6.5.1 Transfer of User Data procedure

25.4153GPPRelease 17TSUTRAN Iu interface user plane protocols

6.5.1.1 Successful operation

The purpose of the Transfer of User Data procedure is to transfer Iu UP frames between the two Iu UP protocol layers at both ends of the Iu interface. Since an Iu UP instance is associated to a RAB and a RAB only, the user data being transferred only relate to the associated RAB.

The procedure is controlled at both ends of the Iu UP instance i.e. SRNC and the CN. Exceptions in case of TrFO, where the partner peer entity does not reside within the serving CN node are described in subclause 4.1 and TS 23.153 [13].

The Transfer of User Data procedure is invoked whenever user data for that particular RAB needs to be sent across the Iu interface.

The procedure is invoked by the Iu UP upper layers upon reception of the upper layer PDU and associated control information: RFCI.

The upper layers may deliver a frame quality classification information together with the RFCI.

The NAS Data Streams specific functions makes the padding of the payload (if needed) so that the Iu UP frame payload will be an integer number of octets. Then the NAS Data Streams specific functions perform, if needed, CRC calculation of the Iu frame payload and passes the Iu UP frame payload down to the Frame Handler function together with the RFCI.

The Frame Handler function retrieves the frame number from its internal memory, formats the frame header and frame payload into the appropriate PDU Type and sends the Iu UP frame PDU to the lower layers for transfer across the Iu interface. If the UTRAN initialises the RAB it shall base the selection of the PDU type (in both directions) on the reliability attributes (see TS 25.413 [3]) for the RAB. If the reliability attribute Delivery of Erroneous SDU IE equals "no-error-detection-consideration" for all subflows then PDU Type 1 shall be used, otherwise PDU Type 0 shall be used.

For RABs with the traffic class conversational or streaming the frame number shall be based on time (stepped at each ITI). For RABs with another type of traffic class the frame numbering shall be based on sent Iu UP PDU (stepped at each sent Iu UP PDU). See description of Frame Number IE.

Upon reception of a user data frame, the Iu UP protocol layer checks the consistency of the Iu UP frame as follows:

– The Frame Handler function checks the consistency of the frame header and the consistency of the frame number. If a frame loss is detected due a gap in the sequence of the received frame numbers (for a RAB where frame numbers does not relate to time), this shall be reported to the Procedure Control function. If correct, the Frame Handler function stores the frame number and passes the Iu UP frame payload and associated CRC, if any to the NAS Data Streams specific functions. The received RFCI is passed to the Procedure Control function;

– The NAS Data Streams specific functions check the payload CRC, if any. If the RFCI is correct (i.e. RFCI is used at Initialisation) and matches the Iu UP frame payload (i.e. frame payload is not too short for the RFCI) as indicated by the Procedure Control functions, the NAS Data Streams specific functions removes the padding bits and the spare extension field when present from the Iu UP frame payload based on the RFCI information. Then the NAS Data Streams specific functions forwards to the upper layers the RFCI and the payload.

Figure 7: Successful Transfers of User Data

6.5.1.2 Unsuccessful operation

If the Iu UP frame carrying the user data is incorrectly formatted or cannot be correctly treated by the receiving Iu UP protocol layer, the Iu UP protocol layer shall either discard the frame or pass it to the upper layers with a frame classification indicating a corrupted frame. This decision is based on configuration data of the Iu UP instance for that particular RAB (i.e. if the RAB requests delivery of corrupted frame).

If the Iu UP protocol layer detects a frame loss because of a gap in the received frame number sequence while the frame number does not relate to time (see description of Frame Number IE), the receiving Iu UP protocol layer shall report this to the Procedure Control function.

Figure 8: Unsuccessful Transfers of User Data: 1) Corrupted Frame, 2) Detection of Frame loss