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WIRELESS LAN STANDARD

Wireless LAN: an overview

Introduction
This economic projection for wired LANs that require costly installations have caused a growing interest in wireless local area network (WLANs) that can offer, in principle, portability and lower installation costs. Wireless systems can be installed in different environment, such as offices, manufacturing floors, research laboratories, hospitals or universities, and they offer a set of application that includes communication between terminals and connections to the telephone network.
Technologies
There are several methods of establishing a wireless link between two points: ultrasound, carrier currents through main installations, radio frequency waves, and unguided optical signals. Only two radio and optical signals are capable of supporting the high-speed data transmission necessary on indoor wireless LANs.
Requirements
These requirements have been classified into:

Reliability
The new medium should be as reliable as a wire system. LAN communication relies on an almost errorless link. When error is detected in a data packet, it is not corrected but packet is resent. Wireless LANs must try to keep the error rate at the same level as cabled LANs.
Transparency
WLANs are not going to replace cabled LANs in indoor environments. They will share the same environment, so the existing software has to work with both types.
Throughput
For the sake of transparency, WLANs should be able to work at the same data rate as cabled LANs. High-speed LANs are far from the technological possibilities of WLAN.
Security
Using WLANs, nobody wants the data flowing around without control. Data encryption is mandatory for WLANs. To avoid degrading the performance, this has to be done by hardware using encryptions codes, or by the same method of transmission.
Mobility
There are two different types of mobility. The first, full mobility, are the ability to send and receive information while moving inside the area covered by the WLAN. The second type, weak mobility, is the capability of having a connection to the network by placing a terminal within the area covered by the WLAN, but working at rest.
Network Topology
In WLANs, bus-based topologies are preferred. Nevertheless, if terminals are grouped in clusters and in fixed places, a wireless physical ring can be made using point to point links between clusters. These links require precise alignments between transmitters and receivers.
Flexibility
The number of the active nodes in WLANs can change while the network is working, so the protocols for the inclusion or exclusion of a terminal should be minimized.
Price
Equipment for WLANs is more complex and more expensive than LANs, but the advantage of WLAN is an almost zero reconfiguration cost.
Safety and Regulations
As intended for office environments, the power levels must be innocuous to human beings and interference with other systems has to be avoided.

Network Topologies
There are three basic network topologies: ring, bus, and star. Any topology can be implemented using nay of these configurations, so the following sections are applicable to both strategies.
Standardization Efforts
In 1985, the Federal Communications Commission (FCC) allocated the so-called industrial, scientific, and medical (ISM) bands for LANs using spread-spectrum techniques. These bands are: 902-928 MHz, 2400-2483.5 MHz, and 5725-5850MHz. No license was needed, subject to no more than 1W being emitted. The covering range is about 800ft (240m). These bands are not enough for high-speed LANs and new frequencies are under study. These include 17 GHz and 61GHz bands, which are well suited for RF WLAN. These bands are mainly allocated to radio localization, but the interference between them is difficult. On the other hand, WLANs are intended to be used in buildings in metropolitan areas. It is important that these bands (17 and 61GHz) are restricted to these applications and no other service uses them. The FCC has allocated a narrow unlicensed band (1910-1930MHz) for mobile users, subject to low power emission. Although 20MHz bandwidth is not enough for very loaded areas like office buildings, hospitals, and so forth, it is a good starting point. If it succeeds, other bands can be allocated for WLANs.
In Europe, the standardization is carried out by the European Telecommunications Standard Institute (ETSI) and The Conference of Europenne des Postes etdes Telecommunications (CEPT). In March of 1992, ETSI approved the Digital European Cordless Telecommunications (DECT) standard. A new standard for high performance data networks is under development: High-Performance European Radio LAN (HIPERLAN).
The Institute of Electrical and Electronic Engineers (IEEE) has also focused the presumed necessity of a standard in WLAN. The IEEE standards for LANs have been accepted by International Organization for Standardization and its International Electro-technical Commission (ISO/IEC), and accepted worldwide. The family of IEEE 802 standards has been a real success in the international standardization of LANs. Within this family, a new working group, known as P802.11, is developing a standard for WLAN.
Conclusions
There are many cases where the benefits of having a LAN where the nodes are not tied by the network cables are worth the cost of a wireless technology. We have seen the most important requirements to be fulfilled by WLANs in order to get a place in the LAN market. Two technologies (RF and IR) can be used to develop these systems under the previously stated conditions. But the efforts and the applicability of both technologies are not equally balanced. RF methods are more complex, but they provide a full mobility and a wide covering range. While IR is easier to implement and no license is needed, its coverage is restricted to a single room or line of sight, point-to-point links.
In conclusion, WLANs will provide enough options to the LAN designer to suit the user requirements with the optimal solution. Cabled LANs should be kept as the backbone of the network.

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