The D-channel provides a connection which enables the ISDN user terminal to access a PH function within the ISDN by establishing a link layer connection (SAPI=16) to that function which can then be used to support packet communications according to X.25 layer 3 procedures as defined in 6.3. The X.25 packet layer uses the acknowledged information transfer service (i.e. I-frames) provided by LAPD (see Recommendation Q.920 [45]). Consequently, Q.931 procedures are not required to provide D-channel access.
A number of packet mode user equipment can operate simultaneously over the D-channel, each using a separate ISDN layer 2 data link identified by an appropriate address (see Recommendation Q.921) in frames transferred between the user and the PH.