el at 3 X 108 m/sec or 186,000 mi/sec; fiber-optic light guide, about 2 X 108 m/sec or about 125,000 mi/sec; on heavily loaded wire-pair cable, about 0.22 X 108 m/sec or 14,000 mi/sec; and 19-gauge nonloaded wire-pair cable, about 0.8 X 108 m/sec or 50,000 mi/sec. So we see that the velocity of propagation is very dependent on the types of transmission media being employed to carry a signal. Distances covered by network connectivities are in hundreds or thousands of miles (or kilometers). It is thus of interest to convert velocities of propagation to miles or kilometers per millisecond. Let's use a typical value for carrier (multiplex) systems of 105,000 mi/sec or 105 mi/msec (169 km/msec). First let's consider a country of small geographic area such as Belgium, which could have a very simple transmission-loss plan. Assume that the 4-dB minimum loss for singing is inserted in all four-wire circuits. Based on Figure 8.19, a 4-dB loss will allow up to 4 msec of round-trip delay. By simple arithmetic, we see that a 4-dB loss on all four- wire circuits will make echo tolerable for all circuits extending 210 mi (338 km) (i.e., 2 X 105). This could be an application of a fixed-loss type transmission plan. In the case of small countries or telephone companies covering a rather small geographic expanse, the minimum loss to control singing controls echo as well for the entire system. LOCAL AND LONG-DISTANCE NETWORKS Let us try another example. Assume that all four-wire connections have a 7-dB loss. Figure 8.20 indicates that 7 dB permits an 11-msec round-trip delay. Again assume that the velocity of propagation is 105,000 mi/sec. Remember that we are dealing with round- trip delay. The talker's voice reaches the far-end hybrid and some of the signal is reflected back to the talker. This means that the signal traverses the system twice, as shown in Figure 8.20. Thus 7 dB of loss for the given velocity of propagation allows about 578 mi (925 km) of extension or, for all intents and purposes, the distance between subscribers