l drawback of FDM systems is noise accumulation. At every modulation- demodulation point along a circuit, noise is inserted. Unless the system designer was very careful, there would be SO much noise accumulated at the terminal end of the system that the signal was unacceptable and the signal-to-noise ratio was very poor. Noise does not accumulate on digital systems. We incorporated this section on frequency division multiplex SO that the reader would understand the important concepts of FDM. Many will find that frequency division tech- niques are employed elsewhere such as on satellite communications, cellular, and PCS systems. Also keep in mind that FDM as described herein is still widely used in emerging nations. However, these networks are also starting to phase out FDM in favor of a digital network based on TDM (PCM). TDM techniques are covered in Chapter 6. REVIEW EXERCISES 1. Define switching in light of transmission. Define signaling. 3. Define calling rate and holding time. 4. Define lost calls using the terms offered traffic and carried traffic. 5. Why are call attempts SO important in the design of modern SPC switches? Suppose the average holding time of a call is 3.1 minutes and the calling rate in the busy hour is 465 on a particular work day. What is the traffic flow? 7. Under normal operating conditions, when can we expect blockage (of calls)? 16 Duty cycle refers to how long (in this case) a signal is "on." A 100% duty cycle means the signal is on all the time. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK 8. The statistics during the BH for a particular exchange is 5 lost calls in 841 offered calls. What is the grade of service? 9. When dealing with traffic formulas and resulting traffic tables, we have to know how lost calls are "handled." What are the three ways that lost calls may be handled? 10. On a probability-distribution curve, define the mean. 11. What percentage of events are encompassed in one standard deviation? 12. What traffic formula (and resulting traffic tables) would be u