Wireless networks: unplugging the cables

Wireless networks: unplugging the cables

For as long as there have been computer networks, cabling has been the essential element tying it all together. However, as computing has evolved beyond desk-bound PCs, cable has become a tether for mobile users seeking freedom of movement, rather than limitations. Additionally, the reliance on cabling for network connectivity has restricted the places in which networks can be set up, such as historic buildings or outdoor installations.

One solution to this problem involves making the network wireless - an option becoming more popular as the number of solutions available grows.

According to international research organisation Dataquest, the revenue from sale of wireless LAN equipment will grow from $US135 million in 1996 to an expected $458 million in 1998. The number of new nodes is expected to increase from 285,000 to 2,475,000 over the same period, with the anticipation that unit costs will halve.

A Dataquest report from 1995 exemplifies the trend towards mobile computing. "The mobile worker is taking many forms; mobile executives constantly on the move to unpredictable locations; telecommuters from home to office; locker room professionals; ad hoc workgroups; corridor roamers.

A common thread through all of these is the increasing use of portable computing devices that cannot be permanently attached to the network. Ethernet over ether cannot be the end-all solution for all networking needs of these mobile workers, but it can be enough of a solution to dramatically expand the market."

The report says that since half of these professionals confine themselves to a defined and limited number of buildings, the use of Ethernet over ether is a sensible choice. Indeed, the report even suggests that wireless LANs may find practical applications in home environments, with every third home in the US owning at least one PC, and 15 per cent of those owning two or more.

There are currently two main technologies competing in the arena of wireless networking. The first is infra-red transmission, a technology already used in communication between peripheral devices and PCs, such as downloading from notebooks to printers or PCs.

While this technology is cheap and efficient, it has the limitation of requiring line-of-site transmission between the two devices involved.

Photonics (distributed by 1World Systems) has found a way around this by using diffused infra-red transmission, which allows signals to be bounced off the ceiling or walls, and even through open doors in connecting rooms. This still has limitations, though, in being unable to penetrate walls or floors, and can be affected by the quality of light or the materials it is being bounced off. However, infra-red has the advantage of not being regulated in the same way that radio frequency technology is.

The second technology involves spread spectrum low power radio frequency transmission, a technology more commonly used in the products available.

In this category there are two separate transmission methods available, each with various assets and drawbacks. The first is direct sequence transmission, which was originally developed for military battlefield communication in the 1950s.

Direct sequence transmission involves simultaneous transmission of data on eleven separate frequencies, the reasoning being that if even as many as five of the frequencies drop out the message will still get through. This renders the transmission a high level of immunity from interference and radio background noise.

The second method of transmission involves frequency hopping, whereby the transmission hops from one frequency to another at high speed. Data is transmitted a number of times on two or three different frequencies, and then mapped to determine whether it's getting through. If the transmitter finds it is failing to get through on a particular frequency it is removed from the hopping sequence for a period of time.

There are advantages in both technologies, and their pricing is approximately the same. Direct sequence has double the throughput of frequency hopping, at 1Mbit/sec versus 500 Kbit/sec. However, in the event of deteriorating signal penetration, frequency hopping maintains its throughput for longer.

Frequency hopping also has the advantage of lower battery usage, a factor that comes into prominence on notebook computers. For corridor wanderers its 500Kbit/sec throughput compares favourably with that of 28.8Kbit/sec modems they may be using now. A number of solutions have been developed that take advantage of this, such as the PC Card products available through Xircom and IBM.

Apart from the limitations on throughput compared to standard LANs, wireless LANs are also at the mercy of environmental conditions when it comes to transmission range. These vary greatly from product to product, and are themselves subject to environmental limitations from background noise and obstacles.

Clover, which distributes the AT&T range of WaveLAN products, has developed a number of antenna devices to extend distance, and claims to have achieved reliable transmission up to ten kilometres in any conditions, which can be extended to 20 kilometres in favourable conditions.

Stand-alone products, however, cannot claim such distances, with most claiming ranges up to 600 feet dependent on conditions.

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