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.)

Diagram of a DLE and links

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:

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.

Diagram of a tandem network

We can see that there are four kinds of call:

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.

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).


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:

Diagram showing OLO interconnections

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
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).

Diagram showing DLEs and DSLAMs

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

Back Back to the telephones index. CDWF Back to Clive's home page.