Failure of D-channel one is determined by the receipt of a DL-RELEASE indication primitive from the data link layer. At this point, optionally additional attempts to re-establish this D-channel maybe initiated. Otherwise, it is assumed that D-channel one has failed.
Two states are defined for any D-channel in a backup arrangement. A D-channel is termed out-of-service when layer 2 remains in the TEI-assigned state, after being periodically requested by layer 3 to establish multiple-frame operation. A D-channel is termed maintenance busy when layer 2 is held in the TEI-assigned state by layer 3. While in the maintenance busy condition, the response to an invitation for link establishment is met with the transmission of a DM (Disconnected Mode).
When the D-channel one has failed and if D-channel two is not in an out-of-service condition, the layer 3 shall place D-channel one in a maintenance busy condition, start timer T321 and then issue a DL-ESTABLISH request primitive to re-initialize SAPI 0 link 0 of D-channel two. Upon receipt of this primitive, the data link layer issues an SABME command. Timer T200 is started. The end receiving the SABME command on D-channel two follows the remainder of the Q.931 procedures for establishing logical link with DLCI = 0.
Once the logical link with DLCI = 0 in D-channel two is in the Link Established state, the procedure to establish layer 3 call control signalling can begin on the link.
To establish the backup D-channel for carrying call control signalling, layer 3 should issue an appropriate layer 3message (e.g. a STATUS ENQUIRY on sTable call reference numbers). Once a response to that layer 3 message is received, D-channel two is declared to be the active D-channel, normal layer 3 call control signalling may proceed,timer T321 is stopped, and D-channel one is moved to the out-of-service condition. If the maintenance busy timer T321 expires before a response is received to the layer 3 message, D-channel one is moved to the out-of-service condition and an attempt is made to establish the logical link with DLCI = 0 on D-channel one and D-channel two.
If the logical link with DLCI = 0 of both-channel one and D-channel two are initialized simultaneously, the designated primary shall be chosen as the D-channel for carrying call control signalling.The designated primary D-channel is agreed upon at subscription time by both sides of the interface.
After a switchover, old D-channel two becomes the new D-channel one and old D-channel one becomes the new D-channel two.
Upon completion of appropriate maintenance activity to D-channel two, the logical links for SAPI = 0 and 63 are made active at layer 2 and the D-channel is removed from the out-of-service condition.
D-channels may only be switched again by a failure of D-channel one or a routing or maintenance request from a peer entity.