Digital Private Network Signalling System - Overview of The Protocol

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. 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. 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 signaling. 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. DPNSS carries its protocol messages as short strings of IA5 text. It is therefore much easier to interpret in its native form than Q931/Qsig or H323/H450 and a pre-cursor to the plain language format of SIP.