Signal Types Modulation Transmission

20 Communication Networks Copyright © 2005 PragSoft

1. Simplex. This is a unidirectional connection, i.e., data can only travel in one

direction. Simplex connections are useful in situations where a device only receives or only sends data e.g., a printer.

2. Half-duplex. This is a bidirectional connection, with the restriction that data can

travel in one direction at a time.

3. Full-duplex. This is a bidirectional connection in which data can travel in both

directions at once. A full-duplex connection is equivalent to two simplex connections in opposite directions.

2.2. Transmission

Transmission is the act of transporting information from one location to another via a signal. The signal may be analog or digital, and may travel in different media.

2.2.1. Signal Types

All signals are either analog or digital. An analog signal is one in which information appears as a continuous variation of some property. Human speech is an example: it produces a continuous variation of air pressure. A digital signal, on the other hand, is one in which information appears as a sequence of binary values 0 and 1. To represent these two values, a signal is used in which only two wave shapes are allowed, one representing the binary value 0 and the other representing the binary value 1. By definition, therefore, a digital signal is a restricted form of an analog signal. A human speaker who only utters the two words zero and one is a crude example of a digital signal. In electrical terms, signals appear as variation of some electrical property e.g., voltage. Figure 2.10 illustrates. In the analog signal example, the voltage freely varies between 0 and 5 Volts. In the digital signal, the voltage may assume only two values: 0 Volts to represent digital value 0 and 5 Volts to represent digital value 1. Figure 2.10 Analog and digital signals. 5v 0v 5v 0v time time 1 1 1 analog digital Since digital computers play a central role in data communication, in nearly all cases, digital signals are used. Analog signals are used in cases of equipment which date back to before the advent of digital technology. Existing analog telephone networks are a good example of the latter. www.pragsoft.com Chapter 2: The Physical Layer 21

2.2.2. Modulation

Transmission of digital data over an analog line is achieved using a technique called modulation , where the digital bit stream is modulated over an analog carrier signal. A modem modulator and demodulator is a commonly used device which employs this technique. As illustrated in Figure 2.11, a modem converts the outgoing digital bit stream from a device into an analog signal and converts the incoming analog signal into a digital bit stream. Figure 2.11 Role of modems. modem modem Three basic types of modulation are possible see Figure 2.12 for a visual comparison:

1. Amplitude Modulation AM. In AM, the carrier signal’s amplitude is