An ADSL loop extender or ADSL repeater is a device that a telephone company can place midway between the subscriber and central office to extend the distance and increase the channel capacity of their DSL connection. ADSL repeaters are aggressively deployed by rural telephone companies trying to provide rural Internet service to farms and small towns where it's impractical to place the DSLAM closer. The typical distance improvement with a loop extender is shown in the diagram below, with rate in Megabits per second and distance in thousands of feet.[1]
A repeater can either be an amplifier or a re-generator. Amplifiers increase the signal level of the analog transmission signal; re-generators demodulate the signal to binary, then re-modulate it into the original transmission frequency. Because regeneration restores the signal to binary, an indefinite number of re-generators can be placed on a line and is the preferred choice for services like T1 (Digital Signal 1) that have no distance limits. Because of the simplicity of the amplifier circuits, amplifiers are of lower cost than re-generators. It's the cost of re-generators that drives the price differences between T1 at $800 per month and ADSL at $40 per month. Because ADSL is a low-cost service, it's generally felt that the additional cost of regeneration cannot be supported. ADSL repeaters all work through amplification instead of regeneration.
Before the development of ADSL loop extenders and remote DSLAMs, ADSL was limited to 3–6 miles (5-10 km) from the Central Office depending on the wire gauge used. An ADSL Loop Extender works as an amplifier, boosting the signal level so it can travel longer distances. In some cases, service can now be established as far as 10 miles from the Central Office.
In 2006, US telco promoted Fiber to the Home. This was driven by a rapidly growing housing sector that was creating the “greenfield” customers that are needed to make fiber to the home profitable. Later, with the housing sector in a serious recession, that "greenfield" seems to be drying up fast.[2] With most of the “brownfield” market already tapped for ADSL[3], Telcos finally are interested in extending ADSL to those semi-rural areas that have never been important before[4].
Some ADSL loop extenders aren't repeaters, but instead convert to a different signal (like G.shdsl) that can be repeater-ed. If additional amplification in the C.O. were a good idea, the DSLAM and modem makers would just run their products at higher power. This additional amplification creates noise on adjacent pairs and is not compliant with T1.417 spectrum management. Converting to G.shdsl or other technologies has problems too. These technologies have limited downstream speed, thus are less useful except to extend services to the most distant customers. Because the technology has so many components (special C.O., re-generators, CPE), it's much more expensive than ADSL amplifiers.