MAC Service Definition

Asynchronous Data Service

This service provides peer LLC entities with the capability to exchange 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 WM, broadcast and multicast MSDUs may experience a lower quality of service compared to that of unicast MSDUs. All STAs will support the asynchronous data service. Because the operation of certain functions of the MAC may cause the reordering of some MSDUs, as discussed in more detail in the following section, there are two service classes within the asynchronous data service. By selecting the desired service class, each LLC entity initiating the transfer of MSDUs is able to control whether MAC entities are or are not allowed to reorder those MSDUs.

Security Services

Security services in IEEE 802.11 are provided by the authentication service and the 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 service located within the MAC sublayer, as shown in the reference model Figure 5-9. Actual implementations of the WEP service are transparent to the LLC and other layers above the MAC sublayer. The security services provided by the WEP in IEEE 802.11 are as follows:

• Confidentiality

• Authentication

• Access control in conjunction with layer management

During the authentication exchange, parties A and B exchange authentication information. The MAC sublayer security services provided by WEP rely on information from non-layer-2 management or system entities. Management entities communicate information to WEP through a set of MIB attributes.

MSDU Ordering

The services provided by the MAC sublayer permit, and may in certain cases require, the reordering of MSDUs. The MAC does not intentionally reorder MSDUs except as may be necessary to improve the likelihood of successful delivery based on the current operational (power management) mode of the designated recipient station(s). The sole effect of this reordering (if any) for the set of MSDUs received at the MAC service interface of any single station is a change in the delivery order of broadcast and multicast MSDUs, relative to directed MSDUs, originating from a single source station address. If a higher-layer protocol using the asynchronous data service cannot tolerate this possible reordering, the optional StrictlyOrdered service class should be used. MSDUs transferred between any pair of stations using the StrictlyOrdered service class are not subject to the relative reordering that is possible when the ReorderableMulticast service class is used. However, the desire to receive MSDUs sent using the StrictlyOrdered service class at a station precludes simultaneous use of the MAC power management facilities at that station.