Wireless Local Area Network Technology

WLAN - Wireless Local Area Network - is at present probably the most popular and the most often used wireless way to access the Internet by the net users. However, this solution is not one of those that guarantees top quality of the services (QOS) but the price one must pay investing in the devices totally makes up for this drawback. The equipment, especially the 2,4 GHz, is relatively cheap and is still getting cheaper. That is why it is willingly used by minor Internet providers to solve the last-mile problem.

The IEEE 802.11 standard is promoted by the Local and Metropolitan Area Networks Standards Committee, IEEE Computer Society. Before it was approved in June 1997, it was preceded by six draft versions. Only its final shape was recognized by both IEEE and ISO/IEC standards. It enabled a big group of producers and salesmen to develop a wide range of devices for ISM (Industrial, Scientific and Medical) and UNII (Unlicensed National Information Infrastructure) bands on general release.

At present a few standards of WLANs are dominant. The most popular ones are the following: 802.11a, 802.11b and 802.11g. Due to this a few physical layers are defined, which give the designer the possibility to chose one of them, depending on the system requirements and future users� needs. The IEEE 802.11 standard defines two lowest layers of the wireless computer network model operating with the throughput in the radio linkup to 2 Mbit/s. It presumes two types of radio interface: operating within 2,4 GHz band and one using the IR. The original specification of IEEE 802.11 was characterized by low bandwidth and problems with co-cooperativeness.

802.11b specification, published in 1999, determined a new PHY layer which guarantees a greater bit speed with application of DSSS (Direct Sequence Spread Spectrum) in 2,4 GHz range. Devices operating in this standard can dispatch data with speed up to 11 Mb/s. 802.11a specification was presented in 2001 and it defines PHY layer operating in 5 GHz band. The maximum bandwidth was increased up to 54 MB/s, thanks to, among others, application of a new modulation method, OFDM (Orthogonal Frequency Division Multiplexing). 802.11g is the latest PHY specification, operating in 2,4 GHz range and using OFDM spectrum dispersing technique.

WLAN TOPOLOGY

a) The temporary net ad-hoc in the group of 802.11x standards is called IBSS net (Independent Basic Service Set). In order to created IBSS at least two devices are required (ex. computers) equipped with wireless network adapter cards. Such a web is not connected to a wireless net and, therefore, data exchange with the framework net is impossible (ex. access to sources, ex. Internet). Ad-hoc net does not require application of access points.

b) Dependent net (BSS - Basic Service Set) uses devices called access points, Aps. Their task is to reinforce and regenerate received signals, to manage movement and provide access to the wireless part of the infrastructure. The reach of a dependent network is limited to one access point within which the mobile station may move without breaking the connection.

c) Compound network (ESS - Extended Service Set) is created when at least two BSS sub-networks are connected to LAN; it is the most developed example of a combined network which can be successfully used for creating vast, combined, local computer networks.

See also: Standards of Wireless LAN