WiMAX MAC Layer

- an overview of the essentials of the WiMAX MAC layer - the media access control layer for the IEEE 802.16 system.

The WiMAX MAC layer, or IEEE 802.16 MAC is an essential elements within the overall WiMAX software stack.

The WiMAX MAC layer is a form of MAC used for the WiMAX system.

WiMAX MAC layer basics

A MAC layer or Media Access Control data communication protocol sub-layer may also be known as a Medium Access Control layer.

A MAC layer is a sub-layer of the Data Link Layer. This is defined in the standard seven-layer OSI model as layer 2. The MAC layer provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multi-point network, typically a local area network (LAN) or metropolitan area network (MAN).

The WiMAX MAC has been designed and optimised to enable point to multipoint wireless applications and the WiMAX MAC layer provides an interface between the physical layer and the higher application layers within the stack.

The WiMAX MAC layer has to meet a number of requirements:

  • Point to multipoint:   One of the main requirements for WiMAX is that it must be possible for a base station to communicate with a number of different outlying users, either fixed or mobile. To achieve this, the IEEE 802.16, WiMAX MAC layer is based on collision sense multiple access with collision avoidance, CSMA/CA to provide the point to multipoint, PMP capability.
  • Connection orientated:  
  • Supports communication in all conditions:   The WiMAX MAC layer must be able to support a large number of users along with high data rates. As the traffic is packet data orientated it must be able to support both continuous and" bursty" traffic. Most data traffic is "bursty" in nature having short times of high data rates then remaining dormant for a short while.
  • Efficient spectrum use:   The WiMAX MAC must be capable of supporting methods that enable very efficient use of the spectrum.
  • Variety of QoS options:   To provide the support for different forms of traffic from voice data to Internet surfing, etc, a variety of different classes and forms of QoS support are needed. Support for QoS is a fundamental part of the WiMAX MAC-layer. The WiMAX MAC utilises some of the concepts that are embedded in the DOCSIS cable modem standard.
  • Multiple WiMAX / IEEE 802.16 physical layers:   With different variants, the WiMAX MAC layer must be able to provide support for the different PHYs.

WiMAX MAC layer operation

The WiMAX MAC layer is primarily an adaptation layer between the physical layer and the upper layers within the overall stack.

One of the main tasks of the WiMAX MAC layer is to transfer data between the various layers.

  • Transmission of data - reception of MAC Service Data Units, MSDUs from the layer above. It then aggregates and encapsulates them into MAC Protocol Data Units, MPDUs, before passing them to the physical layer, PHY for transmission.
  • Reception of data - the WiMAX MAC layer takes MPDUs from the physical layer. It decapsulates and reorganises them into MSDUs, and then passes them on to the upper-layer protocols.

For the different formats: IEEE 802.16-2004 and IEEE 802.16e-2005, the WiMAX MAC design includes a convergence sublayer. This is used to interface with a variety of higher-layer protocols, such as ATM, Ethernet, IP, TDM Voice, and other future protocols that may arise.

WiMAX defines a concept of a service flow and has an accompanying Service Flow Identifier, SFID. The service flow is a unidirectional flow of packets with a particular set of QoS parameters, and the identifier is used to identify the flow to enable operation.

WiMAX Protocol Stack
WiMAX Protocol Stack

There is an additional layer between the WiMAX MAC itself and the upper layers. This is called the Convergence Sublayer. For the upper protocol layers, the convergence sublayer acts as an interface to the WiMAX MAC. Currently the convergence sublayer only supports IP and Ethernet, although other protocols can be supported by encapsulating the data.

The WiMAX MAC layer provides for a flexible allocation of capacity to different users. It is possible to use variably sized MPDUs from different flows - these can be included into one data burst before being handed over to the PHY layer for transmission. Also, multiple small MSDUs can be aggregated into one larger MPDU. Conversely, one big MSDU can be fragmented into multiple small ones in order to further enhance system performance. This level of flexibility gives significant improvements in overall efficiency.

WiMAX MAC Connection Identifier

Before any data is transferred over a WiMAX link, the user equipment or mobile station and the base station must create a connection between the WiMAX MAC layers of the two stations. To achieve this, an identifier known as a Connection Identifier, CID, is generated and assigned to each uplink / downlink connection. The CID serves as an intermediate address for the data packets transmitted over the WiMAX link.

There is another identifier used within the WiMAX MAC layer. Known as the Service Flow Identifier, SFID, this is assigned to unidirectional packet data traffic by the base station. It is worth noting that the base station WiMAX MAC layer also handles the mapping of the SFIDs to CIDs to provide the required quality of service.

The WiMAX MAC layer also incorporates a number of other features including power-management techniques and security features.

The WiMAX MAC layer has been developed to provide the functionality required for a point to multipoint system. The WiMAX MAC layer is also able to provide support for the different physical layers needed for the different flavours of WiMAX that are in use.

By Ian Poole


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