fers to the fact that loss in a four-wire circuit may depart from its nominal value for a number of reasons: Variation of line losses and amplifier gains with time and temperature. Gain at other frequencies being different from that measured at the test frequency. (This test frequency may be 800, 1000, or 1020 Hz.) Errors in making measurements and lining up circuits. The band of frequencies most important in terms of echo for the voice channel is that from 300 Hz to 3400 Hz. A good value for echo return loss for toll telephone plant is 11 dB, with values on some connections dropping to as low as 6 dB. For further information, the reader should consult CCITT Recs. G.122 and G.131 (Refs. 5, 6). Echo and singing may be controlled by: Improved return loss at the term set (hybrid). Adding loss on the four-wire side (or on the two-wire side). Reducing the gain of the individual four-wire amplifiers. The annoyance of echo to a subscriber is also a function of its delay. Delay is a func- tion of the velocity of propagation of the intervening transmission facility. A telephone signal requires considerably more time to traverse 100 km of a voice-pair cable facil- ity, particularly if it has inductive loading, than it requires to traverse 100 km of radio facility (as low as 22,000 km/sec for a loaded cable facility and 240,000 km/sec for a carrier facility). Delay is measured in one-way or round-trip propagation time measured in milliseconds. The CCITT recommends that if the mean round-trip propagation time exceeds 50 msec for a particular circuit, an echo suppressor or echo canceler should be used. Practice in North America uses 45 msec as a dividing line. In other words, where echo delay is less than that stated previously here, echo can be controlled by adding loss. An echo suppressor is an electronic device inserted in a four-wire circuit that effec- tively blocks passage of reflected signal energy. The device is voice operated with a sufficiently fast reaction time to "reverse" the direction of transmission, depending