Voip Blog
Cable? DSL? It's All Shared, Part 2
Friday, May 16, 2008
In my last post, we were discussing the difference between cable Internet and DSL provided by a local exchange carriers. Cable has a couple of places where the bottleneck can occur: in the neighborhood due to the shared bandwidth, and at the headend where the cable signals are turned over the Internet.
DSL is deployed a bit differently. Although people old enough to remember party lines might disagree--and I remember them as a child--but there is a dedicated pair of copper between you and the central office. If you happen to be close enough to a central office with a DSL access multiplexer (DSLAM)--18,000 feet by wire--you're very lucky and might be able to get the best speeds DSL has to offer.
However, with suburbia continuing to spread, people have gotten farther and farther away from the central offices. Voice is relatively easy to extend past 18,000 feet, DSL requires more specialized equipment than voice. Remote DSLAMs became available, making it possible to deploy DSL deeper and deeper into the neighborhoods.
Up until about two years ago, my previous house was unable to get DSL because of the lack of a DSLAM in my neighborhood. Once that changed, I was able to get DSL--at a pokey 1.5 megabits down and 864 kilobytes up. Even two years ago, cable modems were faster in my neighborhood.
While DSL could do better--and I was close enough to the DSLAM that I could do it--the main problem was that the DSLAM was connected to the outside world by multiple T1 lines versus being fed by a fiber link. This meant that the available bandwidth by all the people on my DSLAM was limited by those T1 lines--how many T1s, I don't know, but each one gives you 1.544mb/s of symmetrical bandwidth. I don't know how many subscribers they had hooked up to that DSLAM, but I'm sure it didn't take many before that DSLAM didn't have enough bandwidth!
So when the DSL folks tell you that your bandwidth isn't shared, remember that depending on how it's deployed in your neighborhood, it could be even worse than the sharing that goes on with cable!
