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DADDY BOB'S COMPUTER Q & A

 

What Is DSL?

DSL (Digital Subscriber Line) is a technology that brings high-bandwidth information to homes and small businesses over ordinary copper telephone lines. There are different variations of DSL, such as ADSL, HDSL, G.Lite and SDSL explained herein. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, (usually 3.5 miles or 18,000 feet) you may be able to receive data at rates up to 6.1 Mbps with a theoretical max rate of 8.448 Mbps, enabling continuous transmission of motion video, audio, and even 3-D effects.

Most locally typically, individual connections will provide from 512 Kbps to 6.0 Mbps and about 128 to 512 Kbps upstream. (The current 56K modem in wide use has a downstream speed of about 50 Kbps, and an upstream speed of 28 Kbps.) One Mbps (Megabit per second) is equal to 1024 Kbps (Kilobits per second) or 1,048,576 bits per second. Eight (8) bits equal a Byte, which can roughly be compared to a character or number. Therefore, the downstream speed of 512 Kbps to 6.0 Mbps mentioned above is roughly equivalent to 66.000 to 786,000 characters per second.

To put this in better perspective, consider that I am located about 3 miles from the local exchange, and my current ADSL connection is averaging 6.0 Mbps downstream speed. This is some 122 times faster than my 56K dial-up modem. This means my granddaughter can download an average size MP3 file (2.5 –3 MB) in about 5 seconds.

 A DSL line can carry both data and voice signals and the data part of the line is continuously connected. This is sometimes referred to as being “Always On”.

A Brief Description of How It Works

Traditional phone service (called POTS for "plain old telephone service") connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair. Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal.

To better understand the difference between Digital and Analog, consider this scenario. A man is standing on a high cliff and then jumps off. When he is on the cliff before the jump, and when he is on the ground after the jump, he is digital. While he is falling, he is analog. Another comparison could be this. Answering a question with “Yes or No” is digital, while answering with “I don’t know or I don’t care” is analog.

An input device such as a phone set takes an acoustic signal, which is a natural analog signal, and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's equipment is already set up for this analog wave transmission, it's easier for it to use it as the way to get information back and forth between your telephone and the telephone company.

However, your computer needs digital data, and that's why your computer has to have a modem (MOdulate DEModulate)- so that it can demodulate the analog signal and turn it into the string of 0 and 1 values that is called digital information. So, in the simplest of terms, the modem is a D-A (Digital to Analog) converter on upstream data, and an A-D (Analog to Digital) converter on downstream data.

Analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, so this, and other reasons we won’t go into here, the maximum amount of data that you can receive using ordinary modems is about 52 Kbps (thousands of bits per second).

The ability of your computer to receive information is constrained by the fact that the telephone company converts information that arrives as digital data, into analog form for your telephone line, and then your modem is required to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck. You must remember that the telephone system was designed for analog voice transmissions, and we are forcing it to do things it wasn’t designed to do, namely transmit digital data.

Digital Subscriber Line is a technology that recognizes that digital data for the computer does not require changing into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting it to you. In most applications, the signal is separated so that some of the bandwidth is used to transmit an analog signal along with the digital signal, allowing you to use your telephone and computer on the same line and at the same time.

Factors Affecting the Transmission Data Rate

DSL modems follow the data rate multiples established by North American and European standards. In general, the maximum range for DSL without a repeater is 18,000 feet, or about 3.5 miles. As distance decreases between you and the telephone company exchange, the data rate increases. Another factor is the gauge of the copper wire. The heavier 24-gauge wire carries the same data rate farther than 26-gauge wire. It is hard to believe that all this data and voice communications comes into you home on just 2 wires, not much bigger than a sewing thread.

Types of DSL

ADSL

The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth.

Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice, so you can hold phone conversations without requiring a separate line. A special filter usually called a “Z” filter is used in the home to separate the voice from the data. Without this filter, the telephone would have a severe buzz on it making it unusable.

Unlike a similar service over your cable TV line, using ADSL, you won't be competing for bandwidth with neighbors in your area. In many cases, your existing telephone lines will work with ADSL. In some areas, they may need upgrading.

HDSL

HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions.

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