Introduction to Wireless LANs

Wireless LANs (WLANs) use electromagnetic radio waves to transport data between computers in a Local Area Network (LAN), without the limitations set by "hard wired network cable or phone wire connection". Whilst simple optical links are commercially available, radio is presently more useful since it is not strictly restricted to line-of-sight paths.

Radio waves are often called radio carriers when they are used to carry information. The data to be transported is superimposed on the radio carrier by various modulation techniques which allow the data to be faithfully reconstructed at the receiving end. Once data is superimposed (modulated) onto the radio carrier, this combined "radio signal" now occupies more than a single frequency since the frequency components or spectra of the modulating data add frequency bandwidth to the basic carrier (in direct proportion to its information content or bit rate). The frequency range which is needed to accommodate a radio signal with any given modulation bandwidth is called a channel. Radio receiver techniques can select one radio channel while efficiently rejecting signals on other frequencies. Many radio signals to and from many users can thereby co-exist in the same place and time without interfering with each other if the radio waves are transmitted at minimum necessary power within different radio channels.

In a typical commercial WLAN configuration a radio transmitter/receiver (transceiver) called a Wireless Access Point (WAP), connects to a wired network from a fixed location using a standard Ethernet (IEEE 802.3) cable connection. At a minimum, the access point receives, buffers, and transmits data between the WLAN and the wired network infrastructure.

A single commercial wireless access point can support a group of simultaneous users typically with (specification IEEE 802.11) at a range of not more than a couple of hundred metres in free space and much less via obstructions at the very restricted power levels therein specified for licence free users.

The Wireless Access Point (WAP) (or antenna attached with co-axial cable fed, free antenna versions) is usually mounted high and may be mounted essentially anywhere that is practical as long as the desired radio coverage is obtained.

In the United Kingdom, use of IEEE 802.11 equipment is covered by the standard Amateur Radio Operator Licence’s 2.4 GHz frequency allocation which allows unlimited antenna gain and the notional use of self-built linear transmitter power amplifiers, receiver pre-selectors and low noise mast-head amplifiers up to 2450MHz.

With practical and inexpensive improvements to the antenna systems alone, distant users can anticipate reliable working up to 25 Km (18 miles) line-of-sight distance even from a WAP using a low gain (13dBd omni-directional) antenna. Much better performance (over obstructed paths) may be had with (sector directional) gain antenna systems and amateur licenced power amplification.

End users access the WLAN through WLAN Adapters, which are implemented as PCIMCIA cards in notebook computers (Figure 3), or use ISA or PCI adapters in desktop computers (Figure 4) or fully integrated devices within hand-held computers.

Figure 4.

Standard PCI bus Adapter (Planet type WL-2400)

WLAN adapters provide an interface between the client network operating system (NOS) and the airwaves (via an antenna). The nature of the wireless connection is transparent to the NOS.

The simplest configuration is an Independent (or peer-to-peer) WLAN that connects a group of PCs with wireless adapters.

Any time, two or more wireless adapters are within range of each other, they can set up an ad hoc independent network (Figure 5). These on-demand networks typically require no administration or preconfiguration.

This mode of operation will be familiar to all radio amateurs who have used point-to-point AX25 Packet Radio.

Access points can extend the range of independent WLANs by acting as a repeater(see Figure 6), effectively doubling the distance between wireless PCs.

In infrastructure WLANs, multiple access points allow commercial users to efficiently share local network resources linking the WLAN to wired and long distance Wide Area Networks (WANs) and the Internet.

The access points not only provide communication with the wired network but also manage traffic in the immediate wireless neighbourhood. Despite the restricted range at the very restricted power levels due to (IEEE 802.11) Multiple access points can still provide wireless coverage for an entire building complex, small village, university campus or housing estate.

Only the lower 50 MHz or so of IEEE 802.11 WLAN channels (60%) overlap the 13cms Amateur band. Also U.K. amateur radio licence holders will be generally prohibited from accessing the Internet (or any other non-amateur network) via Internet Access Providing WLANs.

However some concessions regarding limited internet access have recently been announced and the use of unlimited antenna gain and up to 26 dBW (400 peak watts) offers some quite extraordinary performance possibilities for amateurs using this mode.

This 500 mW amplifier (Fig.8) is a ready-made commercial product made by Breezecom which combines a RF switched low noise receiver pre-amp with a small PA to increase the range of outdoor links. These kinds of units can be used at mast-head to compensate for cable loss (of say 0.5 dB per metre) due to mounting the antenna far from the WLAN transceiver at the remote top of a tall building or mast. Breezecom’s AMP-500 comes with a DC power injector (for powering the unit up the coaxial downlead), five foot cable and two, "N" male-to-male adapters.

Strictly for the more adventurous, modern solid state devices offer the realization of high power amateur linear amplification up to the full legal limit.

Typical of the potential of smaller Satellite Dish amateur conversions, the Aironet AIR-ANT3338 Solid Dish Antenna is a commercial 24 inch unit designed for outdoor directional connections and according to manufacturer’s data offers a H/V beam width of 12.4° and gain of 21 dBi with approximate outdoor range at 11Mbps of 11.5 miles (18.5 km) using 50 ft (15 m) low loss cable and the same type of antenna at each end.

Bluetooth is a forthcoming Wireless Personal Area Networking (WPAN) technology which has gained significant industry support and will coexist with most wireless LAN solutions. The Bluetooth specification is for a 1 Mbps, small form-factor, low-cost radio solution that can provide links between mobile phones, mobile computers and other portable handheld devices and connectivity to The Internet. This technology, embedded in a wide range of devices to enable simple, spontaneous wireless connectivity is a complement to wireless LANs - which are designed to provide continuous connectivity via standard wired LAN features and functionality.