Unless properly designed, 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, 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 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 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 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。

3。 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 inpidual 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 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 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 802。11 committee。 The terminology and some of the specific features of 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 consists 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 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:

No-transition。 

A station of this type is either stationary or moves only within the direct communication range of the communicating stations of a single BSS。