Return loss (RL) at an impedance discontinuity in a transmission path is the ratio (in dB) of the power level of an incident signal to the power level of the resulting reflected signal. The general expression for return loss is:
where ZI and ZR are the input and reference impedances respectively.
Single-frequency return loss (SFRL) is the lowest value of non-weighted return loss occurring in the frequency range 200 to 3200 Hz.
Single-frequency transhybrid loss (SFTHL) is the lowest value of loss, from the input pair to the output pair of the same 4-wire interface, occurring in the frequency range 200 to 3200 Hz.
C.3 Transhybrid Loss (THL)
Transhybrid loss (THL) is the loss from the input pair to the output pair of the same 4-wire interface.
One of the objectives of the voice gateway transmission standard is the harmonization of loss and level planning between the Pan-European and the North American regions. An ETSI/STQ Pan-European standard, ETSI ES 202 020, was developed in parallel with this standard, with close liaison between the two standards committees.
The Pan-European standard is limited in scope to a half-channel loss plan, with a global view that this form of standard may be subject to adoption in the future by other regions.
Table D1 shows the voice gateway half-channel loss plan for the Pan-European region, given in equivalent loudness ratings and respective loss.