Voip Blog
Cable? DSL? It's All Shared, Part 1
Thursday, May 15, 2008
Chances are before you got VoIP, you got a broadband connection. And chances are, you had to choose: cable or DSL? Which do you choose?
This commercial from Pacific Bell in the 1990s pokes fun at the fact that cable is a shared line versus DSL, which is not, or so they say:
In some respects, this Pacific Bell commercial is correct: cable is a shared line. However, what they aren't entirely honest on is the fact that DSL is also shared. The sharing just happens at a different point in the network.
Cable TV lines run from house to house to house. Depending on the neighborhood, the lines go in a sort of daisy chain, or it can be a star topology. No matter how the local, physical plant is laid out, it ultimately all comes from or goes to the headend, or the cable modem termination system (CMTS). The CMTS is responsible for routing traffic between the cable modems and the larger Internet.
Each cable modem in a network is given a time slice during which they can speak using a technique called time division multiple access (TDMA), so they do not all try and speak at once. They are all speaking to--and can only be heard by--the CMTS. For packets coming the other way, there's only one speaker--the headend. There is only one intended recipient for a given packet, but the CMTS broadcasts that data to all the cable modems. Only the one it is intended for forwards the packet onward to the subscriber.
There are a couple of different bottlenecks here: the "shared" bandwidth to the CMTS--particularly in larger neighborhoods--and the bandwidth the CMTS has to the larger Internet. Which ones causes cable Internet slowdown? Depends on the area, but given the amount of bandwidth available even in a shared network, I'd be willing to bet the CMTS is having issues.
So is DSL the way to go? We'll talk about that next time.
