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What is DSL

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What is DSL?

DSL stands for Digital Subscriber Line, which is not very descriptive. It does not tell you what it is, how it works, or what it does. DSL is like your standard home telephone line. It is a very small electric charge that is sent down two pieces of wire, usually copper. When the phone rings a different type of electric charge is sent through which causes your phone to ring. When you pick up, different electrical charges come in and cause the speaker in your receiver to vibrate in a certain way so you can hear sound come through.

DSL takes a large electrical charge and shoots it down the line, creating a digital connection. Your standard telephone is an analog connection with sound, where the digital connection works by sending 0’s and 1’s back and forth to your computer. This is why DSL can run so much faster than a regular modem. It is not dependent on sounds. Sounds have a limitation to what kind of range they can have. Digital is based on numbers, and a "0" is always a "0" and a "1" is always a "1."

A limitation with DSL is how far this electrical current can travel. It is a fairly large electrical current, at least when compared to the current of a standard telephone line. The strength of an electrical current goes down with the square of the distance you are from where the electrical charge starts. For example, if you send out a charge which is 100 units. At distance 1, it is still 100. At distance 2 it doesn’t cut in half, it goes to 1/4, or the square of the distance (eg.2 2) which is 4. One divided by 4 is .25, so you multiply that by the signal strength of 100. At distance 3, it is about 11 (3 2), which is 9. 1 divided by 9 is about .1111, so 11. There is a range limitation on DSL based on the distance from where the signal starts.

DSL equipment that puts the signal over the line is also very expensive. This is one of the reasons why so many DSL companies are having problems. They put all this money into placing pieces of equipment all over the country. The actual reasonable distance for DSL is about 3 miles from the piece of equipment. If you think about the coverage that you would like to have for DSL, which is practically everywhere, then you have to put a piece of $100,000 equipment every three miles. The costs add up dramatically. DSL will NOT run over fiber optical networks. It requires an electrical current to work, so it cannot take advantage of advances in speed and networking.

Many of the phone companies put their equipment in high population areas. They can afford large losses on the unregulated portion of their business because of the sheer volume they are getting from dial-up connections and e-mail, acting as pseudo ISPs. They can charge very low rates for their DSL, which makes it very hard to compete for other DSL companies since they have to put equipment into the same central offices, hook it into the local telecommunication company’s equipment, pay the telecommunication company as well as pay for the equipment and bill you for what is coming through.

There are really only a couple of DSL companies other than the old "Baby Bells" that are available today. Rhythms and CoVad come to mind. However, at the time of this writing (August 2001), neither is profitable and it is conceivable that either or both could go bankrupt any time - as several others have already done this year.

 

There are a number of different types of DSL services. There is ADSL, SDSL, and RADSL, for example. There are others, but these are the 3 basic types. Most of the telecommunication companies (telco’s) put in ADSL or RADSL. ADSL is generally the type of connection that Aim High!, Inc. installs.

ADSL is Asynchronous Digital Subscriber Line. Asynchronous means that you have one speed in one direction that is higher than the speed in the other direction. Most ADSL is twice the download speed as upload speed. This is how most ADSL works, which is what most of the baby bells started with, and, in Colorado, Qwest can still provide. When you ask for information from the internet, you would upload that at speed, for example, 5, and your information would go out to the internet. You would then get information back at speed 10. So, your download speed was twice your upload speed. If you asked for a 256k link, in general, you got 256 upload speed. So if you were sending to the Internet (something such as an e-mail or request for web information), it would upload significantly faster than a dial-up line, and it would download at 512, twice as fast as your upload speed. This really works well for small businesses or individuals and connections that don’t require a lot of uploading.

Many telco’s also offer RADSL (Rate Adaptive DSL.) With RADSL, you have a base upload speed, but the download speed is dependent on different portions of your connection, the amount of traffic that is existent on the provider’s network, and how many people in your local neighborhood might be connected to it.

Most of the telco’s have moved to systems with less expensive forms of DSL. You may notice that your speeds get a little slower in the evenings because you get a lot more people on at that time and you don’t really have a dedicated line, you are sharing it with other people.

The other DSL service is SDSL (Synchronous Digital Subscriber Line). This means that your upload and download speeds are the same. It is more expensive on a monthly basis. The advantage of SDSL is if you have a web server or a mail server in-house. You want to make sure your upload and download speeds match so that if you are trying to send something out from your mail server, or somebody is trying to get to your web page, the information goes out as fast as it is coming in. That is, as fast as people are making requests from outside. With ADSL, because the download speed is twice the upload speed, people are going to be asking for information twice as fast as you can send it out. You will get time outs, access errors, etc.

DSL is not a technology that has really lived up to its promise. It has been a very expensive prospect and, really, the telco’s are taking advantage of the position that they already have in providing the last mile of connectivity to circumvent some of the competitiveness of an open market.

The speed and reliability of DSL is really not there for the higher end connections, which is where the SDSL people would really shine. CoVad and Rhythms are really SDSL providers. Because of the reliability and availability of DSL, it is really more cost effective for a company to put in a T1 or partial T1 line than to put in a DSL line. The costs of T1 on a monthly basis are more expensive than DSL. You get into the range of the speeds of a full T1 (1.54 MB/second) or even half that (756MB/second) which is the equivalent of your committed information rate on a frame relay T1. You are in the same ballpark as you are with a 1.544 DSL line, it is going to run better and more reliably, and you can put it in just about anywhere in the county. Again, with DSL, you have to be 3 miles from the last point. With frame relay or full T1, they can put in a repeater every couple of miles and take it 300 to 400 miles if they need to. Most telco’s are tariffed to provide frame relay or direct T1 access anywhere in their service area.

CIR and DSL

Frame relay (a T1, higher reliability based connection) also works by sharing your line with other people in your area, but not as much as DSL shares it. T1 based connections are considered more reliable and have better access than DSL lines. T1 information is included here for comparison purposes.

CIR is Committed Information Rate. CIR on a dedicated T1 is the speed of the T1. CIR on a Frame Relay T1 is 1/2 the speed of the T1. For example, if you get 1/2 a T1 on a Frame Relay circuit (768kb/second of throughput), your CIR on the 768 line is 384kb/second. That means at no time will you fall below 384kb/second of simultaneous transmission and receive speed. If nobody else is on the line and if you have large packets and large volume that you are trying to send, you can get up to the full 768. This is called your burst speed. There are times when you will get that fast, but your CIR is 1/2 that.

On DSL (or cable), your CIR is 0, which means if there are a lot of people sharing that same circuit that you are on, you may not get any throughput at all. This is really inconvenient if you are trying to run a web server or high volume e-mail. Nobody can get to you and you can’t send anything out. That is another reason why DSL does not live up to the promise of inexpensive high-speed connectivity. It is inexpensive, but it is not reliable for critical production items. It is not something that an ISP can safely use, and not something that businesses dependent on getting information to their web server or who have a web server in-house can use. For e-mail and for most web browsing you are not going to have an issue with DSL. If it goes down for awhile, you can wait 10 minutes and get back on. On the other hand, if your web server goes down for 10 minutes, that could be a real problem and more reliable broadband solutions are necessary.

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