Digital Private Network Signalling System: Difference between revisions

Revision as of 11:57, 20 April 2013

Revision as of 02:09, 7 December 2013

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==History==

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.

The support for DPNSS as BT's own signaling protocol also differentiated BT's private circuit’s services from those of its emerging rival [[Mercury Communications]].

DPNSS was an active (and successful) collaboration between PBX manufacturers and BT which started relatively slowly (BT & Plessey) but quickly snowballed with [[Mitel|MITEL]], [[General Electric Company plc|GEC]], [[Ericsson]], Phillips[[Philips]] and eventually Nortel all joining to create a powerful and feature rich protocol.

BT and some of the UK manufacturers championed DPNSS into ECMA and CCITT (ITU) but it was eventually deprecated by the standards bodies in favour of Q931 and QSig. Nevertheless, the elegance of the protocol and its compatibility with PBX features ensured the adoption DPNSS actually grew in Europe, compared to the much slower take-up of Qsig.

There were also attempts (during 1984) to take DPNSS into North America. Unfortunately the structures for the creation of standards in North America seemed to prevent manufacturer collaboration as a route forward and [[ANSI]] were not interested in creating PBX interworking standards.

Version 1 of BTNR188 (DPNSS) was issued in 1983; the last version of DPNSS to be released 6 in 1995 included compatibility with ISDN features released in V5. A lightweight version of DPNSS 'APNSS' was developed using analogue trunks (Sometimes compressed) and a modem to support D channel signalling.

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==DPNSS and VoIP==

For a protocol that began life in the 1980s, DPNSS is natively a long way from [[VoIP]]. However, many of the hybrid VoIP PBXs available from manufacturers worldwide provide on-board DPNSS trunk cards. Where they do not, a protocol converter is necessary. Commercially available equipment offers the ability to convert from DPNSS to Q.Sig. Note that it is also possible to tunnel DPNSS and its associated PCM (G711) over an IP network. This can be point to point where the IP network carries packetised voice N x 64 Kbs speech and a separate IP signalling channel to carry the notional 64 Kbs of DPNSS signalling. A more sophisticated solution uses intelligence on the edge of the IP network to route voice to the correct node. This is a [[Voice VPN]].

Note that this should not be confused with the pre-VOIP 'Voice VPN' deployed by routing calls intelligently in a TDM switching platform, often Nortel DMS100 and customers PBX nodes.

==Criticisms==

Some critics of DPNSS suggest that it is too loosely defined and allows too much latitude in its interpretation of message formats and timers. It is also sometimes mistakenly believed that DPNSS is semi proprietary and that it is only possible to connect PBXs from the same manufacturer. i.e. [[Siemens]] will connect to Siemens, Mitel to Mitel etc. Experience indicates that this is not the case and BT's FeatureNet platform (Nortel's DMS100) running DPNSS, has interconnected successfully to many PBX types available in the UK. In addition, as part of the first commercial implementation of DPNSS (in the Government Telephone Network or GTN in 1983), BT insisted that the core of the network be made from PBXs of different manufacture to prove the interoperability in real life.

==See also==