1、 WIRELESS LAN In just the past few years, wireless LANs have come to occupy a significant niche in the local area network market. Increasingly, organizations are finding that wireless LANs are an indispensable adjunct to traditional wired LANs, as they satisfy requirements for mobility, relocation,
2、ad hoc networking, and coverage of locationsdifficult to wire. As the name suggests, a wireless LAN is one that makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used； the reasons for this included high prices, low data rates, occupational safety conce
3、rns, and licensing requirements. As these problems have been addressed, the popularity of wireless LANs has grown rapidly. In this section, we first look at the requirements for and advantages of wireless LANs, and then preview the key approaches to wireless LAN implementation. Wireless LANs Applica
4、tions There are four application areas for wireless LANs: LAN extension, crossbuilding interconnect, nomadic access, and ad hoc networks. Let us consider each of these in turn. LAN Extension Early wireless LAN products, introduced in the late 1980s, were marketed as substitutes for traditional wired
5、 LANs. A wireless LAN saves the cost of the installation of LAN cabling and eases the task of relocation and other modifications to network structure. However, this motivation for wireless LANs was overtaken by events. First, as awareness of the need for LAN became greater, architects designed new b
6、uildings to include extensive prewiring for data applications. Second, with advances in data transmission technology, there has been an increasing reliance on twisted pair cabling for LANs and, in particular, Category 3 unshielded twisted pair. Most older building are already wired with an abundance
7、 of Category 3 cable. Thus, the use of a wireless LAN to replace wired LANs has not happened to any great extent. However, in a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN. Examples include buildings with large open areas, such as manufacturing plant
8、s, stock exchange trading floors, and warehouses； historical buildings with insufficient twisted pair and in which drilling holes for new wiring is prohibited； and small offices where installation and maintenance of wired LANs is not economical. In all of these cases, a wireless LAN provides an effe
9、ctive 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
10、 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. Cross-Building Interconnect Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wi
11、reless LANs. In this case, a point-to-point wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single point-to-point link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN. Nomadic Access Nomad
12、ic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop computer or notepad computer. 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 computer t
13、o 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 computers and may wish access to the servers on a wired LAN from various locati
14、ons. Ad Hoc Networking An ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees li
15、nk their computers in a temporary network just for the duration of the meeting. Wireless LAN Requirements A 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 cap
16、ability. 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 cap
17、acity. 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 accomplished through the use of control mo
18、dules that connect to both types of LANs. There may also need to be accommodation 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 battery-powered workstatio
19、ns 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 de
20、signed, a wireless LAN may be interference-prone 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 become more popular,
21、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. License-free operation. Users would prefer to bu
22、y 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 asp
23、ects 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 802.11 standard: Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850
24、and 950 nm. Direct-sequence spread spectrum operating in the 2.4-GHz ISM band. Up to 7 channels, each with a data rate of 1 Mbps or 2 Mbps, can be used. Frequency-hopping spread spectrum operating in the 2.4-GHz ISM band. The details of this option are for further study. Wireless LAN Technology Wire
25、less 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. Sp
26、read 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 U.S. Narrowband Microwave. These LANs operate at microwave fr
27、equencies 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 802.11 committee. The terminology and some of the specific features of
28、802.11 are unique to this standard and are not reflected in all commercial 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 cons
29、ists of some number of stations executing the same MAC protocol and competing 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
30、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. T
31、ypically, 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: No-transition. A station of this type is either stationary or moves only withi
32、n the direct communication range of the communicating stations of a single BSS. BSS-transition. 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 lo
33、cation of the station. ESS-transition. 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 upper-layer connections supported by 802.11 cannot be guaranteed. In fact, disruption of
34、service is likely to occur. details of this option are for further study. The 802.11 working group considered two types of proposals for a MAC algorithm: distributed-access protocols which, like CSMAICD, distributed the decision to transmit over all the nodes using a carrier-sense 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 network of peer workstations and may also be attractive in other wireless LAN
