What is DSL?

Digital Subscriber Line (DSL) refers to several types of advanced modems that enable mega-fast access at speeds 300 times faster than most analog modems. Since DSL works on regular telephone lines (unlike, for example, cable modems) DSL systems are considered a key means of opening the bottleneck in the "last mile" of the existing telephone infrastructure, as telephone companies seek cost-effective ways of providing much higher speed to their customers.

 What Can DSL Be Used For?

is basically a high speed data "pipe" that can be used to transmit any high speed data application, such as video conferencing, fast Internet access, interactive multimedia, on-line home banking, remote office or remote LAN applications. For example, an advertising agency might use a DSL modem in order to send heavy graphic files back and forth for client approvals. They could then submit the print advertisement to the publication in the same way, in a matter of seconds. Another application is known as telecommuting, or work-from-home. An employee with a DSL modem at home will be able to be connect at nearly LAN speeds to their office network, using their regular telephone linewith POTS (plain old telephone service) remaining available for telephone calls, faxes, and so on.

How is DSL Installed?

In order to install DSL, you must have access to the copper infrastructure. This generally refers to telecom operators (PTT's) and in some countries, Internet Service Providers. DSL Modem transceivers must be installed at both ends of the copper line -- one transceiver is placed at the customer premises, and another at the telephone company Central Office (CO) building (or curb cabinet in certain cases).

Unlike previous copper line technologies, the DSL system when installed, does not need manual adjustment. The DSL modem automatically analyzes the line and adapts itself to start up the link within seconds. This process continues once the link is started, as the modem compensates for ongoing changes (such as those due to temperature). The modems contain advanced digital signal processing (DSP) algorithms that produce mathematical models of the distortions caused by the line and produce automatic corrections, so that data is passed (in both directions) on the line. There is usually a performance trade-off; the faster the data rate on the line, the shorter the transmission distance. Other factors that affect range are the gauge (thickness) of the cable, and the amount of interfering noise present.

How is DSL different from today's analog modems?

In order to achieve data transmission rates of up to 300 times faster than analog modems, DSL technologies use a wider band of frequencies. Also, because DSL uses a digital signal, unlike today's analog modems, DSL transmissions do not pass through the regular analog voice telephone network. This element of DSL can clear the "congestion" that a lot of dial-up Internet traffic causes (the cause of delayed dial tone in certain high-Internet use areas).

What are the different varieties of DSL?

Several different types of DSL have been developed, and each provides different benefits. The table below summarizes the information in the narrative that follows it.


What is HDSL and why was it developed?

Following the completion of ISDN technology design (which provides 128 kbps connections), attention was turned to developing a higher speed DSL for the delivery of high speed data lines, leased by telephone companies to businesses. These traditional leased data line services operate at T1 (1.54 Mbps) rate in the USA and E1 (2 Mbps) in Europe. In order to achieve 4 km range with the technology at the time, two copper pairs had to be used. The data stream is divided into two streams each of half rate (i.e. 784 kbps), and these are independently transmitted over two cables and then the data is recombined at the receiving end. Today HDSL is the most well proven and heavily deployed DSL. It is used as an alternative for providing T1 or E1 type leased lines and is usually cheaper and faster than other traditional methods, which previously involved the deployment of special cables with repeaters at 1 km intervals or optical fiber cables.

What is SDSL?

Symmetric Digital Subscriber Line (SDSL) is a derivative of HDSL that uses only one of the two cable pairs. It transmits only on a single-pair, usually at the 784 kbps (half-T1) data rate. Although not originally intended to operate simultaneously with POTS, some firms have managed to achieve POTS sharing so that residential or small office users can share the same telephone for data transmission and voice or fax telephony.

What is HDSL2?

HDSL2 is an advanced form of HDSL that has the same features as regular HDSL, but uses only a single twisted copper pair.

What is ADSL?

Asymmetric Digital Subscriber Line (ADSL) was conceived originally by researchers at telephone companies for video-on-demand type applications, but has since become focused on providing higher speed Internet services, such as the World Wide Web. ADSL is faster in the downstream (towards the customer) direction and slower in the upstream direction. Some applications, such as downloading from databases using control commands (browsing on the Internet), do not always demand symmetric data rates and can take advantage of an asymmetric system.

As a minimum, ADSL provides T1 rate or higher in the downstream direction and 64 kbps or higher in the upstream. The "enhanced performance" ADSL Standard provides for multiple channels with total downstream rates as high as 8 Mbps, plus bi-directional channels up to 768 kbps. Since ADSL was designed for residential or small-office, home-office (SOHO) type services, is was designed from the outset to operate with POTS simultaneously on the same line, so that an additional copper line would not need to be installed at each location.

Services which deliver their data in packets (e.g. Internet) require that packets delivered downstream are acknowledged before more will be sent and therefore the upstream data rate (maximum acknowledgment rate) will often dictate the maximum useful downstream throughput. A maximum ratio of downstream-to-upstream of 10-1 is a generally accepted rule of thumb.

What is ADSL II?

ADSL II is an advanced form of ADSL that achieves the same throughput as regular ADSL, yet with lower power consumption and other important technical features.

What is RADSL?

The typical fixed rate ADSL can also be configured to be rate-adaptive. This means that the modem can automatically adjust its transmission speed to the quality and length of the line. If it is connected to a line of a long length, it will automatically reduce the bit-rate in order to provide a connection at the highest speed possible. This is useful for lines that are longer than the typical 4 km supported by the 8 Mbps fixed rate ADSL modems. As you can see from the following graph, fixed rate ADSL can reach about 90 percent of customers in Europe. When you consider RADSL, telcos can reach nearly 100 percent of their customers worldwide.

What is VDSL?

As telephone companies try to extend their optical fiber backbone networks closer to the customer it becomes prohibitively expensive to engineer a dedicated optical fiber cable for the last few hundred meters into each customer location. New communications infrastructure costs increase as they are provided closer and closer to the customer, since the resources become less shared. The last few hundred meters are the most expensive, since they will often be dedicated to a single customer. Economic reasons therefore dictate the consideration of a strategy of fiber-to-the-neighborhood (FTTN) or fiber-to-the-curb (FTTC) or for larger buildings fiber-to-the-basement (FTTB), combined with the re-use of the existing copper cable for the last few hundred yards.

In these network architectures an optical fiber cable would be installed to new remote stationed equipment. The remote equipment will tap onto the existing copper line (at frequencies above those already in use for POTS or even ISDN) and transmit at very high bit-rates using VDSL modem technology. Data rates of 13, 26 and even 52 Mbps are possible, since only a short segment (the final few hundred meters) of the cable is used.

The data rates and transmission methods for VDSL have yet to be finalized by the international standards bodies. Using today's 10:1 rule of thumb, upstream data rates of typically 6 Mbps are being considered. However, 13 Mbps symmetric is also available. The world's first fully working VDSL prototype was demonstrated  at Telecom Geneva in June 1995.

What are the specific benefits of DSL technologies compared with competitive technologies (HFC, fiber-optics, cable)?

The key benefit of DSL is that by transmitting on an existing telephone line, it reuses the existing infrastructure of installed copper cables. This saves the costs (as much as $1000-1500 per home) of installing a new dedicated wide bandwidth fiber optic cable. DSL is therefore very attractive for those with access to existing copper telephone lines.

Unlike most alternatives, DSL technology does not require a large upfront expenditure. Individual modem links can be provided as new customers demand service, and therefore costs are incremental. Some competitive methods such as hybrid-fiber coax (HFC, or cable) demand a high-up front expenditure to deploy head-end and cable infrastructure. This is often a high fixed cost regardless of how many customers are initially connected. Such systems are therefore highly sensitive to service penetration (i.e. the percentage of homes that take a service).

From the technical standpoint, HFC cable networks have high capacity, although this capacity is shared by all the customers connected, meaning that the actual datarate is much lower. Also, most cable networks are designed for broadcast and are therefore one-way networks. For example, today less than 10% of cable networks in the USA are able to provide two-way communication, and extensive upgrades (such as exchanging one-way amplifiers for two-way amplifiers) are needed.

How much do DSL modems cost?

The costs to the telecom operator depend greatly on the type and quantity of modems ordered, and the features that are required. In the case of ADSL and VDSL, the technology is only now reaching maturity and costs will continue to fall over the next few years. With the expanding market volumes, manufacturers are increasingly integrating the systems into smaller chips and smaller systems, enabling lower costs. This is similar to the trend that was seen with analog modems which rapidly evolved from large, expensive boxes to very small units and PC card plug-ins that are available from a variety of outlets at low cost. Since DSL services are currently only available through the local or competitive telephone company or Internet Service Provider, the cost to the end-user is dictated by their service provider.

Are there any problems with using DSL over old copper?

No, there are no problems with using DSL over old copper - that's the beauty of DSL. DSL technology is designed to cope with worst-case interference from adjacent cables (crosstalk) and has spare margins in the design. However, a telephone region may contain a percentage of very old, very poor quality cables with poor connections and poor deployment practice. Some DSL transceiver technologies, such as Discrete-MultiTone (DMT) which was adopted as the Standard line code for ADSL, are able to adapt the signals sent over the line. DMT type systems can therefore avoid frequency regions that are not suitable for transmission. DSL modems have integrated diagnostics and maintenance features which provide useful indicators to the operators of line conditions and trends in cable loss. These features help with operations and management of the link. If a geographic region is of particularly poor quality it may already be affecting the quality and reliability of the regular telephone service and will often be targeted for rehabilitation with new cables.

How effective are DSL technologies over ATM and IP networks?

ATM and IP are types of data protocol. A protocol is an agreed set of rules for passing and transferring pieces of data. IP is the basic protocol used by the Internet. ATM is a new protocol that will enable different types of service such as voice, data and video, to be integrated onto the same link, and has begun to be deployed for large high-speed backbone networks. ATM and IP do not alone provide a network, since the protocols have to apply to a communications link. DSL technology is one way of providing that transport link. Both ATM and IP protocols can therefore be used to set the rules for transporting data over a DSL link.

Does the US Telecom Bill and the deregulation in European telecoms market affect DSL deployment?

The passing of the Telecom Bill and European deregulation open up intense competition by enabling service providers to compete with each other's existing services. Cable companies are able to compete with local telephone companies by offering telephone and data services over the cable network. Telephone companies can reciprocate by offering video services over the copper telephone wires. In adding these new services, both players are seeking to exploit the massive investment already made in their existing installed infrastructures, and make incremental upgrades as needed.

Telephone companies currently have access to more than 600 million existing copper lines worldwide. These are already connected into homes and business locations. By applying DSL technology these lines can be upgraded to high-speed digital links that can carry such new services.

Will DSL become outdated?

No, DSL will not become outdated for quite some time. As long as copper telephone wires are used for transporting data, and voice , DSL will be a viable technology. Although advances in digital signal processing research and chip fabrication continually increase the power of DSL chips and make new features possible, the real world useful life-cycle of the end-products is much slower and longer. It often takes years from concept and prototype demonstration, to reach high volume network deployments of approved products, and once approved for use, such products typically have useful lifetimes of more than a decade. Furthermore as the overall costs reduce during the product lifetime, new applications of the mature technology can sometimes emerge. For example, when copper telephone lines were installed decades ago, no one ever dreamed that these lines would remain in use for more than just voice service, and yet the services that they can carry have been continuously and incrementally upgraded over the years.


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