4.1. Data LinksA data link is the physical path between a data transmitter and data receiver. Figure 4.1 shows the range of electromagnetic-spectrum frequencies for various applications in data communications. Figure 4.1. Frequency range for various data communications applications
Data transmission may use either wired links or wireless links , depending on the application and the available bandwidth of links. Transmission links can also be classified as guided , or directional, and unguided . A wired link is normally considered a guided medium for the propagation of data. A wireless medium can be designed to propagate signals in more than one direction, causing the medium to become unguided. Signals travel on the link in the form of electromagnetic waves. Let c be the speed of electro-magnetic waves, f be the frequency of the traveling signal, and » be the wavelength of the signal. Then: Equation 4.1
A guided link, whether wired or wireless, can be full-duplex , whereby two bitstreams in opposite directions can be transmitted, or half-duplex , whereby only one bitstream in one direction can be carried at any time. The following section explores the most commonly used data links. |