Digital Private Network Signalling System: Difference between revisions

Revision as of 23:32, 5 August 2019

Revision as of 23:33, 5 August 2019

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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 signalling. Speech is carried as G711.

Layer 2 Timeslot 16, 64Kbs operates as [[HDLC]] [[LAPB]], to support up to 60 PVCs or [[Data link connection identifier|DLC]]s (data link connections) (30 directly associated with the bearer channels and 30 for unrelated messages) as the specification describes them. Therefore, at maximum operation, each potential traffic channel can have two simultaneous data channels available for messages. Note that HDLC operates as a statistical [[multiplexing]] system. When traffic deltas are low, a single call establishment message will have access to the full 64Kbs (allowing for overheads).

DPNSS is a layer 3 protocol functioning as common channel signalingsignalling. The functionality is divided into Levels (confusingly nothing to do with OSI layers.) Levels 1-6 deal with simple call establishment (make call/break call) and are the minimum requirements by which a PBX can be said to be DPNSS compatible. The remaining levels are allocated to telephony features, supplementary services or to administrative features. Note that support of 'levels' by a PBX is not necessarily incremental. Some levels are interdependent but a PBX may omit support of some levels (above 6) and support others.

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). This means that when interworking two PBXs features invoked on PBX A must be acknowledged by PBX B even if that feature is not supported.