DSL TECHNOLOGIES
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The widespread use of the Internet and especially the World Wide Web have opened up a need for high bandwidth network services that can be brought directly to subscriber's homes. These services would provide the needed bandwidth to surf the web at lightning fast speeds and allow new technologies such as video conferencing and video on demand.
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Digital Subscriber Line A Digital Subscriber Line makes use of the current copper infrastructure to supply broadband services. A DSL requires two modems, one at the phone companies end and one at the subscribers end. The use of the term modem is not entirely correct because technically a DSL modem does not do modualtion/demodulation as in a modem that uses the normal telephone network. DSL's also have the added benefit of transmitting telephone services on the same set of wire as data services. For years it has been believed that the upper limit for transmitting data on analog phone lines was 56 kb/s. This limit is set using the maximum possible bandwidth and no compression. The reason for this limit is that POTS or Plain Old Telephone Service uses the lower 4 Khz only. The limit imposed by the POTS lines does not take advantage of all the bandwidth available on copper, which is on the order of 1 Mhz. The xDSL technologies take advantage of this difference and uses the upper frequencies for data services. Previously this was not possible because of the interference that the data services would cause in the POTS band. Advances in digital signal processing have eliminated the near-end crosstalk that results from the use of the upper bandwidth for data. The new DSP technologies allow data and POTS to be transmitted on the same set of copper wires without interfering with each other.
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Asymmetric Digital Subscriber Line ADSL is asymmetric, meaning that the downstream bandwidth is higher than the upstream bandwidth. Downstream refers to traffic in the direction towards the subscriber, and upstream refers to data sent from the subscriber back to the network. This is done because of the kinds traffic that ADSL is designed to carry. Asymmetry is used to increase the downstream bandwidth. This works because all of the downstream signals can be of the same amplitude thus eliminating crosstalk between downstream channels. Upstream signals would have to put up with more interference because the amplitude of the upstream signals would be of smaller amplitude because the are originating from different distances. To achieve the asymmetry ADSL divides its bandwidth into four classes of transport.
Transmission on the high bandwidth simplex channel and the lower bandwidth duplex channel do not interfere in any way with the POTS channel. So ADSL can carry both data a POTS on the same medium, which makes it ideal for residential and small office use. The actual rates depend on factors such as wire gauge, local loop length, and line condition. In this case, the local loop length is the distance from the central office to the subscriber.
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High-data-rate Digital Subscriber Line The most common DSL deployed today is HDSL. HDSL is mostly used to provision other services by telephone companies. HDSL symmetrically delivers 1.544 Mb/s over two sets of copper twisted pair. Which is the same rate as a T-1 type connection. This allows telco's (short for telephone companies) to use HDSL to deliver T-1 services. HDSL's operating range is about 12,000 feet, and it is possible to extend that by using repeaters along the line to the customer. HDSL is mostly used to deploy PBX network connections , interexchange POP's (Point Of Presence), and directly connecting servers to the Internet.
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Single-line Digital Subscriber Line also know as Symmetric Digital Subscriber Line Similar to HDSL, SDSL delivers the same 1.544 Mb/s, but it does it on a single set of twisted pair of copper. This limits SDSL's reach to 10,000 feet. SDSL could take hold in niche markets like residential video conferencing or connecting LAN's over short distances.
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Very-high-rate Digital Subscriber Line VDSL technology operates on a single set of copper twisted pair, and delivers data in the range of 13 Mb/s to 52 Mb/s. This high bandwidth does not come without a price, the range of VDSL is limited to between 1,000 and 4,500 feet. The VDSL standard is still in the works but there are already applications for the technology. One use for it is in getting high data rate services from the telephone companies central office to the subscriber via a FTTN (Fiber To The Neighborhood) network.. FTTN encompasses the Fiber To The Curb technologies and uses VDSL as the customers connection to the telephone companies fiber based network.
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Rate Adaptive Digital Subscriber Line RADSL is derived from ADSL technologies with some added features. RADSL automatically adjusts line speed based on the condition of the line. In areas where there is a large variance in the distance between the central office and the subscribers RADSL helps to provide a more consistent service for it's subscribers by taking the uncertainties of line conditions out of the equation when setting up a DSL connection.
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2.6 Comparison of DSL Technologies
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| 3. Glossary
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ADSL - Asymmetric Digital Subscriber Line. DSL with higher bandwidth in one direction than the other. ANSI - American National Standards Institution. ATM - Asynchronous Transfer Mode. Digital switched network, transfers in fixed length 53 byte cells. ATU-C - ADSL Terminal Unit Central Office. Terminal nearer the central office or remote network node. ATU-R - ADSL Terminal Unit Remote. ADSL terminal nearer to the subscriber. CAP - Carrierless Amplitude Modulation/Phase Modulation. A possible technology used in ADSL. Current standards put emphasis on DMT technology. DMT - Discrete Multitone. A version of multi-carrier modulation that allows allocation of payload data bits and transmitter power among more than one subchannel depending on loss, and interference among each subchannel. A candidate technology for ADSL. Drop Wire - Last part of the loop connecting distribution cable to the customer premises. DS1 - Digital Signal 1 : 1.544 Mb/s with a payload of 1.536 Mb/s bi-directional. DS2 - Digital Signal 2 : 6.312 Mb/s , which can transport 4 DS1's asynchronously. ECH - Echo Canceler with hybrid. used in DSL and HDSL systems. FTTC - Fiber To The Curb. Fiber optic lines that run to a remote electronics node close to the subscribers location. FTTH - Fiber To The Home. Fiber optic line that run to the subscribers home. POTS - Plain Old Telephone Service. Telephone service as we know it today. RBOCS - Regional Bell Operating Companies. Local exchange carriers. RFI - Radio frequency interference. SONET - Synchronous Optical Network. Synchronous multiplexing for fiber optical transport. Supports synchronous as well as asynchronous payloads. Signals come in multiples of OC-1, 51.84 Mb/s STB - Set Top Box. Performs decoding for broadband services. Examples are devices that connect the network to a TV. telco - Short for telephone company. VDSL - Very-high-rate Digital Subscriber Line. Capable of transporting 50 Mb/s payload or greater. Asymmetric or Symmetric
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