【文章內(nèi)容簡介】 res of are unique to this standard and are not reflected in all mercial products. However, it is useful to be familiar with the standard as its features are representative of required wireless LAN capabilities. The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and peting for access to the same shared medium. A basic service set may be isolated, or it may connect to a backbone distribution system through an access point. The access point functions as a bridge. The MAC protocol may be fully distributed or controlled by a central coordination function housed in the access point. The basic service set generally corresponds to what is referred to as a cell in the literature. An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. Typically, the distribution system is a wired backbone LAN. The extended service set appears as a single logical LAN to the logical link control (LLC) level. The standard defines three types of stations, based on mobility: Notransition. A station of this type is either stationary or moves only within the direct munication range of the municating stations of a single BSS. BSStransition. This is defined as a station movement from one BSS to another BSS within the same ESS. In this case, delivery of data to the station requires that the addressing capability be able to recognize the new location of the station. ESStransition. This is defined as a station movement from a BSS in one ESS to a BSS within another ESS. This case is supported only in the sense that the station can move. Maintenance of upperlayer connections supported by cannot be guaranteed. In fact, disruption of service is likely to occur. details of this option are for further study. The working group considered two types of proposals for a MAC algorithm: distributedaccess protocols which, like CSMAICD, distributed the decision to transmit over all the nodes using a carriersense mechanism。 and centralized access protocols, which involve regulation of transmission by a centralized decision maker. A distributed access protocol makes sense of an ad hoc work of peer workstations and may also be attractive in other wireless LAN configurations that consist primarily of bursty traffic. A centralized access protocol is natural for configurations in which a number of wireless stations are interconnected with each other and with some sort of base station that attaches to a backbone wired LAN。 it is especially useful if some of the data is timesensitive or high priority. The end result of the is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed accesscontrol mechanism with an optional centralized control built on top of that. Figure illustrates the architecture. The lower sublayer of the MAC layer is the distributed coordination function (DCF). DCF uses a contention algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination function (PCF) is a centralized MAC algorithm used to provide contentionfree service. PCF is built on top of DCF and exploits features of DCF to assure access for its users. Let us consider these two sublayers in turn. Distributed Coordination Function The DCF sublayer makes use of a simple CSMA algorithm. If a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit。 otherwise, the station must wait until the current transmission is plete before transmitting. The DCF does not include a collisiondetection function (., CSMAICD) because collision detection is not practical on a wireless work. The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish ining weak signals from noise and the effects of its own transmission. To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that