ADSL loop extender
Encyclopedia
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.
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. With most of the “brownfield
” market already tapped for ADSL, Telcos finally are interested in extending ADSL to those semi-rural areas that have never been important before.
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.
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
can place midway between the subscriber and central office to extend the distance and increase the channel capacity
Channel capacity
In electrical engineering, computer science and information theory, channel capacity is the tightest upper bound on the amount of information that can be reliably transmitted over a communications channel...
of their DSL connection. ADSL repeaters are aggressively deployed by rural telephone companies trying to provide rural Internet
Rural Internet
Rural Internet is the access to the Internet from rural areas , which are settled places outside towns and cities. Inhabitants live in villages, hamlets, on farms and in other isolated houses...
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.
A repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
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
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
) 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
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
depending on the wire gauge
Wire gauge
Wire gauge is a measurement of how large a wire is, either in diameter or cross sectional area. This determines the amount of electric current a wire can safely carry, as well as its electrical resistance and weight per unit of length...
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
Greenfield project
In many disciplines a greenfield is a project that lacks any constraints imposed by prior work. The analogy is to that of construction on greenfield land where there is no need to remodel or demolish an existing structure...
” 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. With most of the “brownfield
Brownfield land
Brownfield sites are abandoned or underused industrial and commercial facilities available for re-use. Expansion or redevelopment of such a facility may be complicated by real or perceived environmental contaminations. Cf. Waste...
” market already tapped for ADSL, Telcos finally are interested in extending ADSL to those semi-rural areas that have never been important before.
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.