s. Nearly all long-distance (toll) telephone connections traverse four-wire links. From the near-end user the connection to the long-distance network is two-wire or via a two- wire link. Likewise, the far-end user is also connected to the long-distance (toll) network via a two-wire link. Such a long-distance connection is shown in Figure 4.14. Schemat- ically, the four-wire interconnection is shown as if it were a single-channel wire-line TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Direction of transmission Two-wire Two-wire connection connection Four-wire side Figure 4.14 A typical long distance (toll) connection model. with amplifiers. However, it would more likely be a multichannel multiplexed configu- ration on wire/fiber-optic cable or over radio. Nevertheless, the amplifiers in Figure 4.14 serve to convey the ideas this section considers. As illustrated in Figure 4.14, conversion from two-wire to four-wire operation is carried out by a hybrid, which is a four-port, four-winding transformer. 4.4.2.3 Operation of a Hybrid. A hybrid, in terms of telephony (at voice frequency), is a transformer with four separate windings. Based on a simplified description, a hybrid may be viewed as a power splitter with four sets of wire-pair connections. A functional block diagram of a hybrid device is shown in Figure 4.15. Two of the wire-pair connections belong to the four-wire path, which consists of a transmit pair and a receive pair. The third pair is a connection to the two-wire link, which is eventually connected to the subscriber subset via one or more switches. The last pair of the four connects the hybrid to a resistance-capacitance balancing network, which electrically balances the hybrid with the two-wire connection to the subscriber subset over the frequency range of the balancing network. Balancing, in this context, means matching impedances-that is, the impedance of the two-wire side to the hybrid two-wire port. Signal energy entering from the two-wire subset connection divides equally. Half of it dis