9.4 3G Services
9.6 Air-Interface Architecture and Processing
The Uu interface is the air interface between a mobile station and a radio access network.
The Uu interface consists of three layers:
• Physical layer (Layer 1)
• Data-link layer (Layer 2) contains sublayers – Medium access control (MAC)
– Radio link control (RLC)
• Network layer (Layer 3)
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L3
controlcontrol
control
control
Logical Channels
Transport Channels C-plane signalling U-plane information
PHY
L2/MAC
L1 RLC
DC Nt GC
L2/RLC
MAC
RLC RLC RLC
RLC
RLC RLC RLC Duplication avoidance
UuS boundary
BMC L2/BMC
control
PDCPPDCP L2/PDCP
DC Nt GC
Radio Bearers RRC
Figure 9.20 Air-interface protocol reference architecture [9].
The protocol architecture is also divided into the control plane responsible for trans- mission of signalling information and the user plane with dedicated user data transfer.
Figure 9.20 shows the reference architecture for a radio interface protocol stack.
The control plane includes application protocols and signalling bearers for transport- ing the application protocol messages. The application protocols include:
• Radio resource control (RRC)
• Radio link control (RLC)
• Medium access control (MAC)
The user plane consists of transport channels and physical channels allocated for the connection and the data carried over the connection. Essential protocols are:
• Packet data convergence protocol (PDCP), which is used with the packet data.
• Broadcast/multicast control (BMC), which is used for broadcast services.
• Radio link control (RLC).
• Medium access control (MAC).
The layered structure of air interface implies message transfer services between layers.
Such a transfer of information requires the definition of adequate interfaces between adjacent layers specifying the path that information follows depending on its nature.
9.6.1 Physical Layer (Layer 1)
The physical layer offers transport channels to the higher layers. The physical layer sup- ports information transfer to the MAC and higher layers through transport channels. A radio link is a bi-directional connection between a mobile station and a base station cell.
Each radio link is comprised of one or more traffic channels that are associated with the same physical layer control channel (DPCCH).
The physical layer is controlled by a data-link layer that has two sublayers: MAC and RLC. MAC realizes transport channel management and RLC realizes flow control. RRC manages the physical layer and its activities.
The main functions of the physical layer are:
• channel coding, interleaving and rate matching
• measuring
• macrodiversity splitting or combining and executing soft handovers
• handling cyclic redundancy check on transport channels
• multiplexing/demultiplexing of transport channels and of coded composite transport channels
• mapping of coded composite transport channels on physical channels
• modulating and spreading, or demodulating and despreading of physical channels
• frequency and time synchronization
• closed-loop power control
• power weighting and combining of physical channels
• radio frequency processing
9.6.2 Medium Access Control (MAC) on Layer 2
The MAC layer maps the logical channels to the transport channels. The layer also selects an appropriate transport format for each transport channel depending on the instantaneous source rates of the logical channels. The transport format is selected with respect to atransport format combination set(TFCS) that is defined by the admission control for each connection. Some of the main functions of MAC are:
• transferring data and providing unacknowledged transfer of MAC Service Data Units (SDU) between peer MAC entities
• reporting measurements: local measurements like traffic volume, quality indication and so on to RRC
• selecting appropriate transport format from the TFCS for each transport channel depending on instantaneous source rate
• priority handling, between data flows of a MS and between users, by means of dynamic scheduling
• multiplexing or demultiplexing higher-layer protocol data units (PDU) into/from transport blocks delivered to/from the physical layer on common transport channels
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• multiplexing or demultiplexing higher-layer PDUs into/from transport-block sets delivered to/from the physical layer on dedicated transport channels
• mapping between logical channels and transport channels, monitoring traffic volume and dynamic transport channel type switching.
9.6.3 Radio Link Control (RLC) on Layer 2
The RLC protocol provides segmentation and retransmission services for both the user and control data to the higher layers. In the control plane, the RLC provides a service called thesignallingradio bearer. In the user plane to circuit-switched core network, the RLC provides a service called the radio bearer (PDCP protocol is not used by the service). In the user plane to the packet-switched core network, PDCP above the RLC provides the radio bearer.
Each radio link control instance is configured by the RRC to operate in one of three modes:
1) Transparent mode (TM): no protocol overhead is added to higher-layer data and erroneous data can be discarded or marked.
2) Unacknowledged mode (UM): no retransmission protocol is used or data delivery guaranteed. Erroneous data is discarded or marked.
3) Acknowledged mode (AM): automatic repeat request is used for error correction, notification is sent in case data is not delivered to the destination.
The main functions of RLC are:
• establishing and releasing data-link layer connections
• segmenting and reassembling packet data units, concatenation and padding
• transferring user data, which is controlled by QoS setting
• error correction, detecting and recovering protocol errors
• sequenced delivery of higher-layer packet data units, controlling data flow
• ciphering.
9.6.4 RRC on Layer 3 in the Control Plane
Control signalling consists mostly of RRC messages in the Uu interface. The control interfaces between the RRC and all the lower layer protocols are used for:
• configuring characteristics of the lower layer protocol entities, including parameters for the physical, transport and logical channels
• reporting measurement results and errors to the RRC.
The RRC layer handles the main part of control signalling between the Mobile Switch- ing System and RAN. The main functions of RRC are:
• broadcasting system information
• paging
• initial cell selection and reselection in idle mode based on system information broad- casted by RNC and on measurements from mobile station (MS)
• establishing, maintaining and releasing RRC connections between mobile station and radio access network
• controlling radio bearers, transport channels and physical channels
• controlling security functions (ciphering and integrity protection)
• RRC connection mobility functions, such as cell or UTRAN registration area (URA) updates and handovers
• supporting SRNS relocation (information to the target SRNS)
• performing downlink outer loop power control and open-loop power control
• higher-layer signalling with core network (direct transfer).