Q&A: Wireless LANs

The switch from the 2.4GHz frequency band used by the IEEE's original 802.11b wireless Ethernet standard seemed like a good idea at the time. Quite a few other devices use the crowded 2.4GHz band, including Bluetooth, some remote controls and microwave ovens.

In addition, congestion was beginning to be a problem in areas where wireless LANs (WLANs) had become popular. Although you can set different wireless access points (APs) to use different channels, the channels overlap so you cannot usually overlay more than three WLANs.

The suggested replacement was the 802.11a standard, which not only increases bandwidth from 11Mbit/s to 54Mbit/s, but operates in the 5GHz frequency waveband, above the worst of the interference. Transmissions at this higher frequency have shorter range and less ability to penetrate walls, but this could even be seen as a benefit, as it reduces the extent to which outsiders can listen in on the network.

The problem was that by the time 802.11a-based WLAN kit began to appear, 802.11b was already well entrenched - and worst of all, there was no backward compatibility between the two. Moving to 802.11a therefore required new APs and network interface cards (NICs), just at the time when the price of 802.11b equipment was tumbling.

Where does the 802.11g standard fit in?

Recognising the need for backwards compatibility, vendors first developed proprietary double-speed versions of 802.11b, and then moved on to 54Mbit/s versions. Work on the latter was then standardised by the IEEE as the 802.11g specification, with the benefit that 802.11g cards will also run on 802.11b networks and vice versa.

Early versions of what eventually became the 802.11g standard were not quite right. In particular, the presence of one 802.11b client could force all other devices on an 802.11g network to drop to the lower 802.11b speeds.

Fortunately, the reputable manufacturers have fixed these problems in their standards-compliant kit, and each device should now run at its own speed.

Manufacturers have also begun introducing triple-mode products. For example, 3Com recently announced a range of APs and NICs that support 802.11a, 802.11b and 802.11g, in effect allowing any client to attach to any network.

What about the 802.11n standard?

The proposed 802.11n specification is currently being defined by an IEEE workgroup and is not expected to be ratified until 2006, although vendors are expected to ship pre-standard products in 2005. It aims to double the speed of current 54Mbit/s 802.11a/g WLAN equipment over the same range of around 25m to 50m, or boost bandwidth to over 200Mbit/s across much shorter distances. The IEEE has not yet declared which radio frequency waveband 802.11n will use for data transmission, however.

Many wireless products carry the Wi-Fi logo. What does it stand for?

Wireless Fidelity, or Wi-Fi, is actually a brand name invented by a trade association called the Wireless Ethernet Compatibility Alliance (WECA), itself subsequently rebranded as the Wi-Fi Alliance.

The Wi-Fi logo on a WLAN product demonstrates that it has been independently tested for compatibility with other certified products that operate on the same frequency.

As such, it represents a good guide to hardware interoperability, and the Wi-Fi Alliance has now begun promoting it for hotspots too. A programme called Wi-Fi Zones is designed to provide a list of compatible public-access WLANs.

All three types of wireless Ethernet can be Wi-Fi certified. The testing process is not cheap, but it also includes invaluable checks for Wi-Fi Protected Access (WPA) security compatibility, for example.

Can wireless networks support voice over IP applications?

The relevant standard here is 802.11e, which is due for approval soon. It is intended to replace proprietary protocols such as Spectralink Voice Priority (SVP) before they get too entrenched.

As with 802.1p for wired Ethernet, 802.11e adds quality of service (QoS) mechanisms to support delay-sensitive applications such as voice and video. It defines eight traffic categories or priority levels, in effect adjusting the period of time that a network station waits after a collision before trying to transmit again.

Combining VoIP with a more predictable form of 802.11 will allow a single wireless infrastructure to support both voice and data, as well as voice-enabled devices such as PDAs and notebook PCs.

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