Technical data

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Layer 3 – Network layer
The network layer for ISDN is specified by ITU in Q.930 to Q.939. Layer 3 has func-
tions to establish, maintain, and terminate a logical connection between two devices.
The structure of the information field on layer 3 has a variable length and the different
fields are specified by Q.931:
The information field’s message header has the following appearance:
Protocol Discriminator (1 octet)
The field identifies the protocol type used to handle layer 3 messages. When Q.931 is
used this field is 0816.
Length (1 octet)
Length of the subsequent field.
Call Reference Value (CRV) (1 or 2 bytes)
The field is used to identify the call/connection that the signal message belongs to. The
value is used in all signalling as long as the current call is in progress.
Message Type (1 octet)
The field states the type of message sent. Four groups of messages can be discerned:
connection, information, disconnection and other messages. SETUP and CONNECT
belong to the first group. Information Elements (variable length)
The contents of this field consist of a number of information elements. The type of
information element sent depends on the previous field Message Type. Elements for
B-number information, additional services and transmission requirements on the net-
work, etc. are found here.
Information Field
876543 2 1
Protocol Discriminator
0 0 0 0 Length of CRV
Call Reference Value (1 or 2 octets)
0 Message Type
Mandatory & Optional Information Elements (variable)
B A C K