The Digital Private Network Signalling System (DPNSS) is a network protocol used on digital trunk lines for connecting to PABX. It supports a defined set of inter-networking facilities.
DPNSS was originally defined by British Telecom. The specification for the protocol is defined in BTNR188. The specification currently comes under the Network Interoperability Consultative Committee.
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.