The D-channel provides a connection which enables the ISDN PH to access an ISDN user terminal or vice versa. This access is accomplished by establishing an ISDN link layer connection (SAPI=16) to the terminal or network which can then be used to support packet communications according to X.25 [5] layer 3 procedures as defined in 6.3.
The layer 2 procedures shall be in accordance with Recommendation Q.921 [3]. The D-channel provides a semi-permanent connection for packet access since all D-channel layer 2 frames containing a packet-mode SAPI (16) are routed automatically between the user and the PH function.
When an incoming call is offered to packet-mode user equipment at the user interface, the channel selection procedures described in 6.2.2.3 shall be used.
A number of packet mode terminals can operate simultaneously over the D-channel, each using a separate layer 2 link identified by an appropriate TEI (see Recommendation Q.921) in frames transferred between the terminal and the network.