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
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The first incarnation DASS I is now obsolete and has been
replaced by DASS II.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.
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.
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.
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.