 in poor condition. Because of the fixed conditions on the four-wire side, we can generally depend on holding a good impedance match. Our concern under these conditions is the impedance match on the two-wire side-that is, the impedance match between the compromise network (N) and the two-wire side (L). Here the impedance can have high variability from one subscriber loop to another. We measure the capability of impedance match by return loss. In this particular case we call it balance return loss: Balance return Let us say, for argument's sake, that we have a perfect match. In other words, the impedance of the two-wire subscriber loop side (L) on this particular call was exactly 900 S and the balancing network (N) was 900 S. Substitute these numbers in the preceding formula above and we get 900 + 900 Balance return Examine the denominator. It is zero. Any number divided by zero is infinity. Thus we have an infinitely high return loss. And this happens when we have a perfect match, an ideal condition. Of course it is seldom realized in real life. In real life we find that the balance return loss for a large population of hybrids connected in service and serving a large population of two-wire users has a median more on the order of 11 dB with a 14 Characteristic impedance is the impedance that the line or port on a device is supposed to display. For most subscriber loops it is 900 So with a 2.16-uF capacitor in series at 1000 Hz for 26-gauge wire pair or 600-50 resistive. The notation for characteristic impedance is Zo. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Line amp. Two/four-wire hybrid 900 D Four- Adjustable Precision balance balance network network Line amp. Figure 4.16 Schematic diagram of two-wire to four-wire conversion using a hybrid. (From Figure 5-9, p. 104, Ref. 5. Reprinted with permission of the IEEE Press.) standard deviation of 3 dB (Ref. 4). This is valid for North America. For some other areas of the world, balance return loss median may be lower with a larger standard deviation. Whe