16a Support of transparent bearer services in Iu mode
3GPP48.020Rate adaption on the Base Station System - Mobile-services Switching Centre (BSS - MSC) interfaceRelease 17TS
The Iu UP protocol is used in transparent mode, see 3GPP TS 25.415. The payload of the Iu frames will consist of user data bits only for synchronous data, and RA0 synchronous bit streams for asynchronous data.
On the Iu interface, the payload (SDU) size is fixed, determined by the bit rate. Following table shows SDU sizes defined. AAL2 is used. The AAL2 SSCS layer shall be supported for segmentation and re-assembly.
Table 13: Relationship between the bit rate and the SDU size
Bit rate |
SDU size (= RLC PDU payload size) |
28.8 kbit/s |
576 bits |
33.6 kbit/s |
672 bits |
32 kbit/s |
640 bits |
56/64 kbit/s |
640 bits |
The primitive Iu-UP-UNIT-DATA-REQUEST is invoked at regular intervals in order to have a constant bit rate (every SDU).