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Learn about DSL
MANY of us now get connected to the internet without using a phone line and without getting our parents to yell at us for blocking their means of communications. We do this via cable, or what is known as DSL.
But what exactly is DSL? Digital Subscriber Line is a technology for bringing high bandwidth information to homes and small businesses over ordinary telephone lines. Assuming that your home or small business is close enough to a telephone company’s main facility which offers a DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second, enabling continuous transmission of motion video, audio, and even 3-D effects.
More typically, individual connections will provide from 1.544Mbps to 512Kbps (thousands of bits per second) downstream and about 128Kbps upstream. A DSL line can carry both data and voice signals — the data part of the line is continuously connected. DSL installations began in 1998 and are expected to continue at a greatly increased pace throughout the next decade.
DSL is beneficial in more ways than one. For one thing, you can leave your internet connection intact and still use the phone line for calls. Then, the speed is much higher than a regular modem (1.5Mbps vs 56Kbps). Also, DSL doesn’t necessarily require new wiring. It can use the phone line you already have. Another advantage is that the company that offers DSL will usually provide the modem as part of the deal.
However, there are a few caveats as well. A DSL connection works better when you are closer to the provider’s main facility. In addition, the connection is faster for receiving data than it is for sending data to the internet. Lastly, the service is not available everywhere.
As already mentioned, DSL uses the same wires as a regular telephone line. A standard telephone setup consists of a pair of copper wires that the phone company installs at your home. The wires have room for carrying more than just your phone conversations. Generally, they are capable of handling a much greater bandwidth than needed for voice transmission.
The underutilization of the wires’ bandwidth is historic. Remember that telephone systems have been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, telephone systems can pack a lot of wires into a very small space without worrying about interference.
Modern equipment that send digital rather than analog data can safely use much more of the telephone line’s original capacity. DSL exploits this “extra” capacity to carry information on the wire without disturbing the line’s ability to carry conversations.
The entire plan is based on matching particular frequencies to specific tasks. A traditional phone service connects your home or small business to a telephone company office over wires that are wound around each other and called a twisted pair. Traditional phone services were 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.
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 signaling is already set up for this analog wave transmission, it’s easier to use that as a means to get information back and forth between your telephone and the telephone company. That’s why your computer has to have a modem — so that it can demodulate the analog signal and turn its values into a string of 0 and 1 values, called digital information.
Because analog transmission only uses a small portion of the available amount of information that could be transmitted over wires, the maximum amount of data that you can receive using ordinary modems is about 56Kbps. The ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into an analog form for your telephone line, and requires your modem to change it back into a digital form. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck.
The concept of DSL holds that digital data do not require to be changed into an analog form and back. Digital data are transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting the same to you.
If you want, the signals can be separated so that a portion of the bandwidth is used to transmit analog signals only, enabling you to use your telephone and computer on the same line simultaneously.
Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies on the assumption that most internet users look at, or download, much more information than they send or upload. Under this assumption, if the connection speed from the internet to the user is three to four times faster than the connection from the user back to the internet, then the user will be better off, most of the time.
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 5.5kms.
As distance between the user and the telephone company office decreases, the data rate increases. A related factor is the gauge of the copper wire. The heavier 24 gauge wire carries the same data rate farther than the 26 gauge wire. If you live beyond the 5.5km range, you may still be able to have DSL, if your phone company has extended the local loop with opticfibre cable.
To interconnect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer (DSLAM). The DSLAM connects to an Asynchronous Transfer Mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections. ADSL uses two pieces of equipment, one on the customer end and one on the side of the ISP, telephone company or other provider of DSL services. At the customer’s location, there is a DSL transceiver, which may also provide other services.
DSLAM at the access provider is the equipment that really allows DSL to happen. DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the internet. DSLAMs are generally flexible and are able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation in the same type of DSL. In addition, DSLAM may provide additional functions, including routing or dynamic IP address assignment for the customers.
DSL is the next step in what has come to be known as the “communications century.” It represents the man’s quest for knowledge and faster communications. Getting connected to the world is now as easy as ABC, or should I say DSL?
The writer
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