Why the difference in speed with my cable modem?
You have encountered one of the fundamental problems with cable modems. Each cable modem is part of a loop that begins at the cable company's central office, goes through a certain neighborhood or group of neighborhoods, and comes back to the central office.
When a cable company offers Internet access over the cable, Internet information can use the same cables because the cable modem system puts information on separate channels:
  • Downstream data -- data sent from the Internet to an individual computer -- into a 6 MHz channel. On the cable, the data looks just like a TVchannel. So Internet downstream data takes up the same amount of cable space as any single channel of programming.
  • Upstream data -- information sent from an individual back to the Internet -- requires even less of the cable's bandwidth, just 2 MHz, since the assumption is that most people download far more information than they upload.
Putting both upstream and downstream data on the cable television system requires two types of equipment: a cable modem on the customer end and aCable-Modem Termination System (CMTS) at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place.
A CMTS enables as many as 1,000 users to connect to the Internet through a single 6 MHz channel. Since a single channel is capable of 30 to 40 megabits per second of total throughput, this means that users may see far better performance than is available with standard dial-up modems. The single channel aspect, though, can also lead to the performance variance some users experience with cable modems.
If you are one of the first users to connect to the Internet through a particular cable channel, then you may have nearly the entire bandwidth of the channel available for your use. As new users, especially heavy-access users, are connected to the channel, you will have to share that bandwidth, and may see your performance degrade as a result. It is possible that, in times of heavy usage with many connected users, performance will be far below the theoretical maximums.
As more people in your neighborhood subscribe to the cable modem, the amount of bandwidth available per user decreases. This means that if you and a lot of your neighbors all get online at the same time, then you will notice a significant performance hit. The good news is that this particular performance issue can be resolved by the cable company adding a new channel and splitting the base of users. Most cable companies monitor performance regularly and add another channel when the bandwidth per user hits a certain point.
As a user, you can observe when the speed of your connection is sluggish and try to avoid that particular time of day. Often, just by changing the time you are online, you can gain substantial improvement in speed. For example, my cable modem connection averages about 500 Kbps between 7 p.m. and 10 p.m. eastern time, but is more than 2.4 Mbps from 5 a.m. to 12 p.m.!