【正文】 this results in a foreshortened superframe period for the next cycle.UWB technology and other shortrange wireless munications technology is paredAs personal munication consumer electronics industry in the rapid development, shortrange wireless munication field of all kinds of new skills ,art, new methods to emerge in endlessly, toward faster and more convenient and more safe and effective etc. The new technology in the development of the Intel access, information home appliances, mobile office, industrialization and other fields has been widely used. Among them, ultrawideband (UWB) Wide Band, the mkultra technology is in after 199039。 and small offices where installation and maintenance of wired LANs is not economical. In all of these cases, a wireless LAN provides an effective and more attractive alternative. In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations. For example, a manufacturing facility typically has an office area that is separate from the factory floor but which must be linked to it for networking purposes. Therefore, typically, a wireless LAN will be linked into a wired LAN on the same premises. Thus, this application area is referred to as LAN extension.CrossBuilding InterconnectAnother use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a pointtopoint wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single pointtopoint link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN.Nomadic AccessNomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop puter or notepad puter. One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a personal portable puter to a server in the office. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move around with their portable puters and may wish access to the servers on a wired LAN from various locations.Ad Hoc NetworkingAn ad hoc network is a peertopeer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop puter, may convene in a conference room for a business or classroom meeting. The employees link their puters in a temporary network just for the duration of the meeting.Wireless LAN RequirementsA wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs:Throughput. The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.Number of nodes. Wireless LANs may need to support hundreds of nodes across multiple cells.Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily acplished through the use of control modules that connect to both types of LANs. There may also need to be acmodation for mobile users and ad hoc wireless networks.Service area. A typical coverage area for a wireless LAN may be up to a 300 to 1000 foot diameter.Battery power consumption. Mobile workers use batterypowered workstations that need to have a long battery life when used with wireless adapters. This suggests that a MAC protocol that requires mobile nodes to constantlymonitor access points 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 network 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 network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users.Physical Medium SpecificationThree 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 TechnologyWireless 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 penetrate opaque walls.Spread Spectrum LANs. This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate