What is the difference between DSL and T1 (besides the price?)

From Bandipedia

It’s obvious by the vast difference in price between DSL and T1 that they are not the same. From the outside looking in, however, it is hard to define exactly what sets them apart. This article attempts to make sense of the major difference between T1 and DSL.

A T1, weighing in at over $500 per month, has a throughput of one and a half megabits per second (1.5 Mbps). DSL, with an average price point of less than $100 per month, also claims 1.5 Mbps speeds. If they are both the same speed, why aren’t they the same price?

There are several factors creating a premium price for the T1 over DSL, but the most significant factor is the quality of the portal through which each connects to the internet. In the case of a T1, a local access line connects the premise of installation to that of the Internet service provider being used. The local access line on their end will interface with a dedicated portal (also called a port) which is measured at 1.5 Mbps. This creates the most efficient and true delivery of 1.5 Mbps speed.

This method of delivery contrasts greatly with that of a DSL line, most notably because with DSL, the access line doesn’t connect to a 1.5 Mbps port like the T1 does. Instead, it connects to a DSLAM. This creates a bottle-neck effect in which the subscribers compete for the throughput available on the port.

Thus, the most common difference between a 1.5 Mbps DSL and a 1.5 Mbps T1 is that a T1 delivers consistent, uninhibited throughput via a dedicated 1.5 Mbps port. DSL may give you a speed of 1.5 Mbps over the access loop, but actual throughput onto the Internet backbone will vary based on how much traffic there is on the DSLAM. As general rule of thumb, the cheaper the price of the DSL, the more traffic will probably be experienced on the DSLAM. DSL providers drive cost down by putting more and more subscribers on a single DSLAM connecting to a single port. This is called oversubscription and it is a widespread practice among low-cost ISPs boasting cheap and fast DSL.

In addition to the price difference and type of delivery, a T-1 comes with a stronger commitment to deliver service. Almost all carriers provide a SLA (service level agreement) that guarantees uptime of your T-1. The SLA generally outlines a credit structure for any time that your T-1 is down. While some carriers provide an SLA for DSL as well, it is often not as assuring as a T-1 SLA. For example, if you were a customer with "Carrier A" and both your DSL and T-1 service were down, you would most likely get a higher level and faster response from tech support for your T-1.

Finally, keep in mind that DSL is a distance-sensitive service. There's a saying, "not as the crow flies, but how the copper lies," which conveys that the distance for DSL is measured in cable length to a carrier's Central Office. There are different limitations, depending on the ADSL and SDSL, but in general it is 15,000-18000 feet with some variation. T-1, on the other hand is readily available in almost all areas, business or residential where there is a telco or phone box. In the small percentage of remote areas of the country, it may be increasingly difficult to deliver a T-1 without incurring a cost for the buildout of the proper facilities. But in general, T-1 is available whether it is 10, 20, 30, 60 plus miles, whereas you will have little luck beyond 3 miles with DSL.

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