4 Layered overview of radio interface

3GPP44.060General Packet Radio Service (GPRS)Mobile Station (MS) - Base Station System (BSS) interfaceRadio Link Control / Medium Access Control (RLC/MAC) protocolRelease 17TS

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

Figure 4.2 shows how the RR sublayer provides services to the MM and LLC sublayers in EC operation. The RR sublayer utilises the services of the Data Link layer (signalling layer 2) and the Physical Link layer. The packet logical channels EC-PACCH and EC-PDTCH, are multiplexed onto the packet data physical channels on a per radio block basis or, in case a Coverage Class other than CC1 requiring the use of blind physical layer transmissions, on a predetermined number of radio blocks, using one or four PDCH(s). Blind physical layer transmissions on EC-PDTCH and EC-PACCH channels in uplink and downlink are mapped on to either 4 consecutive PDCH resources or 2 consecutive PDCH resources. The number of consecutive PDCH resources used for EC TBF operation in an EC-GSM-IoT capable cell is controlled via the EC System information type 2 message (refer to 3GPP TS 44.018). The mapping of EC logical channels onto physical channels shall be as specified in 3GPP TS 45.002.

Figure 4.2: Protocol architecture of Radio Resource (RR) sublayer and RLC/MAC function in EC operation

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 three modes of operation:

– unacknowledged operation;

– acknowledged operation; and

– non-persistent operation.

Only acknowledged operation is supported in EC 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 reassembly 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 EC-GSM-IoT or 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.

When the PS handover procedure is supported the RLC and MAC functions provide the procedures for directing a mobile station to a new cell and continuing packet switched services without the use of contention resolution procedures (i.e. RACH/PRACH) in the new cell.

When the DTM handover procedure is supported the procedures for directing a mobile station to a new cell and continuing both circuit switched and packet switched services are provided by the RLC and MAC functions, under the control of the RR function (see 3GPP TS 44.018).

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 24.007.

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 44.004.

The RR sublayer may use the services provided by the data link layer as defined in 3GPP TS 44.005. Moreover, the RR sublayer directly uses services provided by the physical layer such as BCCH searching, as defined in 3GPP TS 44.004.