10 Time Alignment Handling
25.4023GPPRelease 17Synchronisation in UTRAN Stage 2TS
The purpose of the time alignment procedure over Iu is to minimise the buffering delay in SRNC by controlling the DL transmission timing in the CN node. The time alignment procedure is controlled by SRNC and is invoked whenever the SRNC detects the reception of Iu User Plane PDU at an inappropriate timing that leads to an unnecessary buffering delay. The SRNC indicates to the CN node by means of a TIME ALIGNMENT control frame. The necessary amount of the delay or advance adjustment is indicated by expressing a number of (+/-) 500 µs steps (see Figure 23).
Figure 23: Time Alignment Handling
A supervision timer TTA is started after sending the Time Alignment control frame in order to supervise the reception of the TIME ALIGNMENT ACKNOWLEDGEMENT control frame.
The requested CN node adjusts the transmission timing by the amount as indicated by SRNC and sends a time alignment acknowledgement frame (ACK). Upon reception of a time alignment acknowledgement frame, the SRNC stops the supervision timer TTA.
The procedure can be signalled at any time when transfer of user data is not suspended by another control procedure.
If the TIME ALIGNMENT control frame could not be handled by the requested CN node, a TIME ALIGNMENT NEGATIVE ACKNOWLEDGEMENT control frame (NACK) is sent with a corresponding cause. When the SRNC receives a NACK with cause "Time Alignment not supported", then the SRNC shall not send additional Time Alignment frames for that RAB (unless the Iu User Plane conditions change for that RAB). The cause value "Requested Time Alignment not possible" is used to indicate that the requested time alignment was not possible at that moment. At a later moment the SRNC may initiate a new Time Alignment procedure when needed.
If the SRNC detects that the time alignment command has not been correctly interpreted or received, i.e. NACK received or timer expires, and the time alignment need still persists, the SRNC should re-trigger a Time Alignment procedure. If after "k" repetitions, the error situation persists, the SRNC take appropriate local actions.
Upon reception of a NACK, the SRNC stops the supervision timer TTA.
In order to avoid oscillation in the time alignment handling over Iu, it is beneficial to avoid initiating a new Time Alignment procedure too early after successful completion of a Time Alignment procedure.