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4. Discussion of Load Coils
Load coil detection is essential in determining if a loop is capable of supporting xDSL services. No xDSL services can be supported on a loop with load coils. Ideally, xDSL loop-prequalification testing should determine the presence and location of the load coils. Load coils were originally added to subscriber loops longer than 18,000 feet to improve voice quality. When load coils are added, the attenuation in the voiceband is reduced, whereas that of the higher frequencies is dramatically increased (i.e., low pass filter). As xDSL transmission techniques rely on frequencies above the voiceband, the transmission of any xDSL on a loop with load coils is impossible. It has also been reported that load coils impair the operation of 56–kbps and 33.6–kbps modems for similar reasons.
The accepted practice for engineering local loops is revised resistance design (RRD). These rules specify that all loops longer than 18,000 feet must be loaded. A code letter defines the loading arrangements that specify the distance between the loading coils, and a number defines the inductance value. For example, the designations for two commonly used loading arrangements are H88 and D88. The number "88" refers to the inductance in milli-Henrys (mH) of the actual loading coils; H is the designation for 6,000-foot spacing between coils, and D indicates 4,500-foot spacing. Thus, H88, which is by far the predominant arrangement, is 88 mH coils with 6,000-foot spacing (see Figure 3).
If the rules for installing load coils are so clear, then why is it difficult to determine which loops are loaded? That question can be answered in part by examining statistics from the Bellcore local-loop survey.1 The report estimated that 24 percent of loops are loaded, but only 12 percent of loops are 18,000 feet in length—implying that only 12 percent of loops should be loaded at all. A likely cause is the rise in the number of loop reconfigurations as a result of the installation of new COs or DLCs. Figure 4 shows a 4,000-foot loop located off a DLC with a load coil. This load coil is not necessary but was left in place as the OSP changed with the addition of the DLC. Thus, the use of the LEC's OSP records is inadequate to determine the presence of load coils because of the age of the plant and the significant amount of plant changes in recent years.
It is mandatory for the LECs to determine if a loop is loaded prior to provisioning an xDSL–based service. The use of plant records or loop-length data is not sufficient to determine if a loop is loaded. For this reason, xDSL prequalification must start with the capability to detect if load coils are present in a circuit.
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