In the more modern network configuration, subscriber loops may terminate in an analog concentrator before two-wire/four-wire conversion in a PCM channel bank. The concentration ratio may be anywhere from 2:1 to 10:1. For example, in the 10:1 case a hybrid may connect to any one of a group of ten subscriber loops. Of course, this is much better than selecting any one of a population of thousands of subscriber loops as in condition 1, above. Turning back to the hybrid, we can keep excellent impedance matches on the four-wire side; it is the two-wire side that is troublesome. So our concern is the match (balance) between the two-wire subscriber loop and the balancing network (N in Figure 8.17). If we have a hybrid term set assigned to each subscriber loop, the telephone company (adminis- tration) could individually balance each loop, greatly improving impedance match. Such activity has high labor content. Secondly, in most situations there is a concentrator with from 4:1 to 10:1 concentration ratios (e.g., AT&T 5ESS). With either condition 1 or condition 2 we can expect a fairly wide range of impedances of two-wire subscriber loops. Thus, a compromise balancing network is employed to cover this fairly wide range of two-wire impedances. Impedance match can be quantified by return loss. The higher the return loss, the better the impedance match. Of course we are referring to the match between the balancing network (N) and the two-wire line (L) (see Figure 8.17). Return LossdB = 20 log10(ZN+ZL)/(Z -ZL) (8.1) 8.5 TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY If the balancing network (N) perfectly matches the impedance of the two-wire line (L), then ZN = ZL, and the return loss would be infinite. 13 We use the term balance return loss (Ref. 5) and classify it as two types: Balance return loss from the point of view of echo. 14 This is the return across the band of frequencies from 300 to 3400 Hz. 15 2. Balance return loss from the point of view of stability. This is the return loss between 0 and 4000 Hz. "Stability" re