1.2.3. Network Layer (Layer 3) Frames

The Data Link Layer prepares the way for the transmission of Network Layer (Layer 3) frames of data (see Figure 2).

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

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