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February 03, 2010
Understanding the BT Tandem Network in the UK
In the United Kingdom calls are routed through 4 different types of tandem switches that exist on what is known as the DM Network or the Tandem Network. What kind of tandem switch depends on where the call originates and terminates. Unlike the CLLI codes that are prevalent in North America, the tandem switches of the UK are named after their location and an identifier of some significance.
Digital Main Switching Units (DMSUs)
There are currently 76 DMSUs in use. These are the original long distance exchanges and each DMSU has the capacity to handle 30,000 calls at any one time. Every DMSU has a connection to every other DMSU within the network.
DMSUs are named after their location as well as a local theme. Examples of DMSU names include Edinburgh Bruce (named after the infamous king of Scotland Robert Bruce), Nottingham Arrow (an acknowledgment of the Robin Hood legend) and Shrewsbury Darwin (Shrewsbury was the birth place of Charles Darwin).
Wide Area Tandems (WATs)
There are currently 16 WATs in use in the UK. As opposed to the long distance calls routed through the DMSUs, the WATs are used to connect calls within a particular region. While a WAT might be connected to the DM Network (and DMSUs) via a connection or two, they are not connected to all of the DMSUs or even all of the WATs. They route their calls from one Digital Local Exchange (DLE) to another DLE within the same region.
Similar to DMSUs, the WATs are named after a local theme. For instance one of the WATs is named Reading White Knights, after White Knights Park - home to the University of Reading (the park itself was named after 13th century knight John De Erleigh IV who was otherwise known as the “White Knight”) and the WAT in Liverpool is named the Liverpool Cavern after the famous Cavern Club where Brian Epstein first heard the Beatles play.
Digital Junction Switching Units (DJSUs)
There are 13 DSJUs in use. DSJUs are additional switches within the area of London that allow regional London traffic to be routed from one DLE to another. This saves on the capacity of the DMSU and WAT network. Essentially DSJUs are a tandem network within a tandem network that just serves the London area.
Unfortunately, DJSUs are named only after their location within London, so their names are a little less evocative – such as North Paddington A.
Next Generation Switches (NGSs)
There are currently 63 NGSs being used within the DM Network. These are newer digital switches capable of routing both regional and long distance calls and they are being gradually deployed to replace the aging DMSUs and WATs that have reached their capacity and usefulness. Depending on how it is configured an NGS can handle anywhere from 60,000 to 120,000 simultaneous calls.
NGSs are named after minerals, precious metals, stones, etc. Examples of NGS names are Leicester Opal, London Topaz and Edinburgh Ruby.
Advanced Services Units (ASUs)
In addition to the four types of tandem switches listed above, there is another set of switches used to handle additional services like FeatureNet (a BT product that offers a portfolio of VPN services). These switches are known as ASUs and there are currently 26 of them in use.
ASUs are named after mythological creatures or astronomical features. Examples of ASU names are Leeds Atlas, Leicester Pluto and Guildford Venus.
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