ey were used at 50-mi intervals on 19-gauge loaded cable in the long-distance (toll) plant. Today they are seldom found on long-distance circuits, but they do have application on local trunk circuits where the gain requirements are considerably less. Trunks using VF repeaters have the repeater's gain adjusted to the equivalent loss of the circuit minus the 4-dB loss to provide the necessary singing margin. In practice, a repeater is installed at each end of the trunk circuit to simplify maintenance and power feeding. Gains may be as high as 6-8 dB. Another repeater commonly used on two-wire trunks is the negative-impedance repeater. This repeater can provide a gain as high as 12 dB, but 7 or 8 dB is more common in practice. The negative-impedance repeater requires an LBO at each port and is a true, two-way, two- wire repeater. The repeater action is based on regenerative feedback of two amplifiers. The advantage of negative-impedance repeaters is that they are transparent to dc signaling. On the other hand, VF repeaters require a composite arrangement to pass dc signaling. This consists of a transformer by-pass (Ref. 7). REVIEW EXERCISES 1. Define the voice channel using the band of frequencies it occupies from the CCITT perspective. What is its bandwidth? Where does the sensitivity of a telephone set peak (i.e., at about what frequency) from a North American perspective, from a CCITT perspective? A local serving switch provides a battery emf source for the subscriber loop. What is the nominal voltage of this battery? Name at least three functions that this emf source provides. A subscriber loop is designed based on two limiting conditions (impairments). What are they? What is the North American maximum loss objective for the subscriber loop? 6. What happens when we exceed the "signaling" limit on a subscriber loop? What are the two effects that a load coil has on a subscriber loop or a metallic VF trunk? 8. Using resistance design or revised resistance design, our only concern is resistance. What about loss? REF