcom.telecom
archives at [].  It is US orientated, but some of
the general principles remain the same.

[A UK version of this file would be gratefully received]

Q: Where can I get technical information about UK telecommunication

You need Supplies Information Notes (SINs): dial 0800 318601. For a
full technical report upon how BT's proposed Caller ID (CLID) service
works, ask for SIN 227. You can also ask for the latest SIN index with
lots of interesting stuff, and ask to be added to a mailing list for
all new SINs.  (SIN 227 may also be found at

Q: What do those wires do?

[last update 27/3/95;
Answers courtesy Alan J Flavell <>]

(Unless otherwise stated, this answer only deals with ordinary BT
telephone lines using the British-type modular socket system.

N.B the wiring up to and including your master socket is the property
of BT and the rules say you mustn't interfere with it.  Therefore these
details are here for educational value only.

British regulations prohibit the use of equipment that has not been
type-approved for use in Britain.  Therefore, any mention in here of
equipment brought from elsewhere must be treated as purely hypothetical
and for educational value only...)

At the point where your telephone line comes into the house, only two
wires (1 pair) are used.  If you have more than that, the remainder are
spare.  The wire costs almost nothing, compared to the cost of installing 
it, so the wire that they lay often has several pairs in it.

The wire pair does several things: it carries speech and bell-ringing
current etc., and it brings a limited amount of DC power from the exchange;
see the "How phones work" writeup for a general description, which
is, however, expressed in US terminology.

The two wires of a telephone line pair are called A and B.  These are 
connected to your master socket or linebox (everything said in here about 
a master socket is also true of a linebox, unless a specific difference
is mentioned).  Nominally, line A is at 0V and line B at -50V. 

(The 50V comes from a big battery at the exchange, which is kept
charged from the public electricity supply ("the mains").  This 
is used to power ordinary telephones, thus allowing calls to be made
even during mains failures.  More sophisticated phones (e.g cordless) 
need a local mains supply, but the customer is advised to own at
least one telephone that does not.)

Secondary sockets are connected to the master socket via two-pair or 
three-pair cable, in accordance with the instructions that accompany 
the kits.  However, only three of these 4 or 6 wires are really used: 
namely, the two lines, and the bell wire (also known as "shunt", see 
later point).

The British telephone plug (type 431A) is rectangular: it has ways for 
six contacts along one side, and a latch (and keyway) on one end.
Only the middle four contacts are actually present, with one blank
position at each end, but the contact ways are numbered the same 
irrespective of whether pins are actually present. However, bizarre
as it may seem, it turns out that there are two different schemes of
numbering the pins: the original BT one, starting with pin 1 at the
latch end, and the "BS" one, with pin 6 at the latch end.  The physical
wiring, thankfully, is the same in any event - it is only the contact
numbering convention that is different.

 BT plug, contact face up,          pin number   installation wire
 with cable to the left              (BT) (BS)   colour (Body/stripe)

            no pin      not used      6    1      (White/green)
 .......... ---         line A        5    2       White/blue
 .......... ---         not used(E)   4    3       White/orange 
 .......... ---         bell shunt    3    4       Orange/white
 .......... ---         line B        2    5       Blue/white
            no pin      not used      1    6      (Green/white) 

There are some situations not covered by this FAQ (PABX Earth Recall
for example) in which the pin marked (E) would be connected to a local

The master socket contains the following circuit (SP=Surge Protection)

   ------------------------------o pin BT2 (BS5) ->
    line B     |      |
               |      = 1.8uF
 from          |      |
           SP  X      |----------o pin BT3 (BS4) -> to secondary socket(s)
 exchange      |      >         bell wire
               |      < 470kohms
    line A     |      >
   ------------------------------o pin BT5 (BS2) ->

In the normal UK telephone, the bell is connected between the bell
wire and line A.

(There are some types of telephone approved for UK use in which the ringer 
is connected between the two lines, just as it would be elsewhere.)

(Many modems will be like this too, detecting ringing current from the
line pair and disregarding the bell wire.  If the modem also offers
a socket for connecting a telephone, UK modems can be expected to supply
the telephone with bell current correctly; but foreign modems might 
not do so, with the result that the phone, if it uses the "British" 
bell arrangement, will not ring.)

(There exists also a plug which is the mirror-image of the 431A, and
which, of course, won't work in this socket: thankfully, most people
will never meet one.)

Q: Why a third wire for the bell?

The system was designed at a time when pulse (loop disconnect) dialling
was common, and telephones still had real bells.

During pulse dialling, there is a tendency for the bells on the
associated telephones to tinkle.  By using a separate wire for the
bell, it's possible for the telephone that's doing the dialling to
shunt out the bells, preventing them from tinkling.  Nowadays this is
much less of an issue, what with tone dialling and electronic

Q: What's this about master sockets and lineboxes?

A: The original design of "master socket" was wired by the installer, 
and the customer was not allowed to interfere with it.  The approved 
way to wire a secondary socket, if you still have such an 
installation, is to purchase an approved extension kit which comes 
equipped with an adapter.  After fitting the extension socket, you 
unplug the phone from the master socket, plug the adapter into the 
master socket in its place, and, assuming you still want to have a
telephone at the master socket location, you plug it into the adapter.

The "linebox" is a much neater solution.  It can be recognised by
having a sub-panel on the front, which the customer can remove by 
undoing two screws.  On removing it, one finds that the sub-panel 
is, in effect, an adapter, but with the advantage that the customer is
allowed to punch down their own wiring to it.  When investigating
a fault, the engineer can remove the panel, together with the 
customer's wiring, and test the internal socket, free of confusion 
from customer additions.  It is a good idea, before reporting a fault, 
to take off the subpanel yourself, and try a phone plugged into the 
internal socket in order to eliminate your own wiring - also try a
second phone in case the first is defective - both kinds of fault
would incur a charge if you call for a repair (line rental covers
free repair only of BT's line).

There are, consequently, two different kinds of kit for installing
a secondary socket.  The one with an adapter is meant for the original
"master socket"; it can be used with a linebox too, but it would be
pointless to do so.  The other kit has no adapter - only a length of
telephone wire (two or three pair, it doesn't matter) and a secondary
socket: this type of kit is for use only with a linebox.

Q: My equipment (telephone, modem, computer...) has an RJ11 socket
for connecting the line cord.  What are the connections?

It is clear from the answers given by uk.telecom readers that there ~IS
NOT ONE UNIQUE STANDARD~.  The two situations most likely to be met in
practice are these

- The middle pair of the RJ11 (pins 3,4) carries the phone line.  
  The line cord must connect that pair to pins 2 and 5 of the BT socket.
  This would also be the situation with equipment brought from the USA.

- The line cord between the RJ11 and the BT plug is a straight cord.  
  In this case, the manufacturer has arranged to put the line onto pins
  2 and 5 of the RJ11.  It follows that UK models of this type of 
  equipment necessarily differ from the US models (in the USA, it is
  standard to use pins 3,4 of the RJ11 for the phone line).

Some contributors asserted that one specific arrangement was ALWAYS
used.  But on the evidence, they must be mistaken.

Some contributors say that the first is more common, while others have
more often met the second: so one can only guess what the statistics

At least one contributor implied that there are yet other pin
assignments that have been used at the equipment end.  (There is,
however, only one arrangement at the BT plug end.  Phew!).

Note that in the USA, the middle pair of the RJ11 (pins 3 and 4)
carries the phone line.  Therefore, equipment that has been brought from
the USA will assuredly be like that, and will need a line cord that
connects the RJ11 middle pair to BT pins 2 and 5.  It does not follow,
however, that equipment supplied in the UK by US companies will 
necessarily be like this.  Several readers have UK versions of gear 
made by US companies, that definitely has the phone line on pins 2 
and 5 of its RJ11 socket.

Specialist shops such as Maplin or Tandy should be able to supply
either kind of RJ11-to-BT line cord.

A US traveller unable to get hold of a correct cord could, say, bring
a US phone cord with them, buy a British-style extension cord (readily
available here), cut both cords in half and splice the appropriate
halves together.

Q: does it matter if A and B are interchanged?

Interchanging A and B on the incoming line will merely reverse the
polarity of the lines.  Since normal telephone equipment is designed to
work on either polarity, this would not matter.  (But the installer
**should** get it right, even so.)

(One contributor asserts that ALL phone equipment will work irrespective
of polarity.  Reference was made to the UK (BABT) approvals procedure, 
which lays down that equipment must be unaffected by polarity in order
to be approved for UK use.  Some informants seem to know of equipment 
that will only work on one polarity, although it was not claimed that
this equipment had been approved for UK use: the best that can be said 
with confidence is that such gear would be unusual.)

Interchanging A and B between the master socket and a secondary socket,
or on the line cord between socket and equipment, will prevent the bell
from working (if the ringer is connected in the "British" fashion), and
can cause other problems.

Q: Why that 470 kohm resistor in the master socket?

This allows the exchange's tester or "routiner" to test the line, even
if all the customer's telephones are unplugged.


Subject: BABT

Q: Who or what are BABT? How do I contact them? 

The British Approvals Board for Telecommunications.

BABT (Northern Office): Now closed .

BOIS System Administrator, Claremont House, 34 Molesey Road, Hersham,
Surrey, England. KT12 4RQ. Tel: 01932 222289; Fax: 01932 229756;
modem (database dial-up access): 01932 248032 and 01932 248062.
Telex: 267478 BABT G.

The BABT On-Line Database, based upon data held on the OFTEL Register
of Approved Apparatus, is now situated at the Surrey office.

Free public access is permitted to this database. It contains details
of apparatus approved since 1st January 1984 & a list of of BABT
Approval-related technical & administrative documents held by the

Database (free) dial-up access details:
Terminal emulation: VT-52, VT-100, VT-102, ANSI;
Protocol: 2400/1200/300/1275 -> V32terbo full duplex; 8, 1, N

[my original correspondent had not attempted access, neither have I.  I
am informed by another that access is made through a guest account. 
Some services were due to be moved over to a subscriber only basis.]

If hard copies of BABT documents are required on a regular basis, one may
join the BABT Subsription Service.

Write to Mr J Malcolm-Coe, Senior Technical Administrator [valid
1993], at the Surrey office.

BABT is not connected to BT. It is the British Approvals Board for
Telecommunications, which awards "green circles" to telecoms

Note: BABT can only provide "approved" or "not approved" details of
"apparatus". They will not comment upon the progress of a given
apparatus through the BABT approvals process.


Subject: PSTN

(Public Switched Telephone Network)

Q: How can I simulate the PSTN?

Black Box (01734 866800) sell a 'FREELINK line simulator' which should do
what you want.  It costs UKP123.

You can also buy a cheap PABX. Maplin Electronics sell a few commercially
produced domestic PABX's The spec is quite good, allowing 4 extentions,
call blocking and stuff, for about UKP140.


Subject: PSDN

(Public Switched Data Network)

Q: What is the PSDN?

In this country, PSDN exclusively refers to BT's PSS (originally Packet
Switch Stream (and any Mercury competitor?), which is (are) an X.25-based
network(s).  From BT you can buy (at exorbitant cost) leased-line
connections to PSS at various speeds - contact their sales people. You can
also dial-up PSS at slow speeds - PSS DialPlus.


Subject: ISDN

(Integrated Services Digital Network)

Q: What is ISDN?

Simply speaking, ISDN provides the customer with the 64kbit/s that PCM
(Pulse Code Modulation) digital representation of speech occupies, but
doesn't insist that it be used for speech.  The simplest connection you can
buy offers `2B+D' (BT's ISDN-2, or 'basic-rate' ISDN), which is 2x64kbit/s
with a 16kbit/s signalling channel.  In principle, you can send rather
wimpy video with 128kbit/s, but it would have to be highly compressed, at
low resolution and with a low frame rate (like in a picture-phone).

Q: Say I want to access my University computer from home, using an
ISDN connection, in order to get fast terminal access and file
transfer.  What would I need to connect a PC to a workstation over

You will need a terminal adaptor at the Unix end, and a PC card in the PC.
Terminal adaptors are around UKP1000.  Software is dependent upon what you
wish to run over the link.  An async terminal connection will probably not
utilise the full bandwidth available.  It is possible to run IP over the

Adaptors for Sun Microsystems currently available: [consult relevent
comp.sys.sun and uk.sun lists for more upto date info]

Diehl have an Sbus card with SunOS-4.1.3 and UK approval.  UK distributor
is Controlware, Thatcham, UK, (01635 871636) or try  They
also have PC cards for DOS and Unix.

Bintec have a range of cards for different backplanes and operating
systems, including an Sbus card, approved for UK usage and with
drivers. Contact, Tel: (01911) 99675-0, Fax: (01911) 

NetBlazer PN and an external TA, allowing ISDN access from Ethernet.  The
Controlware CITAM unit was also recommended in this context.

Sparc 10s, LXs and later models have on-board ISDN.  However, Sun are not
planning to release SunOS drivers for this at the moment.  They say that
BABT approval of the hardware is imminent and Solaris drivers will be

I am told some models of DEC AXP also have on-board ISDN. Driver
status is unknown.

Q: What's the difference in equipment at the exchange switch between
an ISDN line and that for an analogue line?

The lines have an identical concentrator, but have different line
termination.  Instead of converting analogue signals to PCM format for
transmisson to the switch, an ISDN Line Terminator will process the
signal differently and pass it on directly to the switch. 

The subscriber's ISDN unit can talk directly to the switch's control
units; thereby reducing call setup times to an absolute minimum. Which
is great for ethernet ISDN bridges as they can raise and drop the line
on demand.

Subject: Leased lines

Q: We ordered a 14.4k leased line, and BT are installing a 64k line with
convertor. Why, and surely it costs them the same?

This is not unusual.

Don't forget that you're paying for the bandwidth used on the trunk
network. The capacity of the local loop has very little to do with it.
When you want more BT will only have to change the MUX at your end.


Subject: Exchange features

Q: Is it possible to set up a three way call, then have one of the other
guys introduce another caller.. ad infinitum?

Yes and no.

With Network Services three way calling, it is possible to introduce a
third caller.  There is supposed to be a facility (it was mentioned in
one of the BT network guides) to introduce 8 callers.  If everyone
has three way calling, you can go on adding people to the call; this
can even be done with several people in the US and several in the UK
all linked through 3-way. The only problem is the voltage on the line
keeps droping with every line added so eventually it gets very hard to
hear everyone: it's ok for 2-6 people, for anymore it is better to set
up a conference call.

However, if you have a PABX, you may be able to do this anyway using the
conference call facility. You can also use BT's conference call service for
three to sixty participants- call 0800 282429 for information.

Q: How does Caller Display ('Caller ID') work?

[For a more detailed discussion see 'Caller Display and Call Return' by
William Dangerfield, Simon Garrett and Melv Bond in British
Telecommuncations Engineering; Volume 12 part 3 (October 1993).
Also See Supplies Information Note (SIN) 227, available on 0800 318601.
BABT have issued a specification, BABT/SITS/94/53 to replace the draft
specifications BABT/TC/128 and BABT/TC/131.  This specification is
identical to the 131 draft and is shorter and easier to meet than the
128 draft.  A copy can be obtained from BABT on (01932) 222289.]

The system described here is that developed by BT for use on the UK
PSTN. It is based on the Bellcore 'CLASS' standard. This has the benefit
of allowing CPE manuafacturers to base their UK models on those
developed for the North American Market.

Most of BT's customers are connected to System X, AXE 10 or TXE4
exchanges and these exchanges are digitally interlinked using CCITT
[now ITU] C7 signalling. C7 provides a way of passing the number of
the calling number to the distant exchange (this information is used
during call tracing).

When a call is made to a customer with Caller Display the distant
exchange requests the number of the caller originating the call from
the exchange at the other end of the C7 link.

If the call is not routed totally over C7 links (e.g. the caller is on
an old analogue exchange), or the caller is on an interconnected network
for which no agreement for the exchange of additional call information
is in force, the number will not be complete. In this case customer with
Caller Display will get a 'Number Incomplete Message'

If on the other hand the caller has deliberately withheld the number, by
use of the 141 prefix the Caller Display Customer will get a 'Number
Withheld' Message.

If the number is complete, and not withheld by the caller the number is
routed on to the Caller Display customer over the local access network.
For this purpose a V.23 sender has to be installed at every exchange

When a line is about to receive a call the polarity of the line is
reversed prior to the ringing current being applied. If the customer has
Caller Display additional messages are interspersed between the polarity
reversal and the application of the ringing current.

First a tone alert signal is sent and then an alternating series of '0's and
'1's lasting 250ms is sent by the V.23 sender to assist the CPE in detecting
the imminent arrival of the Caller Display message.

The Caller Display message itself contains the following information:

- The number of the caller
- Reason for absence of number (e.g. number withheld)
- Time and Date (Can be used to auto-set CPE clocks)
- Caller/Name Text (Intially only used for designating calls from payphones)
- Reason for absence of caller name
- Call type

The Caller Display message takes roughly 0.75 seconds to send, after
which the normal ringing current is applied to the line.

Q: I hear that PABXs with two or more incoming lines require a 
maintainance contract.

There are now at least three two line systems which do not require either
PCI or a maintenance contract:

- Smartalk 208 (2+8) - featurephone system at around UKP170 per ext.
  (also available in 3+8)

- Olympus Olycom 2000 (2+8) - featurephone system at around UKP160 per

- Southwestern Bell Omnicom (2+8) - Uses any phone. CCU is around


Subject: Telephones

Q: Why does my pre-socket phone 'tinkle' in the night ?

There have been postings regarding telephones `tinkling'. It has also been
posted in reply that this is attributable to line testing.  They are tested
by detecting the capacitor in the phone socket. Pre-socket phones are still
tested OK, but the capacitor is actually in the phone. Hence it tinkles
when tested.

Phones are **not** tested every hour, the current target is once a day, but
in most places it'll be once every few days. New systems coming on stream
in the future will permit the lines of people who complain about tinling to
be excluded from the test cycle.

Q: What is the difference between Timed Break Recall (the Recall button)
and flashing the hook?

Conceptually Timed Break Recall (TBR) and Flash Hook are very similar;
Flash Hook is used worldwide, TBR was invented by BT (GPT) for use in the
UK.  The difference is in the duration.  TBR is 90ms, Flash Hook is
anywhere between 200ms and a couple of seconds.

Cable telephones tend to use Flash Hook as they are using North American
exchange equipment (for example, Cambridge Cable use the NT DMS100).

Q: How does the BT Chargecard system work ? 

[{*} This has now been noted as being obsolete. I'll remove this
from the next version- but am open to anyone who has any info
on the current system in operation.]

This may interest those who are trying to guess how Chargecard works.  It
is wholly cribbed from "The Cashless Services System" by N.G. Pope in
"British Telecommunications Engineering" Vol 9, July 1990.

Customer dials 144.  The call is routed to the nearest digital exchange,
which adds 3 digits identifying the charging area of the caller.  It
routes it to the trunk exchange (Digital Main Switching Unit or DMSU)
which routes it to one of 30 Cashless Services Processing Units (CSPUs)
where it terminates. As far as conventional charging is concerned it is
effectively a free call.

There is now a through transmision path from telephone to CSPU.  Payphones
are (internally) programmed to switch to MF after receiving 144.  The CSPU
sends a burst (1 second) of 1600Hz tone.  If the originating phone is a
payphone it will send its identity (9 MF digits) to the CSPU - (for stats
on public payphone usage perhaps, it doesn't say?).  Ordinary phones will
not respond to the tone.

CSPU returns prompt to dial a/c number and PIN.  12 MF digits are received
and stored.  An X25 link connects CSPU to the Cashless Services Database
(CSDB), and the incoming PIN is encrypted and compared with the encrypted
PIN on the CSDB stored against the customer's actual record on the
database.  Validation takes about 100ms.  CSPU returns a message to the
caller to enter the required telephone number.  The CSPU stores enough
digits to route the call which is routed back to the DMSU and on through
the PSTN (Public Switched Telephone Network).  The CSPU remains in circuit
throughout the call.

At the end of the call the CSPU sends an itemised billing record (from
where to where, start time, duration etc) to the CSDB which then performs
the billing calculation and passes it to the telephone input billing
system (TIBS).

The article goes on to explain the hardware etc.

Earlier on it says

>> "There are various ways in which an automatic Chargecard service [could]
>> be introduced into the network.  Ideally the software would be built into
>> the software of an intelligent local exchange with account number/PIN
>> validation being carried out in an intelligent network database (INDB).
>> This is considered to be the ultimate solution to the problem of providing
>> the service.  However in 1986 [when the service was envisaged] this
>> solution was not a practical proposition and so an alternative had to be
>> found. [The present system]"

(All information is from the public domain)

Subject: Modems

Q: I use Network Services to bar incoming calls, but when the barring is
in force, the dialtone is different, and hence my modem won't autodial.
Is there a way round this?

With most Hayes compatible modems you can use the ATX command to prevent
the modem from looking for dialtone before it dials.  This solves the
problem of having to worry about it finding the dialtone.

Q: Can you use ordinary dial-up modems over a leased line?

It depends on the modems.  Some require a dial tone and other things to
originate a call.  However most 'quality' modems seem to work.  USR
Couriers over some of our copper pairs (not BT) have been reported as
being successful.

Note that not all BABT Approved modems will have approval that applies for
both PSTN and analogue 'private' circuits.  Also don't get smart and attempt
to use an EPS-8 (four wires) as two EPS-9s (two wires) over long distances.
Finally, if the presentation uses the wrong pins, you can always get an
extension socket kit and wire it up the way you want.

Q: How can I use my analogue cellular phone with a computer modem? 

Spectrum Cellular Corp. offer a product product, AXCELL, which
interfaces with quite a few TACS (Analogue) phones including NEC P3
and OKI 900. The cost of the interface is UKP299, and you basically
get a BT socket with dialtone.

It will not work with car kits (just in case you wanted the extra power
boost) because they use the same socket as the car kit. However, they
reckon they can get a consistent 4800 carrier over an average quality
connection, and the beauty is you could plug in other BT-jack equipment,
including fax machines...

AXCELL is distributed in the UK by Tele-Adapt and they can be reached on
(0181) 421 4444; Fax (0181) 421 5308 (email <> or
<>) Tele-Adapt are a company specialising in
modem connectivity and also offer kits for connecting in various other

Also, the following units have external jacks for faxes, etc.

NEC 9a, P3, Nokia - most models, Motorola - some models. Most
Transportables have adapters that fit between the handset and base
unit that emulate key presses.  Almost all car phones (ie genuine
class 2 portables) can be 'modified' for use with modems.

A recommended phone for data is an Oki model [number, anyone?] which gives
an RJ-11 jack on the bottom of the phone. 

Q: Can I use a modem with GSM/PCN? (otherwise known as "Can I get
free internet access on Mercury One-2-one?") 

The Short answer- No.

Longer answer- A modem is a device for converting digital information
into an analogue wave-form for transmission (normally) over the
telephone network.

GSM networks send all information in digital form, so there is no need
for a modem.

But why wouldn't one work?  Why not simply connect a modem into the
GSM phone's audio circuitry and let it reconvert the analogue signal
into digital form?

The reason is that a very clever speech coding algorithm is used in
GSM.  It has to be clever, in order to squeeze intelligible speech
into 13 kbit/s.  The coding scheme is called Regular Pulse
Excitation-Long Term Prediction (RPE-LTP).  RPE-LTP only works because
speech contains a lot of redundant information, so a lot of the signal
can be thrown away.  Subjecting a non-speech signal (like a modem
tone) to RPE-LTP will result in a distorted mess.

Try listening to music via a GSM phone... pretty unmusical, isn't it?

So in theory it won't work.  In practice, there are unconfirmed
reports of some success by strapping an acoustic coupler to a Mercury
One-2-one phone.  But I stress that these reports are unconfirmed, and
personally I remain sceptical.

[ Someone emailed the following

>> We tried this by direct wiring into a handset! The results were awful! :-)
>> A 300bps/baud connect was possible but with so much noise it just wouldn't
>> hold. The main problem actually seemed to be RF breakthrough from the
>> transmitter but the coding couldn't have helped much! :-) Anyway it's
>> not worth thinking much about.. :-( If you live under a base station so the
>> phone drops to very low power it might just work... Anyway most ISP's
>> won't let you login at less than 2400 :-/

so that seems to clear it up]

Having told you what won't work... what is the answer to what will
work?  What you need is a data adaptor that enables the digital stream
from your PC to be fed directly into the GSM phone.  Such devices are
currently very expensive (eg the Nokia PCMCIA card), but prices will
surely fall.  Equipped with such a device it is possible to send 9600
bit/s error-protected data via a GSM network (if your network offers a
data service).  The network will have an "Interworking Function",
otherwise known as a bank of modems, that will enable you to make
calls to normal PSTN modem-based services.

Q: I'm having a problem with my modem. Intermittently it fails to connect. 

Problems have been experienced with misbehaviour of Modems on Mercury and
not BT and vice versa. The problem can be due to synchronisation errors.
The clocks free-run until a little slice of time disappears as they
resynchronise; that does for V.32bis modems.

Other reasons involve not automatically adjusting levels (auto gain
control- AGC).

If you are **sure** that it is not your modems or set up (and BT via
150/152 doesn't help), try contacting one of the following for more
information Derek Woodroffe (Mercury) on (0116) 265 2068, or try Andy
Lichfield (0171) 356 9324, Mike Jennings (0171) 356 9169, or Mick Eames
(01426) 923607 (last three people BT).  Be NICE to them! They've
successfully helped others in the past.

You may also experience problems on international connections.
Normally, speech channels on international circuits are cut from 64K
to 16K - this is ok for speech: you are unlikely to notice any
degredation. However a fast modem would see a lot of information
disappear. BT do have some equipment installed at Madley
Communications Centre near Hereford (where over 30% of international
traffic is handled) that recognises Fax and Modem traffic and routes
it through non-compressed channels, and the detection of Modems and
Faxes is now full automatic on most (if not all) compressed routes.

Additionally there is a new (V.8) protocol used with V.34 to clearly
announce to the network that this is a V.34 modem call, and hence to
allow it to optimise the call setup.

BT's national network uses 64K channels everywhere so you shouldn't get
any problems added on long distance routing - any noise should just be
from the local loop or an old local exchange.


Subject: Faxes

Q: How do FAX switchers work?

The calling FAX machine starts sending a (broken) tone as soon as it
finishes dialing. The switch answers the line and listens for this
tone. If the tone is there, it routes the call to the FAX. If it isn't it
rings the voice instrument.

Of course by this time the caller is paying for the call so it's good
manners to have an answering machine on the line so that they won't be
left paying to listen to a ringing signal.


Subject: Answering machines

Q: How do I stop my answering machine recording the "Please replace the
handset & Try Again" message when someone hangs up without leaving a

This message can also be "[Tones] The other person has cleared."

Ring BT on 151 or 154 if this happens.  They have a VERY simple way of
turning off that recording so that it doesn't happen again !


Subject: Radio Pagers

Q: Is it possible to use a computer and modem to send messages to UK

There are two numbers, for BT paging services

   0345 010144 (300bd)
   0345 010155 (1200/75)

There may be better speeds available now and you can also use PSS with

You can have both modem and X.25 access to Mercury pagers too (one of the
few areas where Mercury have actually managed to maintain any quality of
service), and also at least modem access for Hutchison and Vodapage,
possibly others.

A more computer oriented interface is implemented using the TAP (Telocator
Access Protocol) service. Both Hutchison and BT use this. TAP has an
American cousin called IXO, TAP is a superset of IXO.

Info from BT paging can be obtained by calling 0800 860860, ask for
Technical Support and request them to send you doc ref PNO/600/0003,
Technical Interface Spec for TAP. Someone reports success at producing a
simple Visual BASIC interface.

The 'tpage2' package, with a few mods for BT specific bits will handle all
the queuing of messages, and management of callout rotas, and the dialogue
with the remote paging centre, etc.

'Sendpage' (<URL:>) runs on
most Unix systems and will send pages via a TAP interface (and SMS


Subject: Cellular/ Mobile phones

Q: What are the new digital phone networks capabilities for data
communications? [ie. Can I use my One2One/Orange phone?]

GSM systems support a single 9600 baud data connection or two simultaneous
4800 baud data connections, theoretically.

You'll be VERY lucky to get a normal modem to work over GSM/PCN voice
channels, the compression etc squishes the signal nicely (and adversely
affects the modem's encoding). You also get problems with the TDMA signals
squirting out the aerial knocking out nearby electrical equipment! (The GSM
set is not the problem, but the nearby equipment isn't RF immune. This
works both ways- if something is vulnerable to RF pickup it is likely to
also be generating RF noise).

For two simultaneous half-rate channels (Lm + Lm) the phone needs half
rate codecs - they don't seem to be available yet.  When they do become
available, you might be able to use speech on one channel while sending
data on the other ... the specs allow for this.

For a single full rate channel (Bm) the specs support data rates at 9600,
4800, 2400 ... but no other simultaneous use.  I.e. using a data-rate of
4800 baud does not leave any spare capacity for use at the same time.

There are modems available for GSM, but are proprietary, use weird
encoding, and are slow ...

The issue here is they all use clever speech-encoding schemes to turn
voice into the lowest possible bit rate. ISDN encodes speech using
conventional PCM into a 64kbps data stream, and CT2 networks squeeze it
tighter into 32kbps using ADPCM. The encoding used on GSM and PCN networks
is smarter still, and codes speech into a 13kbps channel.

The problem with all this super-efficient coding is that it preserves an
analogue speech signal, but wreaks havoc with the analogue signal modems
turn your data into. For example, the phase-encoding used with any modem
faster than 1200bps is highly likely to be lost.  Quite what the limits
are I don't know - I have heard that you can get 2400 V.22bis and possibly
9600 V.32 through a 32kbps ADPCM link, but no chance for 14.4kbps
V.32bis. Information theory clearly constrains you from sending 14.4kbps
through a 13kbps channel as well.  On the other hand, 64kbps ISDN
preserves V.32bis data no problem.  I would be surprised if you can get
more than 2400bps from a GSM/PCN phone.

What is really needed is the mobile equivalent of ISDN access - instead of
turning data into an analogue signal using a modem, and then feeding it
through a codec to turn it into a higher rate digital signal again, you
want to be able to feed data straight into the network, bypassing the
modem and codec. That way you get the full data rate, like ISDN.

A large proportion of the GSM specifications deal with this.  GSM IS 
designed as a sort of mobile equivalent of ISDN.  It is not the
specifications that cause problems, it is the priorities of the marketing
people (which are based on their perception of the priorities of the
customers).  This then influences the implementors, although, in practice,
they have to provide speech, so this gets first priority.

Something to bear in mind is that such access is still circuit- switched,
simulating a telephone call across the air. Radio waves are a broadcast
medium, just like ethernet, however, and there is a strong case that such
media are better suited to datagram traffic rather than
circuit-switched. This would appear to suit Internet access, as IP very
much datagram-oriented. This is where pure mobile data networks come in,
with providers such as RAM, Cognito, Paknet and Hutchison.

Q: What do One2One PCN aerials look like?

They look like normal radio masts, but half way up the mast (not at the top)
there are horizontal arms coming out, with vertical aerials on top of the

Q: How do mobile phones work in tunnels?

Using 'leaky feeders'. Basically run a length of thick coax and cut
sections of the outer shielding off (about 6" to a foot) every now and
again. This is being done in the Channel Tunnel too.

Q: How does the network find the mobile phone?

With TACS (Total Access Communication System) and ETACS (Extended TACS), as
used on Vodafone and Cellnet, when a mobile is switched on it sends a
registration message to a local cell (usually the one with the strongest
signal). Data about where that mobile is can then be sent to (& stored in)
a special database called a "home location register", usually kept at the
Network Control Centre. This database can be interrogated by any switch on
the network that is trying to call that mobile.

When mobiles move from one cell to another, they can detect that the control
channel has changed and then they re-register on the new cell - and the Home
Location Register is updated.

At regular intervals (between 15 minutes and an hour - depending on how busy
the local area is) the network sends out a data message asking all phones to
re-register: this ensures that the Home Location Register is fairly uptodate.
When a call is made to a mobile, calling signals will be sent out to the area
in which the mobile last registered. If that signal isn't acknowledged by the
mobile, the network then tries to find the mobile by calling in all areas.
Obviously if it had to do this on every call, there would be a tremendous
overhead of unnecessary signalling !

Q: How does the network distinguish between 'switched off' and 'not
responding' when giving messages to the caller?

In the GSM system, when the power is switched off, the mobile sends a
detach message to the Mobile Switching Centre (MSC) that you were connected
to, before the power actually disappears. If your mobile is switched on,
but you don't pick it up, a 'not responding' message is given?

When an ETACS phone is turned on it transmits a power-up message to
the network. However when its turned off it **doesn't** transmit a
shutdown message so the network has to try and 'guess' when you've
turned your phone off.

It does this via the periodic 'pages' which it sends to each phone.  If the
network doesn't receive a response to a number of these pages then it
assumes your phone is off.

Q: How do I program my XXX brand of mobile phone?

For the Nokia 101:

 To enter programming mode, key in *60312#12345
 Key in M
 Key in 00
 If it's worked it'll say STORE NOT DONE
 If it hasn't, it'll say NOT ALLOWED

The information is edited and stored in the same way as for normal code
memories - the power-up name is in memory location 02.
(If you intend to change this, write down the information in memory 01-05, in
case you mess things up!)

The telephone number in memory location 02 is your full mobile number...
By using ABC as per normal memory locations, you can enter a power-up name, too.
Store the information back in memory location 02.

Then exit programming mode by turning the 'phone off.


Subject: CT2 (Phonepoint, Hutchison Rabbit, etc)

Q: How do I unlock a new Hutchison Rabbit handset?

Enter the sum of the last four digits of the serial number, padded with
leading zeros (eg, 65234 -> 5+2+3+4 = 0014).

Q: What are the little switches in a rabbit base-station battery compartment

  RESET:       reset on  <-  .  x 3  ->  reset off (normal operation)
  RECALL:   timed break  <-  .  x 2  ->  earth loop
  DIALLING:  LD (pulse)  <-  x  . 1  ->  DTMF (tone)

Q: Where can I get batters for a Rabbit handset from?

Video Plus
70 Ballards Lane,
Finchley Central
N3 2BU
Tel: 0181 349 4242

UKP3.50 each, plus 1 pound postage

Also, try Varta, (tech support: 01460 733 66 ask for W. Wells).  For
small numbers they suggest Powerpacks: (0121 711 3360) who supply
them at UKP3.38 + VAT each. {*}

Subject: Telephone exchanges

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

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

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...

 A replacement for both 174 and 175 has also been mentioned, which is

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."

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

        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).


Subject: International calls

Q: How do I get a guaranteed non-satellite circuit to the USA ?

To get a guaranteed non-satellite circuit to the USA, useful for certain
data transmission requirements which involve a lot of handshaking, dial
00 1 83 + area code + number.

The code 84 allows you to obtain a satellite link, if you really want one
for any reason.

Q: Is non-satellite link to the US helpful when using transfer protocols
such as XModem?

Simple answer. Yes.

Xmodem, YModem, and (old versions of) Kermit wait for acknowledgement
after sending a block. When using satellite links the lag between end of
block and acknowledgement appearing can be up to a second. With small
block sizes (XModem uses 128 byte blocks; Kermit 96 byte blocks) this can
impact on your throughput. New releases of Kermit permit sliding windows
and the use of long (eg 2kB) packets these days. This is specifically
designed for slow ack links.

Using a streaming protocol, like ZModem, gets around this problem.  ZModem
only sends acknowledgements back if there is an error, or at the end of a


Subject: Phone numbers/ DTMF 

Q: What are the frequencies for DTMF (Dual-tone multi-frequency) digits?

           1209 Hz   1336 Hz   1477 Hz   1633 Hz

 697 Hz       1        2         3         F0/A
 770 Hz       4        5         6         F/B
 852 Hz       7        8         9         I/C
 941 Hz       *        0         #         P/D

The right hand column (obviously) is not normally available but has
certain applications. The functions are labelled either F0, F, I, P or A,
B, C, D.

Q: How can I identify a 'phone number from a recording of the DTMF digits?

IF you have Star services (it works on system X)

- dial the number, hangup
- dial *52#
  you'll hear "telephone number 123456 is stored"
- *54# will dial this number

This is an undocumented feature that has been around for at least 4 years.

[end of uk.telecom FAQ part 3/3]

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