6.5.4 Time Alignment procedure
25.4153GPPRelease 17TSUTRAN Iu interface user plane protocols
6.5.4.1 Successful operation
The purpose of the Time Alignment procedure is to minimise the buffer delay in RNC by controlling the downlink transmission timing in the peer Iu UP protocol layer entity.
The Time Alignment procedure is controlled by SRNC.
The Time Alignment procedure is invoked whenever the SRNC detects the reception of Iu UP PDU at an inappropriate timing that leads to an unnecessary buffer delay. The actual detection of the trigger in SRNC is an internal SRNC matter and is out of the scope of the present document.
The Iu UP protocol layer entity in SRNC indicates the peer entity the necessary amount of the delay or advance adjustment in the number of 500 µs steps.
A supervision timer TTA is started after sending the Iu UP TIME ALIGNMENT control frame. This timer supervises the reception of the time alignment acknowledgement frame.
The requested Iu UP protocol layer entity in the peer node adjusts the transmission timing by the amount as indicated by SRNC.
If the TIME ALIGNMENT control frame is correctly formatted and treated by the receiving Iu UP protocol layer and the time alignment is treated correctly by the upper layers, this latter sends an time alignment acknowledgement frame.
Upon reception of a time alignment acknowledgement frame, the Iu UP protocol layer in 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.
Figure 15a: Successful Time Alignment
Figure 16: Void
6.5.4.2 Unsuccessful operation
If the TIME ALIGNMENT control frame could not be handled by the peer side, the peer side should send a NACK with a corresponding cause. When the Iu UP protocol layer in the SRNC receives a NACK with cause "Time Alignment not supported", then the SRNC shall not send additional TIME ALIGNMENT control frames for that RAB (unless the Iu UP 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 control frame when needed. If the TIME ALIGNMENT control frame is received by the RNC, it shall respond with a NACK with the cause "Time Alignment not supported".
If the Iu UP protocol layer in the SRNC detects that the TIME ALIGNMENT control frame has not been correctly interpreted or received, i.e NACK received or timer expires, and the time alignment need still persists, the Iu UP should retrigger a Time Alignment procedure. If after N TA repetitions, the error situation persists, the Iu UP protocol layers take appropriate local actions.
Upon reception of a TIME ALIGNMENT NEGATIVE ACKNOWLEDGEMENT control frame, the Iu UP protocol layer in the SRNC stops the supervision timer TTA.
Figure 16a: Unsuccessful Time Alignment: 1) N TA negative acknowledgements
or 2) N TA expiries of timer TTA
Figure 16b: Time Alignment received by the RNC