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The Analog Problem
DSL was the solution developed in the mid 1990s to solve the analog program. When you speak on the phone, the microphone senses the vibrations and causes an electrical voltage to go up and down with each vibration. Your phone is on the end of a pair copper wires that stretches from your house to the central office normally located in the center of the nearest town to you, or perhaps in a nice discrete location in a neighborhood near you.
Early on, the phone company realized that most people use the telephone most of the time to talk to another person. The human voice does not vary much in its pitch, so a phone call sounds quite normal if the phone company filters out the high pitched sounds. The reason this is important to the phone company is that when sending your phone call long distance (before the introduction of digital phone equipment), the smaller the range of frequencies that were in a phone call, the more phone calls at one time that could be squeezed on the very expensive high capacity coaxial cables that connected the country's long distance phone system.
With the introduction of the transistor and high speed electronics, the long distance portion of your phone call is digitized and no longer uses this analog technology. However, that final mile or two from your phone company central office to your telephone still operates in analog mode with high frequency filtering, the same way it did when your grandfather used the telephone as a young man. At various times, the phone company has considered making the last mile digital, but the costs are very high and with very little obvious benefit. (Many businesses are all digital, since they have a large number of phone lines in one place)
An ordinary phone line is capable of carrying a high speed digital signal, but there are a few "gotchas". The first is that your phone line doesn't run by itself straight to the central office. It generally runs to a phone company box somewhere close to your house and is combined in a cable with hundreds of other phone lines. Because the original wires were only intended to carry analog voice calls, they normally do not contain shielding to prevent signals on one wire from jumping to another wire, as at the low power and low frequencies, this doesn't normally happen. However, when a high frequency signal is placed onto an unshielded copper pairs, it can easily bleed over to the copper pair next to it.
The other problem is that the farther you are away from the central office, the more the loudness of the signal drops off due to the electrical resistance of the copper. If you try to overcome this signal loss by turning up the volume, the result is that you increase the problem with the signals on wires in the same cable being heard on the other wire. Making the problem yet more complicated, the behavior of high frequency signals on a phone circuit changes based on the temperature.
The Digital Solution
DSL became possible due to the improvements and reduction in cost of sophisticated communications processors. DSL talks to and from a special device at the central office called a DSLAM (Digital Subscriber Line Access Multiplexer). When the phone company installs DSL, they take the phone company end of your copper pair and install a signal splitter that feeds the existing voice signal into the existing voice phone system, and feeds the high frequency digital signals into the DSLAM. The high frequency digital traffic is routed through the DSLAM to your ISP (which may or may not be your local phone company). The DSLAM and your DSL modem (actually not an accurate term) adjust frequencies and voltages during the day to prevent the cross talk between lines. Keep in mind that the high frequency signals are not audible on your phone, as they are much higher than your ears can hear. On your end, a splitter and filters are placed on the existing phones (usually by you if you elect a self-install) to keep the voice and data signals from interfering with each other.
As clever as DSL is, it still has some limitations that could mean you can't get DSL, or the speed will be slower than you would hope. The higher the frequency of a digital signal, the more it wants to jump between wires. If you are close to the central office, they can use a high frequency signal at a low voltage, and keep it quiet. But the lower the voltage, the harder it is for your DSL modem to hear (and vice versa). So, the speed possible with DSL is very much related to your distance from the Central Office. If you are more than about two miles from the central office, you won't be able to get high speed DSL. Most businesses, of course are located "downtown" and are within a few thousand feet or less, which is why DSL is great for many businesses.
If your phone company wants to spend the money, they can install the DSLAM out in your neighborhood as a remote DSLAM. This means that the 2 mile limit starts from the remote DSLAM instead of downtown. Of course, the phone company is only going to do this if there are enough people in the neighborhood to justify the costs - the DSLAM needs electric power and a secure environment, so therefore is not a cheap undertaking.
Another problem area with DSL is if your phone line isn't pure analog. This mostly comes into play in rural areas, where the population density is thin (or at least it was rural when the phone lines were put in). When the phone company runs out of copper lines into a neighborhood, rather than putting in another expensive cable, they'll often install a device called a DLC (Digital Loop Carrier). This is similar to the remote DSLAM but is much older technology and an incompatible technology with DSL (unless you choose IDSL - described later)
Other reasons that you may not get DSL even though your are within range of the central office are Load Coils - a device to extend the range of a solid voice signal, but it blocks the DSL signal, or a bridge tap (you are spliced into the middle of an existing copper line rather than having your own copper pair). The bridge tap picks up noise that interferes with the DSL signals. These two problem areas may be solvable, if your installer is motivated to solve the problem and has the clout (legal and/or regulatory) to get the ILEC's attention to fix the problem.
- xDSL - just a shorthand way of saying "some type of DSL"
- ADSL - Asymmetrical (unequal) DSL.
Most home users have most of their internet traffic in one direction - they are downloading information FROM the internet, not putting information into the internet. ADSL has very high speeds downloading and much slower speeds uploading. This is perfect for most home users, who generally are not running web servers or realaudio servers or devices that push a lot of information into the internet
- SDSL - Symmetrical (equal speed) DSL.
On the other hand, most businesses generate a lot of traffic into the internet. They run web servers, send a lot of email, Video Conferencing, Voice over IP for their internal calls, Virtual Private Networks (VPN), Remote Access Services (RAS) or similar services that send traffic out into the internet. SDSL is rarely needed by a residential customer, and is the normal class of service for a business. As such, it is generally priced much higher. ADSL typically costs about $50 a month, where SDSL can run $150 to $500 or more per month. The traditional T1 circuit, which is the alternative for a business, generally runs about $700-$1000 per month, depending on distance and the amount of competition.
- IDSL - DSL over ISDN.
ISDN
was the phone company's first attempt (1996ish) at making the last mile digital. Due to complexity and equipment that was difficult to set up, ISDN was very slow to catch on. Single channel ISDN is only slightly faster than a 56k modem, and generally a lot more expensive. Two channel ISDN is 128Kb up and down. If you've been told your only DSL option is IDSL, it is probably because you are more than about 15,000 feet from the central office and/or your neighborhood runs from a DLC (Digital Loop Carrier). If this is the case, there really is very little advantage to IDSL over just signing up for ISDN, unless you find out there is a pricing advantage (IDSL is "always on" service, where ISDN is frequently a per-hour charge)
In almost all cases, the local phone company (your ILEC) provides the copper pair, and they want to be paid for the use of the circuit and the space in their switching center for the DSLAM.
The second party is the company that installed the DSLAM at the phone company and routes the digital traffic from the DSLAM into the internet. This is your DSL service provider. If you get DSL from your ILEC (Verizon, SBC, BellSouth, Qwest, etc..), your ILEC is your DSL provider.
The third party is the ISP. The ISP provides you with email service, newsgroups, technical support, billing, web space, etc... Frequently this is outsourced by the ILEC, or the ISP is the one that arranges the deal with the DSL Provider (either the ILEC or a DSL Provider)
If you go with the DSL offered by your phone company, all three services may provided by them (although they may outsource the ISP services - for instance, SBC uses Prodigy as their ISP, Qwest is using MSN).
The independent DSL providers have faced resistance from the ILECS in installing the DSL connections or clearing up the provisioning problems (load coils, noisy lines, etc...) and many of these companies are now in bankruptcy.
Be sure you understand whether the price for DSL quoted by an ISP includes the line charges from the local phone company.
We've noticed a pattern lately of ISPs offering "DSL for only $17.95" - they are only talking about the ISP services, not the charges for the DSL circuit. The local phone company might be charging you an additional $40 a month.