netic field generates current to the wires of other coils around the same iron core. Figure 2.7 shows the 2W/4W hybrid in a subscriber interface of the telephone exchange. Two separate transformers are needed in the hybrid and both of them consist of three similar, tightly coupled windings. In each trans- former an alternating current in one coil generates alternating current to all other coils of the same transformer. Spots of coils indicate the direction of the current flow (polarity of the coil). In Figure 2.7 we see that the current of the receive pair generates two currents with opposite polarity through the two coils of transformer T2. These currents have opposite directions in trans- former T1; they, or actually their magnetic fields in the iron core, cancel each other, and the signal from the receive pair is not connected to the transmit pair, or at least it is much attenuated. In practice, the balance is not ideal and attenuated signal is connected back, which is heard as an echo from the far end of the telephone circuit if two-way propagation delay of the circuit is long enough. Dashed lines in Figure 2.7 show the main signal paths for received and transmitted speech. Satellite connections have long propagation delays because of the long propagation distances. Also speech from the digital cellular network to the fixed telephone network suffers long delays because of speech coding (A/D and D/A conversion). The round-trip delays of these connections are longer than 50 to 100 ms, causing a disturbing echo. Hence, in the case of these connections, we have to use special equipment known as echo cancellers in the network to eliminate the echo. Introduction to Telecommunications Network Engineering Telephone set Local loop Local exchange subscriber Earphone 2W/4W hybrid 4W Black spots indicate the direction of Transmit pair transformer wiring. The signal current generates a current Microphone of the same polarity through other wirings Receive pair inside the same transformer. Main signal paths Example of curre