ices difficult to use. Some tele- phones that have displays are more user friendly, but subscribers still have to memorize command sequences to use the new services offered by a modern telephone network. 2.3.4 Local Loop and 2W/4W Circuits Any use of telephone channels involves two unidirectional paths, one for transmission and one for reception. The local loop, which connects a tele- phone to a local exchange is a two-wire (2W) circuit that carries the signals in both transmission directions (Figure 2.6). Even ISDN and asymmetrical digi- tal subscriber lines (ADSLs) (described in Chapter 6) use this same 2W local 2W-circuit 4W-circuit A pair of wires Two pairs of wires, one for each direction Bidirectional amplifier Attenuation Potential oscillation Figure 2.6 2W/4W circuits. The Telecommunications Network: An Overview loop. Subscriber loops are and will remain two-wire circuits, because they are one of the biggest investments of the fixed telephone network. Early telephone connections through the network were two-wire cir- cuits. Longer connections attenuate the speech signal and amplifiers are needed on the line. In two-wire circuits, amplification of a signal may cause oscillation or ringing if the output signal of an amplifier loops back to the input circuit of another transmission direction (Figure 2.6). The operating principle of electronics in the network is unidirectional and inside the network we use two wires for each direction, or four-wire (4W) connections. Four-wire connections are also much easier to maintain than 2W connections because transmission directions are independent from each other and potential oscillation, as shown in Figure 2.6, is avoided. To connect a 2W local loop to a 4W network a circuit called a 2W/4W hybrid is needed. We explain the operating principle of the 2W/4W hybrid with the help of transformers. A transformer consists of coils of wires wrapped around an iron object. When an alternating current flows through one coil, it produces a magnetic field in the iron core. This mag