|
| |  | |
Principal Sponsors:
 | Digital Subscriber Line (DSL) Testing |
5. Measurement of Line Length
The accurate measurement of line length is essential when determining if a loop is capable of supporting xDSL transmission. All xDSLs are sensitive to the line length and the gauge of wire between the central office and the customer's premises. If a carrier is offering a fixed-rate service or using xDSL to provide voice services, knowing loop length is important; the transmission rate of all xDSLs is inversely proportional to loop distance. The recent introduction of rate-adaptive ADSL and the trend towards offering lower bit-rate services has helped increase the acceptable line lengths. The wire gauge is also important; typically the distance supported for 26AWG is only two-thirds of the distance supported for 24AWG.
As reported by Bellcore, approximately 88 percent of loops are less than 18,000 feet and 65 percent comply with carrier service area design rules (CSA), requirements that prescribe loop lengths of less than 12,000 feet for 24AWG and less than 9,000 feet for 26AWG.1 In addition, Bellcore reported that over two-thirds of DLC loops conform to CSA rules.1 The mix of 24AWG or higher is 59.6 percent, although 26AWG represents 40.4 percent. Table 1 provides a summary of the relationship between xDSL transmission and loop length for the popular DSL variants.
The legacy POTS testing systems measure loop length using simple tip-to-ring capacitance measurements. The loop length is calculated from that measurement by using .083 µF/mile. The tip-to-ring capacitance is a well-controlled parameter and is primarily dependent on the twist of wires, varying 6 percent from .078 to .086 µF/mile, with a nominal value of .083 µF/mile.
While the specifications for xDSL transmission are typically given for either twenty-four or twenty-six gauge cable, it is difficult to determine what percentage of the loop plant is of uniform gauge (i.e., all 26AWG). It may be possible to estimate the smallest gauge by applying either RRD or CSA design rules. Using the smaller gauge yields the slowest transmission speed (lower bound) that can be supported.
|
