4 General

25.4153GPPRelease 17TSUTRAN Iu interface user plane protocols

4.1 General aspects

The Iu UP protocol is located in the User plane of the Radio Network layer over the Iu interface: the Iu UP protocol layer.

The Iu UP protocol is used to convey user data associated to Radio Access Bearers.

One Iu UP protocol instance is associated to one RAB and one RAB only. If several RABs are established towards one given UE, then these RABs make use of several Iu UP protocol instances.

In general, Iu UP protocol instances exist at Iu access point as defined in TS 25.410 [2] i.e. at CN and UTRAN. However, as described in TS 23.153 [13], if TrFO is possible and the Iu UP protocol instances operate in support mode the Iu UP protocol instance in CN may resume performing Iu UP specific functions or vanish completely during stable call states. In this case the partner peer entity actually interacting with the UTRAN Iu UP protocol instance (i.e. performing all Iu UP specific functions except UP initialisation) may either be located within another UTRAN or within a CN node that is not the serving CN node of the UTRAN.

Whenever a RAB requires transfer of user data in the Iu UP, an Iu UP protocol instance exists at each Iu interface access points. These Iu UP protocol instances are established, relocated and release together with the associated RAB.

Whether these peer protocol instances perform some RAB related function depends on the mode of operation of the Iu UP as defined below.

The following figure illustrates the logical placement of the Iu UP protocol layer and the placement of the Data Streams sources outside of the Access Stratum.

Figure 1: Iu UP protocol layer occurrence in UTRAN overall architecture (User Plane View)

4.2 Operational and Functional Aspects

4.2.1 Iu UP protocol modes of operation

The Iu UP protocol operates in mode according to the concept described in earlier subclause.

Modes of operation of the protocol are defined:

1) Transparent mode (TrM);

2) Support mode for predefined SDU size (SMpSDU).

Determination of the Iu UP protocol instance mode of operation is a CN decision taken at RAB establishment based on e.g. the RAB characteristics. It is signalled in the Radio Network layer control plane at RAB assignment and relocation for each RAB. It is internally indicated to the Iu UP protocol layer at user plane establishment.

The choice of a mode is bound to the nature of the associated RAB and cannot be changed unless the RAB is changed.

4.2.2 Transparent mode (TrM)

The transparent mode is intended for those RABs that do not require any particular feature from the Iu UP protocol other than transfer of user data.

The following figure illustrates the transparent mode of operation of the Iu UP protocol layer.

Figure 2: Iu UP protocol layer in transparent occurrence over Iu interface

In this mode, the Iu UP protocol instance does not perform any Iu UP protocol information exchange with its peer over the Iu interface: no Iu frame is sent. The Iu UP protocol layer is crossed through by PDUs being exchanged between upper layers and transport network layer.

For instance, the transfer of GTP-U PDUs could utilise the transparent mode of the Iu UP protocol.

This release of the specification defines transparent mode version 1, the same as in the release 99 specification.

4.2.3 Support mode

The support modes are intended for those RABs that do require particular features from the Iu UP protocol in addition to transfer of user data. When operating in a support mode, the peer Iu UP protocol instances exchange Iu UP frames whereas in transparent mode, no Iu UP frames are generated.

The following figure illustrates the functional model of the Iu UP protocol layer in support mode of operation.

Figure 3: Iu UP protocol layer in support mode occurrence over Iu interface

Some RABs requesting Iu UP protocol support, constrain the Iu UP protocol and possibly the radio interface protocols in specific ways. For instance, certain RABs can have variable predefined rates.

The Iu UP support mode is prepared to support variations.

The only support mode defined here is the:

– Support mode for predefined SDU size (SMpSDU).

For instance, the transfer of AMR speech PDUs would utilise the support mode for predefined SDU size of the Iu UP protocol because it requires some Procedure Control functions and some NAS Data Streams specific functions while the sizes of the user data being transferred can vary in a predefined manner.

This release of the specification defines the Support mode for predefined SDU sizes version 2. The Support mode for predefined SDU sizes version 1 (see release 99 of this specification) shall also be supported by a 3GPP implementation compliant to this release of the specification in order to be backward compatible with release 99.