's talk about the terms analog and digital and how they come into play in the transmission of telecommunications signals. Some telecommunications signals are sent in an analog form, which is represented by a sine wave. The analog signal implies a continuous flow on the line. A digital signal is discrete, repre- senting the presence or absence of current on the line. Samples of the analog signal can be converted to binary numbers that can then be transmitted in a digital form. You may hear that a circuit is digital or analog. These terms do not really apply to the circuit, but rather to the ability to trans- mit the form in which information is sent over the circuit. Your basic telephone at home has two wires, completing the circuit or loop back to the central office of the local telephone company. The incoming voice and your voice are both carried by this loop. Your voice typically travels over the loop in an analog form. Most telecommunications equipment is designed around this elemental concept. If you have a PC with a modem, you can connect it to this same telephone line. Since the computer is sending out information The Telecom Handbook : 245 TRANSMISSION in binary form as either a zero or a one, the purpose of the modem, is to convert these zeroes and ones into acoustic tones. These tones travel over the line until they reach the other end where another compatible modem converts them back into ones and zeroes. Digital lines can be used to send voice communications as well as data, but in some cases it is necessary to convert the ana- log voice signal into a digital form. This is done with a variety of sampling techniques, but the basic concept is the same. The analog signal (sine wave) is sampled at frequent intervals and given a numeric value based upon the amplitude (height) of the wave at each point. This num- ber is then converted to a binary number. Binary numbers represent all values using only ones and zeros. Thus, the numeric value of each point sampled on the analog signal is converted to ones and z