The packet layer procedures of Recommendation X.25 will be used for layer 3 call setup and release. The packet layer procedures will additionally be able to control and monitor the established or released state of the link layer.
In Case B, the PH may maintain a timer T320 (defined in this Recommendation). T320, if implemented, is started:
a) upon clearance of the last virtual call; or
b) upon transmission of a CONNECT message by the network in case of an outgoing B-channel access connection; or
c) upon transmission of a CONNECT ACKNOWLEDGE message by the network in case of an incoming B-channel access connection; or
d) upon establishment of the link layer for D-channel access connections.
T320 is cancelled upon:
1) establishment of the first (next) virtual call; or
2) receipt of a Q.931 clearing message from the user; or
3) disconnection of the SAPI=16 link on the D-channel.
Upon expiry of timer T320, the PH will release the link layer and, in the case of B-channel access, initiate clearing of the B-channel.
X.25 logical channels are associated with their underlying logical link. Specifically, in the case of the use of the B-channel for packet communication there is an association between the logical channels and the LAPB logical link below them. Thus, the same logical channel number may be used simultaneously on each different B-channel.