Many technologies suffer from a proliferation of different formats and standards, each of which is backed by different vendors and regulatory bodies, and wireless LAN buyers are facing exactly these difficulties.

There are currently six different WLAN standards either already in existence or on the brink of ratification: 802.11b, 802.11a, 802.11h, 802.11g, HiperLan and HiperLan/2.

This problem is exacerbated by the fact that each country has different laws concerning the use of the radio frequency wavebands required by WLAN equipment to transmit data.

The most well established and widely deployed standard is 802.11b, a specification which was ratified in 1999. After a slow start, 802.11b kit has had significant success in the enterprise market.

Client devices and access points based on 802.11b transmit data at maximum throughput of 11Mbit/s, although tests show rates of 5.5Mbit/s to be more usual.

Signals sent and received by 802.11b equipment are transmitted in the 2.4GHz radio frequency spectrum. This part of the waveband is also used by short-range Bluetooth and Dect wireless devices, among other hardware, and this therefore raises the spectre of signal congestion and interference in environments where many different types of devices are used.

The few enterprises that have deployed significant amounts of WLAN and Bluetooth hardware in the same office so far report minimal problems. However, experts say that WLAN throughput can be adversely affected by signal interference if devices are situated too closely to one another, and connections can be blocked in worst case scenarios.

It was the fear of signal congestion that led vendors and the Institute of Electrical and Electronics Engineers (IEEE) to develop the 802.11a WLAN standard, which transmits data in the relatively clear 5.15-5.35GHz radio frequency waveband. The 802.11a specification also increases the maximum data rate to 54Mbit/s, although proprietary compression methods make 108Mbit/s bandwidth possible.

A wide range of 802.11a products are already available in the US, where it is legal for the 5.15 to 5.35GHz spectrum to be used for corporate data transmissions. But the same is not yet true in Europe.

Both the European Telecommunications Standards Institute (ETSI) and the UK Radiocommunications Agency (RA) object to 802.11a's use of certain parts of the 5.15 to 5.35GHz waveband, because it could interfere with military and government communications.

European objections to the use of 802.11a have led the IEEE to develop a new WLAN standard, 802.11h. This is a revised version of the 802.11a standard that operates in the same waveband, but includes new features to avoid interference with government communications.

One of these features is transmit power control (TPC) and the other is dynamic frequency selection (DFS), sometimes called dynamic channel selection (DCS). The purpose of these functions is to minimise the possibility of interference by reducing power output and detecting the clearest transmission channels. However, the final 802.11h standard is not likely to be ratified by the IEEE until the beginning of 2003 at the earliest.

An extension to the 802.11b standard, 802.11g, which increases the data rate to 54Mbit/s, is set to be ratified at the end of this year. The advantage here is that 802.11g transmits signals in the 2.4GHz frequency band, meaning that 802.11g and 802.11b equipment can be mixed and matched on the same wireless network, although one or two interoperability problems with 802.11b remain.

The 802.11g standard does not have widespread vendor support, however, and final products are unlikely to be available before 2003. More crucially, it does not avoid the key problem of congestion in the 2.4GHz waveband, which means it is likely to be largely ignored.

The same is true for the 11Mbit/s HiperLan standard, which also suffers from a lack of vendor backing and has a far smaller user base than 802.11b.

The upgraded version of HiperLan, HiperLan/2, has much to offer, however, and could pose a real threat to the predicted dominance of 802.11a if it gets the backing of equipment manufacturers and if products reach the market quickly.

HiperLan/2 provides the same 54Mbit/s maximum data transfer rates, transmits in the same 5GHz waveband and has its own DFS scheme built in. It also offers better quality of service (QoS) than 802.11a, but appears to have lost the support of all the major vendors except Ericsson and Nokia. Even so, HiperLan/2 may surface as the technology of choice for carriers and mobile operators wanting to integrate their mobile cellular networks with public-access WLANs and hotspots.

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