Rec. ITU-T G.121 (11/1988)  – LOUDNESS RATINGS (LRs) OF NATIONAL SYSTEMS
Preamble
1 Nominal LRs of the national systems
    1.1 Definition of nominal LRs of the national systems
    1.2 Traffic-weighted mean values of the distribution of send and receive loudness ratings, SLRs and RLRs
2 Maximum Send and Receive Loudness Ratings, SLR and RLR
    2.1 Values for each direction of transmission
    2.2 Difference in transmission loss between the two directions of transmission in national systems
3 Minimum SLR
4 Determination of nominal Loudness Ratings
5 Sidetone
    5.1 General
    5.2 Talker's sidetone STMR
    5.3 Listener's sidetone LSTR
6 Incorporation of PCM digital processes in national extensions
    6.1 Effect on national transmission plans
    6.2 Transmission loss considerations
    6.3 The designation of relative levels and digital pads
ANNEX A – Evaluation of the nominal differences of loss between the two directions of transmission
ANNEX B – Transmission considerations for a 4-wire loop inserted in a 2-wire circuit
B.1 General
B.2 Attenuation
B.3 Impedance
B.4 Sidetone considerations
ANNEX C – Examples of values of R and T pads adopted by some Administrations