4 Layered overview of radio interface

04.603GPPGeneral Packet Radio Service (GPRS)Mobile Station (MS) - Base Station System (BSS) interfaceRadio Link Control / Medium Access Control (RLC/MAC) protocolRelease 1999TS

The Radio Resource sublayer provides the functions necessary for

– Radio Resource (RR) management of packet data physical channels (PDCHs); and

– Radio Link Control and Medium Access Control (RLC/MAC) on packet data physical channels.

As shown in Figure 4.1, the RR sublayer provides services to the MM and LLC sublayers. The RR sublayer utilises the services of the Data Link layer (signalling layer 2) and the Physical Link layer. The packet logical channels PBCCH, PCCCH (including PPCH, PAGCH and PRACH), PACCH and PDTCH, are multiplexed onto the packet data physical channels on a per radio block basis.

Figure 4.1: Protocol architecture of Radio Resource (RR) sublayer and RLC/MAC function

4.1 Layer services

The RR sublayer provides services for the transfer of upper layer PDUs using a shared medium between multiple mobile stations and the network. Direct communication is only possible between the network and one or more mobile stations. The RLC/MAC function supports two modes of operation:

– unacknowledged operation; and

– acknowledged operation.

The RR sublayer further provides services for the paging of mobile stations.

4.2 Layer functions

The RLC function defines the procedures for segmentation and reassemble of LLC PDUs into RLC/MAC blocks and, in RLC acknowledged mode of operation, for the Backward Error Correction (BEC) procedures enabling the selective retransmission of unsuccessfully delivered RLC/MAC blocks. In RLC acknowledged mode of operation, the RLC function preserves the order of higher layer PDUs provided to it.

The RLC function provides also link adaptation.

In EGPRS in RLC acknowledged mode of operation, the RLC function may provide Incremental Redundancy (IR).

The MAC function defines the procedures that enable multiple mobile stations to share a common transmission medium, which may consist of several physical channels. The function may allow a mobile station to use several physical channels in parallel, i.e. use several timeslots within the TDMA frame.

For the mobile station originating access, the MAC function provides the procedures, including the contention resolution procedures, for the arbitration between multiple mobile stations simultaneously attempting to access the shared transmission medium.

For the mobile station terminating access, the MAC function provides the procedures for queuing and scheduling of access attempts.

4.3 Service primitives

Information flow between layers is performed by the use of Service Primitives. Service Access Points (SAP) and their corresponding Service Primitives for the RR sublayer are defined in 3GPP TS 04.07.

4.4 Services required from lower layers

The RLC/MAC function uses the services provided by the physical link layer as defined in 3GPP TS 04.04.

The RR sublayer may use the services provided by the data link layer as defined in 3GPP TS 04.05. Moreover, the RR sublayer directly uses services provided by the physical layer such as BCCH searching, as defined in 3GPP TS 04.04.