7.5 Wireless Links and Transmission

Nowdays there are people who need to be on-line all the time. For these mobile users, twisted pair and fiber optics are of no use. They need to get data for their laptop or wristwatch computers without being tethered to the terrestrial communication infrastructure. For these users, wireless communication is the answer. Computer networks can take advantage of the wireless infrastructure where physical wires cannot be laid out.

Some people believe that the future holds only two kinds of communication: fiber and wireless. All nonmobile computers, telephones, printers, and so on will use fiber, and all mobile ones will use wireless.

One of the key challenges in wireless networking is the efficient utilization of the available transmission spectrum. Because the frequency spectrum available for wireless communication is limited, frequencies must be reused within the same geographic area. The spectrum used for wireless communications typically ranges up to several GHz. Security is also a concern in wireless networks. The open-air interface makes it difficult to prevent snooping.

The link-level design techniques involve making trade-offs among the various parameters relevant to the link layer. The optimum design would involve the use of minimum bandwidth and transmit power while maintaining a high data rate, low latency, and low bit error rates (BER). These design challenges must be achieved in the presence of channel imperfections, such as interference.

Wireless links use devices as an antenna for transmitting signals through vacuum, space, air, or substances. Electromagnetic waves can be propagated through the first three, as well as through water and wood. The frequency range depends on the type of substance.

Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver don't have to be carefully aligned physically.

The properties of radio waves are frequency dependent. At low frequencies, radio waves pass through obstacles well, but the power falls off sharply with distance from the source, roughly as 1/r2 in air. At high frequencies, radio waves tend to travel in straight lines and bounce off obstacles. They are also absorbed by rain. At all frequencies, radio waves are subject to interference from motors electrical equipment.

Due to radio's ability to travel long distances, interference between users is a problem. For this reason governments tightly license the use of radio transmitters.

Fiber Optics The Physical Layer