6.4 Functions of the Iu UP Protocol Layer in Support mode

25.4153GPPRelease 17TSUTRAN Iu interface user plane protocols

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.4.1 Functional model of the Iu UP Protocol Layer in Support mode

Figure 5: Functional model of the Iu UP protocol layer in Support mode

The Iu UP protocol layer in Support mode is made of three sets of functions:

1) Frame Handler function;

2) Procedure Control functions;

3) Non Access Stratum Data Streams specific functions.

6.4.2 Frame Handler function

This function is responsible for framing and de-framing the different parts of an Iu UP protocol frame. This function takes the different parts of the Iu UP protocol frame and set the control part field to the correct values, including the handling of the frame number. It also ensures that the frame control part is semantically correct. This function is responsible for interacting with the Transport layers. This function is also responsible for the CRC check of the Iu UP frame header. The Iu UP frame with header CRC check error is discarded.

6.4.3 Procedure Control functions

This set of functions offers the control of a number of procedures handled at the Iu UP protocol level. These functions are responsible for the procedure control part of the Iu UP frames.

Namely, these procedures are:

– Rate Control: is the procedure which controls over the Iu UP the maximum rate that is allowed to be sent downlink among the rates that can be controlled. The set of rates is represented by RFCI indicators. The function controlling this procedure interacts with functions outside of the Iu UP protocol layer.

– Initialisation: is the procedure which controls the exchange of initialisation information that is required for operation in support mode for predefined SDU size. Such information can contain the RFCI Set to be used until termination of the connection or until the next Initialisation procedure. This procedure is also used for negotiating the version of the Iu UP Mode among the versions the CN requested for the related RAB.

– Time Alignment: is the procedure that controls the timing of the downlink data to the RNC over Iu. The function controlling this procedure interacts with functions outside of the Iu UP protocol layer.

– Handling of Error Event: is the procedure that controls the information exchanged over the Iu related to detection of a fault situation. The function controlling this procedure interacts with functions outside of the Iu UP protocol layer.

6.4.4 Non Access Stratum Data Streams specific function(s)

These functions are responsible for a "limited manipulation" of the payload. These functions are responsible for the CRC check and calculation of the Iu UP frame payload part. These functions are also responsible for the Frame Quality Classification handling as described below.

These functions interact with the upper layers by exchanging Iu data stream blocks of Iu UP frame payload. These functions also handle the padding and depadding of the Iu UP frame payloads when needed.

These functions interact with the Procedure Control functions.

These functions provide service access to the upper layers for the Procedure Control functions.

6.4.4.1 Frame Quality Classification function

6.4.4.1.1 General

On the Iu UP in Support Mode the frames are classified with the Frame Quality Classifier (FQC). This classifying is based on the radio frame classification and the setting of the RAB attribute Delivery of erroneous SDU IE. The RAB attribute Delivery of erroneous SDU IE tells if erroneous frames shall be delivered or not.

Figure 6 shows the main input and output information for frame quality classification function on the Iu UP.

Figure 6: Frame quality classification in Iu UP

6.4.4.1.2 Handling of FQC information in uplink path

6.4.4.1.2.1 Handling of FQC information at RNC

In SRNC on the sending side, the Support Mode Functions takes as input the radio frame quality information together with the frame. Based on this, the Frame Quality Classification (FQC) IE is set for the frame, a CRC is or is not added (depending on PDU type) and the frame is sent to CN. The following steps shall be sequentially applied to derive the SRNC behaviour and the Frame Quality Classification (FQC) IE setting:

a) If there is at least one subflow with the Delivery of erroneous SDU IE set to "No" and for at least one of those subflows the radio frame classification is "Bad" then the Iu UP frame shall not be sent;

b) Otherwise, if there is at least one subflow with the Delivery of erroneous SDU IE set to "Yes" and for at least one of those subflows the radio frame classification is "Bad" then the Iu UP frame shall be sent with Frame Quality Classification (FQC) IE set to "frame bad due to radio";

c) Otherwise the Iu UP frame shall be sent with Frame Quality Classification (FQC) IE set to "frame good".

6.4.4.1.2.2 Handling of FQC information at CN

The Support Mode Functions in CN on the receiving side makes a CRC check of the frame payload, if CRC is present and passes the appropriate frame and the appropriate frame quality classification information through the RNL-SAP. The following steps shall be sequentially applied to derive the CN behaviour and the FQC field setting:

a) If a CRC is available and the CRC check indicates that the Iu UP frame payload part is corrupted and at least one subflow has the Delivery of erroneous SDU IE set to "No", then the Iu UP frame shall be dropped;

b) Otherwise, if a CRC is available and the CRC check indicates that the Iu UP frame payload part is corrupted and at least one subflow has the Delivery of erroneous SDU IE set to "Yes", then the Iu UP frame shall be forwarded with the FQC set to "frame bad";

c) Otherwise the Iu UP frame shall be forwarded with the FQC as set by UTRAN.

6.4.4.1.3 Handling of FQC information in downlink path

The Support Mode Functions in CN on the sending side adds a CRC, if necessary to the frame payload and passes it together with the FQC. If the payload stems from a transcoding unit of the NAS within the CN the FQC is always set to "frame good". Otherwise it may be set by a partner peer entity residing in another RNC.

The Support Mode Functions in SRNC then makes a CRC-check, if the CRC is present. Based on the CRC check, a decision is made whether to deliver the frame or not based on the following sequential steps:

a) If a CRC is available and the CRC check indicates that the Iu UP frame payload part is corrupted then the frame shall be dropped;

b) Otherwise, if the FQC value of the Iu UP frame is set to "frame bad" or "frame bad due to radio" then the frame shall be dropped, regardless of the CRC check indication;

c) Otherwise, the frame shall be passed to radio interface protocols.

NOTE: The case where SRNC receives a frame with the FQC set to "frame bad due to radio" (respectively: "frame bad"), corresponds to a TrFO (respectively: TFO) case. The frame is then trashed by the receiving RNC since there is currently no means to pass the frame quality indicator down to the UE.