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       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 electronic, 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
 centres 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 accessed 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 order to
 cope.