Digital Private Network Signalling System: Difference between revisions

Revision as of 22:41, 22 June 2010

Revision as of 20:21, 12 September 2010

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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.

==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. It may seem odd now that BT would invest in the development of a signaling protocol for PBXs in which it had a minority interest and in competition with its PSTN services. 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 ROLM/Northern Telecom SL1, and Plessey PDX, 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 for the PSTN was complex and would not support sufficient features.

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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 it's compatibility with PBX features ensured the adoption DPNSS actually grew in Europe, compared to the much slower take-up of Qsig.

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 for smaller PBXs.

==Overview of the Protocol==

Layer 1(CCITT) ITU-G703 defines the physical and electrical interface. G704 defines the Frame structure of the 2.048 Mbs sent across the link. G732 defines the allocation of that frame structure into the 32 discrete 64Kbit 'channels', of which 0 is used for alignment of the frames and 16 is (by convention only) allocated to common channel signaling. Speech is carried as G711.

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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 manufacturer’s world wide, 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 it's associated PCM (G711) over an IP network. This can be point to point where the IP network carries packetised voice N x 64Kbs speech and a separate IP signalling channel to carry the notional 64Kbs 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 [http://en.wikipedia.org/wiki/Voice_VPN[VoIP (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.

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[[Category:Network protocols]]

[[Category: Telephony signals]]

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[[ro:Digital Private Network Signalling System]]