3.2. Framing

• 3.2.1. Data Link Layer (Layer 2) Frames
• 3.2.2. Network Layer (Layer 3) Frames

Voice data from each time slot in a configuration is routed to a separate B channel. The signaling information for all B channels is routed to the D channel of the device. Information from the B and D channels is transmitted in frames. A single frame contains information from each of the channels, providing a "snapshot" of the data being transmitted at any given time.

The frames contain the eight bits of information about each time slot or channel. Each frame includes a series of bits used for synchronization, error checking, and diagnostics. A frame can be in one of several formats. In all formats, there is a flag bit between each frame to show where the frames begin and end. Different frame formats are supported in different networks to provide a variety of added features or benefits. The following frame formats are supported by the Dialogic ISDN software:

• ESF frame (Extended Superframe)
• D4 frame (Superframe)
• CEPT multiframe (with or without CRC4)

3.2.1. Data Link Layer (Layer 2) Frames

The frames that are transmitted over the Data Link Layer (Layer 2) contain information that controls the setup, maintenance and disconnection between the two physically connected devices (see Figure 1).

Figure 1. Layer 2 Frame (D Channel)

3.2.2. 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

Refer to the Digital Network Interface Software Reference for more on framing.

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