Last Updated: 03/03/2005
Introduction MAC is a Layer 2 protocol and it resides between Physical Layer (L1) and RLC. The internal configuration of MAC is done by the RRC layer (L3). The interface between PHY and MAC are the transport channels where as the interface between RLC and MAC are the logical channels. MAC provides services to both User plane as well as Control plane. MAC takes care of circuit switched as well as packet switch traffic and signalling traffic as well. ![]() Fig 1: Radio Interface Protocol Architecture (Taken from 25.301, figure 2) One important thing to remember is that the MAC is not symmetrical protocol. It is different in UL and DL. MAC Services MAC sublayer provides following services to the upper layers:
MAC Functions MAC Functions include:
Channel Structure Before discussing anything further, we should look at the channel structures for Layer 1, MAC and RLC. The Transport Channels are interface between MAC and Layer 1, while Logical Channels are interface between MAC and RLC. Transport channels can be further subdivided into Common Transport Channels (where there is need for inband identification of the UEs when particular UEs are addressed); and dedicated transport channels (where the UEs are identified by the physical channel, i.e. code and frequency for FDD and code, time slot and frequency for TDD). Common transport channel types are:
Dedicated transport channel types are:
A general classification of logical channels is into two groups; Control Channels (for the transfer of control plane information) and Traffic Channels (for the transfer of user plane information). Control Channels:
Traffic Channels:
The mappings of Logical channels to transport channels is shown in the diagram below.
![]() Fig. 2: Logical channels mapped onto transport channels, seen from the UE side (Taken from 25.301, figure 4) ![]() Fig. 3: Logical channels mapped onto transport channels, seen from the UTRAN side (Taken from 25.301, figure 5) MAC Entities As mentioned earlier, the MAC protocol is not symmetrical in UL and DL. The MAC entities might be present in the UL and/or DL. The functional entities are as follows:
Discussion of Functions of MAC 1. Ciphering of Transparent Mode Radio Bearers In case of TM RBs, RLC acts like a pipe that transfers the data to the MAC layer without adding any headers. Thus in this case Ciphering has to be performed in the MAC layer. In all other cases (AM and UM RBs), Ciphering is performed in the RLC layer. The part of MAC PDU that is ciphered is the MAC SDU as shown in the fig below.
![]() Fig. 4: Ciphered part of MAC PDU (Taken from 25.321, figure 11.5.1) 2. Traffic Volume Measurement for Dynamic RB Control Dynamic radio bearer control is performed by RRC, based on the traffic volume measurements reported by MAC. Traffic volume information is measured in MAC layer and the results are reported from MAC layer to RRC layer. This concept can be explained with the help of the diagram below.
UE-RRC UE-RLC UE-MAC UTRAN ------- -------- ---------- ------- | | | | | Measurement Control Message (Traffic Volume Meas) | |<--------------------------------------------------------------------------| | | | | | CMAC-Measure-REQ | | | |------------------------------------------------->| | | | MAC-Data-REQ | | | |------------------------->| | | | MAC-Data-REQ | | | |------------------------->| | | CMAC-Measure-IND | | |<-------------------------------------------------| | | | | | | Measurement Report (Traffic VOlume Meas) | | |-------------------------------------------------------------------------->| | | | | | | | | At least every TTI, the MAC layer shall receive from each RLC entity the value of its Buffer Occupancy (BO), expressed in bytes. RRC can configure MAC to keep track of statistics (i.e. raw BO, average of BO and variance of BO) on the BO values of all Radio Bearers mapped onto a given transport channel. When the average or variance are requested, an averaging interval duration will also be provided. Every time the BO values are reported to MAC, the UE shall verify whether an event was triggered or if a periodic report is required. If reporting is required (multiple reports may be triggered in a single TTI), the MAC shall deliver to RRC the reporting quantities required for the corresponding RBs. In the case of average and variance of BO, the averaging should be performed for the interval with the configured duration ending at the time when the event was triggered.
3. Access Service Class Selection for RACH Transmission The physical RACH resources (i.e. access slots and preamble signatures for FDD) may be divided between different Access Service Classes in order to provide different priorities of RACH usage. It is possible for more than one ASC or for all ASCs to be assigned to the same access slot/signature space. Access Service Classes are numbered in the range 0 £ i £ NumASC £ 7 (i.e. the maximum number of ASCs is 8). An ASC is defined by an identifier i that defines a certain partition of the PRACH resources and an associated persistence value Pi. A set of ASC parameters consists of NumASC+1 such parameters (i, Pi), i = 0, …, NumASC. The PRACH partitions and the persistence values Pi are derived by the RRC protocol from system information. The set of ASC parameters is provided to MAC with the CMAC-Config-REQ primitive. The ASC enumeration is such that it corresponds to the order of priority (ASC 0 = highest priority, ASC 7 = lowest priority). ASC 0 shall be used in case of Emergency Call or for reasons with equivalent priority. At radio bearer setup/reconfiguration each involved logical channel is assigned a MAC Logical channel Priority (MLP) in the range 1,…,8. When the MAC sublayer is configured for RACH transmission in the UE, these MLP levels shall be employed for ASC selection on MAC. The following ASC selection scheme shall be applied, where NumASC is the highest available ASC number and MinMLP the highest logical channel priority assigned to one logical channel:
When an RRC CONNECTION REQUEST message is sent RRC determines ASC by means of the access class. The ASC to be used in these circumstances is signalled to MAC by means of the CMAC-CONFIG-REQ message. If MAC has knowledge of a U-RNTI then the ASC is determined in the MAC entity. If no U-RNTI has been indicated to MAC then MAC will use the ASC indicated in the CMAC-CONFIG-REQ primitive.
4. Transport format combination selection in UE RRC can control the scheduling of uplink data by giving each logical channel a priority between 1 and 8, where 1 is the highest priority and 8 the lowest. TFC selection in the UE shall be done in accordance with the priorities indicated by RRC. Logical channels have absolute priority, i.e. the UE shall maximise the transmission of higher priority data. Hence higher priority data flows will be given higher bit rate combinations while lower priority data flows will have to use low bit rate combinations. Note that zero bit rate is a special case of low bit rate combination. MAC has to derive the priority parameters from two sources:
A given TFC can be in any of the following states:
![]() Fig. 5. State transitions for the state of a given TFC (Taken from [1], fig. 11.4.1) UEs in CELL_FACH state may estimate the channel path loss and set to excess power state all the TFCs requiring more power than the Maximum UE transmitter power. All other TFCs shall be set to Supported state. Every time the set of supported TFCs changes, the available bitrate shall be indicated to upper layers for each logical channel in order to facilitate the adaptation of codec data rates when codecs supporting variable-rate operation are used. The details of the computation of the available bitrate and the interaction with the application layer are not further specified. At Radio Bearer Setup (or Reconfiguration), each logical channel is assigned MAC logical channel prioroty (MLP) in rage 1-8 by RRC. MAC has to then select a TFC that can transmit as much as and as high priority data as possible. In a noisy cell, a high priority TFC can be blocked if its use would cause the UE to transmit more power than the UEs current maximum transmitter power.
References: [1] 3GPP TS 25.321: Medium Access Control (MAC) protocol specification [2] 3GPP TS 25.301: Radio Interface Protocol Architecture [3] 3GPP TS 23.110: UMTS Access Stratum; Services and Functions [4] 3GPP TS 25.401: RAN Overall Description [5] Introduction to 3G Mobile Communications - Juha Korhonen |
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