educational

Access Angles: Part 2

Karl LaFong

In our first installment, we looked at the basics of computer data transfer, along with dial-up modem and high-speed cable modem based 'broadband' Internet access. In this installment, we'll look at arguably the most popular form of high speed access: Digital Subscriber Lines:

DSL — Speed for the Masses
In the middle-90's, technicians at Bell Laboratories discovered a new way to use existing phone lines to transfer data at a faster rate. They used a combination of existing technologies and the fact that analog phone lines only use 2% of the bandwidth available to them. This technology is generically called DSL.

What inventors did was to essentially figure out a way to use the 98% of the bandwidth available on your phone for data transfer. If you remember our dial-up modem section, POTS (Plain Old Telephone Systems) work on an analog signal. The wires that carry the signal work for both analog and digital signals, and all the engineers had to do was to find a way to transfer both signals at once, and of course they did, by using what is known as a 'packet sniffer' to separate data packets from analog signals, and then route the signals to the processing equipment.

There are several different forms of Digital Subscriber Lines (DSL), including ADSL (Asymmetric DSL), XDSL, VDSL, ADSL Lite, SDSL, and others. Essentially they use the same basic technology with slightly different methods of transferring data. The most common form of DSL is ADSL, which is offered by most of the major phone companies and DSL providers, and is the main form of DSL we will be discussing here.

How DSL Access Works
DSL uses an un-used section of your existing phone line traffic to send it's information. There are three unique sections to the data stream from your telephone line, each of which has a distinct sound bandwidth that it operates in. Common phone systems use the section below 4 kilowatts. ADSL uses the other two sections of bandwidth above 4 kilowatts. That means that while you're talking in that 4 kilowatt range, your DSL connection can still be connected to the net using the sound range between 4 kilowatts and 1Megahertz, giving it over 97% of the available bandwidth of your existing phone line.

Essentially, digital information passes through your phone line the same way it does with a regular modem, the only difference is the equipment used. The modem used to transfer data is a specific DSL modem, and connects to a splitter box that combines it with the rest of the phone system in the house. This is necessary, as the DSL system uses a part of the phone line's capacity not used by the regular phone line. This allows the subscriber to use a single phone line for both Internet and phone systems without the need for a second line.

Using the same phone line (even while you're talking on the phone), the data information from the Internet is passed to a "central office," where it is split from the telephone information, and sent to the Internet via a system called Asynchronous Transfer Mode (ATM). This allows the information to be sent to the location on the Internet where it is required, and then sent from the Internet back to your computer. DSL speed is dependant on how far you are from your phone company's nearest DSLAM Multiplexer (the splitter for phone and data information).

Normal 56k Internet access lines offer speeds of about 5Kbps (kilobits per second), and by comparison, DSL offers transfer rates of between 64Kbbps up to 3Mbps (1 Mbps = 1024 Kbps) upstream and then between 1.5Mbps and 52Mbps downstream. Upstream is your connection speed that you can send information from your computer, and downstream is the rate at which to can receive information. The average connection rate of ADSL is about 256kpbs, the same as cable. This means a 100K file takes about 3.1 seconds to download.

DSL speed is dependant on how far you are away from your telephone company's nearest DSLAM Multiplexer (the splitter for phone line and data information). Most estimates place the distance from this center at about 3 miles, but some forms require you to be within 1,000 feet. The closer you get, the faster your service can theoretically be, but the faster your service is, the more you can expect to pay as well. This also means that 50% of us can't use DSL service, so check with your phone company to find out if you fall into their range.

DSL currently seems to be by far the most popular form of consumer level high-speed Internet access due to its widespread availability, very low cost, and reliability; but like cable access, it's not available everywhere, and does have its own peculiar set of limitations. Still, for many users, DSL is the best choice, especially for those within close range of a 'central office.'

Range becomes less of an issue with our final modes of Internet Access: wireless and satellite - Stay tuned!

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