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Medium Access Control
The MAC model maps the transport channels it receives from the physical layer to the logical channels it passes on to the Radio Link Control protocol and vice versa.
MAC takes each RLC PDU from the logical channel and constructs a MAC PDU (also known as transport block) according to the Transport Format defined for the transport channel. Each transport channel can have different bit rates. Thus, the MAC model is responsible for transporting blocks of data according to the specified channel bit rate. The illustration shows the position of the MAC protocol.
Physical Layer (PHY) Medium Access Control (MAC) RLC RLC RLC RLC RLC RLCRLC RLC BMC PDCP DCP control control GC Nt DC
Radio Resource Control (RRC) control Layer 3 L2/PDCP L2/BMC L2/RLC Logical Channels L2/MAC Transport Channels L1 Physical Channels C-plane signaling U-plane information
UTRAN Signaling
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Functions and Services
The MAC sublayer provides the following functions and services: Function and service Description Mapping between logical channels and transport channels.
The MAC is responsible for mapping logical channel(s) onto the appropriate transport channel(s).
Selection of appropriate Transport Format for each Transport Channel depending on instantaneous source rate.
Given the Transport Format Combination Set assigned by RRC, MAC selects the appropriate transport format within an assigned transport format set for each active transport channel depending on source rate. The control of transport formats ensures efficient use of transport channels.
Priority handling between data flows of one UE.
When selecting between the Transport Format Combinations in the given Transport Format Combination Set, priorities of the data flows to be mapped onto the corresponding Transport Channels can be taken into account. Priorities may be given according to attributes of Radio Bearer services and RLC buffer status. The priority handling is achieved by selecting a Transport Format Combination for which high priority data is mapped onto layer 1 with a ″high bit rate″ Transport Format, at the same time letting lower priority data be mapped with a ″low bit rate″ (could be zero bit rate) Transport Format. Transport format selection may also take into account transmit power indication from layer 1.
Identification of UEs on common transport channels.
When a particular UE is addressed on a common downlink channel, or when a UE is using the RACH, there is a need for inband identification of the UE. Since the MAC layer handles the access to, and multiplexing onto, the transport channels, the identification functionality is naturally also placed in MAC. Traffic volume
monitoring.
Measurement of traffic volume on logical channels and reporting to RRC. Based on the reported traffic volume information, RRC performs transport channel switching decisions.
Ciphering This function prevents unauthorised acquisition of data. Ciphering
is performed in the MAC layer for transparent RLC mode.
Data transfer This service provides unacknowledged transfer of MAC SDUs
between peer MAC entities. This service does not provide any data segmentation. Therefore, segmentation/reassembly function should be achieved by upper layer.
UTRAN Signaling Medium Access Control
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Function and service
Description
Reallocation of radio resources and MAC parameters.
This service performs on request of RRC execution of radio resource reallocation and change of MAC parameters, i.e. reconfiguration of MAC functions such as change of identity of UE, change of transport format (combination) sets, change of transport channel type.
MAC entities
MAC is structured into the following dedicated MAC entities: • Dedicated MAC (MAC-d) terminates in the SRNC
• Common MAC (MAC-c/sh) terminates in the CRNC
• MAC-b is the entity that handles the broadcast channel (BCH). There is one MAC-b entity in each UE and one MAC-b in the Node B.
Data transfer type
The MAC protocol can be one of two data transfer types: • transparent MAC
• or non-transparent MAC.
The data transfer type depends on whether a MAC header is attached to the packet. The case where no MAC header is required is referred to as ″transparent″ MAC transmission.
Parameters of the MAC header
The following fields are defined for the MAC header:
• The C/D field is a single-bit flag that provides identification of the logical channel class on FACH and RACH transport channels, i.e. whether it carries CCCH or dedicated logical channel information.
• The C/T field provides identification of the logical channel instance when multiple logical channels are carried on the same transport channel. The C/T field is used also to provide identification of the logical channel type on dedicated transport channels and on FACH and RACH when used for user data transmission. • The UE-Id field provides an identifier of the UE.
Depending on the logical to transport mapping relationship, all, a selection or none of the above parameters may be used.
UTRAN Signaling Medium Access Control
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Here are some examples:
• DTCH or DCCH mapped to DCH, no multiplexing of dedicated channels on MAC: No MAC header is required.
• DTCH or DCCH mapped to DCH, with multiplexing of dedicated channels on MAC: C/T field is included in MAC header.
• DTCH or DCCH mapped to RACH/FACH: C/D field and UE-Id are included in the MAC header. C/T field is included if multiplexing on MAC is applied.
UTRAN Signaling Medium Access Control
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