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:
- Asynchronous data service
- Security services
- 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.