the distance a remote switch is from the parent exchange can be as much as 100 miles (160 km). Satellite exchanges range in size from 300 to 2000 lines. Concentrators are cost effective for 300 or less subscribers. However, AT&T's SLC-96 can serve 1000 subscribers or more. 4.4 ESSENTIAL CONCEPTS IN TRANSMISSION 100 Subscriber loops To/from parent exchange Crossbar 20 Trunks matrix +2 Control trunks Control identification Signaling Power supply Figure 4.13 A typical concentrator. 4.4 ESSENTIAL CONCEPTS IN TRANSMISSION 4.4.1 Introduction In this section we discuss two-wire and four-wire transmission and two impairments that are commonly caused by two-wire-to-four-wire conversion equipment. These impairments are echo and singing. The second part of this section is an introduction to multiplexing. Multiplexing allows two or more communication channels to share the same transmission bearer facility. 4.4.2 Two-Wire and Four-Wire Transmission 4.4.2.1 Two-Wire Transmission. A telephone conversation inherently requires trans- mission in both directions. When both directions are carried on the same pair of wires, it is called two-wire transmission. The telephones in our homes and offices are connected to a local switching center (exchange) by means of two-wire circuits. A more proper def- inition for transmitting and switching purposes is that when oppositely directed portions of a single telephone conversation occur over the same electrical transmission channel or path, we call this two-wire operation. 4.4.2.2 Four-Wire Transmission. Carrier and radio systems require that oppositely directed portions of a single conversation occur over separate transmission channels or paths (or use mutually exclusive time periods). Thus we have two wires for the transmit path and two wires for the receive path, or a total of four wires, for a full-duplex (two-way) telephone conversation. For almost all operational telephone systems, the end instrument (i.e., the telephone subset) is connected to its intervening network on a two-wire basi