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【正文】 ts or to engage in frequent handshakes with a base stationis inappropriate. Transmission robustness and security. Unless properly designed, a wireless LAN may be interferenceprone and easily eavesdropped upon. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping. Collocated work operation. As wireless LANs bee more popular, it is quite likely for two of them to operate in the same area or in some area where interference between the LANs is possible. Such interference may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN. Licensefree operation. Users would prefer to buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN. HandoWroaming. The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another. Dynamic configuration. The MAC addressing and work management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users. Physical Medium Specification Three physical media are defined in the current standard: Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm. Directsequence spread spectrum operating in the ISM band. Up to 7 channels, each with a data rate of 1 Mbps or 2 Mbps, can be used. Frequencyhopping spread spectrum operating in the ISM band. The details of this option are for further study. Wireless LAN Technology Wireless LANs are generally categorized according to the transmission techniquethat is used. All current wireless LAN products fall into one of the following categories: Infrared (IR) LANs. An individual cell of an IR LAN is limited to a single room, as infrared light does not perate opaque walls. Spread Spectrum LANs. This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands, so that no FCC licensing is required for their use in the . Narrowband Microwave. These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands. A set of wireless LAN standards has been developed by the IEEE mittee. The terminology and some of the specific features 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
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