5.1 Overall protocol structure

25.3013GPPRadio interface protocol architectureTS

The radio interface is layered into three protocol layers:

– the physical layer (L1);

– the data link layer (L2);

– network layer (L3).

Layer 2 is split into following sublayers: Medium Access Control (MAC), Radio Link Control (RLC), Packet Data Convergence Protocol (PDCP) and Broadcast/Multicast Control (BMC).

Layer 3 and RLC are divided into Control (C-) and User (U-) planes. PDCP and BMC exist in the U-plane only.

In the C-plane, Layer 3 is partitioned into sublayers where the lowest sublayer, denoted as Radio Resource Control (RRC), interfaces with layer 2 and terminates in the UTRAN. The next sublayer provides ‘Duplication avoidance’ functionality as specified in [13]. It terminates in the CN but is part of the Access Stratum; it provides the Access Stratum Services to higher layers. The higher layer signalling such as Mobility Management (MM) and Call Control (CC) is assumed to belong to the non-access stratum, and therefore not in the scope of 3GPP TSG RAN. On the general level, the protocol architecture is similar to the current ITU-R protocol architecture, ITU-R M.1035.

Figure 2 shows the radio interface protocol architecture. Each block in Figure 2 represents an instance of the respective protocol. Service Access Points (SAP) for peer-to-peer communication are marked with circles at the interface between sublayers. The SAP between MAC and the physical layer provides the transport channels (cf. subclause 5.2.1.1). The SAPs between RLC and the MAC sublayer provide the logical channels (cf. subclause 5.3.1.1.1). The RLC layer provides three types of SAPs, one for each RLC operation mode (UM, AM, and TM, see [8]). PDCP and BMC are accessed by PDCP and BMC SAPs, respectively. The service provided by layer 2 is referred to as the radio bearer. The C-plane radio bearers, which are provided by RLC to RRC, are denoted as signalling radio bearers. In the C-plane, the interface between ‘Duplication avoidance’ and higher L3 sublayers (CC, MM) is defined by the General Control (GC), Notification (Nt) and Dedicated Control (DC) SAPs.

NOTE: The SAPs shown in Figure 2 are examples. For details on the definition of SAPs refer to the respective radio interface protocol specification.

Also shown in the figure are connections between RRC and MAC as well as RRC and L1 providing local inter-layer control services. An equivalent control interface exists between RRC and the RLC sublayer, between RRC and the PDCP sublayer and between RRC and BMC sublayer. These interfaces allow the RRC to control the configuration of the lower layers. For this purpose separate Control SAPs are defined between RRC and each lower layer (PDCP, RLC, MAC, and L1).

The RLC sublayer provides ARQ functionality closely coupled with the radio transmission technique used. There is no difference between RLC instances in C and U planes.

The MAC sublayer is made up of several different MAC entities, MAC-d, MAC-c/sh/m, MAC-hs/MAC-ehs, MAC-es/MAC-e, MAC-i/is and MAC-m.

The MAC-hs/MAC-ehs entity provides Hybrid ARQ functionality, and is only used on the HS-DSCH.

The MAC-es/MAC-e and MAC-i/is entities provide Hybrid ARQ functionality, and are only used with E-DCH.

The MAC-m entity provides selection combining functionality for MTCH from different cells. MAC-m is only used for FACH carrying MTCH and MSCH.

The UTRAN can be requested by the CN to prevent all loss of data (i.e. independently of the handovers on the radio interface), as long as the Iu connection point is not modified. This is a basic requirement to be fulfilled by the UTRAN retransmission functionality as provided by the RLC sublayer.

However, in case of the Iu connection point is changed (e.g. SRNS relocation, streamlining), the prevention of the loss of data may not be guaranteed autonomously by the UTRAN but relies on ‘Duplication avoidance’ functions in the CN.

There are primarily two kinds of signalling messages transported over the radio interface – RRC generated signalling messages and NAS messages generated in the higher layers. On establishment of the signalling connection between the peer RRC entities three or four UM/AM signalling radio bearers may be set up. Two of these bearers are set up for transport of RRC generated signalling messages – one for transferring messages through an unacknowledged mode RLC entity (see subclause 5.3.2. for details on RLC modes) and the other for transferring messages through an acknowledged mode RLC entity. One signalling radio bearer is set up for transferring NAS messages set to "high priority" by the higher layers. An optional signalling radio bearer may be set up for transferring NAS messages set to "low priority" by the higher layers. Subsequent to the establishment of the signalling connection zero to several TM signalling radio bearers may be set up for transferring RRC signalling messages using transparent mode RLC.

Figure 2: Radio Interface protocol architecture (Service Access Points marked by circles)

5.1.1 Service access points and service primitives

Each layer provides services at Service Access Points (SAPs). A service is defined by a set of service primitives (operations) that a layer provides to upper layer(s).

Control services, allowing the RRC layer to control lower layers locally (i.e. not requiring peer-to-peer communication) are provided at Control SAPs (C-SAP). Note that C-SAP primitives can bypass one or more sublayers, see Figure 2.

In the radio interface protocol specifications, the following naming conventions for primitives shall be applicable:

– Primitives provided by SAPs between adjacent layers shall be prefixed with the name of the service-providing layer, i.e. PHY, MAC, RLC, PDCP, BMC or UUS.

– Primitives provided by SAPs to an application shall be prefixed with the name of the service-providing layer, i.e. RRC.

– Primitives provided by Control SAPs, in addition to the name of the service-providing layer, shall be prefixed with a "C", i.e. CPHY, CMAC, CRLC, CPDCP or CBMC.

This principle leads to the following notations, where <Type> corresponds to request, indication, response or confirm type of primitives:

Primitives between PHY and MAC:

PHY- <Generic name> – <Type>

Primitives between PHY and RRC (over C-SAP):

CPHY- <Generic name> – <Type>

Primitives between MAC and RLC:

MAC- <Generic name> – <Type>

Primitives between MAC and RRC (over C-SAP):

CMAC- <Generic name> – <Type>

Primitives between RLC and upper layers, between RLC and RRC for data transfer and between RLC and PDCP:

RLC- <Generic name> – <Type>

Primitives between RLC and RRC for control of RLC (over C-SAP):

CRLC- <Generic name> – <Type>

Primitives above Uu Stratum:

UUS- <Generic name> – <Type>

Primitives between PDCP and non-access stratum:

PDCP- <Generic name> – <Type>

Primitives between PDCP and RRC (over C-SAP):

CPDCP- <Generic name> – <Type>

Primitives between BMC and upper layer:

BMC- <Generic name> – <Type>

Primitives between BMC and RRC for control of BMC (over C-SAP):

CBMC- <Generic name> – <Type>

In this model, some UUS primitives map directly to RLC primitives without intervening function.