Meeting Pearls 1/Meeting Pearls Vol 1 (1994).iso/installed_progs/text/faqs/uk-telecom.part3

Internet Message Format | 1994-05-09 | 26.2 KB

Subject: uk.telecom FAQ, Part 3/3 - Technical matters
Newsgroups: uk.telecom,news.answers
From: [e-mail] (James R Grinter)
Date: Sat, 7 May 1994 23:00:00 +0000

Archive-name: uk-telecom/part3
Posting-Frequency: monthly
Last-modified: Time-stamp: <1994/05/06 12:34:49 james>
Version: 2.1

          Frequently Asked Questions for uk.telecom

              Part 3 - Technical matters

               Original FAQ Compiled by
         Christopher Samuel <[e-mail]>

   Now Compiled/ Posted by James Grinter <[e-mail]>

~See <[e-mail]> 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 how.phones.work in the comp.dcom.telecom
archives at lcs.mit.edu [18.26.0.36].  It is US orientated, but some of
the general principles remain the same.

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

----

Subject: BABT

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

The British Approvals Board for Telecommunications.

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

----

Subject: PSTN

(Public Switched Telephone Network)

Q: How can I simulate the PSTN?

Black Box (0734 866800) sell a 'FREELINK line simulator' which should do
what you want.  It costs 123 pounds.

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 UKP150.

----

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', 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).

Some BT exchanges offer ISDN (to find out whether _your_ exchange can
offer it, you need to talk to their sales droids again), and it works
out rather more expensive by way of line rental than does an ordinary
line, but call rates (apart from special offers) are the same.  At
least some other carriers also offer ISDN - try asking those that are
to be found in your area (for example, I believe Cambridge Cable do
round here).  Note that an ISDN connection to Mercury via 131 (even if
Mercury have ISDN capability) is probably a no-hoper. Mercury now do
ISDN access via 132. It needs to be signed for seperately from the
normal Mercury service.


Q: What are the costs of ISDN connection and calls?

For ISDN2 (Basic Rate Access), the connection charge is UKP400 and the
quarterly rental is UKP84.  Call charges are the same as the normal PSTN.


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 Sun over this ?

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
link.

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, (0635 871636) or try [e-mail].  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 [e-mail], Tel: [phone], Fax: [phone]

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

Sparc 10s and LXs has 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 released.

----

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.

However, if you have a PABX, you may be able to do this anyway using the
conference call facility.

----

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.  Apparently
lines are now tested every hour or so [complete rumour/ urban legend?].
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.

My friendly engineer told me that when BT get complaints about this, they
just tell the customer 'We are just testing your phone line automatically
to ensure continued good service, etc..' then follow with 'Your equipment
is out of date, how about us coming round so that we can offer you an
improved service, it will cost you... etc.'


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).

----

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 '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 cellular phone with a computer modem?

Spectrum Cellular Corp. offer a product product, AXCELL, which interfaces
with quite a few 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 Teleadapt and they can be reached on
081 421 4444 [or try 081 429 0479 - two different numbers have been
posted]. (email <[e-mail]>)

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.

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


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).

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 0533-652068, or try Andy Lichfield 071 356
9324, Mike Jennings 071 356 9169, or Mick Eames 0426 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 will see a lot of information disappear. BT do have some
equipment installed at Madley Communications Centre in Wembley (where over
30% of international traffic is handled) that recognises Fax and Modem
traffic and routes it through non-compressed channels, but this Israeli
kit is apparently the only such stuff in the UK.

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
message ?

Ring BT on 150 or 152 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
pagers?

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
23422220102800.

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.

----

Subject: Cellular/ Mobile phones

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

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.

At present, I don't think you can buy digital mobile phones with data
sockets to let you do this. Nor am I aware of ISDN standards for these
nets.  There is also the issue how you get the data across the non-mobile
part of the net, and out the other end if it is not the same kind of
service.  I have heard various promises of such services, though, and
think Cellnet have some prototypes.

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
poles.


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.

----

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
for?

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

----

Subject: Telephone exchanges

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

The types are:

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

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.

Dial *#001#. If you get "No services 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.

At 31st March 1992, all traffic on the trunk network was handled
by 63 Digital Main Switching Units (4 in London). The last analogue trunk
exchange was taken out of service in June 1990. Four international
exchanges are in operation. Over 80% of customers were 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 ?

~Quoting Raymond Okonski (known in the business as "Buzby")~

Post Office Telephones designed a modular, digital telephone system with
Plessey.  Other manufacturers complained that it was unfair a monopoly was
abusing its power by precluding other suppliers.  A complaint was made to
the Government and they agreed - at least TWO suppliers to the PO/BT would
be required.  Ericsson's made the AXE10 which offered practically the same
flexibility in a smaller box - mainly because System X was designed by a
committee, it was already old before the first release model was
available.  The AXE10 was renamed by BT to 'System Y' just to keep things
'simple'.

If anything the AXE10 is more fully featured than the 'X' but BT only took
the basic software package one each, so both offer practically the same.
Incidentally, Cellnet tried to use System X as their mobile switched but
dumped it as it was very old tech.  Vodafone tried Ericsson, and that is
why Vodafone runs totally on AXE10's!


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.

[This is the fullest list available so far - Ed.]

System X Subscribers Apparatus Line Test (SALT) facility

 DIGIT 1 (Dial Test)     - Expects "123456789*0#" to be dialled
 DIGIT 3 (CCB Test)      - Sends 'open slots' + pay tone to line under test
               Expects coin insertion indication from line
               (Measures coin insertion signal characteristics
                and checks result)
               Sends coin pulse ACK signals to line
 DIGIT 4 (SPM Test)      - If SPM is provided on the line
               50Hz SPM
                 5 long SPM pulses sent to line followed
                 by NU tone. 1 second timeout then 5 short
                 SPM pulses.
               16KHz
                 10 pulses of nominal characteristics sent to
                 line without burst on NU tone

 DIGIT A (MF4 extra)     - Expects "ABCD" to be dialled.


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.  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
0101 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
file.

----

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]