he two. Therefore, at various points in a network, it is necessary to convert between the two signal types. The devices that handle these conversions are codecs and modems (see Figure 1.11). Analog and Digital Transmission Table 1.1 Characteristics of Analog and Digital Networks Feature Analog Characteristics Digital Characteristics Signal Continuously variable, in both ampli- Discrete signal, represented as either tude and frequency changes in voltage or changes in light levels Capacity Hz (e.g., a telephone channel is Bits per second (e.g., a T-1 line carries measurement 4KHz) 1.544Mbps, and an E-1 line transports 2.048Mbps) Bandwidth Low bandwidth (4KHz), which High bandwidth that can support high- means low data transmission rates speed data applications that involve video (up to 33.6Kbps) because of limited and multimedia channel bandwidth Network Low; one conversation per telephone High; multiplexers enable multiple conver- capacity channel sations to share a communications channel and hence to achieve greater transmission efficiencies Network Poor; a lot of labor is needed for net- Good; smart devices produce alerts, alarms, manageability work maintenance and control traffic statistics, and performance measure- because dumb analog devices do not ments, and technicians at a network control provide management information center (NCC) or network operations center streams that allow the device to be (NOC) can remotely monitor and manage remotely managed the various network elements Signal High; the signal contains a wide range Low; only two discrete signals-the one and structure of frequencies and amplitudes the zero-need to be transmitted Security Poor; when you tap into an analog Good; encryption can be used circuit, you hear the voice stream in its native form, and it is difficult to detect an intrusion Error rates High; 10-5 bits (i.e., 1 in 100,000 bits) Low; with twisted-pair, 10-7 bits (i.e., 1 in 10 is guaranteed to have an error million bits) has an error; with satellite, 10-9 bits (i.e., 1 in 1 billi