5 Radio Interface Protocol Architecture

3GPP43.051GSM/EDGE Overall descriptionRelease 17Stage 2TS

The radio interface protocol architetcture when GERAN connects to A or Gb interface is the same as defined in earlier releases. The clause below describes the protocol architecture when connecting through an Iu interface to the CN. Whether the protocol structure described in this clause (5) is also applicable for an evolved A interface is for further study. The multiplexing principles of data coming from the different CN interfaces (A, Gb and Iu) are illustrated in clause 5.3.

5.1 Protocol Structure when connecting through Iu

The radio interface is layered into three protocol layers:

– the physical layer (L1);

– the data link layer (L2);

– the network layer (L3).

Layer 2 is split into the following sublayers: Radio Link Control (RLC), Medium Access Control (MAC) protocol and Packet Data Convergence Protocol (PDCP). RLC/MAC is used as layer 2 in the control plane below RRC, except for operation on the BCCH, where DL is used. Whether the Broadcast/Multicast Control (BMC) protocol described in 25.301 is needed is for further study.

The protocol architecture is divided into Control (C-) and User (U-) planes. The RLC and MAC protocols and the physical layer carries data from both C- and U-plane. PDCP exists 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 GERAN. The next sublayer provides ‘Duplication avoidance’ functionality as specified in 3GPP TS 24.007. 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) are assumed to belong to the non-access stratum, and therefore not in the scope of 3GPP TSG GERAN. On the general level, the protocol architecture is similar to the current ITU-R protocol architecture, ITU-R M.1035.

Figure 10 shows the radio interface protocol architecture. Each block in Figure 10 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 logical channels when Flexible Layer One is not used, and transport channels when Flexible Layer One is used. 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. A description of these SAPs can be found in 3GPP TS 23.110.

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. 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 GERAN can be requested by the CN to prevent loss of data according to the quality of service requirements [UMTS 23.107] of the bearer in question (i.e. independently of the handovers on the radio interface), as long as an inter-BSS handover does not take place. This is a basic requirement to be fulfilled by the GERAN retransmission functionality as provided by the RLC sublayer. However, in case of the inter-BSS handover, the prevention of the loss of data may not be guaranteed autonomously by the GERAN but relies on ‘Duplication avoidance’ functions in the CN.

Figure 10: Radio Interface protocol architecture

Figure 10 reflects the radio interface protocol architecture when connecting to the Iu interface and the Flexible Layer One is not used.

Figure 11 below represents the radio interface protocol architecture in relation to the Flexible Layer One only.

Figure 11: Radio Interface protocol architecture with FLO

5.2 Multiplexing Principles

5.2.1 Multiplexing of different types of radio access bearers for one MS

GERAN can allocate multiple dedicated and shared basic physical subchannels to a mobile station. The allocation shall be consistent with the mobile station’s capability (See GSM 05.02 [11]).

Different types of Radio Access Bearer Services can be multiplexed for one MS using functionality of the MAC and/or the physical layers on one or more shared and/or dedicated basic physical subchannels. One radio bearer can only be mapped either on dedicated or shared basic physical subchannels.

5.2.2 Multiplexing of user plane data from different core network interfaces

Figure 12 shows the multiplexing principles on the network side of user plane data coming from different type of core network interfaces. User data from the Iu interface and user data from the Gb interface are multiplexed on MAC level, through shared basic physical subchannels, or on the physical layer, through different basic physical subchannels. User data coming from the A interface is multiplexed with user data from the Gb interface and user data from the Iu interface on the physical layer through different basic physical subchannels.

Figure 12: Multiplexing principles on the network side of user plane data flows coming
from different interfaces

5.3 Iu vs A/Gb mode selection

5.3.1 Introduction

A GERAN cell can support either A/Gb mode only, or Iu mode only, or both modes. The support of each mode depends on the interfaces via which the GERAN is connected to the Core Network nodes.

The support of each mode of operation by a GERAN cell is indicated in the broadcast system information messages, see subclause 6.3.1.

Similarly, the mode(s) of operation a mobile station supports is indicated in information concerning radio aspects of the mobile station, made available to the radio access network, see [25]. Iu mode support may also be indicated implicitly at radio access by the mobile station. A mobile station can only operate either in A/Gb mode or in Iu mode at a given time.

5.3.2 PLMN, cell and mode (re-)selection in GERAN

The procedures for PLMN selection apply independently of the mode(s) supported by the cells belonging to the available PLMNs.

The procedures for cell re-selection defined in A/Gb mode apply also in Iu mode. The cell re-selection may be under the control of the network or the mobile station, see [26].

In case cell re-selection is under control of the mobile station, cell re-selection shall only be based on radio criteria and not on which mode is supported by the neighbour cells, see [27]. The Iu mode shall be selected in the target cell if supported by both the cell and the mobile station unless otherwise ordered by the network.

NOTE: The above text outlines the default mechanism of mode of operation selection and does not prohibit the introduction of a more flexible solution, which avoids unnecessary mode of operation changes, at a later stage.

In case cell re-selection is under control of the network, the mode to apply in the target cell is provided by the network.

NOTE: When a mobile station is allocated dedicated physical sub-channels, the maintenance of the radio resources is handled via handover procedures by the network. Irrespective of the mode of operation in the current cell, the network will select the mode of operation to apply in the target cell.

Once the mode of operation has been chosen in the new cell, the relevant procedures to A/Gb or Iu mode apply. Procedures will be defined in A/Gb or Iu mode to allow for changing mode without changing cell.