r-wire paths must have some amount of loss. Once they go into a gain condition, and we refer here to overall circuit gain, positive feedback will result and the amplifiers will begin to oscillate or "sing." For an analog network, Round-trip delay (ms) Figure 8.19 Talker echo tolerance for average telephone users. 8.5 TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY North American practice called for a minimum of 4-dB loss on all four-wire circuits to ensure against singing. CCITT recommends 10 dB for minimum loss on the national network (Ref. 5, p. 3). The modern digital network with its A/D (analog-to-digital) circuits in PCM channel banks provides signal isolation, analog-to-digital, and digital-to-analog. As a result, the entire loss scenario has changed. This new loss plan for digital networks is described in Section 8.5.7. 8.5.6 Introduction to Transmission-Loss Engineering One major aspect of transmission system design for a telephone network is to establish a transmission-loss plan. Such a plan, when implemented, is formulated to accomplish three goals: 1. Control singing (stability). 2. Keep echo levels within limits tolerable to the subscriber. 3. Provide an acceptable overall loudness rating to the subscriber. From preceding discussions we have much of the basic background necessary to develop a transmission-loss plan. We know the following: A certain minimum loss must be maintained in four-wire circuits to ensure against singing. Up to a certain limit of round-trip delay, echo may be controlled by adding loss (i.e., inserting attenuators, sometimes called pads). It is desirable to limit these losses as much as possible, to improve the loudness rating of a connection. National transmission plans vary considerably. Obviously the length of a circuit is impor- tant, as well as the velocity of propagation of the transmission media involved. Velocity of Propagation. A signal takes a finite amount of time to traverse from point A to point B over a specific transmission medium. In free space, radio signals trav