Page "Digital Private Network Signalling System" Paragraph 2

DPNSS was developed in the early 1980s by BT, or its forerunner, Post Office Telecommunications in recognition that the emerging Digital Private Circuit Primary Rate product ' Megastream ' had to address the market for both data and voice, the latter being significantly greater because of the market for PBXs.

At the time, BT dictated what signalling could be used on its leased lines and, whilst it only had a minority interest in the sales of PBXs, was requested by the PBX manufacturers to produce a standard to prevent the creation of a plethora of conflicting PBX protocols being developed.

Under the liberalization rules of the day ( 1979 ), BT was barred from manufacturing, selling or supplying PBXs of more than 200 extensions.

Digital ( PCM-based ) PBXs were just starting to come into the marketplace with the Plessey PDX ( a licensed version of the ROLM CBX ) and the GEC SL1 ( a licensed version of the Northern Telecom SL1 ).

It was recognised that corporate customers would wish to network these systems across the country.

At the time, ' CAS ' inter node signaling was slow and inter-register signaling MF5, developed from PSTN signalling protocols, was complex and would not support sufficient features.