m CLR Circuit loudness rating OLR Overall loudness rating RLR Receive loudness rating SLR Send loudness rating Figure 3.2 Designation of LRs in an international connection. 3.2.2.4.1 Determination of Loudness Rating. The designation with notations of loud- ness rating concept for an international connection is given in Figure 3.2. It is assumed that telephone sensitivity, both for the earpiece and microphone, have been measured. Overall loudness rating (OLR) is calculated using the following formula: OLR = SLR + CLR + RLR. (3.2) The measurement units in Eq. (3.2) are decibels. The overall loudness rating (OLR) is defined as the loudness loss between the speaking subscriber's mouth and the listening subscriber's ear via a telephone connection. The send loudness rating (SLR) is defined as the loudness loss between the speaking subscriber's mouth and an electrical interface in the network. The receive loudness rating (RLR) is the loudness loss between an electrical interface in the network and the listening subscriber's ear. The circuit loudness rating (CLR) is the loudness loss between two electrical inter- faces in a connection or circuit, with each interface terminated by its nominal impedance (Refs. 1, 2). 3.2.3 Data Circuits Bit error rate (BER) is the underlying QoS parameter for data circuits. BER is not sub- jective; it is readily measurable. Data users are very demanding of network operators regarding BER. If a network did not ever carry data, BER requirements could be much less stringent. CCITT/ITU-T recommends a BER of 1 X 10-6 for at least 80% of a month. 1 Let us assume that these data will be transported on the digital network, typ- ical of a PSTN. Let us further assume that conventional analog moderns are not used, 1 See CCITT Rec. G.821. 3.3 THE THREE BASIC IMPAIRMENTS AND HOW THEY AFFECT THE END-USER and the data are exchanged bit for bit with "channels" on the digital network. Thus, the BER of the data reflects the BER of the underlying digital channel which is acting as its transport. BERs encount