The M.E.D's intermediate guide to British Telecommunications
         \//ritten, Typed & Conceived by iNFERNO.. 24^11^95


The oldest telephone services that need concern any phreak are the ALSUs, 
ALSU stands for Analogue Local Switching Unit, the exchange types these 
include are; 

 1. Strowger (TXS) 
2. Crossbar (TXK1) 
3. Electronic (TXE2,TXE4) 

 The first two, Strowger and Crossbar, are extremely rare in the modern
network, although deviations of them may exist (i think Hatch End in London
runs on a TXK3 which is a slightly modernised crossbar system). The
electronic exchanges, TXEx, are fairly common, TXE4 more so than the TXE2
because it can enhanced to provide digital exchange customer facilitesand
work in union with digital exchanges.

 4. Digital (System X, AXE10, 5ESS, UXD5, DMS) 

 The most common digital exchanges are System X and AXE10 (also knownas
SYSTEM Y). The other three are used in more specific situations for
specific facilites. A digital system is basically totally trnic, which
means there are no moving parts and the infomation is transferred digitally
(when you speak into the phone, you voice is processed into binary form at
the exchange, and coverted back to analogue at the other end, which gives a
very good quality sound and reduces the need for repeater stations) The
older TXS and TXK1 systems work upon a mechanical system, which are purely
analogue, whilst their brother electronic analogue TXE systems,are
obviously, electronic in their function, although, as I mentioned before,
they can be enhanced, not to offer a digital service but the same sortof
facilites that a digital exchange may give. I will now go into more detail
with some of the exchanges :

 -[* The Electronic TXEs *]------------------------------------------------- 

 Types of TXE ; TXE - Telephone Exchange - Electronic 
TXE4 - Telephone Exchange - Electronic (4) 
TXE4A - TXE4 - Cost reduced 
TXEE - TXE Enhancement 

 TXE4s cannot support a high bit rate (for example, high-speed fax cannot
be used across a TXE4), neither do they support ISDN functions, for these
reasons, British Telecom are quickly updating all TXE4s. TXEx use a system
called COMFORTE, which stands for Computers For Telephone Exchanges, this
centers around an exchange processor which stores infomation and commands
parts of the TXE switch. It runs on a ICL DRS300 mircocomputer situated
inside the exchange. It is connected to a local-area network in the
building (so it can be access from any computer in there), the Public
Telephone Network and the Packet Switched Network. I could go into great
detail about how this system works, but it's not too relavant to this
phile.

 -[* The DiGITAL Systems *]------------------------------------------------- 

 Types of digital system; SYS X - System X, manufactured by GEC/Plessy. 
AXE10 - System Y, manufactured by Ericsson's of Sweden in Britain.
5ESS - An AT&T/Phillips system, used for the Digital Derived Services
Network (which control the 0800,0345,0898 etc numbers) and as an
international exchange.
UXD5 - Small digital exchange used in rural areas, it's manufactured by GPC
and is derived from the `MONARCH' PABX.

 A simple theoretical digital system only needs; 

1. Customer Access - Connects any customer to the exchange,
2. Junction Access - Connects any junction to the exchange, 
3. Switch - Connects any combination of customers and junctions together,
4. Control - Ensures the correct operation of all the above. 

 System X systems are built up from a number of modules, called subsystems. 
This is refered to as `Modular Design'. Each subsystem carries a particular 
function and is connected to the other subsystems. Modules can be upgraded 
individually without the need to upgrade the rest of the system in orderto 
cope.