 Telecommunication Union, Geneva, 1965. REFERENCES 2. Principles of Electricity Applied to Telephone and Telegraph Work, American Telephone and Telegraph Co., New York, 1961. 3. R. L. Freeman, Telecommunication Transmission Handbook, 4th ed., Wiley, New York, 1998. 4. "Time and Frequency: Theory and Fundamentals," NBS Monograph 140, U.S. Dept. of Com- merce, Washington, DC, May 1974. 5. Clint Smith, LMDS, McGraw-Hill, New York, 2000. QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS 3.1 OBJECTIVE Quality of service (QoS) was introduced in Section 1.4. In this chapter we will be more definitive in several key areas. There are a number of generic impairments that will directly or indirectly affect quality of service. An understanding of these impairments and their underlying causes are extremely important if one wants to grasp the entire picture of a telecommunication system. 3.2 QUALITY OF SERVICE: VOICE, DATA, AND IMAGE 3.2.1 Signal-to-Noise Ratio Signal-to-noise ratio (S/N or SNR) is the most widely used parameter for measurement of signal quality in the field of transmission. Signal-to-noise ratio expresses in decibels the amount that signal level exceeds the noise level in a specified bandwidth. As we review the several types of material to be transmitted on a network, each will require a minimum S/N to satisfy the user or to make a receiving instrument function within certain specified criteria. The following are S/N guidelines at the corresponding receiving devices: Voice: 40 dB Video (TV): 45 dB Data: ~15 dB, based upon the modulation type and specified error performance To illustrate the concept of S/N, consider Figure 3.1. This oscilloscope presentation shows a nominal analog voice channel (300-3400 Hz) with a 1000-Hz test signal. The vertical scale is signal power measured in dBm (see Appendix C for a tutorial on dBs), and the horizontal scale is frequency, 0-3400 Hz. The S/N as illustrated is 10 dB. We can derive Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045