# Call transfer and NI2

NI2 user-side specifications describe interactions between an application and the network from a user's point of view. In an NI2 configuration, the network performs actual transfer operations. The application's role in a call transfer is to send a request to the network to perform TBCT using the enhanced explicit call transfer supplementary service. This topic describes:

- Invoking two B channel transfer (NI2)

- TBCT notification (NI2) and the ACU_NOTIFY_IN message

The following terms are used in the illustrations in this topic:

This term... | Refers to...
--- | ---
Call #1 | The call between the controller and user A.
Call #2 | The call between the controller and user B.
Controller | The application requesting TBCT.
User A | The first user connected with the controller that is to be transferred to user B. User A does not need to be an ISDN terminal or connected to an ISDN exchange.
User B | The second user connected with the controller that the transferred user (user A) is to be connected to upon completion of TBCT. User B does not need to be an ISDN terminal or connected to an ISDN exchange.

### A controller requests TBCT for two calls

TBCT enables a controller on a PRI
network to request that the switch connect two independent calls. The two calls can be served by the same PRI trunk or by two different PRI trunks that both serve the application, as shown in the following illustration:

### The switch accepts and processes the TBCT request

If the switch accepts the request, the controller is released from the calls and the other two users are directly connected, as shown in the following illustration:

TBCT works only when all of the following conditions are met:

- The controller has subscribed to TBCT.

- The controller has at least two independent calls.

- At least one call to be transferred has been answered.

  If the other call is... | Then the call...
  Outgoing from the controller | Is alerting or has been answered.
  Incoming to the controller | Has been answered.

- The bearer capabilities of both calls are compatible.

Note: Your application is responsible for assuring that the bearer capabilities of both calls are compatible.

The switch notifies a controller when a transferred call is cleared if the controller subscribes to the notification to controller feature.

## Invoking two B channel transfer (NI2)

To perform a two B channel transfer, an NI2 application invokes the enhanced explicit call transfer supplementary service. This service allows an application serving two calls, one in the connected state and other in either the connected state or the alerting state, to request the network to connect the remote parties of these two calls and drop the local connections.

## TBCT notification (NI2)

If the PRI serving the controller subscribed to receive the notification to controller (NTC) feature, the controller receives a call tag for each of the calls corresponding to the transferred parties when these calls are cleared from the PRI. The call tag is associated with the transferred call until it is cleared and is unique to that call for that PRI.

Note: If the controller is served by more than one PRI, the same call tag can be used for different transferred calls on different PRIs.

### ACU_NOTIFY_IN message

The ACU_NOTIFY_IN message contains an acu_ss_notify_tbct_calls_ret_result extended data structure. This message means that the transferred call was cleared and the controller should release the call tag value for re-use.