802.11 MAC Layer
MAC services

One aspect to defining standards for an interoperable wireless network is providing standards for services at the MAC and physical (PHY) layers. This section will discuss standards at the MAC layer, while the next section will discuss physical layer services. Three services are provided by the MAC sublayer in IEEE 802.11. These services are as follows:

  1. Asynchronous data service
  2. Security services
  3. MSDU ordering

Asynchronous Data Service
This service provides peer LLC entities with the ability to exchange MAC service data units (MSDUs). To support this service the local MAC uses the underlying PHY-level services to transport an MSDU to a peer MAC entity, where it will be delivered to the peer LLC. Such asynchronous MSDU transport is performed on a best-effort, connectionless basis. There are no guarantees that the submitted MSDU will be delivered successfully. Broadcast and multicast transport is part of the asynchronous data service provided by the MAC. Due to the characteristics of the wireless medium, broadcast and multicast MSDUs may experience a lower quality of service, compared to that of unicast MSDUs. All STAs support the asynchronous data service.

Security Services
Security services in IEEE 802.11 are provided by the authentication service and the Wired Equivalent Privacy (WEP) mechanism. The scope of the security services provided is limited to station-to-station data exchange. The privacy service offered by an IEEE 802.11 WEP implementation is the encryption of the MSDU. For the purposes of this standard, WEP is viewed as a logical layer service located within the MAC sublayer. The actual implementation of the WEP service is transparent to the LLC and other layers above the MAC sublayer. The security services provided by the WEP in IEEE 802.11 were designed to support the following security goals:

  • Confidentiality
  • Data integrity
  • Access control

WEP and other security services are covered in detail in upcoming modules.

MSDU Ordering
The services provided by the MAC sublayer permit, and may require, the reordering of MSDUs. The MAC will intentionally reorder MSDUs only if necessary to improve the likelihood of successful delivery based on the power management mode of the designated recipient station. The sole effect of this reordering is a change in the delivery order of broadcast and multicast MSDUs. This change is relative to directed, or unicast, MSDUs originating from a single source station address. Unicast MSDUs are given priority over multicast and broadcast.