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The DTN was conceived by the Post Office in 1938, initially to provide the telegraph requirements of the Air Ministry and Admiralty. Its size was colossal, and it was intended to be equal in capacity to the existing civil network. However, by the end of 1944 it had trebled in size.
The backbone of the network consisted of five large switching centres discreetly located around the country. Smaller switchboards and teleprinter terminals were spread around the entire British Isles at military and government establishments. Very often the equipment was installed in hastily erected Nissen Huts. In London and provincial cities, the equipment was located in deep-level accommodation or heavily-reinforced buildings, known as 'citadels'.
Mobile telegraph equipment units were built into vehicles to allow the urgent provision of service in the advance of permanent installations. These units were also used as a backup in the event of air raid damage to permanent installations. In addition, smaller mobile units were built containing a 30-line telegraph switchboard and 6 teleprinters.
These mobile units proved very useful throughout the war, particularly in providing urgent communications prior to D-Day. Many were sent to the Continent along with the invasion forces, providing urgent communications ahead of more permanent installations.
To give some idea of the DTN's size at its peak, it comprised:
1500 Multi-channel voice frequency systems, giving a total of over 12 thousand individual telegraph circuits;
562 terminal stations containing equipment on the premises of the various military and government departments;
10,000 teleprinters and associated equipment. Many circuits terminated in large teleprinter rooms with anything up to 200 machines.
Eight different types of telegraph switchboards were developed to meet various requirements.
To ensure the efficient running of such a vast network, a small advisory group was set up at Post Office Headquarters. Composed of experts in the relatively new field of Multi Channel Voice Frequency Telegraph Equipment, their function was to tour the various DTN terminal stations to help and advise on the operation and maintenance of the equipment. Their efforts helped the network to run efficiently, despite the hardships imposed by the war.