-8 Copyright © 2005 by Roger L. Freeman QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS Signal Noise Figure 3.1 Signal-to-noise ratio. this by inspection or by reading the levels on the oscilloscope presentation. The signal level is +15 dBm; the noise is +5 dBm, then (S/N)db level(signal in dBm) - level(no (noise in dBm) (3.1) Inserting the values from the "oscilloscope example" above, we have S/N = +15 dBm - (+5 dBm), S/N = 10 dB. This expression is set up as shown because we are dealing with logarithms (see Appendix B). When multiplying in the domain of logarithms, we add. When dividing, we subtract. We are dividing because on the left side of the equation we have S/N or S divided by N. Signal-to-noise ratio really has limited use in the PSTN for characterizing speech transmission because of the "spurtiness" of the human voice. We can appreciate that individual talker signal power can fluctuate widely SO that S/N is far from constant during a telephone call and from one telephone call to the next. In lieu of actual voice, we use a test tone to measure level and S/N. A test tone is a single frequency, usually around 800 or 1000 Hz, generated by an audio signal generator and inserted in the voice channel. The level of the tone (often measured in dBm) can be easily measured with the appropriate test equipment. Such a tone has constant amplitude and no silent intervals, which is typical of voice transmission. 3.2.2 Voice Transmission 3.2.2.1 Loudness Rating and Its Predecessors. Historically, on telephone con- nections, the complaint has been that at the receiving telephone the distant talker's voice was not loud enough. "Hearing sufficiently well" on a telephone connection is a subjective matter. This is a major element of quality of service (QoS). Various meth- ods have been derived over the years to rate telephone connections regarding customer satisfaction. The underlying cause of low signal level is loss across the network. Any method to measure "hearing sufficiently well" should incorporate intervening l