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 plants, 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 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。

Cross-Building Interconnect

Another use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless 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

Nomadic 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 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 computers and may wish access to the servers on a wired LAN from various locations。

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 link their computers in a temporary network just for the duration of the meeting。

2。 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 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 accomplished through the use of control modules 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 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。