Over the past five years, the world has become increasingly mobile. As a result, traditional ways of networking the world have proven inadequate to meet the challenges posed by our new collective lifestyle.

If users must be connected to a network by physical cables, their movement is dramatically reduced. Wireless connectivity, however, poses no such restriction and allows a great deal more free movement on the part of the network user.

As a result, wireless technologies are encroaching on the traditional realm of "fixed" or "wired" networks. This change is obvious to anybody who drives on a regular basis. One of the "life and death" challenges to those of us who drive on a regular basis is the daily gauntlet of erratically driven cars containing mobile phone users in the driver's seat.

We are on the cusp of an equally profound change in computer networking. Wireless telephony has been successful because it enables people to connect with each other regardless of location. New technologies targeted at computer networks promise to do the same for Internet connectivity.

The most successful wireless networking technology this far has been 802.11. To dive into a specific technology at this point is getting a bit ahead of the story, though.

Wireless networks share several important advantages, no matter how the protocols are designed, or even what type of data they carry. The most obvious advantage of wireless networking is mobility.

Wireless network users can connect to existing networks and are then allowed to roam freely. A mobile telephone user can drive miles in the course of a single conversation because the phone connects the user through cell towers.

Initially, mobile telephony was expensive. Costs restricted its use to highly mobile professionals such as sales managers and important executive decision makers who might need to be reached at a moment's notice regardless of their location.

Mobile telephony has proven to be a useful service, however, and now it is relatively common in the United States and extremely common among Europeans. Likewise, wireless data networks free software developers from the tethers of an Ethernet cable at a desk.

Developers can work in the library, in a conference room, in the parking lot, or even in the coffee house across the street. As long as the wireless users remain within the range of the base station, they can take advantage of the network.

Commonly available equipment can easily cover a corporate campus; with some work, more exotic equipment, and favorable terrain, you can extend the range of an 802.11 network up to a few miles.

Flexibility is an important attribute for service providers. One of the markets that many 802.11 equipment vendors have been chasing is the so-called "hot spot" connectivity market.

Airports and train stations are likely to have itinerant business travelers interested in network access during connection delays. Coffeehouses and other public gathering spots are social venues in which network access is desirable.

Many cafes already offer Internet access; offering Internet access over a wireless network is a natural extension of the existing Internet connectivity. While it is possible to serve a fluid group of users with Ethernet jacks, supplying access over a wired network is problematic for several reasons.

Running cables is time-consuming and expensive and may also require construction. Properly guessing the correct number of cable drops is more an art than a science. With a wireless network, though, there is no need to suffer through construction or make educated (or wild) guesses about demand.

A simple wired infrastructure connects to the Internet, and then the wireless network can accommodate as many users as needed. Although wireless LANs have somewhat limited bandwidth, the limiting factor in networking a small hot spot is likely to be the cost of WAN bandwidth to the supporting infrastructure.

Flexibility may be particularly important in older buildings because it reduces the need for constructions. Once a building is declared historical, remodeling can be particularly difficult.

In addition to meeting owner requirements, historical preservation agencies must be satisfied that new construction is not desecrating the past. Wireless networks can be deployed extremely rapidly in such environments because there is only a small wired network to install.

Flexibility has also led to the development of grassroots community networks. With the rapid price erosion of 802.11 equipment, bands of volunteers are setup shared wireless networks open to visitors.

Community networks are also extending the range of Internet access past the limitations for DSL into communities where high-speed Internet access has been only a dream.

Community networks have been particularly successful in out-of-the way places that are too rugged for traditional wireline approaches. Like all networks, wireless networks transmit data over a network medium. The medium is a form of electromagnetic radiation.

To be well-suited for use on mobile networks, the medium must be able to cover a wide area so clients can move throughout a coverage area. The two media that have seen the widest use in local-area applications are infrared light and radio waves.

Most portable PCs sold now have infrared ports that can make quick connections to printers and other peripherals. However, infrared light has limitations; it is easily blocked by walls, partitions, and other office construction.

Radio waves can penetrate most office obstructions and offer a wider coverage range. It is no surprise that most, if not all, 802.11 products on the market use the radio wave physical layer.

Wireless devices are constrained to operate in a certain frequency band. Each band has an associated bandwidth, which is simply the amount of frequency space in the band. Bandwidth has acquired a connotation of being a measure of the data capacity of a link.

A great deal of mathematics, information theory, and signal processing can be used to show that higher-bandwidth slices can be used to transmit more information. As an example, an analog mobile telephony channel requires a 20-kHz bandwidth.

TV signals are vastly more complex and have a correspondingly larger bandwidth of 6 MHz. The use of a radio spectrum is rigorously controlled by regulatory authorities through licensing processes.

In the U.S., regulation is done by the Federal Communications Commission (FCC). Many FCC rules are adopted by other countries throughout the Americas. European allocation is performed by CEPT's European Radiocommunications Office (ERO).

Other allocation work is done by the International Telecommunications Union (ITU). To prevent overlapping uses of the radio waves, frequency is allocated in bands, which are simply ranges of frequencies available to specified applications.

Here the lists some common frequency bands used in the U.S.

• UHF ISM - 902-928 MHz
• S-Band - 2-4 GHz
• S-Band ISM - 2.4-2.5 GHz
• C-Band - 4-8 GHz
• C-Band satellite downlink - 3.7-4.2 GHz
• C-Band Radar (weather) - 5.25-5.925 GHz
• C-Band ISM - 5.725-5.875 GHz
• C-Band satellite uplink - 5.925-6.425 GHz
• X-Band - 8-12 GHz
• X-Band Radar (police/weather) - 8.5-10.55 GHz
• Ku-Band - 12-18 GHz
• Ku-Band Radar (police) - 13.4-14 GHz 15.7-17.7 GHz

In list above, there are three bands labeled ISM, which is an abbreviation for industrial, scientific, and medical. ISM bands are set aside for equipment that, broadly speaking, is related to industrial or scientific processes or is used by medical equipment.

Perhaps the most familiar ISM-band device is the microwave oven, which operates in the 2.4-GHz ISM band because electromagnetic radiation at that frequency is particularly effective for heating water.

I pay special attention to the ISM bands because that's where 802.11 devices operate. The more common 802.11b devices operate in S-band ISM. The ISM bands are generally license-free, provided that devices are low-power.

How much sense does it make to require a license for microwave ovens, after all? Likewise, you don't need a license to set up and operate a wireless network.

Wireless networks do not replace fixed networks. The main advantage of mobility is that the network user is moving. Servers and other data center equipment must access data, but the physical location of the server is irrelevant.

As long as the servers do not move, they may as well be connected to wires that do not move. The speed of wireless networks is constrained by the available bandwidth. Information theory can be used to deduce the upper limit on the speed of a network.

Unless the regulatory authorities are willing to make the unlicensed spectrum bands bigger, there is an upper limit on the speed of wireless networks. Wireless-network hardware tends to be slower than wired hardware.

Unlike the 10-GB Ethernet standard, wireless-network standards must carefully validate received frames to guard against loss due to the unreliability of the wireless medium. Using radio waves as the network medium poses several challenges.

Specifications for wired networks are designed so that a network will work as long as it respects the specifications. Radio waves can suffer from a number of propagation problems that may interrupt the radio link, such as multipath interference and shadows.

Security on any network is a prime concern. On wireless networks, it is often a critical concern because the network transmissions are available to anyone within range of the transmitter with the appropriate antenna.

On a wired network, the signals stay in the wires and can be protected by strong physical-access control (locks on the doors of wiring closets, and so on). On a wireless network, sniffing is much easier because the radio transmissions are designed to be processed by any receiver within range.

Furthermore, wireless networks tend to have fuzzy boundaries. A corporate wireless network may extend outside the building. It is quite possible that a parked car across the street could be receiving the signals from your network.