Asymmetric Digital Subscriber Line


Team Members:

Hugh Hamill

Catherine Delaney

Emily Furlong

Kieran Gantley

Keith Gardiner

Research Method

Project Report




In this project we hope firstly to define exactly what ADSL is, give a brief history of ADSL, illustrate ADSL'S system architecture, describe ADSL'S various applications and benefits and forecast what the future holds for ADSL.

We hope after reading this article one would fully understand ADSL and how it will revolutionize Information Technology.

Finally we will perform a oral presentation illustrating ADSL.

Asymmetric Digital Subscriber Line


Research Method

As ADSL is such a new technology it is not surprising that most of the research was done on the World Wide Web. Search Engines were queried and Forums were visited. There is a lot of information readily available on the internet and the whole team were busy gathering information and filtering the good from the bad. When the information was all downloaded it was time to sort it out into useable and not useable. The library was searched for a good technical book on ADSL, with this in hand it was easy to differentiate between factual information from the 'net and "exadurations" made by ISP's eager to sell their product. Also on one occasion a video lecture on ADSL was borrowed from the library.

The internet was indeed though the most beneficial source, and we would highly reccommend it in future use of any sort of technilogical research. It was essential, however, that the information integrity remained intact and this was achieved with the old fashioned techinical manual from the library. During the course of research, old sources and methods were mixed with new ones to produce a thorough and well rounded result.

click here for adsl.mpp the Microsoft Project version of the project schedule

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Asymmetric Digital Subscriber Line


1. Introduction
2. A Brief History of ADSL
3. Various Applications of ADSL
4. Technology Description
5. The Future of ADSL


The purpose of this project is to discuss Asymmetric Digital Subscriber Line (ADSL). ADSL is a standard for digital communications between client and remote network host. ADSL generally offers downstream rates of about 512Kbps to 6Mbps. This is a revolutionary increase in subscriber speed with the closest commercial match in Europe at the moment being ISDN offering 64Kbps to 128Kbps. ADSL offers high speed remote network connections on conventional phonelines without the need to install expensive backbones. This brings sophisticated client speeds to the home user at a low cost. This is what makes ADSL so special.

Although there is a large buzz about ADSL in the ISP industry in Europe and especially the UK at the moment (both BT and AOL UK are developing ADSL services to be released shortly) it is not new concept. It was developed at Belcore in 1987 although it was more of a concept for delivering high speed multimedia for residential applications than for connection to the world wide web.

In Web applications speed is the key issue at the moment, everyone wants faster connections and cheaper! ADSL meets both these requirements offering amazing speed increase at a relatively low cost, to both provider and consumer. Seemingly ADSL is the magic solution but it does suffer from one major weakness; signal attenuation in the copper, which increases with increasing line length, increasing signal frequency, and decreasing line gauge.

ADSL is going to bring future entertainment to the computer at the same quality and speed as your television. The way will be paved for digital video conferencing and other such applications. ADSL will revolutionize the way we think about the World Wide Web, information superhighway? Not without Asymmetric Digital Subscriber Line.

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A Brief History of ADSL

ADSL was started up as the Phone Company's way to compete with cable TV by delivering both TV and phone service on the plain old copper phone line. Of course now ADSL is also a good candidate for high speed Internet access. ADSL was originally designed so that it could still be used as a regular phone line if the power happened to go out.

The "A" stands for "Asymmetric", meaning the Phone Company can send lots of data to the customer, but the customer can't send much to them at all. At the beginning only a tiny uplink of 16 or 64kbps was supported now ADSL can support up to ten times that much. The idea for ADSL was spawned by Joe Leichleder in 1987, he was a Bellcore researcher and the first idea for it was analog to digital conversion at the subscriber end of an advanced transmission technology over a twisted pair copper line. ADSL was designed to provide megabit access for interactive video (video on demand, video games, delayed TV segments, etc.) and high speed data communications (Internet access, remote LAN access, other network access) but it is also perfect for surfing the web.

In the 1990s, vendors developing video-on-demand products started using ADSL technology, with different speeds for sending and receiving channels. Although the video idea didn't pan out, the Internet did and ADSL emerged as a hot product for residential Web surfing. The only thing missing was the telephone line, so frequency-division multiplexing (FDM) was added to ADSL to reserve a separate 4-kHz POTS (plain old telephone system) channel below the higher frequency data band. ADSL uses a single wire pair. Speeds on the receiving channel can climb to 6 Mbit/s, but actual performance depends on the condition of the line being used.

There was a DSL Forum formed towards the end of 1994, and this was set up to help telephone companies and their suppliers realize the great market potential of ADSL. This forum has two forms of assistance which are technical and marketing. The Forum's marketing programs attempt to uncomplicate the technical complexity of it and to spread the news concerning ADSL. The output of the forum to the public mixes the tutorial with the promotion of this device.

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Various Applications of ADSL

The success of new multimedia services on the information highways with ever-present access by residential customers including faster internet, faster access to multimedia, e.g. VoD(Video on Demand) and digital TV, home shopping and home access to corporate networks will depend on cost effective solutions and the ability to reach as many subscribers as possible.

ADSL provides high-speed digital services and is being adopted by many telephone companies around the world.

The following is a comparison of download speeds between ADSL modems and other modems:

Modem Type Download Speed (for a short video – 7.5 MB)

6 Mbps ADSL 10 sec
1.5 Mbps ADSL 40 sec
1.5 Mbps Cable 40 sec
128 Kbps ISDN 7 mins
28.8 Kbps 35 mins
14.4 Kbps 71 mins


With the increase of speed of the internet, more applications arise, like education. The internet promises to revolutionise educational opportunities for future children. However at the moment, to avail of sufficient technology for this can be expensive and slow.

ADSL offers a fast on-ramp to the internet, other schools, communities colleges and universities, libraries at a low cost.

Multi-Service Selection is the ability to access information (e.g. financial and medical records) regardless of physical location using the internet, Corporate LANs and on-line databases. It is very beneficial to businesses. ADSL will increase the benefits furthermore, mainly because it increases the data services speed by up to 300 times.

Real Estate professionals need better access to market and property specifications so they have the latest listing information and can provide this information to their clientele. Agents will then be able to better manage the time spent with clients and match properties to the specific needs of the buyer. Much of this information is very large and therefore would be difficult to access easily and take an very long amout of time to download over conventional modems.

An ADSL system delivers high speed downloading and allows the selling agent to provide a more detailed graphical content of listed properties. So the buyer can see each property - from several angles; external views, internal rooms and features – and determine if the property is suitable.

Video Conferencing meets the need to meet face-to-face in business. It removes the barriers of location and enhances the customer experience while creating saving in travel cost and time. Video conferencing requires access to high speed communications media. ADSL is the optimum media solution for video conferencing due to its ability to make use of the approximately 750 million lines currently available to virtually every location on the planet.

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Technology Discription

ADSL is a major advance in the information technology field. The main concept was to be able to work in larger bit rates with more efficiency over the present copper line technology. It is being viewed as a quick way to deliver data e.g. MPEG videos and live video conferencing over the existing copper lines. With an embedded base of over 125 million local loops in the United States alone, it makes sense to find a way to reuse this infrastructure.

There are however few basic requirements that ADSL had to satisfy:

* Full use of the copper line frequency spectrum (1.1Mhz).

* An advanced coding/decoding method.

* Ability to work simultaneously with POTS on the same copper line.

In the simplest version of this architecture a customer would only need a new ADSL modem. This modem would have RJ-11 jacks that would support the existing analog telephone service. (SEE DIAGRAM 1).

Using the existing subscriber line already installed from the central office to the customer's premises, two ADSL devices are connected to both ends of the local loop. (SEE DIAGRAM 2) The ADSL functions at the central office are handled by the ATU-C (ADSL Terminal Unit). The ATU-C transmits high-speed simplex channels downstream, and sends and receives lower-speed duplex channels. The ATU-C units may either be standalone, or mounted with others in an equipment shelf. In the future, the ATU-C will be integrated into access nodes and remote access nodes.

The ATU-C is paired with its mirror image, an ADSL remote terminal unit (ATU-R). The ATU-R is generally a standalone device, and receives downstream data and transceives corresponding duplex data.

A 'splitter' (which is a filter), one at the user end and one at the exchange end, separates the telephony signal from the ADSL signal (SEE DIAGRAM 1). This means that telephone calls can be made at the same time that data is being sent or received (i.e. a customer can surf the Internet and still make telephone calls).

ADSL Channel Configurations

Frequency Division Multiplexing (FDM) is used for ADSL over a 1 MHz spectrum. Diagram 3 illustrates the general allocation of the frequency spectrum above the voice band.The downstream (high capacity) data rate is largely dependent on the length of the subscriber line from the central office and the gauge of the twisted pair cable. Richard Karpinski outlines the length and gauge requirements in Table 1.

Table 1: Characteristics of ADSL (Karpinski 26)

Diagram3: FDM for ADSL

Besides the 0-3.4 MHz band of voice communications, ADSL provides for a low speed upstream channel (from subscriber to central office) and a high speed downstream channel (from central office to subscriber). The baseband occupied by POTS (Plain Old Telephone Service) is split from the data channels which guarantees POTS services in the case of ADSL system failure (e.g. passive filters).

Depending on the loop length and cable gauge, an upstream channel of 9.6 to 384 kb/s provides for the subscriber's need to transmit data, while a downstream channel of 1.544 to 7.000 Mb/s delivers TV, video on demand and computer network connectivity. ADSL can provide VCR-type functionality, like fast forward, rewind, freeze frame, pause, etc. - on demand. There are two main methods of handling the FDM cinnection, CAP and DMT.


There is a raging debate over the implementation of ADSL at the moment, this debate centres around the two methods for line coding. One of these methods is Carrierless Phase/Amplitude Modulation (CAP) the other is Disctrete Multitone Technology (DMT). DMT is so far winning the debate becoming ADSL line coding standard in ANSI T1.413-1995. Line coding basically means how the 1's and 0's are transmitted down the line.

Carrierless Phase/Amplitude Modulation (CAP)

CAP is a close relative of a coding technique known as Quadrature Amplitude Modulation (QAM). These techniques are almost mathematically identical except that CAP does not transmit a carrier and instead reconstructs it electronically at the destination. For this reason CAP is also known as carrier-suppressed QAM. To understand CAP it is best to gain an understanding of QAM.

Quadrature Amplitude Modulation (QAM)

QAM is a way of fitting information onto a limited frequency line, in the case of ADSL it is copper wire. QAM can split a single signal into 16 by using both phase and amplitude modulation. QAM uses a combination of sine and cosine waves at different phases to each other to produce these signals (sine and cosine ave function always 900 out of phase, i.e. in quadrature). QAM uses four different amplitudes for each of the waves. in this way 16 different signal types are generated using all possible pairs of the amplitudes, e.g. A1sin(Ft)+A1cos(Ft),A1sin(Ft)+A2cos(Ft),A1sin(Ft)+A3cos(Ft),... This creates what is known as the QAM "constellation" a collection of 16 signals all representing 4-bit nibbles. This constellation is differential meaning it is 15 degrees out of phase with the previous 4 bits, it is not referenced to a fixed signal. CAP/QAM is a single carrier technique.

Discrete Multitone Technology (DMT)

DMT like QAM is all about fitting information onto a line. DMT divides the entire bandwidth range on the formerly analog passband limited loop into a large number of equally spaced sub channels called subcarriers. This bandwidth is normally 1.1 MHz and is divided into 256 subcarriers, starting at 0Hz. Each subcarrier occupies 4.13125 kHz, this gives a total bandwidth of 1.104 MHz on the loop. Most DMT systems only use 249 or 250 of these subcarrier for information. Generally the lower subcarriers #1 through #6 are reserved for the 4 kHz passband for analog voice, giving ADSL the distinctive ability to transmit data and carry a voice conversation on the same line. 6 times 4.3125 kHz is 25.875 kHz so it is common that ADSL services start at 25 kHz, this also means that a wide "guardband" exists between analog voice and DMT transmissions. Generally the upper subcarriers exhibit signal loss so those above #250 are not used.

There are 32 upstream carriers, starting at #7 and 250 downstream channels, giving putting the "Asymmetric" in ADSL. If the upstream and downstream channels overlap echo cancellation techniques are used to prevent errors from transmitted signals being mistaken for recieved ones. Some channels are also reserved for special purposes like pilot signals. At activation ADSL devices measures each of these subchannels for signal attenuation and noise, in a complex "handshake" procedure. DMT can also monitor the channels for changing quality allowing a DMT granularity (possible speed drop) of only 32 kbps against CAP's granularity of 340 kbps.

Each subcarrier operates a coding technique based on QAM (ironic considering the CAP/DMT debate). The total throughput of is the sum of all the QAM bits sent on all of the active channels at a given time. The throughput is maintained on a DMT connection by turning off subcarriers that experience outside interference rather than retransmitting the entire signal. DMT devices can be said to be Rate Adaptive DSL (RADSL) as each subcarrier may transmit at a different rate than the others, depending on the quality of the signal in each.

For the moment DMT seems to have won the debate, not least because of this RADSL capability, it is also future proof, in that although CAP/QAM has not yet run its course, DMT still has a long way to go towards realising its full potential.

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The Future of ADSL

A powerful broadband switching infrastructure will deliver next generation services to consumer and business clients. A single access ATM line to end users will effectively handle multiple applications and will allow organizations to better manage the delivery of their services.

A service provider's network design must cover all aspects of new service delivery and support, with particular care given to service provisioning and management. The most profitable networks will offer superior flexibility in defining, provisioning, administering and operating new services and networks. They will also be intuitively easy to manage.

Forward-looking service providers have already designed and built broadband networks. By consolidating their current networks onto a broadband ATM infrastructure, service providers protect their investment, enhance their ability to manage their overall network, and increase their opportunities to introduce new services.



The figure above describes how the current modem operates with ISDN access.

The figure above describes how ADSL operates using dry copper cabling over distances of <1800ft. This does not allow you to be online and use your phone at the same time.

The above diagram describes how in the future the ADSL rack will be located in the phone co. central office. This will enable you to be online and also be able to use your phone at the same time

Demand for Increased Bandwidth Continues

According to a recent survey of more than 2,000 U.S. households conducted by the Yankee Group, a Boston- based communications research firm, nearly two-thirds of the PC-owning households in the U.S. are interested in faster speed Internet access.

In addition to an increase in interest in high speed data (HSD) services, there is greater willingness to pay for such services, the survey found. Thirty-six percent of online households are willing to pay $40 per month for HSD services.

With MTS ADSL Line Enhancement technology, the Internet will be just one electronic avenue on your personal computer. With high-speed data transmission, the door is open for a wide range of communication media to be accessed. In the not-too-distant future, your computer will expand its capabilities as a multi-media center, whereby broadband services such as video on demand - calling up your favorite movie on the Internet without going to the video store, or watching an interesting program at any time without having to program your VCR, CD-ROMS, video catalogues, video games, interactive video and other dynamic broadband services will be downloaded with lightning speed.For business users, features such as video conferencing, corporate LANs (Local Area Network), and audio video presentations will be faster and more dynamic.

ADSL developers and manufacturers already have plans for a 52 Mbp ADSL modem, another 50 times faster than present ADSL high-speed modems, and 4,000 times faster than a standard 14.4 modem.

The Future of the local Loop

The diagram above shows the number of new broadband lines installed over 7 years. As can be seen from the graph there is a very rapid increase in ADSL lines being installed worldwide. More than 25 million lines will have been installed by 2005.

In reality, cable modem users are getting between 200K bit/sec and 1M bit/sec for downstream [because] the modem itself is only one of many bottlenecks out there on the World Wide Web. Data transmission is slowed down at other parts of the network - at the server, or over the Internet - whether modems are cable- or ADSL-based.

In the past, ADSL faced competing standards and there was a lack of interoperability. Additionally, the power consumption of the unit was too high to go inside computers.

Broadband access users will begin to feel the benefits of ADSL in 2000-2001. Support for the G.Lite ADSL standard reads like the 'Who's Who' list of communication, including vendors such as Compaq, Intel, Microsoft, Dell and on. While cable modems are aimed squarely at home users, ADSL modems are targeted at a wider audience, including home business, telecommuters and the residential market.

ADSL's wide acceptance as an industry standard will ultimately drive down the cost of modems and broadband access overall. What is most important is not which technology is the most successful, but that consumers and businesses have broadband access. Broadband access stimulates all kinds of things-e-commerce, new business over the Internet that we have not thought about, more video conferencing, a boost to retail on the Internet.

The growing acceptance of ADSL and its availability to U.S. consumers will cause the broadband access market itself to grow. While ABI's study Broadband Delivery in the Local Loop focuses solely on the U.S. market, ADSL is being investigated and deployed worldwide. Bandwidth shortage is not unique to the United States.

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Asymmetric Digital Subscriber Line



ADSL was born of the need for speed coupled with the desire for low cost dedicated remote network access. There is no doubt that ADSL will revolutionize the way we see the World Wide Web, and quite possibly witness the demise of home entertainment as we know it. As the phoenix from the flames we will see ADSL emerge heralding the coming of a new age of remote multimedia. There is little doubt that ADSL will be around for a long time to come, albeit under another name.

If we are to truly realise the potential of the cyberspace concept we will need to access it with as much convenience as turning on the television. With the internet influencing our lives more and more each day, it will be high speed ADSL connections that power the revolution. In the future people will view ADSL like they view cable TV. That such a small object as an ADSL card may weild such an influence over our lives may seem a little unbalanced, or is that asymmetric!?

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Asymmetric Digital Subscriber Line




ADSL and DSL Technologies by Walter Goralski

Video Dialtone Technology by Daniel Minoli


Web sites:

The ADSL Forum

Epl - Specialist In Telecommunication Test Systems

Westell Technologies, Inc.

Orckit Communications Ltd.

Kimmo K. Saarela
Tampere University of Technology
Telecommunication Laboratory
P.O. Box 553, 33101 Tampere, Finland



ADSL from Computer Channel International presented by Peter Mason


White Papers:

Westell DSL Solutions white paper "Less Truly Is More"

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