DSL Limitations

DSL is a relatively distance-sensitive technology. As the distance between the subscriber and their local CO increases, the signal quality and connection speeds decrease. ADSL service is limited to a maximum distance of 18,000 feet (5460 m) between the DSL CPE and the DSLAM, although many ADSL providers place an even lower limit on the distance to ensure quality.

The 18,000-foot distance limitation for ADSL is not a limitation for voice telephone calls, but for data transmission. Telephone companies use small amplifiers, called load coils, to boost voice signals. Load coils have a rather nasty tendency to disrupt DSL data signals. This means that if there are load coils in the loop between the CPE and CO, those subscribers are likely not within an area that can receive DSL service.

Table 4-2 outlines the distance limitations of some common DSL data-rate offerings.

Table 4-2 DSL Distance Limitations

DSL Technology

Data Rate Downstream/Upstream

Maximum Distance

ADSL

8 Mbps/1 Mbps

18,000 ft

VDSL

55 Mbps/13 Mbps

4,500 ft

IDSL

144 kbps/144 kbps

18,000 ft

SDSL

768 kbps/768 kbps

22,000 ft

G.SHDSL

2.3 Mbps/2.3 Mbps

28,000 ft

As is evident from the table, DSL is offered in a wide range of transfer rates. The table does not represent all available rates. In theory, the maximum throughput potential of any first-generation ADSL technology is 8.448 Mbps. However, that would require a subscriber to be very close (within a few hundred feet) to the local CO.

In 2004, the second generation ADSL, known as ADSL2 (ITU G.992 3/4), was created to offer 12 Mbps downstream at distances less than or equal to 8000 feet. Additionally, ADSL2+ (ITU G.992.5) was created to provide up to 24 Mbps for distances less than 5000 feet.

Throughput capacity and subscriber distance from the CO are inversely proportional in nature. As distance increases, maximum upstream and downstream speeds decrease. Additionally, other impairments or conditions may contribute to reduced functionality. These include the following:

■ AM radio interference—AM radio frequencies can interfere with DSL signal quality, causing throughput reduction. This is particularly problematic with in-house wiring using low-quality cabling (untwisted or poorly twisted-pair wiring).

■ Bridge taps—These are extensions between the CPE and the CO. Essentially, bridge taps are an additional wire with an unterminated cable end that is connected to the local loop. This causes noise, reflections, and potential for power bleed that reduces signal strength (and therefore throughput).

■ Crosstalk—Crosstalk is interference between two wires in a bundle as a result of electromagnetic interference. Crosstalk occurs when there is frequency overlap between channels.

■ Fiber optic cable—ADSL signals cannot pass through the conversion from analog to digital to analog that occurs if a portion of the telephone circuit traverses fiber optic cable in transit.

■ Impedance mismatch—Impedance mismatch in the local loop causes echo, resulting in noise. The mismatch can be caused by changes in wire gauge, splices, or corrosion.

■ Load coils—Load coils are used to extend the range of a local loop for voice operations. They are intended to function as inductors to compensate for parallel capacitance on the line. Essentially, they are inductors that act like a low-pass filter, disallowing higher frequency signals. Because of this, load coils significantly distort DSL frequencies and must be removed for any DSL operation. They are often found within loops extending farther than 12,000 ft.

■ Signal attenuation—Attenuation is the loss or degradation of signal strength (energy) over distance. Attenuation varies with the distance between subscriber and CO.

■ Wire gauge—Variations in wire thickness can affect throughput by introducing impedance mismatches. Also, the use of thicker wire in the local loop can improve signal strength and improve throughput.

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