_     _  _                 _      _            _
   __  __  / /   / /(_)_ __   ___  /\ \ \___ (_)___  ___  \ \ ___  __
   \ \/ (_) |   / / | | '_ \ / _ \/  \/ / _ \| / __|/ _ \  | (_) \/ /
  _ >  < _| |  / /__| | | | |  __/ /\  / (_) | \__ \  __/  | |_ >  < _
 (_)_/\_(_) |  \____/_|_| |_|\___\_\ \/ \___/|_|___/\___|  | (_)_/\_(_)
           \_\                                            /_/

               ++++++++++++++++++++++++++++++++++++++++++
               ++++++Local Digital Switching Systems+++++
               ++++++++++++++++++++++++++++++++++++++++++
               +++++++++++++JF@linenoise.org+++++++++++++
               ++++++++++++++++++++++++++++++++++++++++++

In the late 1990s in the UK there are really only two types of Switching
systems that are used.  Both are Digital systems which are installed over
the majority of the country.  The Electronic switches (TXEs) which were
used in the passt throughout the country have been slowly replaced by the
newer digital systems which at one time were only installed in the major
cities.  There are two types of Digital Systems which are used at
residential level to deal with the majority of calls, System X and System 
Y which is based on AXE10.  There are however other Digital Systems that are
used to control other networks 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.

System X was the first Digital System to be used in the UK in the larger
cities and was suppose to be installed 100% as the premium Digital System
for the UK but this it deamed unfair that one digital system be able to
take over the market and be installed fully that other companies were
enabled to step in.  One notable company that stepped in was the Swedish
company, Ericsson.  Ericsson had designed and developed a digital system
of switching that was based on System X but highly improved and greatly
technologically advanced.  This was mainly due to the fact that System X
was designed by an Independant committee rather than a company and the
committee were rather slow at producing and releasing their final version
of the switch, which by the time it had been released was already outdated
by AXE10 which had become known as the System Y Digital System. 

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.  The detailed workings within the system are seperated into
particular modules, with each module dealing with a specific function and
a certain stage of the switching process.  It all starts on a local level
with smaller digital exchanges performing tasks which make up the modules
within the system to make sure the processes run 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.

I hope this text has enabled you to grasp the concepts of the UK Digital
Systems of Switching and Signalling at the local level.  More texts can be
found on the Linenoise website addressing other issues within the UK fone
system.

JF
JF@linenoise.org
http://www.linenoise.org