DSL or xdsl, is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood

Transcription

1 DSL or xdsl, is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital s ubscriber loop, although in recent years, the term digital subscriber line has b een widely adopted as a more marketing-friendly term for ADSL, which is the most popular version of consumer-ready DSL. DSL can be used at the same time and on the same telephone line with regular telephone, as it uses high frequency, while regular telephone uses low frequency. Typically, the download speed of consumer DSL services ranges from 256 kilobits per second (kbit/s) to 24,000 kbit/s, depending on DSL technology, line conditio ns and service level implemented. Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for the rarer Symmetric Digital Subscriber Line (SDSL). Voice and data DSL (VDSL) typically works by dividing the frequencies used in a single phone li ne into two primary "bands". The ISP data is carried over the high-frequency ban d (25 khz and above) whereas the voice is carried over the lower-frequency band (4 khz and below). (See the ADSL article on how the high-frequency band is subdi vided.) The user typically installs a DSL filter on each phone. This filters out the high frequencies from the phone line, so that the phone only sends or recei ves the lower frequencies (the human voice). The DSL modem and the normal teleph one equipment can be used simultaneously on the line without interference from e ach other. Operation [edit] Regular DSL The local loop of the public switched telephone network (PSTN) was initially des igned to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the p hone system nominally passes audio between 300 and 3,400 Hz, which is regarded a s the range required for human speech to be clearly intelligible. This is known as voiceband or commercial bandwidth. At the local telephone exchange (United Kingdom) or central office (United State s) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, according to the Nyquist theorem, any signal above 4,000 Hz is not passed by the phone network (a nd has to be blocked by a filter to prevent aliasing effects). The laws of physics, specifically the Shannon limit, cap the speed of data trans mission. For a long time, it was believed that a conventional phone line couldn' t be pushed beyond low speed limits (typically under 9600 bit/s). In the 1950s, 4 MHz television signals were often carried between studios on ordinary twisted pair telephone cable, suggesting that the Shannon Limit would allow transmitting many megabits per second. However, these cables had other impairments besides G aussian noise, preventing such rates from becoming practical in the field. In th e 1980s techniques were developed for broadband communications that allowed the limit to be greatly extended. The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 khz upper limit of POTS. Depending o n the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating Hz wide channels starting between 10 and 100 khz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel i s evaluated for usability in much the same way an analog modem would on a POTS c

2 onnection. More usable channels equates to more available bandwidth, which is wh y distance and line quality are a factor (the higher frequencies used by DSL tra vel only short distances). The pool of usable channels is then split into two di fferent frequency bands for upstream and downstream traffic, based on a preconfi gured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual cir cuits, one in each direction. Like analog modems, DSL transceivers constantly mo nitor the quality of each channel and will add or remove them from service depen ding on whether they are usable. One of Lechleider's[2] contributions to DSL was his insight that an asymmetric a rrangement offered more than double the bandwidth capacity of symmetric DSL. Thi s allowed Internet Service Providers to offer efficient service to consumers, wh o benefitted greatly from the ability to download large amounts of data but rare ly needed to upload comparable amounts. ADSL supports two modes of transport: fa st channel and interleaved channel. Fast channel is preferred for streaming mult imedia, where an occasional dropped bit is acceptable, but lags are less so. Int erleaved channel works better for file transfers, where the delivered data must be error free but latency incurred by the retransmission of errored packets is a cceptable. Because DSL operates at above the 3.4 khz voice limit, it cannot be passed throu gh a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, while voice service cannot be maintained past a certain distance with out such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this, p hone companies are endeavoring to remove load coils on copper loops that can ope rate without them, and conditioning lines to avoid them through the use of fiber to the neighborhood or node FTTN. The commercial success of DSL and similar technologies largely reflects the adva nces made in electronics, that, over the past few decades, have been getting fas ter and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the populariz ation of DSL technology: Until the late 1990s, the cost of digital signal processors for DSL was prohibit ive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of VLSI technology, the cost of the equipment associated with a DSL deployment (a DSLAM at one end and a DSL modem at the other end) lowered s ignificantly. A DSL line can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cab le over the same route and distance. This is true both for ADSL and SDSL variati ons. In the case of ADSL, competition in Internet access caused subscription fees to drop significantly over the years, thus making ADSL more economical than dial up access. Telephone companies were pressured into moving to ADSL largely due to c ompetition from cable companies, which use DOCSIS cable modem technology to achi eve similar speeds. Demand for high bandwidth applications, such as video and fi le sharing, also contributed to popularize ADSL technology. Most residential and small-office DSL implementations reserve low frequencies fo r POTS service, so that (with suitable filters and/or splitters) the existing vo ice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires w ith DSL. Only one DSL "modem" can use the subscriber line at a time. The standar d way to let multiple computers share a DSL connection is to use a router that e

3 stablishes a connection between the DSL modem and a local Ethernet, Powerline, o r Wi-Fi network on the customer's premises. Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider. Naked DSL Dry-loop DSL or "naked DSL," which does not require the subscriber to have tradi tional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT &T's merger with SBC,[3] and Verizon's merger with MCI,[4] those telephone compa nies are required to offer naked DSL to consumers. Even without the regulatory mandate, however, many ILECs offer naked DSL to cons umers. The number of telephone landlines in the US has dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has gro wn to 195 million.[5]. This lack of demand for landline service has resulted in the expansion of naked DSL availability. [edit] Typical setup and connection procedures The first step is the physical connection. On the customer side, the DSL Transce iver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone l ine. The telephone company(telco) connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single b ox. The location of the DSLAM depends on the telco, but it cannot be located too far from the user because of attenuation, the loss of data due to the large amo unt of electrical resistance encountered as the data moves between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM. When the DSL modem is powered up, it goes through a sync procedure. The actual p rocess varies from modem to modem but can be generally described as: The DSL Transceiver does a self-test. The DSL Transceiver checks the connection between the DSL Transceiver and the co mputer. For residential variations of DSL, this is usually the Ethernet (RJ-45) port or a USB port; in rare models, a FireWire port is used. Older DSL modems sp orted a native ATM interface (usually, a 25 Mbit serial interface). Also, some v ariations of DSL (such as SDSL) use synchronous serial connections. The DSL Transceiver then attempts to synchronize with the DSLAM. Data can only c ome into the computer when the DSLAM and the modem are synchronized. The synchro nization process is relatively quick (in the range of seconds) but is very compl ex, involving extensive tests that allow both sides of the connection to optimiz e the performance according to the characteristics of the line in use. External, or stand-alone modem units have an indicator labeled "CD", "DSL", or "LINK", wh ich can be used to tell if the modem is synchronized. During synchronization the light flashes; when synchronized, the light stays lit, usually with a green col or. Modern DSL gateways have more functionality and usually go through an initializa tion procedure that is very similar to a PC starting up. The system image is loa ded from the flash memory; the system boots, synchronizes the DSL connection and establishes the IP connection between the local network and the service provide r, using protocols such as DHCP or PPPoE. The system image can usually be update d to correct bugs, or to add new functionality. [edit] Equipment

4 The customer end of the connection consists of a Terminal Adaptor or in layman's terms "DSL modem." This converts data from the digital signals used by computer s into a voltage signal of a suitable frequency range which is then applied to t he phone line. In some DSL variations (for example, HDSL), the terminal adapter is directly con nected to the computer via a serial interface, using protocols such as RS-232 or V.35. In other cases (particularly ADSL), it is common for the customer equipme nt to be integrated with higher level functionality, such as routing, firewallin g, or other application-specific hardware and software. In this case, the entire equipment is usually referred to as a DSL router or DSL gateway. Some kinds of DSL technology require installation of appropriate filters to sepa rate, or "split", the DSL signal from the low frequency voice signal. The separa tion can be done either at the demarcation point, or can be done with filters in stalled at the telephone outlets inside the customer premises. Either way has it s practical and economical limitations. See ADSL for more information about this. At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other netw orking transports. In the case of ADSL, the voice component is also separated at this step, either by a filter integrated in the DSLAM or by a specialized filte ring equipment installed before it. The DSLAM terminates all connections and rec overs the original digital information. [edit] Protocols and configurations Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same lin k. DSL implementations may create bridged or routed networks. In a bridged configur ation, the group of subscriber computers effectively connect into a single subne t. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details. [edit] DSL technologies The line length limitations from telephone exchange to subscriber are more restr ictive for higher data transmission rates. Technologies such as VDSL provide ver y high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Technologie s likes GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based IDSN, ADSL and DSL over f iber optics. Example DSL technologies (sometimes called xdsl) include: ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN. High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technol ogy that uses a higher frequency spectrum of copper, twisted pair cables. Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equ al in both directions. Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardised replaceme nt for early proprietary SDSL.

5 Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other. Asymmetric Digital Subscriber Line 2 (ADSL2), an improved version of ADSL Asymmetric Digital Subscriber Line 2 Plus (ADSL2+), A version of ADSL2 that doub les the data rates by using twice the spectrum. Asymmetric Digital Subscriber Line Plus Plus (ADSL++), technology developed by C entillium Communications for Japan market that extends downstream rates to 50 Mb it/s by using spectrum up to 3.75 MHz. Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and no ise tolerance by sacrificing up stream speed Very High Speed Digital Subscriber Line (VDSL) Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version of VDSL Etherloop Ethernet Local Loop Uni-DSL (Uni Digital Subscriber Line or UDSL), technology developed by Texas Ins truments, backwards compatible with all DMT standards Gigabit Digital Subscriber Line (GDSL), based on binder MIMO technologies[6]. Universal High bit rate Digital Subscriber Line (UHDSL) using fiber optics. Deve loped in 2005 by RLH Industries, Inc. Converts HDSL-1, 2 or 4 copper service int o fiber optic HDSL service. [edit] Transmission methods Transmission methods vary by market, region, carrier, and equipment. 2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL, used for HDSL DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM ( Orthogonal frequency-division multiplexing)