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How DSL Internet Access WorksYour Knowledge Connection2004-05-04 by Gareth Marples Ah yes – the world of digital. It appears to be a world of technological advances all geared towards developing more speed. More speed creates, in turn, better audio, better video, faster downloads. And speaking of downloads, have you ever sat in front of your computer and waited…and waited…and waited…and…well, you get the picture. At least, eventually, you probably got the picture. And the technology that brought you that picture is DSL. So, to give you a better understanding of how DSL Internet access works, we’ll take a simple look at a complex process. DSL is, technologically, quite complex. But we’ll try to create an easy-to-follow picture for you. Here we go.
It all starts with a telephone lineFirst, we’ll look at the telephone line. The wires that connect your home or small business to a telephone company are made of copper. These wires are wound around each other and are called twisted pair. You exchange voice information over the POTS, and the type of signal used for this is called an analog signal. This analog signal is acoustic in nature, but is converted to an electrical equivalent in terms of volume and pitch. The signal is transmitted through the wire on a selected frequency. To properly understand DSL, we first need to know what frequencies are. Think of it this way: Split a telephone wire lengthwise into sections. Each section is a frequency. You can send information along this frequency in the form of a signal. The transmission of the signal is converted into cycles per second, each cycle being known as a Hertz. (For example, a megahertz (MHz) is one million cycles per second.) Each hertz can carry a signal along its separate frequency. The total range of frequencies, expressed in Kilobits per second (Kbps) is called the bandwidth. Now, getting back to your telephone conversation, human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 – 3,400 Hertz ( a very small frequency). Because telephone companies limit the frequencies carried over the lines, the phone system can pack lots of wires into a very small space without worrying about the interference between lines. This is good news for DSL –they can use the large amount of space left on the lines to transmit digital signals.
DSL offers far faster signal speed – but not too farSo now DSL could use the available space on the phone line to transmit high-speed digital signals. As soon as they’d established their basic system, improvements were sought. Different types of DSL were developed. Most DSL users are connected to one of these new developments, an ADSL (asymmetric DSL) line. ADSL divides up the available frequencies in a line. Because most Internet users download more information than they upload, it was logical to make the connection speed from the Internet to the user four or five times faster than the connection from the user back to the Internet. So ADSL offers far faster speed – but, as we mentioned, not too far. Not too far from what? The CO (Central Office). Distance is a major factor in ADSL technology. As you get farther away from the CO, your signal gets weaker and your connection speed gets slower. The limit for ADSL service is 18,000 feet (5,460 meters). Maximum downstream (Internet to customer) speeds are up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds are up to 640 Kbps. So when you’re ordering your DSL, make sure to ask how far you are from your CO.
ADSL systems competeThere are two completely different competing standards for ADSL. American National Standards Institute (ANSI) is the primary organization for fostering the development of technology standards in the United States. The standard they set for ADSL is a system called discrete multitone (DMT). Most ADSL equipment today uses DMT. The earlier standard was the carrierless amplitude/phase (CAP) system, which was more easily implemented. The CAP system divides the signals on the telephone line into three bands:
This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines. DMT also divides signals into separate channels, but instead of using two broad channels for upstream and downstream data, it divides the data into 247 separate channels, each 4 KHz wide. Each channel is monitored and, if the quality is poor, the signal is shifted to another channel. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. DMT also uses some of the lower channels as bi-directional channels. These frequencies can be used to transmit upstream or downstream information. All this information going up and down, along with monitoring the quality of it all, makes DMT more complex to operate than CAP, but it’s much more flexible.
Of course, nothing runs without equipmentADSL used two pieces of equipment, one at the customer end and one at the DSL provider end. At the customer end, there’s a DSL transceiver, commonly known as a DSL modem. The modem is the point where data from the user’s computer is connected to the DSL line. We’ve already determined that the signal sent by the telephone company is an analog signal. The purpose of the DSL modem is to convert this signal back to a digital form that your computer can understand. The DSL service provider has a DSLAM (a DSL Access Multiplexer) to receive customer connections. This piece of equipment is what really allows DSL to happen. A DSLAM takes connections from many customers and combines them into a single, high-capacity connection to the Internet. It’s able to support multiple types of DSL, as well as different types of protocol and modulation, including CAP and DMT, in a single central office.
It all comes together on your computer screenSo now we’ve tracked the journey of the information you see on your computer. ADSL is the form of DSL that’ll become most familiar to home and small business users. Using ADSL, up to 6.1 Mbps 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. Also, a small portion of the downstream bandwidth can be devoted to voice data, and you can hold phone conversations without requiring a separate line. So now you’re connected. Now you know how DSL Internet access works. Now you can sit at your computer, knowing why, occasionally, your computer slows down. But, hey, don’t worry – you can still talk on the phone while you wait!
About The AuthorGareth Marples is a successful freelance writer providing valuable tips and advice for consumers purchasing internet service providers, free long distance and discount web hosting. His numerous articles offer moneysaving tips and valuable insight on typically confusing topics. This article on "How DSL Internet Access Works" reprinted with permission.
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