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 facilites and 
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 known as 
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 sort of 
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 

 -[* 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 order to 

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