5.3.1 MAC Services and Functions
25.3013GPPRadio interface protocol architectureTS
This subclause provides an overview on services and functions provided by the MAC sublayer. A detailed description of the MAC protocol is given in [7].
5.3.1.1 MAC Services to upper layers
– Data transfer. This service provides unacknowledged transfer of MAC SDUs between peer MAC entities. This service provides data segmentation on HS-DSCH and E-DCH but not for other transport channels. Therefore, segmentation/reassembly function should be achieved by upper layer when HS-DSCH or E-DCH is not used and optionally when HS-DSCH or E-DCH is used.
– 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. In TDD mode, in addition, the MAC can handle resource allocation autonomously.
– Reporting of measurements. Local measurements such as traffic volume and quality indication are reported to RRC.
5.3.1.1.1 Logical channels
The MAC layer provides data transfer services on logical channels. A set of logical channel types is defined for different kinds of data transfer services as offered by MAC. Each logical channel type is defined by what type of information is transferred.
A general classification of logical channels is into two groups:
– Control Channels (for the transfer of control plane information);
– Traffic Channels (for the transfer of user plane information).
The configuration of logical channel types is depicted in Figure 3.
Figure 3: Logical channel structure
Control Channels
Control channels are used for transfer of control plane information only.
Broadcast Control Channel (BCCH)
A downlink channel for broadcasting system control information.
Paging Control Channel (PCCH)
A downlink channel that transfers paging information. This channel is used when the network does not know the location cell of the UE, or, the UE is in the cell connected state (utilising UE sleep mode procedures).
Common Control Channel (CCCH)
Bi-directional channel for transmitting control information between network and UEs. This channel is commonly used by the UEs having no RRC connection with the network and by the UEs using common transport channels when accessing a new cell after cell reselection.
Dedicated Control Channel (DCCH)
A point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. This channel is established through RRC connection setup procedure.
Shared Channel Control Channel (SHCCH)
Bi-directional channel that transmits control information for uplink and downlink shared channels between network and UEs. This channel is for TDD only.
MBMS point-to-multipoint Control Channel (MCCH)
A point-to-multipoint downlink channel used for transmitting control information from the network to the UE. This channel is only used by UEs that receive MBMS.
MBMS point-to-multipoint Scheduling Channel (MSCH)
A point-to-multipoint downlink channel used for transmitting scheduling control information, from the network to the UE, for one or several MTCHs carried on a CCTrCH. This channel is only used by UEs that receive MBMS.
Traffic Channels
Traffic channels are used for the transfer of user plane information only.
Dedicated Traffic Channel (DTCH)
A Dedicated Traffic Channel (DTCH) is a point-to-point channel, dedicated to one UE, for the transfer of user information. A DTCH can exist in both uplink and downlink.
Common Traffic Channel (CTCH)
A point-to-multipoint unidirectional channel for transfer of dedicated user information for all or a group of specified UEs.
MBMS point-to-multipoint Traffic Channel (MTCH)
A point-to-multipoint downlink channel used for transmitting traffic data from the network to the UE. This channel is only used for MBMS.
5.3.1.1.2 Mapping between logical channels and transport channels
5.3.1.1.2.1 Mapping in Uplink
In Uplink, the following connections between logical channels and transport channels exist:
– CCCH can be mapped to RACH;
– CCCH can be mapped to E-DCH (in FDD and 1.28 Mcps TDD mode only);
– DCCH can be mapped to RACH;
– DCCH can be mapped to DCH;
– DCCH can be mapped to USCH (in TDD mode only);
– DCCH can be mapped to E-DCH;
– DTCH can be mapped to RACH;
– DTCH can be mapped to DCH;
– DTCH can be mapped to USCH (in TDD mode only);
– SHCCH can be mapped to RACH (in TDD mode only);
– SHCCH can be mapped to USCH (in TDD mode only);
– DTCH can be mapped to E-DCH.
5.3.1.1.2.2 Mapping in Downlink
In Downlink, the following connections between logical channels and transport channels exist:
– BCCH can be mapped to BCH;
– BCCH can be mapped to FACH;
– BCCH can be mapped to HS-DSCH (in FDD and 1.28 Mcps TDD mode only);
– PCCH can be mapped to PCH;
– PCCH can be mapped to HS-DSCH (in FDD and 1.28 Mcps TDD mode only);
– CCCH can be mapped to FACH;
– CCCH can be mapped to HS-DSCH (in FDD and 1.28 Mcps TDD mode only);
– DCCH can be mapped to FACH;
– DCCH can be mapped to DSCH (in TDD mode only);
– DCCH can be mapped to HS-DSCH;
– DCCH can be mapped to DCH;
– MCCH can be mapped to FACH;
– MSCH can be mapped to FACH;
– DTCH can be mapped to FACH;
– DTCH can be mapped to DSCH (in TDD mode only);
– DTCH can be mapped to HS-DSCH;
– DTCH can be mapped to DCH;
– CTCH can be mapped to FACH;
– MTCH can be mapped to FACH;
– SHCCH can be mapped to FACH (in TDD mode only);
– SHCCH can be mapped to DSCH (in TDD mode only).
The mappings as seen from the UE and UTRAN sides are shown in Figure 4 and Figure 5 respectively.
Figure 4: Logical channels mapped onto transport channels, seen from the UE side
Figure 5: Logical channels mapped onto transport channels, seen from the UTRAN side
5.3.1.2 MAC functions
The functions of MAC include:
– Mapping between logical channels and transport channels. The MAC is responsible for mapping of 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 are e.g. given by 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 L1 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.
– Priority handling between UEs by means of dynamic scheduling. In order to utilise the spectrum resources efficiently for bursty transfer, a dynamic scheduling function may be applied. MAC realises priority handling on common transport channels, shared transport channels and for the dedicated and common E-DCH transport channel. Note that for dedicated transport channels other than E-DCH, the equivalent of the dynamic scheduling function is implicitly included as part of the reconfiguration function of the RRC sublayer.
NOTE: In the TDD mode the data to be transported are represented in terms of sets of resource units.
– 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. In FDD and 1.28 Mcps TDD mode only, if a UE is in CELL_FACH, CELL_PCH or URA_PCH state and HS-DSCH is used as a common transport channel, it is possible to identify the UE in physical layer using dedicated H-RNTI on HS-SCCH if the H-RNTI is dedicated for the UE. In FDD mode only, if a UE is in CELL_FACH state or IDLE mode and E-DCH is used as a common transport channel, there is a need for inband identification of the UE.
– Multiplexing/demultiplexing of upper layer PDUs into/from transport blocks delivered to/from the physical layer on common transport channels. MAC should support service multiplexing for common transport channels, since the physical layer does not support multiplexing of these channels.
– Multiplexing/demultiplexing of upper layer PDUs into/from transport block sets delivered to/from the physical layer on dedicated transport channels. The MAC allows service multiplexing for dedicated transport channels. This function can be utilised when several upper layer services (e.g. RLC instances) can be mapped efficiently on the same transport channel. In this case the identification of multiplexing is contained in the MAC protocol control information.
– Multiplexing/demultiplexing of upper layer PDUs into transport blocks delivered to/from the physical layer on HS-DSCH. The MAC allows service multiplexing for HS-DSCH. This function can be utilised to multiplex data from several upper layer services (e.g. RLC instances). In this case the identification of multiplexing is contained in the MAC protocol control information.
– Traffic volume measurement. Measurement of traffic volume on logical channels and reporting to RRC. Based on the reported traffic volume information, RRC performs transport channel switching decisions.
– Transport Channel type switching. Execution of the switching between common and dedicated transport channels based on a switching decision derived by RRC.
– Ciphering. This function prevents unauthorised acquisition of data. Ciphering is performed in the MAC layer for transparent RLC mode. Details of the security architecture are specified in [15].
– Access Service Class selection for RACH transmission and Enhanced Uplink for CELL_FACH and Idle mode. The RACH and E-DCH (as common transport channel) resources (i.e. access slots and preamble signatures for FDD, timeslot and channelisation code for TDD) may be divided between different Access Service Classes in order to provide different priorities of RACH and E-DCH (as common transport channel) usage. In addition it is possible for more than one ASC or for all ASCs to be assigned to the same access slot/signature space. Each access service class will also have a set of back-off parameters associated with it, some or all of which may be broadcast by the network. The MAC function applies the appropriate back-off and indicates to the PHY layer the RACH partition associated to a given MAC PDU transfer.
– HARQ functionality for HS-DSCH and E-DCH transmission. The MAC-hs, MAC-ehs, MAC-e and MAC-i entities are responsible for establishing the HARQ entity in accordance with the higher layer configuration and handling all the tasks required to perform HARQ functionality. This functionality ensures delivery between peer entities by use of the ACK and NACK signalling between the peer entities.
– Data segmentation/re-assembly for HS-DSCH and E-DCH. Higher layer SDUs can be segmented by the transmitting MAC-hs/MAC-ehs or MAC-i/is entity to fit the available radio resource. Reassembly is provided in the receiving MAC-hs/MAC-ehs or MAC-is entity.
– In-sequence delivery and assembly/disassembly of higher layer PDUs on HS-DSCH. In CELL_DCH state the transmitting MAC-hs/MAC-ehs entity assembles the data block payload for the MAC-hs/MAC-ehs PDUs from the delivered MAC-d PDUs. For MAC-hs, the MAC-d PDUs that are assembled in any one MAC-hs PDU are the same priority, and from the same MAC-d flow. For MAC-ehs the MAC-d PDUs that are assembled in any one MAC-ehs PDU may be of the same or different priority, and from the same or different MAC-d flow. The receiving MAC-hs/MAC-ehs entity is then responsible for the reordering of the received data blocks according to the received TSN, per priority and MAC-d flow, and then disassembling the data block into MAC-d PDUs for in-sequence delivery to the higher layers. In FDD and 1.28 Mcps TDD mode in CELL_FACH, CELL_PCH and URA_PCH state the transmitting MAC-ehs entity assembles the data block payload for the MAC-ehs PDUs from the delivered MAC-d PDUs and MAC-c PDUs for CCCH, BCCH and PCCH logical channels.
– In-sequence delivery and assembly/disassembly of higher layer PDUs on E-DCH. The transmitting MAC-es/MAC-e or MAC-i/is entity assembles the data block payload for the MAC-e/MAC-i PDUs from the delivered MAC-d PDUs. For DTCH and DCCH transmission, the receiving MAC-es or MAC-is entity is then responsible for the reordering of the received data blocks according to the received TSN and Node-B tagging information, per re-ordering queue, and then disassembling the data block into MAC-d PDUs for in-sequence delivery to the higher layers. For CCCH transmission, the receiving MAC-is entity is then responsible for the reordering of the received data blocks according to the received TSN and Node-B tagging information, then disassembling the data block into a MAC-c PDU. If more then one MAC-is PDU was received, the added check sum is removed and is verified, before delivery to the higher layers. A failure to verify the checksum results in discarding the MAC-c PDU.