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Broadband through copper

DSL offers high-speed Net access over existing copper wire. Here’s a peak into DSL and its varients. by Mahesh Rathod

Digital Subscriber Line is a technology that works on the presumption that digital data does not require change into the analog form and back

Traditional phone services (also called as POTS or Plain Old Telephone Lines) are meant to carry analog voice signals over copper wires. To transmit anything other than voice, for instance data, and receive the same on your computer, this analog signal has to be converted into digital information. This task is done by the modem, which demodulates the analog signal into digital data. This conversion from analog to digital and back to analog limits the amount of data that can actually be sent or received to 56 Kbps.

DSL (Digital Subscriber Line) is a technology that over comes this bandwidth bottleneck. Additionally, it also provides an always on, fast Internet access.

Fast Net access
DSL works on the presumption that digital data does not require change into analog form and back. Digital data is transmitted to your computer directly in its original form. This allows the phone company to use a much wider bandwidth for transmitting it to you. Also, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal while the remaining can be used to transmit a digital signal. This enables you to chat over the telephone or access the Internet at the same time.

DSL provides secure, reliable high-speed Internet access to homes and offices using high-bandwidth over ordinary copper telephone lines. It is commonly referred to as xDSL due to the different variations such as ADSL, HDSL, RADSL, etc, based on upload/download speeds offered. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may soon be able to receive data at rates up to 6.1 Mbps (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically,

individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line always stays connected to the Internet.

With DSL technology, digital data does not require change into analog form and back. It can be transmitted to your computer directly as digital data and this allows for the use of a much wider bandwidth. This means you can blaze down the information highway with virtually no roadblocks.

How it works?
The several variants of DSL technology use the ordinary, pre-existing copper telephone lines to

deliver high-speed data service to users. DSL services can operate over the same pair of wires as an existing POTS phone connection. At the customer's end, a splitter box separates the DSL and POTS phone signals. Telephones, fax machines, and other devices attach to the POTS phone line as usual. The DSL splitter connects to a DSL modem, which in turn connects to the user's PC via an Ethernet connection.

At the phone company's central office, a device called a DSL Access Module (DSLAM) separates the phone signal and connects it to the normal switched telephone network. The DSLAM also acts as a router. Typically, the router connects to an Internet backbone connection to provide Internet access for DSL users. In some areas, the phone company can route one DSL user directly to another. This allows the DSL connection to be used for remote LAN access.

Drawbacks

Despite its many positive
attributes, DSL is not entirely flawless. For instance, in order to be eligible for DSL, the end user must be geographically within a certain distance from the central telephone office, otherwise the signal degradation is too great and DSL is unfeasible (for ADSL that distance is two miles). In addition, numerous standards still exist for DSL, hardware is still comparatively pricey, and service is available only in limited areas.

Regardless of these drawbacks, DSL is still a quicker alternative to analog modems and ISDN.

The introduction of DSL has fostered a fast growing and meaningful competition in data services, most notably for Internet access. In most major markets today, residences and small to medium-sized businesses (SMBs) have the ability to order DSL service that turns an ordinary copper local loop into a high-speed data pipe for personal or shared use, at a fraction of the cost of a traditional leased line.

Flavors of DSL
In its multiple form DSL offer users a choice of speeds ranging from 32 Kbps to more than 50 Mbps. These digital services can be used to deliver bandwidth-intensive applications like video-on-demand and distant learning. Today DSL is putting high-speed Internet access within the reach of homes, small and medium-size businesses. DSL takes existing voice cables and turns them into a high-speed digital link. Over any given link, the maximum DSL speed is determined by the distance between the customer site and the telephone exchange.

There are several forms of xDSL, each designed around specific goals and needs of the marketplace. Some forms of xDSL are proprietary, some are simply theoretical models and some are widely used standards. They may best be categorized within the modulation methods used to encode data. See table for a brief summary of some of the popular xDSL technologies.

Mahesh Rathod can be reached at rathodmp@hotmail.com