The Care and Feeding of ISDN4BSD
2.3 The D-Channel	2.3 The D-Channel	2.3.2 D-Channel Layer 3

2.3.1 D-Channel Layer 2

The D-channel layer 2 is specified in[2] and [3]. This recommendations specify a protocol which is also known as LAPD - Link Access Protocol for the D-channel. Q.921 contains a very important concept for multiple devices on an S0 bus: the TEI (Terminal Equipment Identifier). The TEI is an ``address'' for a device on the S0 bus which is dynamically assigned, verified and - under certain circumstances - removed without user interaction, more, the user doesn't notice the TEI management procedures at all.

More generally, layer 2 is responsible for providing an error free link for the signaling procedures specified in layer 3 and as such it deals mostly with error detection, correction and recovery. Recommendation Q.921 specifies:

• the frame structure for peer-to-peer communication
• the elements of the procedures for peer-to-peer communication and
• the elements of layer-to-layer communication

The layer 2 HDLC frames are constructed out of layer 1 frames like this: