Issue 2 - 07/00 A3-11
Appendix 3—Introduction to ISDN
Layer 3
Layer 3 routes messages to their destination. The diagram
below shows the structure of the Layer 3 signalling messages.
Protocol Discriminator Identifies the protocol.
Call Reference Value Identifies the call with which a
message is to be associated.
Message Type Describes the intention of the
message: CONNECT, SETUP, etc.
Information Elements A number of these may be included.
Their number and content depend
upon the message type.
Protocols
A protocol is an agreed set of rules for carrying out a particular
function such as the exchange of information.
ETSI
The standard ISDN protocol used within Europe. Based on
ITU-T specifications Q.921 and Q.931.
1TR6
The national protocol for the ISDN environment in Germany.
It is also often used in private networks utilising equipment of
German origin.
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