Digital Subscriber Line 
Frequently Asked
Questions
About DSL
DSL stands for Digital Subscriber Line. It is basically a way of providing high-speed data over regular phone lines and it is currently one of several options for high-speed Internet access at home (the other options includes a cable modem from your cable TV company and satellite). Because DSL runs over regular phone cables and at the same time is a high-speed data service, it has much more stringent requirements than a telephone. DSL will typically not be installed if there is more than 15,000 feet of wire between you and the phone company's CO (central office, or basically the neighborhood location where all the phone wires end up), or if it does get installed, it'll run slower than normal.
There are two types and two flavors of DSL. The flavors are ADSL (asymmetrical) and SDSL (symmetrical). There are technical differences, I'm sure, but it all boils down to whether the transfer rate (speed) of the line is the same both ways. In ADSL downstream transfer (downloading from the internet, browsing the web, getting e-mail, etc.) is going to be significantly faster than upstream transfer (uploading to the internet). In SDSL the transfer rate is the same both ways. Because of this, SDSL is typically limited to a maximum speed of about 1.5 megabits/second, whereas ADSL can go up to about 6 megabits/second download and up to about 384 kilobits/second upload. By comparison, the fastest a dial-up modem can go is 56 kilobits/second.
The two types of DSL are the type you get from your local phone company and the type you get elsewhere. The major difference here is whether your DSL service will be provided over the same wires as your telephone service (local phone company) or over a new set of wires (other companies). It is important to keep in mind that your local phone company is not necessarily the company that provides your phone service, but is actually the company that maintains the wires that run into your house. I am talking about companies like Verizon and Pacific Bell here. Let's say you live in the northeast. While you may have local phone service from AT&T, Verizon is still the only company that has a local office with wires running to your house. This is an important thing to keep in mind, because if you live in an apartment or condominium, there may not be free wires running from the phone company closet in your building to your particular unit. This would significantly complicate and possibly delay and increase the costs of the installation.
Local Phone Company DSL
I have some limited experience with this type of service, so I will describe what I know. Typically you will contact your phone company and they will test to make sure your are eligible for DSL service. If you are, they will connect your line on their end and send you the equipment to plug in on your end. You will get a DSL modem and several line filters. The filters connect to phone jacks where you have regular voice telephones. Because this DSL service runs over the same line as your telephone, the filters will help prevent any DSL noise interference from getting into your phone conversations. Your phone company will be your point of contact for all issues, although you will probably have to deal with separate departments for DSL questions and phone service questions.
Other Company DSL
There are plenty of companies out there that provide DSL service. There are customer stories, ratings and reviews on http://www.broadbandreports.com. What I will try to do here is educate you about how this type of service will be set up.
There are a total of 3 different companies involved in setting up DSL. There is the ISP (internet service provider) the network provider and the phone company. The ISP is the company that will give you an IP address, e-mail address, web space, etc. The ISP will also be your one and only point of contact for technical support. The ISP is also the company that will set the price and contract terms in most cases. The network provider is the company that your ISP contracts with to connect their network and your house. Your only contact with them will be during the installation. An example of a DSL network provider is Covad Communications (http://www.covad.com). The network provider is the company that determines the kind of DSL modem that gets installed in your house. They are the company that has a presence in your local phone company's central office. They will also be the ones to get in touch with your local phone company and arrange for the circuit. The phone company is responsible for allocating a dedicated pair of wires that runs from your house (not inside your house, but from the junction box that is usually located either outside or in a basement or storage closet if you live in a building) to the network provider's equipment at the central office.
The installation will typically go something like this: You will call up an ISP and order service. They will contact their network provider and order service. The network provider will contact the phone company and order service. The phone company will set up their circuit. The network provider will schedule a visit with you. They will do any necessary wiring from the point where the phone company left off to where you want the DSL modem located, they will install the modem and make sure you can connect to the internet. That will typically be the last you hear of either the network provider or your local phone company.
How DSL is hooked up to your computer
The setup is really quite simple. The DSL modem has two connections on the back. One connection will go to the DSL line, and the other will go to the network card in your computer. If your computer did not come with a network card, then you will need to buy one. There are plenty of network cards on the market for as little as $20. In most cases you will not need to install any software on your computer and you will be connected to the internet as soon as your computer boots up. One case where I had to install software was using Verizon's DSL service. This set up required an very cumbersome installation and required me to actually click a connect button and put in a username and password to connect.
DSL modems come in two flavors: routers and bridges. There are important differences to be aware of. In the case of a bridge, your computer will have a real internet IP address and will be directly connected to the internet. In the case of a router, the router will have the real IP address, and your computer will have a fake address. The router will take care of translating all the data between the internet and your computer using NAT (network address translation).
A bridge provides a more direct connection to the internet, but also exposes your computer more. If your computer has a real internet IP address then it is more open to hackers. Putting your computer behind a router and using NAT makes it more difficult to hack in, because all incoming traffic has to go through the router first. The router acts as a very minimal firewall in this scenario. Using a router also means that you can hook up multiple computers to it without much difficulty. If you have a bridge, then the only way to share the connection is by setting up one computer to act as a "proxy" so that all traffic goes through it. You will then use tools such as proxy servers or internet connection sharing (as in Windows 2000) to share this connection.
Reliability
DSL is not the most reliable of services, especially in a residential setting. The theory behind it is that, since it runs on phone wires, it should be as reliable as telephone service. The reality is quite different. Because DSL is much more picky about the condition of the wires than your telephone, it is entirely possible that a pair of wires will work fine for telephone but will not support DSL. The other common problem is, oddly enough, your local phone company. The phone companies have had a really hard time finding qualified technicians. What can end up happening is that a telephone installer will be up on a pole, looking for a pair of wires to give someone new phone service. He will check the wires that run your DSL line, and notice that there is no dialtone. He will assume that it is a free pair, and suddenly your DSL stops working.
There are other hazards to your DSL. Squirrels are known to chew on wires, and while they may not necessarily disconnect a wire, they may get enough insulation off to make the line unusable for DSL. Weather can sometimes be a factor in the performance of a DSL line. Poorly shielded lines can be affected by wet or stormy (lightning) weather, though these types of problems usually go away once the weather changes.
Cost
DSL is significantly cheaper than a dedicated data connection, such as a T1 or a fractional-T1 line that you will typically find in businesses. ADSL service starts at approximately $40/month and SDSL service starts at approximately $150/month. The costs go up from there, depending on how fast you want your line to be. You will have to buy the DSL modem in most cases, and these range in price from about $100 to $300.
This guide was written by Sergey Polak, a computer programmer and network engineer. He has a BS in Computer Science, is a Microsoft Certified Professional and has over 9 years experience in the field.
