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.