ets only and not in the other direction. Another example is a paging system that allows a user to receive only alphanumerical messages. In half-duplex operation the signal is transmitted in both directions but only in one direction at a time. An example of this is a mobile radio system where the person speaking must indicate by saying the word over that she is done transmitting and the other person is allowed to transmit. LANs use a high-speed, half-duplex transmission over the cable even though users may feel that the communication is continuously bidirectional, that is, full duplex. In full-duplex operation signals are transmitted in both directions at the same time. An example of this is an ordinary telephone conversation where it is possible for both people to speak simultaneously. Most modern telecommunications systems use the full-duplex principle, which we call duplex operation for short. Frequency and Bandwidth To understand the requirements of different applications for a telecommuni- cations network, we must understand the fundamental concepts of frequency and bandwidth. The information that we transmit through a telecommuni- cations network, whether it is analog or digital, is in the form of electrical voltage or current. The value of this voltage or current changes through time, and this alteration contains information. The transmitted signal (the alteration of voltage or current) consists of multiple frequencies. The range of frequencies is called the bandwidth of the Introduction to Telecommunications Network Engineering signal. The bandwidth is one of the most important characteristics of analog information and it is also the most important limiting factor for the data rate of digital information transfer. 3.3.1 Frequency We can see the telecommunications signal as a combination of many cosine or sine waves with different strengths and frequencies. The frequency refers to the number of cycles through with the wave oscillates in a second. As an example of the concept of frequency, we hear the oscill