STATION ID - 7047/3.12

9x Datakit Network
FOR OFFICIAL USE ONLY

This is a 9x system, restricted to authorized persons and for
official 9x business only. Anyone using this system, network or data
is subject to being monitored at any time for system administration and
for identifying unauthorized users or system misuse. Anyone using this
system expressly consents to such monitoring and is advised that any
evidence of criminal activity revealed through such monitoring may be
provided to law enforcement for prosecution.


          Local UK Digital Switching Systems and Line Tests
                                          by gr1p <gr1p@linenoise.org>

In the late 1990s in the UK there are really only two types of Switching
systems that are used at local level, however in extreem rural areas, such
as the Remote Scottish Highlands, the old Electronic Switches may sitll be
in use. These Electronic switches (TXEs) which were used in the past
throughout the whole country have been slowly replaced by the newer
digital systems which at one time were only installed in the major cities
where they catered for the increasingly large call volume both from
business and residential lines very efficiently.  There are two types of
Digital Systems which are used at local level to deal with the
majority of calls, System X and the newer variant, System Y which
is based on AXE10 style technology. There are however other Digital
Systems that are used to control other networks which aren't necessarily
on a local level such as 5ESS which is used for the DDSN (Digitally
Derived Service Network) which controls the 0800/0500/0891 service
networks, the freefone numbers, premium rate numbers etc.  0800 and 0500
numbers are the equivalent of US 800/888 numbers, and 0891 numbers are the
equivalent of US 900 numbers.

The Basic principles of working within the Digital Systems are the same
between the two due to the fact that System Y was based on the original
System X.  System X was set to be installed over 100% of the UK as the
prime local digital switching system until this was deamed to be unfair
due to the fact that one company would have a total hold over the
Switching Market and not allow any other companies to compete in the
market so a company called Ericsson produced AXE, a rival digital fone
exchange system, the AXE10 class system was chosen by BT to be installed
throughout the main cities in the UK and this forms the local BT System Y
digital switching system.  System X technology was soon outdated after
release due to the fact that it was designed and produced by a
consultive committee who were slow at releasing its first model and by
this time AXE10 had been designed and unveiled and it saw a vast 
technological improvement on System X incorporating more digitally
advanced extras such a improved CLASS Technology (essentially Caller
ID technology) while at the same time keeping the fundamental backbone on
which SystemX is based.

The detailed workings within the system, whether X or Y, are seperated
into particular modules, with each module dealing with a specific function
at a certain stage of the switching process.  It all starts on a local
level with smaller modules within the exchange process performing tasks
which make up the larger modules within the system ensuring that the whole
process runs smoothly.

A lot of processes make up for the local level of the system, which is
really the most important part of the system.  The lowest level is where
the meeting point for the lines is formed, these meeting places are called
Remote Concentrator Units (RCUs), these are where a very large number of
lines meet and basically form your locally telco depot.  The step up from
RCUs are the Digital Local Exchanges (DLEs) which host all the RCUs in the
area and form for a much wider distribution area, say, over the area of
quite a few miles.  The wide distribution of these DLEs into smaller
areas, with the use of RCUs makes for a much more organised and proficient
system of switching and exchange.

The higher level switching of calls is dealt with by the Digital Switching
Unit (DSU) which deals with a high volume of calls in an area, for
example, the size of a large city.  The Digital Switching Units then
distibution the switching of calls to the smaller Digital Main Switching
Unit (DMSU) which controls the switching of calls on a smaller level and
delegates calls even further down the scale to the Digital Local Switching
Units (DLSUs) which forms the basis of call switching by making sure it
reaches its destination by setting the call along to the right connections
and Subsystem.

A few important factors effect whether a call is switched to its
destination perfectly.  The main factor relies on the Call Processing
Subsystem (CPS) which takes overall control over a local line, registering
its state whether engaged, free or otherwise out of order.  This function
is very sophisticated and a fundamental part of the digital exchange as it
is needed to ensure that harmony is achieved among the fonelines.  The
next needed function, or module, is Common Channel Signalling (CCS) which
is the process British Telecom (BT) use to reserve a speech channel for
switching and to enable the lines to coherantly exist without cross-overs
etc, this is therefore the standard form of signalling on any digital
system and is used successfully in countries that make use of a digital
system of switching.  Although BT have produced a method of packet cutting
for signalling the voice along the channels to ensure perfect arrival.
They use a method called Pulse Code Modulation (PCM) to cut down voice
into smaller packets sending them down the channels, several calls are cut
at the sent time and sent seperatly without confusion, as the modulation
is programmed to be sensitive enough to control the lines and the
differences between each without any problems.

When all the modules that make up the Digital System are combined and put
into functional order they work in harmony automatically with no problems
and with great efficiency.  If one module fails, the whole process fails,
each module relies on another and they all deam equal importance in the
system.

There is a very easy way to find out which digital fone exchange type 
your area falls under.  Just go to your home fone and tap in the following
code.

*#001#

If you get the following answer:-

'No services in operation on this line' or a 'List of Services' then your  
local digital exchange is SYSTEM X.

If you get the answer:-

'Sorry, you have dialled an invalid service c0de' then your local digital  
exchange type is SYSTEM Y.

This System of Digital Switching is the main system which is installed in
the UK today for local use.  5ESS is implemented, but has not been
discussed in this file as it is implemented on the Digitally Derived
Service Network which will be discussed at a later date.
 
[eof]

