Digital Subscriber Loop
DSL stands for digital subscriber loop. It is a method of providing broadband Internet access over a telephone circuit which does not interfere with the normal operation of the line. DSL provides full time high speed internet connectivity without affecting the ability to receive or place calls on your telephone line. At the same time, calls placed or received do not affect your internet connectivity.
On average, DSL provides an Internet experience similar to cable broadband, however, there are distinct advantages and disadvantages to each. The raw speed of transmission over a cable network is usually higher than DSL, however, cable is a shared medium and that speed may be shared with as many as 1000 other users on the same segment. The speed of cable is thus sensitive to how many users are using it. Because the more users a cable company can put on a segment, the more economical it is, cable companies frequently take actions to discourage actually using the service.
For example, one large national capable company returns "fake" torrent packets in order to disable the usage of a popular peer-to-peer file transfer protocol known as Bit Torrent. I won't tell you which but a few seconds with Google will reveal the company or companies in question. Other times cable companies have been known to cancel accounts for transferring too many movies or other large data over the net.
If you are a Linux user, then you most likely downloaded the ISO for your operating system over the Internet and likely do so from time to time as new versions become available. CentOS 5.2 Linux for example, the DVD ISO is presently ONLY distributed as a bit torrent so if you use this company that sends out the fake torrent packets, you can't download an ISO for the DVD to install CentOS 5.2. Personally, if I pay for a service I want to be able to use it so I have real issues with this practice.
DSL by contrast sends only your data over your telephone wire. You aren't competing with anyone else for the bandwidth of that circuit, and for that reason there aren't big economic incentives to keep you from actually using your circuit. The data speeds will tend to be constant across the day providing the servers you are accessing aren't congested and the Internet backbones aren't congested.
The big disadvantage to DSL is that it is distance sensitive. That is to say, the farther you are from the central office, the lower the maximum obtainable speed; and beyond approximately 18,000 cable-feet from the central office DSL service may not be obtainable at all. Please note however that 18,000 feet is not an absolute minimum. This distance is dependent upon a variety of other factors. Larger diameter wire has more reach than a smaller diameter wire. A wire with more twists per foot has more reach than a wire with fewer. Some vendors equipment is capable of more reach than others. There are vendors that make DSL repeaters though how well they work and whether your telephone company will even utilize them varies with circumstances. Remote DSLAMs (the device that terminates the telephone company end of a DSL circuit) are installed by some telephone companies to reach customers farther from a central office.
The bottom line is that which is best for you depends upon your usage habits. If you frequently transmit large files or need maximum transparency, DSL is a better option for you if you are in an area where you can obtain a connection with adequate speed for your needs. If, on the other hand, you are in an area where good DSL speeds are not obtainable, or you are a light user and do not expect to be transmitting large files like movies or operating system disk images, then cable might well be the better choice. Be sure to read your contract, many cable companies place contractual restrictions with respect to how much data downloading or streaming is allowed and will terminate your service or downgrade your speed if those limits are exceeded.
Ordinary phone lines carry voice transmissions in a frequency range from approximately 300Hz to 3600Hz or so. The actual lines are capable of frequencies far higher but the telephone switching network and multiplexing equipment are not. DSL uses frequencies above 4000 Hz all the way up into the Mhz region to carry broadband data to and from the central office. In the central offices instead of being switched like a voice call is, a DSL data connection is created through an ATM packet switched network to an ISP or back haul provider that ultimately connects back to your ISP where host services are provided.
Higher frequencies are more difficult to transmit down copper wire pairs and that is why the maximum obtainable speed rolls off with distance. It also makes DSL more sensitive to any line noise issues than the voice portion of your telephone line. Nearby radio or television stations and other data services may induce noise in your line that impair it's ability to function. In addition, other devices sharing the same line can interfere with the DSL signal even though they do not actively utilize the frequencies utilized for DSL. For this reason, filters are placed between devices other than the DSL modem and the phone line. These filters prevent the higher frequencies used by DSL from entering other devices and prevent other devices from interfering with the DSL signal.
It is also possible to get DSL signal over a pair of wires without an actual telephone line circuit sharing that pair of wires. This is referred to as "Dry DSL" or "DSL over Dry Copper". Often telephone companies will offer a bundled price for having both a DSL circuit and a telephone line circuit since this is making more efficient use of the copper wire but that bundle reduction is often less than you might pay if you were to get a VOIP or "Voice Over IP" telephone line that transmits data across your DSL connection. VOIP may or may not meet your needs. Many VOIP providers have quality issues or incomplete operator and 911 emergency services. I recommend utilizing only a provider that offers enhanced 911 (full featured 911) service since being able to reach the fire department when a loved one is having a heart attack or when your house is on fire is worth far more than saving a few bucks on a marginal service. Alternately, if you have a need for multiple lines, you may wish to retain an analog line and then use VOIP for additional lines. Keep in mind that during a power failure, unless you have a UPS to keep your VOIP modem going, you will lose that telephone service.
For normal consumer DSL, a flavor known as ADSL is normally used. This stands for Asymmetrical DSL and the asymmetry refers to the fact that generally your download speed will be much higher than your upload speed. Because people tend to download much more than they upload, the available bandwidth is intentionally divided in such as way as to give the lions share to the downstream side and a smaller portion upstream. ADSL modems do not use the same frequency range for downloading as they do for uploading and thus the need for expensive equipment to prevent the modem from hearing it's own transmitted signal is eliminated.
Business class DSL services are available that use SDSL or symmetrical DSL which utilize the full bandwidth in each direction providing identical upload and download speeds but these modems tend to be five or six times as expensive as modems for ADSL services and the equipment required on the telephone companies end is also more expensive and this tends to be reflected in the cost of the circuit.
Because DSL is not a shared facility the way cable is; the pressure not to use it usually doesn't exist unless you are somewhere that data transport is very expensive (Canada tends to be this way because deregulation and the existence of competing carriers happened later in Canada and thus there tend to be fewer competitive carriers available to transport data).
There are two protocols commonly used by the telcos to transmit IP data over an ATM virtual circuit over a physical DSL connection. PPPOA in which a PPP connection across an ATM virtual circuit is terminated at the end of the ATM connection in the modem and then a normal TCP/IP over Ethernet connection. This type of connection is used with our DSL service if you have Qwest as a telephone company. We really prefer it because by terminating an ATM connection in the modem, this type of DSL connection is completely insensitive to problems with PPP implementations in the end users operating system or computer.
PPPOE stands for PPP Over Ethernet and it is used by Verizon, ATT, and Southwestern Bell and others. It is more problematic in that it involves the PPP connection going over the ATM virtual circuit, through the modem, over the wire between the modem and the end users computer, and finally terminates in the end users computer requiring a PPPOE protocol be present in the end users computer.
Generally speaking, for customers who are using any operating system other than Windows XP or Vista, I recommend using a DSL Gateway router that terminates PPPOE internally rather than requiring the computer provide it. This reduces headaches considerably, particularly when problems arise. Invariably, telcos prefer not to support anything other than Windows or Mac, so for users running Linux, BSD, or other operating systems, this takes the computer and operating system out of the equation. With a DSL Gateway/Router there is absolutely no difference between a Windows machine and any other operating system on the other end of the DSL connection.
Because DSL is sensitive to distance and other line conditions, prequalification is required to determine if your line is capable of supporting DSL and if so at what speeds. If you are interested in finding out what is available in your location, back-up a level and select the DSL prequalification link and we'll get back to you with what is available at your location.
Switching from dial-up to DSL will not affect your e-mail address or host services. It will only provide faster access to those services.