DESIGN OF LOCAL AREA WIRE-PAIR TRUNKS (JUNCTIONS)

 Introduction

Exchanges in a common local area often are connected in a full-mesh topology (see Section 1.3.7, a definition of full mesh). Historically, depending on distance and certain other economic factors, these trunks used VF (analog wire-pair) transmission over cable. In North America, this type of transmission is phasing out in favor of a digital connectivity. However, analog wire-pair transmission still persists in a number of parts of the world, and especially outside of North America. In the United Kingdom the term junction is used for trunks serving the local area, whether analog or digital. There are notably fewer trunks than subscriber lines for which they serve. This is due to the concentration at a local serving switch. The ratio of trunks to subscriber lines varies from 3 to 25. Because there are fewer trunks, more investment can be made on this portion of the plant. Losses are generally kept around 2 dB and return losses are well over 24 dB because of excellent impedance matches. These low trunk insertion losses can be accomplished by several means, such as using larger diameter wire pairs, employing VF amplifiers, and inductive loading. 

Inductive Loading of Wire-Pair Trunks (Junctions)

The approach to inductive loading of wire-pair trunks is similar to that for loading subscriber loops. The distance (D) between load coils is all important. The spacing (D) should not vary more than ± 2% from the specified spacing. The first load coil is spaced D/ 2 from an exchange main frame, where D is the specified distance between load coils (see Table 5.3).9 Take the case of H loading, for instance. The distance between load coil points is 6000 ft (1830 m), but the first load coil is place at D/ 2 or 3000 ft (915 m) from the exchange. Then if the exchange is by-passed by some of the pairs, a full-load section exists. This concept is illustrated in Figure

8Planning period refers to telecommunication planning. Here we mean advanced planning for growth. Planning periods may be 5, 10, or 15 years in advance of an installation date. 9Main frame is a facility, often a frame at a switching center, where all circuits terminate and where they may be cross connected. In other words, this is a location where we can get physical or virtual access to a circuit and where we may reconfigure assets. Main frames in local serving switches often are extremely large, where 10,000 or more subscriber lines terminate.

Loading of VF trunks (junctions)

Now consider this example. A loaded 500-pair VF trunk cable extends across town. A new switching center is to be installed along the route where 50 pairs are to be dropped and 50 inserted. It would be desirable to establish the new switch midway between load points. At the switch, 450 circuits will by-pass the office (switch). Using this D/ 2 technique, these circuits need no conditioning; they are full-load sections (i.e., D/ 2 + D/ 2 c 1D, a full-load section). Meanwhile, the 50 circuits entering from each direction are terminated for switching and need conditioning so that each looks electrically like a full-load section. However, the physical distance from the switch out to the first load point is D/ 2 or, in the case of H loading, 3000 ft or 915 m. To make the load coil distance electrically equivalent to 6000 ft or 1830 m, line build-out (LBO) is used. LBO is described in Section 

                    Suppose that the location of a new switching center was such that it was not halfway, but at some other fractional distance. For the section comprising the shorter distance, LBO is used. For the other, longer run, often a half-load coil is installed at the switching center and LBO is added to trim up the remaining electrical distance. 

Line Build-Out (LBO). In many instances the first (and last) load coil cannot be placed at a D/ 2 distance from a switch or the separation between load coils cannot be D within tolerance. The reasons for the inability of an installation crew to meet the siting requirements are varied. Buildings could be in the way; the right-of-way requires a detour; hostile cable ground conditions exist; and so forth. In these cases, we install the load coil at a distance less than D/ 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, which 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 (Ref. 7).

Local Trunk (Junction) Design Considerations

The basic considerations in the design of local trunks (junctions) are loss, stability, signaling, 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 Q in series with a 2.16-mF 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-Q tandem switches. 

Simplified block diagram of a VF repeater