The Basics of DSL High Speed Internet Access
Previously, I wrote about how Cable High Speed Internet worked. Well, today I'm going to follow up with with DSL, and how it works. So, how exactly is it that you can surf the net and talk on the phone at the same time, on the same phone line.
A great deal of this is essentially the same as how Cable works, but I'll go over it again anyhow just in case you didn't read the other article. As usual, I am going to simplify this explanation a bit for the sake of expedience (and, well, I'd like it to be understandable too). So basically, you've got a pair of copper wires going into your house that all your telephone calls go over. A whole broad range of frequencies can be carried on this cable at any given point in time. In fact, the wires can handle frequencies up to millions of Hertz. Voice (telephone calls) use only a very small fraction of those frequencies (less than 4000Hz). Now, there is a good reason for all that extra bandwidth going unused. You see, if they used more of the bandwidth, interference would be a problem (the higher the frequency, the more likely the signal is to cause interference, and they run a LOT of cables very close together, so there is a great deal of potential for problems). However, this problem was largely a technological one. Analogue signals are much more prone to this issue than digital (remember, when the phone system was installed, digital didn't really exist), so with digital signals you can get away with using a much much larger portion of the available frequency range. So basically, what happens with DSL is, your DSL modem (and the one at the phone company) send digital signals to each other, using all that bandwidth that your telephone doesn't. This is why you can still use the phone with a DSL connection. Most modern DSL technologies split up the available bandwidth into 4KHz channels (just like your telephone uses) and then uses an appropriate number of those channels.
Of course, the downside is your DSL modem needs to be within a certain distance of a phone company CO - or Central Office - (the exact distance depends on the type of DSL you have). The farther you are from the CO, the more limited the available speeds will be, and the more the quality of your connection will deteriorate.
Cable internet access is pretty much all the same. Sure, you can get different speeds, but the technology is more or less the same across the board. DSL is a little different. There are many different kinds of DSL available (ADSL, SDSL, VDSL, RADSL, etc..). I'll briefly go over a few of them here just to familiarize you with the terms.
The most commonly used form of DSL in the US is ADSL. The "A" stands for Asymmetric. Basically, this means your available bandwidth is not evenly divided between incoming and outgoing - your incoming bandwidth (the data that comes to you) is much much higher than your outgoing (uploading) bandwidth. The idea being that the average user downloads much more than he/she sends out - and this is generally accurate. Of course, this is less than ideal for folks who want to run servers or send files to people, but that is not a problem most people have. The distance limit for ADSL is approximately 18000 feet.
SDSL is Symmetric DSL. Basically, the upload and download bandwidth is evenly split. Generally, this means your download speeds will be much slower than with ADSL, but your upload will be much much faster. This is generally used by small businesses who want to run their own server. The big downside with SDSL is that you lose the ability to use your phone. SDSL is generally much more expensive than ADSL, and is not always available everywhere. The distance limits for SDSL are about the same as for ADSL.
VDSL stands for Very Fast DSL. Really, it works essentially the same way that ADSL does, except it works only over a much shorter distance (4000 ft). It also assumes that the phone company has run Fibre optic cables fairly close by. VDSL is generally considered to be the future of DSL due to its exceptionally high speeds (it is many times faster than ADSL).
RADSL is Rate Adaptive DSL. This is another variation of ADSL, but it has a variable speed. Basically, your DSL modem looks at how far you are from the CO and the quality of your phone line and adjusts the speed accordingly. This is not generally used or advertised. Obviously, RADSL is less distance limited than VDSL, but still falls into approximately the same limitations as ADSL and SDSL.
Why is there a distance limitation? Well, for telephone calls there are these things called Signal Amplifiers, which the telephone company has periodically placed along the phone lines to amplify the signal. Unfortunately, these signal amplifiers are not compatible with the DSL signal - hence the distance limitation. Those same signal amplifiers can actually disqualify you from being able to get DSL at all. Because of the incompatibility, if there is one of those amplifiers between you and your CO, DSL will not function at all.
So why use DSL instead of cable? Well, unlike Cable, DSL does not share bandwidth. The number of your neighbors with DSL will not affect the speed/quality of your connection - this one is really the biggie. Also, if you are a gamer, DSL tends to offer lower ping times, which can be a great asset. Really though, cable and DSL compliment each other nicely. Each has strengths and weaknesses, and offers up an alternative to the other.
Written by : Ravn (5th Jan 2006)
Ravn
is C/O of the CyberArmy Advanced Hardware Projects
This article was originally published by CyberArmy.net in the CyberArmy Library.
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