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3. ISDN Technology Overview
15
Flag
Protocol
Discriminator
Information
Message
Type
Flag
Address FCS
Information Elements
Control
Call Reference
Value
Layer 2
Layer 3
Figure 2. Layer 3 Frame (D Channel)
In general, the message format for Layer 3 frames comprises variable length fields
with the following format:
• Protocol Discriminator – identifies the protocol type used to handle Layer 3
messages
• Call Reference Value (CRV) – a valued assigned to a call, by the network, for
the duration of the call
• Message type – the set of messages used for establishing, controlling and
tearing down a call
• Information elements (IE) – used with the message to provide additional
information on the type and requirements of the call
Refer to the Digital Network Interface Software Reference for more on framing.
3.3. ISDN Call Control States
Each ISDN call that is received or generated by an application is processed
through a series of call control states. Each state represents the completion of
certain tasks and/or the current status of the call. The following table describes the
ISDN call control states, based on standard Q.931 (Layer 3).