DPNSS was an active (and successful) collaboration between PBX manufacturers and BT which started relatively slowly (BT & Plessey) but quickly snowballed with MITEL, GEC, Ericsson, 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.
A lightweight version of DPNSS 'APNSS' was developed using analogue trunks (Sometimes compressed) and a modem to support D channel signalling.
Layer 2 Timeslot 16, 64Kbs operates as HDLC LAPB, to support up to 60 PVCs or DLCs (data link connections) (30 directly associated with the bearer channels and 30 for unrelated messages) as the specification describes them.
When traffic deltas are low, a single call establishment message will have access to the full 64Kbs (allowing for overheads).
DPNSS is a compelled protocol in that each instruction issued must be met with an appropriate response from the other PBX otherwise the message is re-transmitted (until timer expiry).
However, such badly synchronised links were frowned upon because of the problems associated with sending faxes and/or other modem based communication which were not specifically identified within the protocol.
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.
Some critics of DPNSS suggest that it is too loosely defined and allows too much latitude in its interpretation of message formats and timers.
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.