In the speech codec modification procedure BSS informs the peer BSS the speech codec types supported by the sending BSS (within the ongoing call) and tries to align the codecs using implicit rule and BSS internal means to make appropriate handover.
The support of the informing and BSS internal modification procedures are optional for a BSS since it depends on the BSS internal capabilities. This means that TFO should be applicable even if the informing and modification are not supported by one of the two peer entities. In this case, TFO will be available if both BSS happen to use a compatible speech codec type.
The exchanged parameters are the speech codec currently in use and the other possible codecs that can be used within the call Modification procedure.
In the TFO request message (TFO_REQ), the BSS informs the fact that it supports TFO.
The possible modification procedure is started when the TFO request message from the peer BSS has been detected. It is not synchronised explicitly on Call Set-up states.
If both BSS receive a TFO_REQ indicating that a compatible speech codec type is used, the modification is not necessary and the BSS can move to the TFO established mode.
If the speech codec configurations used on both sides are not compatible, each BSS looks at the list of the acceptable speech codec configurations from the other BSS.
There is an implicit rule as to the speech codec configuration that every BSS will try to use based on the current speech codec configuration, the list of locally acceptable speech codec configurations, and the list of remotely acceptable speech codec configuration. Each BSS then tries if it supports it to perform a change of codec configuration via a intra-BSS (normally intra-cell) handover.
When a handover has been performed, each BSS will reply to TFO_REQ by a new TFO_ACK message indicating the newly used speech codec configuration. The process goes on until either a compatible codec configuration is used, or no more change of the codec configuration is possible according to the fixed rule.
There is a timer that protects the TFO establishment phase. The BSS, after the timer has elapsed, stops sending TFO request messages, but continues the monitoring of the peer entity. This is in order not to degrade the communication if TFO cannot be established. To resume the TFO establishment phase, it will be up to the peer entity to send a new TFO_REQ message to initiate a new attempt of establishment/modification of TFO e.g. when a handover has occurred.
This procedure is internal to the BSS. It can be based on the use of O&M frames on the Abis interface or on the use of a specific TRAU to BSC interface or on some other method.