 of measure VU, standing for volume unit. For a 1000-Hz sinusoid signal (simple sine wave signal), 0 VU = 0 dBm. When a VU meter is used to measure the level of a voice signal, it is difficult to exactly equate VU and dBm. One of the problems, of course, is that speech transmission is characterized by spurts of signal. However, a good approximation relating VU to dBm is the following formula: Average power of a telephone talker VU 1.4 (dBm). (3.7) In the telecommunication network, telephone channels are often multiplexed at the first serving exchange. When the network was analog, the multiplexers operated in the fre- quency domain and were called frequency division multiplexers (FDM). Voice channel QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS inputs were standardized with a level of either -15 dBm or -16 dBm, and the out- puts of demultiplexers were +7 dBm. These levels, of course, were test-tone levels. In industrialized and postindustrialized nations, in nearly every case, multiplexers are digital. These multiplexers have an overload point at about +3.17 dBm0. The digital reference signal is 0 dBm on the analog side using a standard test tone between 1013 and 1022 Hz. 3.5 ECHO AND SINGING Echo and singing are two important impairments that impact QoS. Echo is when a talker hears her/his own voice delayed. The annoyance is a function of the delay time (i.e., the time between the launching of a syllable by a talker and when the echo of that syllable is heard by the same talker). It is also a function of the intensity (level) of the echo, but to some lesser extent. Singing is audio feedback. It is an "ear-splitting" howl, much like the howl one gets by placing a public address microphone in front of a loudspeaker. We will discuss causes and cures of echo and singing in the next chapter. REVIEW EXERCISES 1. Define signal-to-noise ratio. 2. Give signal-to-noise ratio guidelines at a receiving device for the following three media: voice, video-TV, and data. Base the answer on where a typical customer says the