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 Figure2).

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 your operating system for more information on framing.

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