asynchronous modems can be used for synchronous data if the receiving terminal can derive the clock signal from the data. Asynchronous transmission is advantageous when transmission is irregular (e.g., when it is initiated by a keyboard operator's typing speed). It is also inexpensive because of the simple interface logic and circuitry required. Synchronous transmission, on the other hand, makes far better use of the transmission facility by eliminating the start and stop bits on each character. Furthermore, synchronous data are suitable for multilevel modulation, which combines two or four bits in one signal element (baud). This can facilitate data rates of 4.8 k or 9.6 k bits per second over a bandwidth of 2.4 kHz. 1.3 A BRIGHT FUTURE WITH FIBER OPTICS A light-bearing finger-sized fiber-optic cable can carry 40,000 phone calls at once in contrast to 20,000 calls with an arm sized copper cable. The conduit for sending these super-quick light pulses is made of glass, with clarity 1000 times greater than ordinary window glass. 10 TELECOMMUNICATIONS Table 1-1. Fiber Optics versus Conventional Cable SOLID FIBER RIBBON OPTIC Physical size Small Medium Large Small Mechanical integrity Excellent Excellent Weight Moderate Heavy Crosstalk immunity Excellent Bandwidth Excellent EMI and noise immunity Excellent Fiber optics systems offer an effective alternative method to trans- mit information (see Table 1-1). Fiber optics provide the advantages of a light weight, smaller size, durable, nonconducting signal path which has no effect on the electrical environment that the signal passes through. In addition, the frequency and distance limits of a fiber optic system are said to be better than twisted pair, coaxial cable, or microwave waveguide systems. Fiber optic systems (Fig. 1-6) have three major components: the light sources in the transmitter, the fiber cable, and the photodetec- tors in the receiver. The sources must be inexpensive, have long life- times, and be easy to modulate at high frequencies. The fiber must be su