The latest technology to obtain high bandwidth over normal telephone lines at low cost is Digital Subscriber Line, commonly referred to as ADSL or xDSL, technology. This new family of technologies has the ability to revolutionize the way Internet access is done for private users and small businesses, provide low cost telecommuting services and movies on demand, and many other low cost high bandwidth services.

Varieties of DSL Technology

The term ADSL is being commonly used as a catch-all term for all of the various technologies designed to provide high bandwidth circuits over existing telephone circuits. This can cause some confusion, as there are actually several different technologies which fall under this "umbrella" description. A more accurate term is xDSL, where the "x" stands for which of several variations a particular term (such as ADSL, HDSL, RADSL, etc,) is referring to. Therefore, in this document, we will use the xDSL term when discussing the entire family of technologies, and only use the ADSL term when discussing that particular variation.

Below are the current types of xDSL technology:

DSL - The original variation, used as the physical layer for ISDN BRI (2B+D) lines. DSL provides 160 kb/s over a distance of 18,000 feet.

ADSL - Asymetric Digital Subscriber Line - Provides an uplink bandwidth from the subscriber to the network of 64 kb/s - 384 kb/s, and a downlink bandwidth of 1.544 Mb/s - 6 Mb/s from the network to the subscriber. Distances range from 5,000 feet to 18,000 feet. This is the technology most likely to be used for high-speed Internet access, as most Web browsing requires a high downlink speed but very little uplink speed.

HDSL - High Speed Digital Subscriber Line - Provides a symetrical (same speed uplink and downlink) bandwidth from 128 kB/s - 1.544 Mb/s, at 5,000 - 18,000 feet.

RADSL - Rate Adaptive Digital Subscriber Line - A variation of ADSL which adapts the bandwidth used to match line conditions. Provides uplink speeds up to 1.5 Mb/s and downlink speeds up to 6.1 Mb/s. Distances range is 5,000 feet to 18,000 feet

SDSL - Single Line Digital Subscriber Line - A 2-wire version of HDSL.

VDSL - Very High Bit Rate Digital Subscriber Line - Provides uplink speeds between 1.6 Mb/s and 2.3 Mb/s, and downlink speeds up to 51 Mb/s. This provides an extremely high bandwidth, however, it is limited in distance to roughly 1000 feet.

Common Properties of xDSL Technologies

The basic premise behind all of the xDSL technologies is to provide faster data services using the existing outside plant telephone cabling system. This is done for reasons of cost and ease of implementation. If telephone companies are forced to rewire to offer some new service, then that service is almost certain to either not be offered, or at best, be offered at a very high cost to the end user. On the other hand, any technology which allows the telephone company to offer a new service without rewiring is likely to be implemented fairly quickly, if it offers both a benefit to the customer and a new revenue source to the telephone company.

All versions of xDSL are designed to provide a high-speed data link over existing telephone lines. There is a growing demand for such services for Internet access, movies on demand, telecommuting, remote LAN access, and Wide Area Networking (WAN) applications. Each version of xDSL has its own niche, and an explaination of each technology and its main applications is provided below.

ADSL - Asymetric Digital Subscriber Line
RADSL - Rate Adaptice Asymetric Digital Subscriber Line

The explosive growth of the Internet, especially for home users dialing in to their Internet Service Providers (ISPs), is fueling demand for higher speed access lines. The limited bandwidth of traditional analog telephone lines, generally 33.6 kb/s - 56 kb/s, combined with the relatively large graphic files commonly used on Web pages, generally result in long wait times for Web pages to load. This phenomenon is commonly referred to as the "World Wide Wait." Also, the long-duration local telephone calls are somewhat costly to provide by the telephone company serving these customers. The Public Switched Telephone Network (PSTN) was designed for relatively short duration phone calls and a large number of long-duration local calls can result in the telephone switch running out of available lines. Therefore, the telephone company is forced to spend large sums of money upgrading the telephone switch to a higher capacity level. A final consideration, from the user's perspective, is that the phone line is "tied up" and unavailable for voice calls while the modem is on-line.

Figure One
Normal Dial-Up Internet Connections

ADSL allows all of the above problems to be solved in a very elegant fashion. Nearly all existing telephone lines are capable of carrying frequencies up to the range of 1 MHz. However, analog telephone service only requires a maximum frequency of 3 kHz. Therefore, a large amount of the wire's capacity is essentially wasted. ADSL is "piggybacked" on the same telephone line as the existing voice service by using the currently wasted frequencies from 4 KHz to 1 MHz and filtering by frequency at each end. Therefore, the telephone company is able to offer both traditional telephone service and a high speed data service over the same existing telephone line (see Figure Two).

Figure Two
ADSL & Analog Voice Integration

From the telephone company's perspective, the fact that ADSL provides high speed connections over the existing copper cabling plant allows them to economically provide high speed Internet service to end users, while taking the load of this Internet access off of the telephone switches. It also frees telephone switch capacity for normal voice calls and reduces the need for switch upgrades. From the customer's perspective, he or she sees a major speedup in Internet access, and the telephone is no longer unavailable for use while he or she is using the Internet.

Figure Three
ADSL Internet Access

Real-World Applications For ADSL

ADSL is an asymetric service. This means that the downstream speed from the telephone company to the user is many times faster than the upstream speed from the user to the telephone company. This is an ideal scenario for most Internet access, as users generally download far more data than they upload. For example, let's look at a typical user accessing a Web page. The user enters an address, for example "", in to his or her Web browser. The browser then looks up the name of the server and sends a "GET" request to retrieve the page. In the page is embedded several graphics. For each graphic, the browser issues a separate "GET" request. These requests are fairly short transmissions, generally under 150 bytes each. However, the file being requested may be several thousand or million bytes in size. Therefore, far more data is being transferred from the network to the user than the user is transferring to the network. A very high bandwidth uplink speed is essentially wasted in these applications, but a high bandwidth downlink speed is essential.

ADSL Implementation Issues

ADSL can be implemented over the vast majority of existing telephone lines, but not all. It does have some practical limitations which must be taken in to account before it can be used. These are:

Distance: 18,000 feet maximum from Central Office to customer premises.
Unloaded Lines Required: The line used for ADSL must not have any loading coils, bridge taps, or repeaters of any kind anywhere along its length.

It is currently estimated that roughly 80% to 85% of the existing telephone lines in the United States are able to be used to provide ADSL service. Before ADSL can be universally installed, however, the lines which are not suitable for it must be upgraded.

One problem with ADSL is that it requires a fair amount of configuration at both the telephone company and customer premises ends of the circuit. This is due to the fact that the maximum data rate is variable with line conditions, and must be set at the time of installation of the circuit. Because of this, a new version of ADSL called Rate Adaptive Asymetric Digital Subscriber Line, or RADSL for short, has been introduced. This version automatically runs at the highest possible rate permitted by current line conditions and speeds up or slows down as the momentary conditions dictate. Therefore, it is far more "plug and play" in installation, and is most likely the version which will see widespread deployment.

HDSL - Symetric Digital Subscriber Line
SDSL - Single Line Digital Subscriber Line

HDSL and SDSL are symetrical versions of ADSL. These variations have similar distance and line condition requirements to ADSL, however they always operate in a symetrical (same speed uplink and downlink) manner. The data rate is between 128 kb/s and 1.544 Mb/s. The only difference between them is that HDSL requires a 4-wire (2 pair) circuit while SDSL only requires a 2-wire (1 pair) line.

These technologies are primarily focused at Wide Area Network (WAN) setups and as a better physical layer driver for T1 services. This is due to the longer distances and lower quality cable they support at T1 data rates.

Another interesting application for HDSL and SDSL is in LAN extention products. Many companies have a requirement to extend their Ethernet networks beyond the normal range it can cover. Generally, this requires the use of bridges or routers and a dedicated DDS or T1 circuit, resulting in a large initial and recurring expense. However, if the two locations to be connected are within three miles (18,000 feet), it is possible to use simple extention products to do the job over normal telephone wires. Therefore, a large cost savings results. Such products now exist and are currently shipping.

VDSL - Very High Bit Rate Digital Subscriber Line

Where up to 6.1 Mb/s is just not enough speed, there is VDSL. This variation of xDSL is designed for extremely high speed (up to 51 Mb/s) applications with distances up to 1000 feet or so. Current thinking is that it could be used as the transport medium for applications such as High Definition Television (HDTV) which require a very large bandwidth. However, due to the relatively short distances it supports, actually implementing VDSL will be far more difficult for the telephone companies to do than ADSL or RADSL. Basically, it will require pulling fiber-optic cables to distribution points in each neighborhood and then pulling short copper cables from these distribution points to each building to be serviced by that distribution point.

Due to the high cost of installation and current lack of applications demanding its incredibly high speeds, VDSL is not expected to be widely implemented for several years, if at all. It may find a place in LAN extention products for Fast Ethernet networks, which are very limited in how much area they can cover over UTP cabling. However, such products have not yet been produced, so such applications are currently only conjecture.