The committee for IEEE 802 standards formed a working group for wireless local network standards 802.11 in 1990. This group undertook development of the general standard for radio equipment and networks working at 2.4 GHz, with access time of 1 and 2 Mbps (Megabits-per-second). The works on development of the standard were completed in 7 years, and in June 1997 the first specification of the 802.11 was ratified. The IEEE 802.11 is the first standard for WLAN products from an independent international organization developing the most of standards for wired networks. But the first established data rate in a wireless network didn't meet the requirements of users by that time. And the developers had to create a new standard to make the Wireless LAN technology popular, cheap and attractive for modern tough business applications.

In September 1999 the IEEE ratified the extension of the previous standard. The new IEEE 802.11b extension (also known as 802.11 High rate) defines a standard for products of wireless networks working at 11 Mbps (similar to Ethernet), what allows using these devices in large organizations. The compatibility of products from different manufacturers is ensured by an independent organization named Wireless Ethernet Compatibility Alliance (WECA). It was founded by the leaders of wireless communication industry in 1999. At present it includes more than 80 companies such as Cisco, Lucent, 3Com, IBM, Intel, Apple, Compaq, Dell, Fujitsu, Siemens, Sony, AMD etc. On the WECA's site you can look through the products which meet the requirement of Wi-Fi (the WECA's term for IEEE 802.11b).

The need in wireless access to local networks grows with the number of mobile devices such as notebooks and PDAs, as well as with the desire of users to be connected to a network without dealing with a network cable. According to forecasts, in 2003 there will be more than a billion of mobile devices, and the WLAN market is estimated to be more than two billion of dollars by 2002.

IEEE 802.11 standard and its extension

802.11b

Like all IEEE 802 standards, the 802.11 works on two lower levels of the ISO/OSI model: a physical and a channel one (fig. 1). Any network applications, a network operating system or a protocol (e.g., TCP/IP) will work perfectly in the 802.11 network as in the Ethernet.

Fig. 1. Levels of the ISO/OSI model and their correspondence with the 802.11 standard.

The basic architecture, peculiarities and services of the 802.11b are defined in the initial standard 802.11. The specification of the 802.11b touches only the physical level, adding only higher access speeds.

Operating modes of the 802.11

The 802.11 deals with two types of equipment - a client which is a computer equipped with a wireless Network Interface Card (NIC), and an Access point (AP) which serves a bridge between a wireless and wired networks. An access point usually contains a transceiver, a wired network interface (802.3) and software for data processing. ISA, PCI or PC Card network cards in the 802.11 standard or integrated solutions (e.g. telephone garniture of the 802.11) can serve a wireless station.

The IEEE 802.11 standard has two operating modes of a network - "Ad-hoc" and Client/Server (or infrastructure mode). In the client/server mode (fig. 2) a wireless network consists of at least one access point connected to a wired network, and a set of wireless terminal stations. Such combinations are called Basic Service Set (BSS). Two or more BSSs which make a single subnetwork form an Extended Service Set (ESS). Since the most of wireless stations require an access to file servers, printers, Internet available in a wired LAN, they will work in the client/server mode.

Fig. 2. Architecture of the "client/server" network.

The "Ad-hoc" mode (