Introduction To Line Telecommunications


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7. Integrated Services Digital Network
  7.1 - Basic Rate ISDN (ISDN 2) / I.420 7.5 - DPNSS Signalling
  7.2 - Primary Rate ISDN (ISDN 30) 7.6 - Q-Sig
  7.3 - DASS Signalling 7.7 - ADSL
7.4 - Euro ISDN (I.421) 7.8 - Cable Modems

7.2 - Primary Rate ISDN (ISDN 30)

The Primary Rate Interface (PRI), or ISDN 30, uses the same technology as a Megastream. In the UK this is a 2Mbps link or ‘pipe’ providing 30 B channels and using 2-D channels for timing and control. The USA and Japan use a similar system with slower data rates and a different data encoding system. On this platform a range of protocols have been and are being developed to allow interfacing between the network and connected equipment.

The high capacity of the PRI means that its use is primarily for PBX connection to the network using one of the standard protocols.

7.3 - DASS Signalling

The Digital Access Signalling System (DASS) is the UK proprietary standard created by BT to provide ISDN services in the UK. The first incarnation DASS I is now obsolete and has been replaced by DASS II. This itself will become obsolete over the coming years as Q.931/I.421, a European standard becomes widely adopted in the EEC.

DASS II is purely a protocol used to control data flow across the ISDN doing a similar job to the protocols used with modems when sending the data over the analogue PSTN. It is more highly specified and controlled and operates at higher speeds.

DASS II is usually presented as G.703, a 75 ohm, twin BNC ( 1 transmit and 1 receive ) connection.

7.4 - Euro ISDN (I.421)

The Euro-ISDN, I.421, is a further protocol development building upon the features and facilities offered by DASS II and developed for implementation on networks throughout the EEC. It will eventually replace DASS II in the UK and the systems used by other EEC member countries to become the single ISDN standard used throughout Europe.

Users of previous protocols will in most cases be able to continue with their current equipment with the necessary conversions being performed by the network equipment to allow their traffic across the Euro-ISDN network.

Presentation is via a 120 ohm , balanced UTP (Unshielded Twisted Pair) using an RJ45 connector. The Q.931 defines this connection. I.421 defines the protocol used on the circuit.

7.5 - DPNSS Signalling

Digital Private Network Signalling System (DPNSS) is, in a way, the ‘odd man out’ in the list of ISDN protocols. It is not formally regulated, but is a voluntary standard developed by the exchange and large PBX manufacturers, in conjunction with BT to allow interconnection between their equipment over the ISDN network.

The specification carries details of approximately 50 services and functions which if followed will operate over systems produced by the involved parties and even allow for equipment from the same manufacturer to be transparently connected over a digital link using ‘private’ functions. In practice any given manufacturer will implement a core of essential services and around 6 optional functions completely plus some ‘private’ ones for their own equipment. Each manufacturer selects a subset, since these facilities are all inter-related and to use all of them would be too difficult to engineer. However the subsets vary between manufacturers and the DPNSS protocol will limit inter-connection facilities to those supported in common between linked PBX equipment.

If one of the involved manufacturers develops a new facility which is considered to be of general interest and practical use it will be included as a standard and controlled function in the DPNSS definition for all parties to use and draw upon.

7.6 - Q-Sig

Q-Sig is a development of Euro ISDN, to provide inter-site connection between PBXs, in a similar fashion to DPNSS.
Though the two types of service are incompatible, the range of features is closely matched. Q-Sig is regulated and will be developed further in the future. DPNSS is not going to be developed further.



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Extracted from

Introduction To Line Telecommunications
Copyright Panasonic Business Systems UK Ltd 2000