ered on digital networks in the industrialized/postindustrialized nations are far improved, some attaining an end-to-end BER of 5 X 10-10 Thus, the data being transported can expect a similar BER. The genesis of frame relay, discussed in Chapter 10, is based on the premise that these excellent BERs can be expected. 3.2.4 Video (Television) Television picture quality is subjective to the viewer. It is based on S/N of the picture channel. The S/N values derived from two agencies are provided below. The first are called "TASO ratings." TASO stands for Television Allocations Study Organization. The TASO ratings follow: TASO PICTURE RATING Quality Excellent (no perceptible snow) 45 dB Fine (snow just perceptible) 35 dB 3. Passable (snow definitely perceptible but not objectionable) 29 dB Marginal (snow somewhat objectionable) 25 dB Snow is the visual perception of high levels of thermal noise typical with poorer S/N values. CCIR developed a 5-point scale for picture quality versus impairment. This scale shown in the following table: CCIR FIVE-GRADE SCALE Quality Impairment 5. Excellent 5. Imperceptible 4. Good 4. Perceptible, but not annoying 3. Fair 3. Slightly annoying 2. Poor 2. Annoying 1. Bad 1. Very annoying Later CCIR/ITU-R documents steer clear of assigning S/N to such quality scales. In fact, when digital compression of TV is employed, the use of S/N to indicate picture quality is deprecated. 3.3 THE THREE BASIC IMPAIRMENTS AND HOW THEY AFFECT THE END-USER There are three basic impairments found in all telecommunication transmission systems. These are: Amplitude (or attenuation) distortion Phase distortion Noise QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS 3.3.1 Amplitude Distortion The IEEE defines attenuation distortion (amplitude distortion) as the change in attenuation at any frequency with respect to that of a reference frequency. For the discussion in this section, we'll narrow the subject to the (analog) voice channel. In most cases a user is connected, through his/her metallic subscriber loop 