5 Transparent mode, version 1

25.4153GPPRelease 17TSUTRAN Iu interface user plane protocols

5.1 General

5.1.1 Operation of the Iu UP in Transparent mode

The Iu UP layer in transparent mode is present in the Iu User plane for transferring data transparently over the Iu interface.

The two strata communicate through a Service Access Point for Non Access Stratum (NAS) Data Streams transfer.

5.1.2 Interfaces of the Iu UP protocol layer in Transparent mode

Interfaces of the Iu UP protocol layer in transparent mode are the transport network layer and the upper layers. The Iu UP protocol layer in transparent mode is an empty layer through which NAS Data Streams PDUs are crossing between the Transport Network Layer and upper layers.

The Iu UP protocol layer in transparent mode is using services of the Transport layers in order to transfer the Iu UP PDUs over the Iu interface.

5.2 Iu UP Protocol layer Services in Transparent mode

The following functions are needed to support this mode:

– Transfer of user data.

5.3 Services Expected from the UP Data Transport layer

The Iu UP protocol layer in transparent mode expects the following services from the Transport Network Layer:

– Transfer of user data.

– Delivery of SDUs in sequence when requested by the RAB parameter Delivery Order IE (TS 25.413 [3]).

5.4 Elements for Iu UP communication in Transparent mode

5.4.1 Frame Format for transparent mode

The following shows the format of the PDU crossing the Iu UP protocol layer in transparent mode. This frame is transferred transparently between the Iu UP protocol upper layers and transport network layer (TNL-SAP).

Figure 4: Frame format for transparent mode

This PDU has a variable length of n octets, whose maximum range depends on the type of user data (e.g. IP packet). No explicit length indication is visible at the Iu UP protocol layer.