The BT Network

BT still provide the majority of telephone services in the UK, so the design
of their network is of interest to all. This page describes the basic topology
of the network.

All details on this page are based on data from October
2000.

Local exchanges

The telephone line from a subscriber runs underground or overhead on poles to
the local BT building. While this building is often known as "the exchange", in
actual fact it might well not be. All the lines in an area terminate on a large
patchboard known as the Main Distribution Frame (MDF). A second set of
wires run from the MDF to the switching unit in the building. For the majority
of lines this unit is a Remote Concentrator Unit (RCU), while for the
remainder it is the Digital Local Exchange (DLE). (Between a third and a
quarter of lines connect directly to a DLE.)

An RCU is a simple switch that runs under the control of a DLE - each RCU
belongs to exactly one DLE. It will have, on average, one connection to the DLE
for every 4 subscriber lines, so an RCU serving 6000 customers will have about
1500 lines connecting it to the DLE. The RCU can connect calls between two
subscribers, or it can connect a subscriber line to a DLE link. There are 6941
RCUs of three different types: 4339 Marconi System Xs, 2137 Ericsson AXE10s, and
465 UXD5s. Most RCUs are located in villages or in small towns, but a number are
in the same building as the DLE that controls them.

There are 770 DLEs around the UK, consisting of 559 System Xs and 211 AXE10s
(note that System Xs and AXE10s perform a number of different rôles in the
network). Each DLE has its own subscribers and also controls one or more RCUs,
for a total of 30 to 40 thousand lines (a few DLEs have no direct subscribers
and only control RCUs). The DLE controls all calls made between those
subscribers, as well as taking part in calls to and from elsewhere. DLEs are not
spread evenly across the country - indeed there are 149 in London but only 21 in
the whole of Northern Ireland. Rather, they are distributed to match the density
of lines. A building in a major town may well have two or three DLEs, each with
its own cluster of RCUs. These will not necessarily divide the locality
geographically - it's possible for adjacent villages to be on different DLEs
while those far apart may be on the same one. Similarly, there is not
necessarily a connection between DLE and dialling code: all four of the DLEs in
Cambridge have both 01223 and 01954 numbers on them, and at least one has 01353,
01440, 01553, 01638, and 01842 numbers as well.

Technology note

A System X switch is a unit that has a MDF and trunks to other switches.
Therefore an RCU is a switch that only connects to its DLE, while a DLE is a
switch that connects to many other switches, including RCUs. This means that
calls from an RCU involve an extra switching stage compared with lines directly
connected to the DLE.

With the AXE10, on the other hand, the MDF is connected to a separate
switching unit called the SSS. The SSS can be either physically attached to the
switch (in which case it is called a CSS) or can be sited remotely (in which
case it is called an RSS). In both cases calls within the SSS can be switched
directly or at the DLE. Thus all calls are switched in the same way whether or
not the MDF is attached to the DLE or an RSS.

The DM or Tandem network

Where a call is being made to a subscriber on a different DLE from the
caller, it needs to travel over some kind of link between DLEs. In theory all
DLEs could be connected to one another, but this would be wasteful of links
(there would be over a quarter of a million such links). Alternatively calls
could be relayed from DLE to DLE (as was done in the days of manual operators),
but this would take up a lot of the processing power of the DLEs.

Instead there is a second network of switches, known as the DM Network
or the Tandem Network. This network is made up of four kinds of
switch:

76 Digital Main Switching Units (DMSUs). These are the original
long-distance exchanges; they are System X switches with the capacity to
handle 30,000 simultaneous calls each. Every DMSU is connected to all the
other DMSUs; obviously this involves a much smaller number of links than would
be required to link together the DLEs.

16 Wide Area Tandems (WATs). A WAT is used to connect calls within
a region, rather than longer-distance calls. While WATs are connected to other
switches in the DM network, this links are less important. In particular, a
WAT is not connected to all the DMSUs or even all the other WATs.

13 Digital Junction Switching Units (DJSUs). These are additional
switches in the London area that allow traffic between London DLEs to be
routed without taking up the capacity of the DMSUs and WATs.

63 Next Generation Switches (NGSs). These are newer switches that
are gradually replacing the DMSUs and WATs, many of which have now reached the
limits of their capacity. An NGS is a modified AXE10 capable of handling
either 60,000 or 120,000 simultaneous calls (depending on
configuration).

Note that the term tandem switch is often used to refer to all four
kinds. A full list
of tandem switches is at the bottom of this page.

As well as all of these, there are also a certain number of inter-DLE links,
particularly between DLEs in the same physical building.

In the following diagram, the lettered circles represent DLEs (and their
associated RCUs), the numbered squares tandem switches, and the triangle a DJSU.
The black lines show the tandems all connected to one another, the blue lines
are DLE-DM links, and the pink lines inter-DLE and DLE-DJSU links.

We can see that there are four kinds of call:

Own exchange calls, where the call remains within a single DLE;

Linked exchange calls that go over an inter-DLE link, such as from
B to C (these two categories are together known as Local exchange
calls);

Single tandem calls, that go DLE-tandem-DLE, such as from A to B;

Double tandem calls such as from A to E.

In fact, a call is normally described in terms of the most optimal route, so
B to C is linked exchange even if the call doesn't go over the link, A to B is
single tandem even if tandem 1 has failed and the call is going via tandems 2
and 3, and A to E is double tandem even if congestion means the call goes
A-1-2-3-4-E.

Each switch (both DLE and tandem) will have a routeing table giving up
to four routes, in order of preference, for each possible call. The tables for
DLE C and for tandem 4 might look like this.

DLE "C"

A

Single tandem

2

3

B

Local exchange

Direct

3

2

D

Single tandem

DJSU

2

3

E

Single tandem

DJSU

3

2

F

Double tandem

3

2

G

Double tandem

2

3

H

Double tandem

3

2

J

Double tandem

3

2

K

Single tandem

2

3

L

Double tandem

3

2

DMSU "4"

A

1

2

6

3

B

3

1

5

6

C

2

3

6

1

D

Direct

2

3

5

E

Direct

3

1

2

F

Direct

5

2

6

G

5

6

1

3

H

6

5

3

2

J

5

6

1

3

K

6

2

5

1

L

Direct

1

2

5

The links in roman are direct links, while those in italics will
involve an additional tandem in the call. In each case the apparently random
order is designed to even out the load overall (for example, the second
preferences for DLE C split equally between the two tandems).

Interconnections

Of course, BT is not the only telephone company in the UK. There are a range
of Other Licensed Operators (OLOs) who also carry telephone calls. In
order that calls can be transferred from BT to the OLO or vice versa, it is
obviously necessary for the two networks to be connected. This is done at
Points of Interconnection (POIs). A POI can be at any tandem switch or,
in some circumstances, at a DLE.

An OLO is not required to connect to every tandem switch. Instead, BT will
route calls through the network to a convenient POI. While payment arrangements
vary, the most common situation is that BT charges the OLO a transit fee for the
call. This fee depends on the length of the call, the time of day, and the
amount of the BT network that the call uses. For this purpose the most efficient
path through the network is selected and the call is divided into one of a few
classes:

connected at the appropriate DLE (inter-DLE links are not used);

single tandem;

double tandem, further divided into "short", "medium", or "long"; every
possible pair of tandems that could process the call is examined, and the
shortest distance is used to determine the classification.

Here we see an OLO with three POIs, two at tandems and one at a DLE. Each DLE
and tandem will have entries for the OLO in its routeing tables, just as with
calls to other DLEs. In this example, the calls would be classified as
follows:

A

Single Tandem

B

DLE

C

Single Tandem

D

Single Tandem

E

Double Tandem

F

Single Tandem

G

Single Tandem

H

Single Tandem

J

Single Tandem

K

Single Tandem

L

Double Tandem

Note, of course, that while the OLO is shown as a single entity in this
diagram, in practice they will probably have their own network of switches.

Local Loop Unbundling and ADSL

Recently Local Loop Unbundling (LLU) and Digital Subscriber
Line (DSL) have become important topics. LLU is an arrangement whereby an
OLO rents the subscriber's line from BT and connects it to their own switch. To
do this, the corresponding link from the MDF to the switch is disconnected and a
new link is made to the OLO's equipment (which may be in space in the exchange
building or may be in another building nearby). It is also possible to
half-unbundle, with the switch continuing to provide voice service over the line
but DSL services being provided by the OLO.

DSL is an arrangement whereby high-speed digital data can be sent down the
link between the subscriber and the exchange building, without affecting the use
of the line for voice. At the exchange end the line is connected to a device
called a DSL Access Multiplexer (DSLAM), using a link from the MDF.
Again, the DSLAM may be in the exchange building or may be in another building
nearby. The DSLAM then connects to the ISP providing service, usually over a
dedicated data link. Note that each ISP requires its own DSLAM at each exchange
building (or has to agree use of another's).

Here we see two DSLAMs for an ISP; the corresponding arrangements for
multiple ISPs or for LLU should be obvious.

Special switches

Finally, there are a number of special switches. The most important of these
are the 26 Advanced Services Units (ASUs) which are Nortel DMS10 (or
DMS100; sources disagree) switches used to implement things like FeatureNet.
These are allocated various number ranges that don't necessarily match their
physical location.

Until recently there was also a mysterious device known as the Colindale MDX,
which may have been a prototype for the NGSs, and "London Front Office", which
handled the 0171 555 XXXX number block.

Lists of switches

Each tandem switch has a name, which usually includes the location of the
switch. In addition, the DMSUs and WATs have names based on local themes, while
the NGSs are named after minerals, precious metals, and precious and
semi-precious stones, and the ASUs after mythological and astronomical
features.

The 76 DMSUs are:

Aberdeen Gordon

Birmingham Cadbury

Birmingham Chamberlain

Birmingham City

Birmingham Mercia

Bishops Stortford Waytemore

Bradford Priestley

Bristol Brunel

Bristol Concorde

Cambridge Kings

Cardiff Taffmead

Carlisle Solway

Chelmsford Tindall

Chester Foregate

Clyde Valley Carrick

Coventry Spires

Crawley Newtown A

Crawley Newtown B

Darlington Stead

Edinburgh Bruce

Edinburgh Saltire

Exeter Buller

Glasgow Kelvin

Glasgow Wallace

Gloucester Civic

Guildford Friary

Ipswich Orwell

Leeds Aire

Leeds Westgate

Leicester Fosse

Leicester Lambert

Liverpool Crystal

London Balfour (Ilford)

London Baynard (City)

London Compton (Colindale)

London Cranmer (Croydon)

London Cromwell (Kingston)

London Gladstone (Colindale)

London Grenville (Eltham)

London North Paddington

London Palmerston (Wood Green)

London Rosebery (Ilford)

London Southbank A (Southwark)

London Southbank B (Southwark)

London Tower A (West End)

London Tower C (West End)

London Walpole (Ealing)

Luton Hatter

Maidstone Courtenay

Manchester Bradley

Manchester Halle

Manchester Lowry

Milton Keynes Phoenix

Newcastle Hadrian

Norwich Wensum

Nottingham Arrow

Oxford Isis

Peterborough Cathedral

Plymouth Drake

Portsmouth Nelson

Preston Guild

Reading Forbury

Reading Kendrick

Salisbury Sarum

Sheffield Charter

Sheffield Sheaf

Shrewsbury Darwin

Slough Upton

Southampton Bargate

Stoke Bennett

Swansea Kilvey

Swindon Gooch

Tunbridge Wells Nash

Warrington Wire

Wolverhampton Express

York Ebor

Note: London Southbank A and B are recently converted from DJSUs. London
Tower C has recently been renamed London Tower DXC, and may be a new kind of
switch.

The 16 WATs are:

Belfast Cromac

Bolton Crompton

Cambridge Trinity

Gloucester Dean

Leeds Tetley

Liverpool Albany

Liverpool Cavern

London Croydon Fairfield

London Ealing Questor

London Eltham Hope

London Kingston Globe

London Wood Green Empire

Luton Wyvern

Newcastle Federation

Reading White Knights

Tunbridge Wells Tyler

The 13 DJSUs (all in London) are:

Baynard C (City of London)

Colindale A

Croydon A and B

Ealing A and B

Eltham A

Ilford A

Kingston A

North Paddington A

Tower A (West End)

Wood Green A and B

The 63 NGSs are:

Birmingham Azurite

Birmingham Jade

Birmingham Silver

Bishops Stortford Lapis

Bradford Tektites

Bristol Bloodstone

Bristol Zircon

Cambridge Celestine

Cardiff Iolite

Chelmsford Citrine

Chester Danburite

Clyde Valley Moissanite

Coventry Agate

Crawley Peridot

Darlington Rhodonite

Edinburgh Ruby

Exeter Gold

Glasgow Emerald

Gloucester Coral

Guildford Almandine

Ipswich Morganite

Leeds Amber

Leeds Turquoise

Leicester Datolite

Leicester Opal

Liverpool Gypsum

London Albite (Central)

London Argonite (South)

London Axinite (North)

London Bixbite (Central)

London Carnelian (South)

London Diamond (East)

London Feldspar (South)

London Garnet (South)

London Pearl (South)

London Pyrope (West)

London Quartz (East)

London Rutile (North)

London Topaz (Central)

Luton Heliotrope

Maidstone Scheelite

Manchester Jasper

Manchester Millennium

Manchester Taylor

Milton Keynes Kyanite

Newcastle Sard

Norwich Serpentine

Nottingham Calcite

Oxford Dravite

Peterborough Lazulite

Portsmouth Ivory

Preston Pyrite

Reading Amethyst

Reading Moonstone

Salisbury Spinel

Sheffield Sapphire

Slough Corundum

Southampton Shell

Stoke Beryl

Swindon Sodalite

Tunbridge Wells Jacinth

Warrington Helidore

Wolverhampton Baryte

The 26 ASUs are:

Birmingham Leda

Bristol Helene

Cambridge Quasar

Clyde Valley Iona

Coventry Ceres

Crawley Zeta

Edinburgh Rhea

Gloucester Galaxy

Guildford Venus

Irving Neptune

Leeds Atlas

Leicester Pluto

London Dione (East)

London Frey (East)

London Jupiter (City)

London Krypton (South)

London Miranda (North)

London Saturn (City)

London Titan (West)

London Ursa (North)

London Xenon (South)

Manchester Elara

Newcastle Bianca

Oxford Oberon

Sheffield Yogan

Warrington Woden

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