23.5.6 Feature Operation
Initially there are at least two calls over the PRI between end-users and the controller. Each call uses a B-channel on the PRI. To be eligible for TBCT transfer, at least one of the calls must be in an answered state; it may have been initiated by the end-user or by the controller. The other call may be in an answered state, or, if initiated by the controller, it may be in a ringing state.
If the controller determines that two of the calls are to be transferred to each other, the controller sends a request for two B-channel transfer to the 5ESS® switch.
When the 5ESS® switch receives the request from the controller, it checks the validity of the message containing the request, the states of the calls to be transferred, the compatibility of the bearer capabilities of the calls, whether the PRI groups associated with the B-channels in the request are provisioned for the feature, whether the PRI groups are members of the same PRI serving group (see below for explanation of PRI serving group), whether interactions with other services or features should prohibit execution of the transfer, and other criteria. If any of the checks fail, an error message is sent to the controller, and the calls remain connected to the controller. If the checks pass, the 5ESS® switch performs the TBCT transfer operations. The two users' calls are then connected together at the switch, and the two B-channels, which were previously occupied on the PRI by the users' calls to the controller, are released. After the transfer, the two B-channels are available for new calls to the controller.
The multiple interface capability allows transfers between B-channels that are on different PRI groups, that is, B-channels that are associated with different D-channels. A PRI serving group is introduced with this feature. A PRI serving group consists of members, each of which is a PRI group. All members of a PRI serving group must be on the same SM.
This feature also provides two counters that are available to control requests and usage of the feature on a per PRI group basis. If the two B-channels are from different PRI groups, one counter is incremented -- that of the PRI group with the B-channel that requests the transfer. One counter records the number of TBCT requests per 10-second interval. If the number of TBCT requests reaches a provisionable limit within the time interval, all additional requests are denied in the current time interval. The second counter records the number of active transfers, that is, the number of transferred calls that are currently active. If the number of active transfers reaches a provisionable limit, all subsequent TBCT requests are denied. (It is also possible for the number of active transfers to exceed the limit, for example if the limit is lowered while there are active transfers on the interface.) When a sufficient number of transferred calls are cleared, such that the number of active transfers no longer equals or exceeds the limit, new TBCT requests will be allowed. An audit is provided to ensure the integrity of the active transfers counter.
Traffic measurements are provided to record feature usage on a requesting PRI group basis. The number of valid TBCT requests, the number of successful TBCT transfers, the number of overflows of the active transfers counter, and the number of overflows of the transfer requests per current interval counter are measured.
As an enhancement to Feature 99-5E-3261 (Two B-Channel Transfer [TBCT] on PRI), the TBCT Notification to Controller feature, 99-5E-7268, sends notification to the controller when the transferred call is cleared. A non-call associated NOTIFY message is used. When the Notification to Controller feature is active, the switch assigns a Call Tag to the TBCT call at the time of the transfer. This Call Tag is unique at the PRI interface serving the controller. If the multiple PRI option is subscribed, the Call Tag is unique across the PRI serving group. The switch retains an association between the Call Tag, the transferred call, and the controller's interface throughout the duration of the transferred call.
The base TBCT feature, 99-5E-3621, passed the number of active and available transfers to the controller in the DISCONNECT message at the time of the transfer. When the TBCT NTC feature, 99-5E-7268, is active on the PRI group, the DISCONNECT message will also include the Call Tag assigned to the transferred call. The switch sends a NOTIFY message to the controller, when the TBCT transferred call clears. The NOTIFY message contains two notifications. One notification contains the Call Tag value to alert the controller that the TBCT call associated with the Call Tag has cleared and a second notification provides the updated number of active and available transfers on that PRI group.
The Notification to Controller feature is provided on a per PRI GROUP basis.
NOTE: Notification to Controller can be subscribed only if TBCT is active.
