ABSTRACT Digital Subscriber Lines (DSL) are used to deliver high-rate digital data overexisting ordinary phone-lines. A new modulation technology called DiscreteMultitone (DMT) allows the transmission of high speed data. DSL facilitates thesimultaneous use of normal telephone services, ISDN, and high speed datatransmission, e.g., video. DMT-based DSL can be seen as the transition fromexisting copper-lines to the future fiber-cables. This makes DSL economicallyinteresting for the local telephone companies. They can offer customers highspeed data services even before switching to fiber-optics. DSL is a newly standardized transmission technology facilitatingsimultaneous use of normal telephone services, data transmission of 6 M bit/s inthe downstream and Basic- rate Access (BRA). DSL can be seen as a FDM systemin which the available bandwidth of a single copper-loop is divided into threeparts. The base band occupied by POTS is split from the data channels by using amethod which guarantees POTS services in the case of ADSL-system failure (e.g.passive filters).
INDEX PAGE NO.CHAPTER 1 INTRODUCTION 061.1 Different variants of DSL 071.2 What makes DSL popular 081.3 What are the benefits. 08CHAPTER 2 ASYMMETRIC DIGITAL SUBSCRIBER LINE (ADSL). 102.1. The components of an ADSL network include a 11TELCO and a CPECHAPTER 3 ADSL CAPABILITIE 13CHAPTER 4 ADSL TECHNOLOGY. 164.1. ADSL transceiver – network 17CHAPTER 5 METHODS TO SPLIT THE SIGNAL. 205.1 Carrier less amplitude/phase(cap). 205.2 Discrete Multi tone 215.3. Splitting the signal: filters 22
1 .INTRODUCTION The past decade has seen extensive growth of the telecommunicationsindustry, with the increased popularity of the Internet and other datacommunication services. While offering the world many more services than werepreviously available, they are limited by the fact that they are being used ontechnology that was not designed for that purpose. The majority of Internet users access their service via modems connects tothe Plain Old Telephone System (POTS). In the early stages of the technology,modems were extremely slow by todays standards, but this was not a majorissue. A POTS connection provided an adequate medium for the relatively smallamounts of data that required transmission, and so was the existing system wasthe logical choice over special cabling. Technological advances have seen these rates increase up to a point wherethe average Internet user can now download at rates approaching 50Kbps, andsend at 33.6Kps. However, POTS was designed for voice transmission, atfrequencies below 3kHz, and this severely limits the obtainable data rates of thesystem. To increase performance of new online services, such as steaming audioand video, and improve general access speed, the bandwidth hungry public musttherefore consider other alternatives. Technologies, such as ISDN or cableconnections, have been in development for sometime but require special cabling.
This makes them expensive to set up, and therefore have not been a viablealternative for most people. 1.1 DIFFERENT VARIANTS OF DSLHDSL is the pioneering high speed format, but is not a commercially viable optiondue to its need for two twisted pairs and does not have support for normaltelephone services.SDSL is symmetric DSL, and operates over a single twisted pair with support forstandard voice transmission. The problem with this system is that it is limited torelatively short distances and suffers NEXT limitation due to the use of the samefrequencies for transmitting and receiving.IDSL stands for ISDN DSL, and is in many ways similar to ISDN technology. Itsdisadvantages are the lack of support for analog voice, and that its 128kbps rate isnot much greater than that offered by standard 56kbps V90 modems.VDSL provides very high bit rate DSL, up to 52Mbps, but requires shorterconnections lengths than are generally practical. It has been used in conjunctionwith an experimental project, FTTC (Fiber to the Curb), but development in thisarea has slowed due to commercial viability issues.ADSL is the most promising DSL technology, proving suitable for personalbroadband requirements and allowing for the same channel to still act as atraditional POTS service.
Rate Adaptive DSL, RADSL, is a further advancement which is able toautomatically optimize the ADSL data rate to suit the conditions of the line beingused.1.2. WHAT MAKES DSL POPULAR Digital Subscriber Line (DSL) technology provides high-speed Internet Accessusing regular telephone lines. It has the ability to move data over the phone linestypically at speeds from 256K to 1.5Mb - up to 25 times quicker than the fastestanalog modems available today (56,000 bits per second).1.3. WHAT ARE THE BENEFITS?DSL is Always On, 24 hrs A DayNo Dial-Up RequiredData SecurityNo Second Phone Line RequiredUse the Phone At The Same Time You Are On-LineNo Dropped ConnectionsSuper Fast SpeedsFlat Rate BillingUpgrade Speed As Your Needs ChangeCost Effective The Bandwidth You Need To Truly Experience The Internet. In addition to itsvery high speed, DSL has many benefits over analog connections. Unlike dial-up
connections that require analog modems to "dial-in" to the Internet ServiceProvider every time the user wants to retrieve e-mail or obtain access to theInternet, DSL connections are always on. CHAPTER- 2 ASYMMETRIC DIGITAL SUBSCRIBER LINE (ADSL)
2. ASYMMETRIC DIGITAL SUBSCRIBER LINE (ADSL) Asymmetric Digital Subscriber Line (ADSL), a modem technology,converts existing twisted-pair telephone lines into access paths for multimediaand high-speed data communications. ADSL can transmit up to 6 Mbps to asubscriber, and as much as 832 kbps or more in both directions. Such ratesexpand existing access capacity by a factor of 50 or more without new cabling.ADSL is literally transforming the existing public information network from onelimited to voice, text and low resolution graphics to a powerful, ubiquitous systemcapable of bringing multimedia, including full motion video, to everyones homethis century. ADSL will play a crucial role over the next ten or more years as telephonecompanies, and other service providers, enter new markets for deliveringinformation in video and multimedia formats. New broadband cabling will takedecades to reach all prospective subscribers. But success of these new serviceswill depend upon reaching as many subscribers as possible during the first fewyears. By bringing movies, television, video catalogs, remote CD-ROMs, corporateLANs, and the Internet into homes and small businesses, ADSL will make thesemarkets viable, and profitable, for telephone companies and application suppliersalike.
Asymmetric Digital Subscriber Line (ADSL) technology is asymmetric. Itallows more bandwidth downstream from an NSPs central office to the customersite than upstream from the subscriber to the central office. This asymmetry,combined with always-on access (which eliminates call setup), makes ADSL idealfor Internet/intranet surfing, video-on-demand, and remote LAN access. Users ofthese applications typically download much more information than they send. ADSL transmits more than 6 Mbps to a subscriber and as much as640 kbps more in both directions (shown in Figure-1). Such rates expand existingaccess capacity by a factor of 50 or more without new cabling. ADSL can literallytransform the existing public information network from one limited to voice, text,and low-resolution graphics to a powerful, ubiquitous system capable of bringingmultimedia, including full-motion video, to every home this century.2.1. THE COMPONENTS OF AN ADSL NETWORK INCLUDE A TELCO AND A CPEFIGURE-1
ADSL will play a crucial role over the next decade or more as telephonecompanies enter new markets for delivering information in video and multimediaformats. New broadband cabling will take decades to reach all prospectivesubscribers. Success of these new services depends on reaching as manysubscribers as possible during the first few years. By bringing movies, television,video catalogs, remote CD-ROMs, corporate LANs, and the Internet into homesand small businesses, ADSL will make these markets viable and profitable fortelephone companies and application suppliers alike. CHAPTER- 3 ADSL CAPABILITIES
3. ADSL CAPABILITIES An ADSL circuit connects an ADSL modem on each end of a twisted-pairtelephone line, creating three information channels: a high-speed downstreamchannel, a medium-speed duplex channel, and a basic telephone service channel.The basic telephone service channel is split off from the digital modem by filters,thus guaranteeing uninterrupted basic telephone service, even if ADSL fails. Thehigh-speed channel ranges from 1.5 to 9 Mbps, and duplex rates range from 16 to640 kbps. Each channel can be sub multiplexed to form multiple lower-ratechannels. ADSL modems provide data rates consistent with North American T1 1.544Mbps and European E1 2.048 Mbps digital hierarchies (see Figure 21-2), and canbe purchased with various speed ranges and capabilities. The minimumconfiguration provides 1.5 or 2.0 Mbps downstream and a 16-kbps duplexchannel; others provide rates of 6.1 Mbps and 64 kbps for duplex. Products withdownstream rates up to 8 Mbps and duplex rates up to 640 kbps are availabletoday. ADSL modems accommodate Asynchronous Transfer Mode (ATM)transport with variable rates and compensation for ATM overhead, as well as IPprotocols.
Downstream data rates depend on a number of factors, including thelength of the copper line, its wire gauge, the presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency,and decreases as wire diameter increases. Ignoring bridged taps, ADSL performsas shown in Table 1.Data Rate Wire Gauge Distance Wire Size Distance1.5 or 2 24 AWG 18,000 ft 0.5 mm 5.5 kmMbps1.5 or 2 26 AWG 15,000 ft 0.4 mm 4.6 kmMbps6.1 Mbps 24 AWG 12,000 ft 0.5 mm 3.7 km6.1 Mbps 26 AWG 9,000 ft 0.4 mm 2.7 kmTABLE-1 Although the measure varies from telco to telco, these capabilities cancover up to 95 percent of a loop plant, depending on the desired data rate.Customers beyond these distances can be reached with fiber-based digital loopcarrier (DLC) systems. As these DLC systems become commercially available,telephone companies can offer virtually ubiquitous access in a relatively shorttime.
4 . ADSL TECHNOLOGY ADSL depends upon advanced digital signal processing and creativealgorithms to squeeze so much information through twisted-pair telephone lines.In addition, many advances have been required in transformers, analog filters,and A/D converters. Long telephone lines may attenuate signals at one megahertz(the outer edge of the band used by ADSL) by as much as 90 dB, forcing analogsections of ADSL modems to work very hard to realize large dynamic ranges,separate channels, and maintain low noise figures. On the outside, ADSL lookssimple -- transparent synchronous data pipes at various data rates over ordinarytelephone lines. On the inside, where all the transistors work, there is a miracle ofmodern technology.