High data rate Digital Subscriber Line
Encyclopedia
High-bit-rate digital subscriber line (HDSL) was the first DSL technology to use a higher frequency spectrum
of copper, twisted pair
cables. HDSL was developed in the US, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier
systems, and also to carry high-speed corporate data links and voice channels, using T1
lines. HDSL service types include HDSL1, HDSL2 and HDSL4 and are typically transmitted over twisted pair cables or over fiber optics
using the RLH Universal UHDSL fiber system.
T-carrier
circuits operate at 1.544 Mbit/s. These circuits were originally carried using a line code
called alternate mark inversion (AMI). Later the line code used was B8ZS. AMI did not have sufficient range, requiring the application of repeaters over long circuits. As with any wire circuit, they were subject to lightning and cable trouble such as inferior splices and backhoe fade. These line codes at this bit rate give a bandwidth of 772 kHz and the repeaters are usually spaced every mile to 1.2 miles depending on conductor gauge and other circumstances.
As in classical T-carrier, HDSL has a positive and negative polarity to the side of the repeater. In splicing this type of service the telcos placed the low voltage side of the repeater cable together and then the High voltage side together in the splice. The telcos have a powering end to the circuit path and this gives the polarity and the repeaters are typically powered up to 130 volts dc. Usually if you see 130 volts there is trouble because the repeaters are running full power to try to compensate for the trouble. They require 60 milliamps and if they cannot get it they try to achieve it by raising the voltage.
The first attempts to use DSL technology to solve the problem were done in the US, using the line code 2B1Q
. This modulation allowed for a 784 kbit/s data rate over a single twisted pair
cable. With two twisted pair
cables, the full 1.544 Mbit/s was achieved. The new technology attracted the attention of the industry, but could not be directly used worldwide, due to the differences between the T1
and E1
standards. A new standard was then developed by the ITU
for HDSL, using the CAP (carrierless amplitude phase modulation
) line code, that reached the maximum bandwidth of 2.0 Mbit/s using two pairs of copper.
HDSL gave the telcos a greater distance reach when delivering a T-1 circuit. It was marketed originally as a Non Repeated T-1, with a distance of 12k feet (3.8 km) over 24-gauge cable. The cable gauge affects the distance. To allow for longer distances, a repeater can be used. The repeater actually terminates the circuit and regenerates the signal. Up to four repeaters can be used for a reach of 60k feet (about 20 km). This reduced the cost of maintenance when compared with AMI-based repeaters that had to be used at every 35 db of attenuation (about 1 mile).
HDSL can be used either at the T1 rate (1.544 Mbit/s) or the E1
rate (2 Mbit/s). Slower speeds are obtained by using multiples of 64 kbit/s channels, inside the T1/E1 frame. This is usually known as channelized T1/E1, and it's used to provide slow-speed data links to customers. In this case, the line rate is still the full T1/E1 rate, but the customer only gets the limited (64 multiple) data rate over the local serial interface. Unlike later ADSL, HDSL did not allow POTS
at baseband.
HDSL gave way to two new technologies, called HDSL2
and SDSL
. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 kbit/s to 2.3 Mbit/s, using a single pair of copper. SDSL is used as a replacement (and in some cases, as a generic designation) for the entire HDSL family of protocols.
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
of copper, twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
cables. HDSL was developed in the US, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier
Local exchange carrier
Local Exchange Carrier is a regulatory term in telecommunications for the local telephone company.In the United States, wireline telephone companies are divided into two large categories: long distance and local...
systems, and also to carry high-speed corporate data links and voice channels, using T1
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...
lines. HDSL service types include HDSL1, HDSL2 and HDSL4 and are typically transmitted over twisted pair cables or over fiber optics
using the RLH Universal UHDSL fiber system.
T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
circuits operate at 1.544 Mbit/s. These circuits were originally carried using a line code
Line code
In telecommunication, a line code is a code chosen for use within a communications system for baseband transmission purposes...
called alternate mark inversion (AMI). Later the line code used was B8ZS. AMI did not have sufficient range, requiring the application of repeaters over long circuits. As with any wire circuit, they were subject to lightning and cable trouble such as inferior splices and backhoe fade. These line codes at this bit rate give a bandwidth of 772 kHz and the repeaters are usually spaced every mile to 1.2 miles depending on conductor gauge and other circumstances.
As in classical T-carrier, HDSL has a positive and negative polarity to the side of the repeater. In splicing this type of service the telcos placed the low voltage side of the repeater cable together and then the High voltage side together in the splice. The telcos have a powering end to the circuit path and this gives the polarity and the repeaters are typically powered up to 130 volts dc. Usually if you see 130 volts there is trouble because the repeaters are running full power to try to compensate for the trouble. They require 60 milliamps and if they cannot get it they try to achieve it by raising the voltage.
The first attempts to use DSL technology to solve the problem were done in the US, using the line code 2B1Q
2B1Q
Two-binary, one-quaternary is a physical layer encoding used for Integrated Services Digital Network Basic Rate Interface implementations...
. This modulation allowed for a 784 kbit/s data rate over a single twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
cable. With two twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
cables, the full 1.544 Mbit/s was achieved. The new technology attracted the attention of the industry, but could not be directly used worldwide, due to the differences between the T1
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
and E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
standards. A new standard was then developed by the ITU
Itu
Itu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
for HDSL, using the CAP (carrierless amplitude phase modulation
Carrierless Amplitude Phase Modulation
Carrierless amplitude phase modulation is a variant of quadrature amplitude modulation . Instead of modulating the amplitude of two carrier waves, CAP generates QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert pair.CAP...
) line code, that reached the maximum bandwidth of 2.0 Mbit/s using two pairs of copper.
HDSL gave the telcos a greater distance reach when delivering a T-1 circuit. It was marketed originally as a Non Repeated T-1, with a distance of 12k feet (3.8 km) over 24-gauge cable. The cable gauge affects the distance. To allow for longer distances, a repeater can be used. The repeater actually terminates the circuit and regenerates the signal. Up to four repeaters can be used for a reach of 60k feet (about 20 km). This reduced the cost of maintenance when compared with AMI-based repeaters that had to be used at every 35 db of attenuation (about 1 mile).
HDSL can be used either at the T1 rate (1.544 Mbit/s) or the E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
rate (2 Mbit/s). Slower speeds are obtained by using multiples of 64 kbit/s channels, inside the T1/E1 frame. This is usually known as channelized T1/E1, and it's used to provide slow-speed data links to customers. In this case, the line rate is still the full T1/E1 rate, but the customer only gets the limited (64 multiple) data rate over the local serial interface. Unlike later ADSL, HDSL did not allow POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
at baseband.
HDSL gave way to two new technologies, called HDSL2
HDSL2
HDSL2 is the 2nd generation of HDSL with a 6dB Noise Margin, put simply it is another way to provision a T-1 line, only this technology relies on fewer wires - two instead of four - and therefore costs less to set up....
and SDSL
Symmetric Digital Subscriber Line
Symmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...
. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 kbit/s to 2.3 Mbit/s, using a single pair of copper. SDSL is used as a replacement (and in some cases, as a generic designation) for the entire HDSL family of protocols.
See also
- Asymmetric digital subscriber lineAsymmetric Digital Subscriber LineAsymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
- Symmetric digital subscriber lineSymmetric Digital Subscriber LineSymmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...
- Digital subscriber lineDigital Subscriber LineDigital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
- Very-high-bit-rate digital subscriber line