on bits) has an error; and with fiber, 10-11 bits (i.e., 1 in 100 billion bits) to 10-13 bits (i.e., 1 in 10 trillion bits) has an error Chapter 1 Telecommunications Technology Fundamentals Digital Signals Transmitted Over an Analog Circuit Computers Computers Modem Modem Modem Modem Modem Analog Analog Analog Modem Multiplexing Transmission Multiplexing Analog Signals Transmitted Over a Digital Circuit Telephones Telephones Codec Codec Codec Codec Codec Digital Digital Digital Codec Multiplexing Transmission Multiplexing Figure 1.11 Codecs and modems A codec (which is a contraction of coder-decoder) converts analog signals into digital signals. There are different codecs for different purposes. For the PSTN, for example, there are codecs that minimize the number of bits per second required to carry voice digitally through the PSTN. In cellular networks, because of the con- straints and available spectrum, a codec needs to compress the voice further to get the most efficient use of the spectrum. Codecs applied to video communication also require very specific compression techniques to move those high-bandwidth signals over what may be somewhat limited channels today. A modem (which is a contraction of the terms modulate and demodulate) is used to infuse digital data onto transmission facilities and vice versa. Some modems are designed specifically to work with analog voice-grade lines. There are also modems designed to work specifically with digital facilities (e.g., ISDN modems, ADSL modems). A modem manipulates the variables of the electromagnetic wave to dif- ferentiate between the ones and zeros. Although it is possible to convert between analog and digital networks, in gen- eral, conversions are a weak link in a network. A conversion is a point at which network troubles can occur, an opportunity for errors and distortions to be intro- duced. Therefore, ideally, we want to move toward an end-to-end digital and end- Multiplexing to-end optical environment. This means that nowhere between the transmitter and th