Media access layer
 


The Media Access Control Layer is one of two sublayers that make up the Data Link Layer of the OSI model. The MAC layer is responsible for moving data packets to and from one Network Interface Card (NIC) to another across a shared channel.

The MAC sublayer uses MAC protocols to ensure that signals sent from different stations across the same channel don't collide.

In the Open Systems Interconnection (OSI) model of communication, the Media Access Control layer is one of two sublayers of the Data Link Control layer and is concerned with sharing the physical connection to the network among several computers. Each computer has its own unique MAC address. Ethernet is an example of a protocol that works at the Media Access Control layer level.

WiMAX MEDIA ACCESS CONTROL, MAC (data link) layer
The WiMAX MAC uses a scheduling algorithm for which the subscriber station needs to compete only once for initial entry into the network. After network entry is allowed, the subscriber station is allocated an access slot by the base station. The time slot can enlarge and contract, but remains assigned to the subscriber station, which means that other subscribers cannot use it. In addition to being stable under overload and over-subscription, the scheduling algorithm can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control Quality of Service (QoS) parameters by balancing the time-slot assignments among the application needs of the subscriber station.


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.

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.

WiMAX QoS or WiMAX Quality of Service is a key element in the delivery of service over the WimAX medium. WiMAX QoS.

With techniques such as Internet Protocol being used, delays or latency and jitter can be introduced into the data transmission arena. While IP techniques provide improved levels of efficiency, this comes at a cost.

To overcome the effects of latency and jitter, the concept of quality of service is used. For WiMAX QoS several techniques and definitions are at the core of the implementation.


WiMAX QoS basics

In an ideal world it would be possible to send data over a network and gain the same performance as that achieved by a circuit switched network. However the nature of packet data means that the same channels are sued for data travelling to and from a variety of different sources and end points.

Within a packet data network, there are three main parameters that are key to the performance of the network, and the WiMAX QoS. These three parameters are:

  • Latency:   Latency is a measure of time delay experienced in a system - in the case of a telecommunications system such as WiMAX it is the time that it takes from the initiation of sending data until it arrives at its destination. In a WiMAX system. The WiMAX system can be split into three major elements as far as latency is concerned:
    • From the user to the base station via the WiMAX radio interface
    • Over the IP network
    • From the base station to the end user over the WiMAX radio interface
    Typically it is found that the maximum latency occurs within the IP network - this may be around 100ms in many instances with the WiMAX radio access network / interface introducing around 5ms.
  • Jitter:   In the context of computer networks and int hei case the WiMAX system, jitter is a measure of the variability over time of the packet latency across a network. A network with constant latency has no variation and hence no jitter. However as the levels of data are constantly varying, it takes a variable amount of time for a packet to arrive at its destination. Packet jitter is expressed as an average of the deviation from the network mean latency. Although the term jitter is often used, it is actually imprecise and a standards-based term, packet delay variation, PDV, is more correctly used. PDV is an important quality of service factor in assessment of network performance.
  • Packet loss:   Packet loss is the term used to indicate the loss of data packets during transmission over a network. Packet loss may occur for a variety of reasons but normally occurs as a result of high network latency or overloading of switches or routers that are unable to process or route all the incoming data.

WiMAX QoS definitions

In order to categorise the different types of quality of service, there are five WiMAX QoS classes that have been defined.

These WiMAX QoS classes are defined in the table below:


WiMAX QoS Class WiMAX QoS Class Details
Unsolicited Grant Service The Unsolicited Grant Service, UGS is used for real-time services such as Voice over IP, VoIP of for applications where WiMAX is used to replace fixed lines such as E1 and T1.
Real-time Packet Services This WiMAX QoS class is used for real-time services including video streaming. It is also used for enterprise access services where guaranteed E1/T1 rates are needed but with the possibility of higher bursts if network capacity is available. This WiMAX QoS class offers a variable bit rate but with guaranteed minimums for data rate and delay.
Extended Real Time Packet Services This WiMAX QoS class is referred to as the Enhanced Real Time Variable Rate, or Extended Real Time Packet Services. This WiMAX QoS class is used for applications where variable packet sizes are used - often where silence suppression is implemented in VoIP. One typical system is Skype.
Non-real time Packet Services This WiMAX QoS class is used for services where a guaranteed bit rate is required but the latency is not critical. It might be used for various forms of file transfer.
Best Effort This WiMAX QoS is that used for Internet services such as email and browsing. Data packets are carried as space becomes available. Delays may be incurred and jitter is not a problem.

WiMAX network basics

The basic WiMAX standard does not define the WiMAX network for end to end connectivity. However the need for a standard WiMAX network architecture is realised and the WiMAX Forum Network Working Group have developed a standard for defining the WiMAX network architecture.

The standard now used is available from the WiMAX Forum as WiMAX Forum Network Architecture, document: WMF - T32-002-R010v04 and it is dated February 03, 2009.


WiMAX network architecture major entities

The WiMAX architecture developed by the WiMAX form supports is a unified network architecture to support fixed, nomadic and mobile operation. The WiMAX network architecture is based upon an all-IP model.

The WiMAX network architecture comprises three major elements or areas.

  • Remote or Mobile stations:   These are the user equipments that may be mobile or fixed and may be located in the premises of the user.
  • Access Service Network, ASN :   This is the area of the WiMAX network that forms the radio access network at the edge and it comprises one or more base stations and one or more ASN gateways.
  • Connectivity Service Network, CSN:   This part of the WiMAX network provides the IP connectivity and all the IP core network functions. It is what may be termed the core network in cellular parlance.

WiMAX network architecture

The overall WiMAX network comprises a number of different entities that make up the different major areas described above. These include the following entities

  • Subscriber Station, SS / Mobile Station, MS :   The Subscriber station, SS may often be referred to as the Customer Premises Equipment, CPE. These take a variety of forms and these may be termed "indoor CPE" or "outdoor CPE" - the terminology is self-explanatory. The outdoor CPE has the advantage that it provides better performance as a result of the better position of the antenna, whereas the indoor CPE can be installed by the user. Mobile Stations may also be used. These are often in the form of a dongle for a laptop, etc.
  • Base Station, BS:   The base-station forms an essential element of the WiMAX network. It is responsible for providing the air interface to the subscriber and mobile stations. It provides additional functionality in terms of micro-mobility management functions, such as handoff triggering and tunnel establishment, radio resource management, QoS policy enforcement, traffic classification, DHCP (Dynamic Host Control Protocol) proxy, key management, session management, and multicast group management.
  • ASN Gateway, ASN-GW:   The ASN gateway within the WiMAX network architecture typically acts as a layer 2 traffic aggregation point within the overall ASN.

    The ASN-GW may also provide additional functions that include: intra-ASN location management and paging, radio resource management and admission control, caching of subscriber profiles and encryption keys. The ASN-GW may also include the AAA client functionality(see below), establishment and management of mobility tunnel with base stations, QoS and policy enforcement, foreign agent functionality for mobile IP, and routing to the selected CSN.
  • Home Agent, HA:   The Home Agent within the WiMAX network is located within the CSN. With Mobile-IP forming a key element within WiMAX technology, the Home Agent works in conjunction with a "Foreign Agent", such as the ASN Gateway, to provide an efficient end-to-end Mobile IP solution. The Home Agent serves as an anchor point for subscribers, providing secure roaming with QOS capabilities.
  • Authentication, Authorisation and Accounting Server, AAA:   As with any communications or wireless system requiring subscription services, an Authentication, Authorisation and Accounting server is used. This is included within the CSN.