2 and use LBO (line build-out). Line build-out networks are used to increase the electrical length of a wire-pair cable section. These networks range in complexity from a simple capacitor that simulates the capacitance of the missing cable length to artificial cable sections. Network complexity increases as the frequency range over which the network has to operate increases. There is no comparable simple means to shorten the electrical length of a cable section. LBO can also be used for impedance matching. 5.5.3 Local Trunk (Junction) Design Considerations The basic considerations in the design of local trunks (junctions) are loss, stability, sig- naling, noise, and cost. Each are interrelated such that a change in value of one may affect the others. This forces considerable reiteration in the design process, and such designs are often a compromise. One major goal is to optimize return loss on trunk facilities. This turns out to be a more manageable task than that required in the subscriber distribution plant. In North America the characteristic impedance of local wire trunks in most cases is 900 S in series with a 2.16-uF capacitor to match the impedance of the local (end-offices) exchanges. It should be pointed out that some tandem and intertandem trunks connect to 600-S2 tandem switches. 5.6 VF REPEATERS (AMPLIFIERS) Voice frequency (VF)9 repeaters (amplifiers) in telephone terminology imply the use of uni directional amplifiers on VF trunks. With one approach on a two-wire trunk, two 9VF or voice frequency refers to the nominal 4-kHz analog voice channel defined at the beginning of this chapter. TRANSMISSION ASPECTS OF VOICE TELEPHONY Ampl. Two-wire VF Hybrid Hybrid Two-wire VF trunk trunk Ampl. N = balancing network (precision) Figure 5.6 Simplified block diagram of a VF repeater. amplifiers are used on each pair connected by a hybrid at the input and a hybrid at the output. A simplified block diagram is shown in Figure 5.6. The gain of a VF repeater can be run up as high as 20 dB or 25 dB, and originally th