Security in Wireless Ad Hoc and Sensor Networks

3

The Wireless Medium

In this chapter we explain the fundamental concepts related to wireless communications. The material provided is introductory and related to the security issues. For interested readers, further details about the physical layer data communication concepts can be found in Stallings (2000).

3.1 Wireless Channel Fundamentals and Security

Electromagnetic signals can be either analog or digital. The intensity of an analog signal varies and the change in the signal strength is usually smooth and continuous. On the other hand, digital signal strength is constant for a period, after which it may change to another discrete level. Analog and digital signals are depicted in Figure 3.1.

Figure 3.1 (a) Analog and (b) digital signal

When the signal strength is the same as the signal strength a certain time period, T, before, and this repeats every T, the signal is called periodic. Figure 3.2 shows sine and square waves, which are analog and digital periodic signals respectively, where

Figure 3.2 (a) Sine and (b) square wave

The frequency (f) of a periodic signal is the rate at which it repeats a complete cycle in every second, i.e. f = 1/T, and this rate is given in Hertz (cycles ...