he receive reference equivalent of the same type subset). In one CCITT recommendation, 97% of all international calls were recommended to have an ORE of 33 dB or better. It was found that with this 33-dB value, less than 10% of users were unsatisfied with the level of the received speech signal. 3.2.2.3 Corrected Reference Equivalent. Because difficulties were encountered in the use of reference equivalents, the ORE was replaced by the corrected reference equiv- alent (CRE) around 1980. The concept and measurement technique of the CRE was essentially the same as RE (reference equivalent), and the decibel remained the measure- ment unit. CRE test scores varied somewhat from its RE counterparts. Less than 5 dB (CRE) was too loud; an optimum connection had an RE value of 9 dB and a range from 7 to 11 dB for CRE. For a 30-dB value of CRE, 40% of a test population rated the call excellent, whereas 15% rated it poor or bad. 3.2.2.4 Loudness Rating. Around 1990 the CCITT replaced corrected reference equivalent with loudness rating. The method recommended to determine loudness rating eliminates the need for subjective determinations of loudness loss in terms of corrected reference equivalent. The concept of overall loudness rating (OLR) is very similar to the ORE concept used with reference equivalent. Table 3.1 gives opinion results for various values of overall loudness rating (OLR) in decibels. These values are based upon representative laboratory conversation test results for telephone connections in which other characteristics such as circuit noise have little contribution to impairment. QUALITY OF SERVICE AND TELECOMMUNICATION IMPAIRMENTS Table 3.1 Overall Loudness Rating Opinion Results Representative Opinion Results Overall Loudness Percent Percent Rating (dB) "Good plus Excellent" "Poor plus Bad" aBased on opinion relationship derived from the transmission quality index (see Annex A, ITU-T Rec. P.11). Source: ITU-T Rec. P.11, Table 1/P.11, p. 2, Helsinki, 3/93. National International National system system syste