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Distance Limitations of DSL
April 1999

Why are there distance limits to getting DSL and just what are they?

When the telephone companies wired residences and offices in the United States they tried to minimize their biggest expense, copper wire. In fact, telcos are the largest consumers of copper in the world. The goal of the telcos in wiring for telephone service was to get the greatest range from a telephone company’s central office (explained later in this article) to the customer at the lowest possible cost.

Voice signals over our phone lines use a frequency range from 400Hz to 4,000Hz. DSL uses frequencies above that range are different DSL technologies use different frequency ranges up to more than 1Mhz. ADSL uses these higher ranges and the frequency places limits on how far the signal can travel over copper wire before it is too weak to be useable. Since some DSL services, like IDSL, use a lower frequency, their range is farther and the Plain Old Telephone service, which uses a very low frequency, can travel the farthest.

The copper wire that runs form the telephone company central office to your home or office is called a local loop. The thickness for wire used for the local loop can vary over it’s path and the thickness affects the strength of the signal. Wire thickness is important since the thickness of the wire has an affect on how much the signal deteriorates when it travels over the wire. This effect, called attenuation is greater with thinner wires and less with thicker wires.

Wire thickness is measured in gauges. A 26 gauge wire is 1/26^th of an inch in thickness. A 24-gauge wire is 1/24^th of an inch thick. The gauge is equal to the fraction of an inch of measurement with the gauge as the bottom half of the fraction. Remember, copper is the telco’s biggest expense so they wired the country with as thin a wire as possible (bigger gauge numbers = thinner wire). Also, when most telco wiring was installed, no one even had heard of the Internet and the goal was to get telephone service to all residences and offices..

The wire that runs from your home or office to the telephone company Central Office is called a Local Loop. Local Loops have standardized range limits for data signals in the United States. The Revised Resistance Distance (RRD) Limit is an industry standard for the distance certain signals (including ADSL) travel on a telco wire pair. The distance is 18,000 feet for a 24 gauge wire (thicker) and 15,000 feet for a 26 gauge wire (thinner). About 80 percent of all telco customers are within the Revised Range Distance Limit. But the actual distance depends on what gauge wire connects you to the central office. Since most telcos use a mix of both wire gauges, the effective range of ADSL is usually somewhere between 15,000 feet and 18,000 feet.

In fact, SBC and PacBell have a stated ADSL range limits of 17,500 feet and GTE Telephone has a range limit of 16,500 feet. That’s the farthest you can be from the Central Office.

 

When you call up an ISP or your local telephone company to see if you can get ADSL service, the first thing that will be checked is to see if your location is within the RRD range limit. If it is, you passed the first step of qualification and will go on to the next. If not, you can still probably get a service called IDSL DSL over ISDN circuits) which has the range of ISDN and can provide service as far as nine miles from the CO to your location.

Your Phone Service Distance

The Carrier Service Area is another range limit used as a standard for how far a POTS signal can travel. The CSA range limit for phone service is 12,000 feet for 24 gauge wire and 9,000 feet for 26 gauge wire. How can you get phone service if you’re farther from the Central Office than 12,000 feet? The telco’s have Remote Offices that are placed in locations necessary to provide coverage for all customers serviced by the central office. The Remote Office has equipment that effectively extends the range of service another 12,000 feet for 24 gauge and 9,000 feet for 26 gauge wire. Increasingly, telcos run fiber connections for new Remote Offices or replace older runs of copper with fiber from the Remote Offices to the Central Office to provide growth capacity.

The equipment necessary to provide ADSL service is usually installed in the central office and the distance range limit is from that location. Since DSL services are still relatively new and no one really can determine demand, it’s unclear when the telcos will start to place equipment in the Remote Offices. When telco’s start to install ADSL equipment in their Remote Offices, then the RRD will be measured from the Remote Office to the customer location, effectively expanding coverage of ADSL services to more users.

 

 

copyright 1999 MM Internet, Inc.

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