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Asymmetric Digital Subscriber Line - INTRODUCTION, BACKGROUND, ADSL TECHNOLOGY, Signal Modulation, Code and Error Correction, Framing and Scrambling, STANDARDS FOR ADSL

access data transmission internet

Leo Tan Wee Hin
Singapore National Academy of Science and Nanyang Technological University, Singapore

R. Subramaniam
Singapore National Academy of Science and Nanyang Technological University, Singapore


The plain, old telephone system (POTS) has formed the backbone of the communications world since its inception in the 1880s. Running on twisted pairs of copper wires bundled together, there has not really been any seminal developments in its mode of transmission, save for its transition from analogue to digital toward the end of the 1970s.

The voice portion of the line, including the dial tone and ringing sound, occupies a bandwidth that represents about 0.3% of the total bandwidth of the copper wires. This seems to be such a waste of resources, as prior to the advent of the Internet, telecommunication companies (telcos) have not really sought to explore better utilization of the bandwidth through technological improvements, for example, to promote better voice quality, to reduce wiring by routing two neighboring houses on the same line before splitting the last few meters, and so on. There could be two possible reasons for this state of affairs. One reason is that the advances in microelectronics and signal processing necessary for the efficient and cost-effective interlinking of computers to the telecommunications network have been rather slow (Reusens, van Bruyssel, Sevenhans, van Den Bergh, van Nimmen, & Spruyt, 2001). Another reason is that up to about the 1990s, telcos were basically state-run enterprises that had little incentive to roll out innovative services and applications. When deregulation and liberalization of the telecommunication sector was introduced around the 1990s, the entire landscape underwent a drastic transformation and saw telcos introducing a plethora of service enhancements, innovations, and other applications; there was also a parallel surge in technological developments aiding these.

As POTS is conspicuous by its ubiquity, it makes sense to leverage on it for upgrading purposes rather than deploy totally new networks that need considerable investment. In recent times, asymmetric digital subscriber line (ADSL) has emerged as a technology that is revolutionizing telecommunications and is a prime candidate for broadband access to the Internet. It allows for the transmission of enormous amounts of digital information at rapid rates on the POTS.


The genesis of ADSL can be traced to the efforts by telecommunication companies to enter the cable-television market (Reusens et al., 2001). They were looking for a way to send television signals over the phone line so that subscribers can also use this line for receiving video.

The foundations of ADSL were laid in 1989 by Joseph Leichleder, a scientist at Bellcore, who observed that there are more applications and services for which speedier transmission rates are needed from the telephone exchange to the subscriber’s location than for the other way around (Leichleider, 1989). Telcos working on the video-on-demand market were quick to recognize the potential of ADSL for streaming video signals. However, the video-on-demand market did not take off for various reasons: Telcos were reluctant to invest in the necessary video architecture as well as to upgrade their networks, the quality of the MPEG (Moving Picture Experts Group) video stream was rather poor, and there was competition from video rental stores (Reusens et al., 2001). Also, the hybrid fiber coaxial (HFC) architecture for cable television, which was introduced around 1993, posed a serious challenge. At about this time, the Internet was becoming a phenomenon, and telcos were quick to realize the potential of ADSL for fast Internet access. Field trials began in 1996, and in 1998, ADSL started to be deployed in many countries.

The current motivation of telcos in warming toward ADSL has more to do with the fact that it offers rapid access to the Internet, as well as the scope to deliver other applications and services whilst offering competition to cable-television companies entering the Internet-access market. All this means multiple revenue streams for telcos.

Over the years, technological advancements relating to ADSL as well as the evolution of standards for its use have begun to fuel its widespread deployment for Internet access (Chen, 1999). Indeed, it is one of those few technologies that went from the conceptual stage to the deployment stage within a decade (Starr, Cioffi, & Silverman, 1999).

This article provides an overview of ADSL.


ADSL is based on the observation that while the frequency band for voice transmission over the phone line occupies about 3 KHz (200 Hz to 3,300 Hz), the actual bandwidth of the twisted pairs of copper wires constituting the phone line is more than 1 MHz (Hamill, Delaney, Furlong, Gantley, & Gardiner, 1999; Hawley, 1999). It is the unused bandwidth beyond the voice portion of the phone line that ADSL uses for transmitting information at high rates. A high frequency (above 4,000 KHz) is used because more information can then be transmitted at faster rates; a disadvantage is that the signals undergo attenuation with distance, which restricts the reach of ADSL.

There are three key technologies involved in ADSL.

Signal Modulation

Modulation is the process of transmitting information on a wire after encoding it electrically. When ADSL was first deployed on a commercial basis, carrierless amplitude-phase (CAP) modulation was used to modulate signals over the line. CAP works by dividing the line into three subchannels: one for voice, one for upstream access, and another for downstream access. It has since been largely superseded by another technique called discrete multitone (DMT), which is a signal-coding technique invented by John Cioffi of Stanford University (Cioffi, Silverman, & Starr, 1999; Ruiz, Cioffi, & Kasturia, 1992). He demonstrated its use by transmitting 8 Mb of information in one second across a phone line 1.6 km long. DMT scores over CAP in terms of the speed of data transfer, efficiency of bandwidth allocation, and power consumption, and these have been key considerations in its widespread adoption.

DMT divides the bandwidth of the phone line into 256 subchannels through a process called frequency-division multiplexing (FDM; Figure 1; Kwok, 1999). Each subchannel occupies a bandwidth of 4.3125 KHz. For transmitting data across each subchannel, the technique of quadrature amplitude modulation (QAM) is used. Two sinusoidal carriers of the same frequency that differ in phase by 90 degrees constitute the QAM signal. The number of bits allocated for each subchannel varies from 2 to 16: Higher bits are carried on subchannels in the lower frequencies, while lower bits are carried on channels in the higher frequencies.

The following theoretical rates apply.

  • Upstream access: 20 carriers x 8 bits x 4 KHz = 640 Kbps
  • Downstream access: 256 carriers x 8 bits x 4 KHz = 8.1 Mbps

In practice, the data rates achieved are much less owing to inadequate line quality, extended length of line, cross talk, and noise (Cook, Kirkby, Booth, Foster, Clarke, & Young, 1999). The speed for downstream access is generally about 10 times that for upstream access.

Two of the channels (16 and 64) can be used for transmitting pilot signals for specific applications or tests. It is the subdivision into 256 channels that allows one group to be used for downstream access and another for upstream access on an optimal basis. When the modem is activated during network access, the signal-to-noise ratio in the channel is automatically measured. Subchannels that experience unacceptable throughput of the signal owing to interference are turned off, and their traffic is redirected to other suitable subchannels, thus optimizing the overall transmission throughput. The total transmittance is thus maintained by QAM. This is a particular advantage when using POTS for ADSL delivery since a good portion of the network was laid several decades ago and is susceptible to interference owing to corrosion and other problems.

The upstream channel is used for data transmission from the subscriber to the telephone exchange, while the downstream channel is used for the converse link. It is this asymmetry in transmission rates that accounts for the asymmetry in ADSL. As can be seen from Figure 1, the voice portion of the line is separated from the data-transmission portion; this is accomplished through the use of a splitter. It is thus clear why phone calls can be made over the ADSL link even during Internet access. At frequencies where the upstream and downstream channels need to overlap for part of the downstream transmission, so as to make better use of the lower frequency region where signal loss is less, the use of echo-cancellation techniques is necessary to ensure the differentiation of the mode of signal transmission (Winch, 1998).

Code and Error Correction

The fidelity of information transmitted on the phone line is contingent on it being coded suitably and decoded correctly at the destination even if some bits of information are lost during transmission. This is commonly accomplished by the use of constellation encoding and decoding. Further enhancements in reliability is afforded by a technique called forward error correction, which minimizes the necessity for retransmission (Gillespie, 2001).

Framing and Scrambling

The effectiveness of coding and error correction is greatly enhanced by sequentially scrambling the data. To accomplish this, the ADSL terminal unit at the control office (ATU-C) transmits 68 data frames every 17 ms, with each of these data frames obtaining its information from two data buffers (Gillespie, 2001).


The deployment of ADSL has been greatly facilitated by the evolution of standards laid by various international agencies. These standards are set after getting input from carriers, subscribers, and service providers. The standards dictate the operation of ADSL under a variety of conditions and cover aspects such as equipment specifications, connection protocols, and transmission metrics (Chen, 1999; Summers, 1999). The more important of these standards are indicated below.

  • G.dmt: Also known as full-rate ADSL or G992.1, it is the first version of ADSL.
  • G.Lite: Also known as universal ADSL or G992.2, it is the standard method for installing ADSL without the use of splitters. It permits downstream access at up to 1.5 Mbps and upstream access at up to 512 Kbps over a distance of up to 18,000 ft.
  • ADSL2: Also known as G 992.3 and G 992.4, it is a next-generation version that allows for even higher rates of data transmission and extension of reach by 180 m.
  • ADSL2+: Also known as G 992.5, this variant of ADSL2 doubles the speed of transmission of signals from 1.1 MHz to 2.2 MHz, as well as extends the reach even further.
  • T1.413: This is the standard for ADSL used by the American National Standards Institute (ANSI), and it depends on DMT for signal modulation. It can achieve speeds of up to 8 Mbps for downstream access and up to 1.5 Mbps for upstream access over a distance of 9,000 to 12,000 ft.
  • DTR/TM-06001: This is an ADSL standard used by the European Technical Standards Institute (ETSI) and is based on T1.413, but modified to suit European conditions

The evolution of the various ADSL variants is a reflection of the technological improvements that have occurred in tandem with the increase in subscriber numbers.


Where the telephone exchange has been ADSL enabled, setting up the ADSL connection for a subscriber is a straightforward task. The local loop from the subscriber’s location is first linked via a splitter to the equipment at the telephone exchange, and an ADSL modem is then interfaced to the loop at this exchange. Next, a splitter is affixed to the telephone socket at the subscriber’s location, and the lead wire from the phone is linked to the rear of the splitter and an ADSL modem. The splitters separate the telephony signal from the data streams, while the modems at the telephone exchange and subscriber location cater for the downstream and upstream data flow, respectively. A network device called digital subscriber line access multiplexer (DSLAM) at the exchange splits signals from subscriber lines into two streams: The voice portion is carried on POTS while the data portion is fed to a high-speed backbone using multiplexing techniques and then to the Internet (Green, 2001). A schematic of the ADSL setup is illustrated in Figure 2.

The installation of the splitter at the subscriber’s premises is a labor-intensive task as it requires a technician to come and do the necessary work. This comes in the way of widespread deployment of ADSL by telcos. A variant of ADSL known as splitterless ADSL (G992.2) or G.Lite was thus introduced to address this (Kwok, 1999).

Speeds attainable on an ADSL link are variable and are higher than that obtained using a 56-K modem. The speed is also distance dependent (Table 1; Azzam & Ransom, 1999). This is because the high frequency signals undergo attenuation with distance and, as a result, the bit rates transmitted via the modem decrease accordingly. Other factors that can affect the speed include the quality of the copper cables, the extent of network congestion, and, for overseas access, the amount of international bandwidth leased by the Internet service providers (ISPs). The latter factor is not commonly recognized.


Any new technology is not perfect, and there are constraints that preclude its optimal use; this has to be addressed by ongoing research and development. The following are some of the advantages of ADSL.

  • It does not require the use of a second phone line.
  • It can be installed on demand, unlike fiber cabling, which requires substantial underground work as well as significant installation work at the subscriber’s location.
  • It provides affordable broadband access at speeds significantly greater than that obtainable using a dial-up modem.

  • Since there is a dedicated link between the subscriber’s location and the telephone exchange, there is greater security of the data as compared to other alternatives such as cable modem.
  • No dial up is needed as the connection is always on.

Some of the disadvantages of ADSL are as follows.

  • The subscriber’s location needs to be within about 5 km from the telephone exchange; the greater the distance away from the exchange, the less is the speed of data transfer.
  • As ADSL relies on copper wires, a good proportion of which was laid underground and overland many years ago, the line is susceptible to noise due to, for example, moisture, corrosion, and cross talk, all of which can affect its performance (Cook, Kirkby, Booth, Foster, Clarke & Young, 1999).

On the balance, the advantages of ADSL far outweigh its disadvantages, and this has led to its deployment in many countries for broadband access, for example, in Singapore (Tan & Subramaniam, 2000, 2001).


Currently, ADSL is used mainly for broadband access, that is, for high-speed Internet access as well as for rapid downloading of large files. Other applications include accessing video catalogues, image libraries (Stone, 1999), and digital video libraries (Smith, 1999); playing interactive games that guzzle bandwidth; accessing remote CD-ROMs; videoconferencing; distance learning; network computing whereby software and files can be stored in a central server and then retrieved at fast speeds (Chen, 1999); and telemedicine, in which patients can access specialist expertise in remote locations for real-time diagnostic advice, which may include the examination of high-quality X-ray films and other biomedical images.

Future applications could include television, Internet telephony, and other interactive applications, all of which can lead to increase in revenue for telcos. There is a possibility that video-on-demand can take off.


The maturation of ADSL is being fueled by technological advances. The number of subscribers for ADSL has seen an upward trend in many countries (Kalakota, Gundepudi, Wareham, Rai, & Weike, 2002). New developments in DMT are likely to lead to more efficient transmission of data streams. The distance-dependent nature of its transmission is likely to be overcome either by the building of more telephone exchanges so that more subscriber locations can be within an effective radius for the deployment of ADSL, or by advances in the enabling technologies. The technology is likely to become more pervasive than its competitor, cable modem, in the years to come since the installation of new cabling will take years to reach more households and also entails further investments.

The higher variants of ADSL such as ADSL2 and ADSL2+ are likely to fuel penetration rates further (Tzannes, 2003). For example, compared to first-generation ADSL, ADSL2 can enhance data rates by 50 Kbps and reach by 600 ft (Figure 3), the latter translating to an increase in area coverage by about 5%, thus raising the prospects of bringing more subscribers on board.

Some of the features available with the new variants of ADSL, such as automatic monitoring of line quality and signal-to-noise ratio, offers the potential to customize enhanced service-delivery packages at higher tariffs for customers who want a higher quality of service.


Twisted pairs of copper wires forming the POTS constitute the most widely deployed access network for telecommunications. Since ADSL leverages on the ubiquity of this network, it allows telcos to extract further mileage without much additional investments whilst competing with providers of alternative platforms. It is thus likely to be a key broadband technology for Internet access in many countries in the years to come. A slew of applications that leverage on ADSL are also likely to act as drivers for its widespread deployment.

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over 3 years ago

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about 7 years ago

hi , can you help me, i want to know : in digital phone like DBC series , what is the type of modulation use in this categories of digital phones , thank you for your helping.