20 Meg Cable Modems Practical Since 1999. (Score 5, Informative)

19 megabits on an unloaded cable modem node is quite practical, especially for brief speed tests.The U.S. cable modem architecture is based around nodes that share 35 megabits or so downstream. RCN even sells a 20 megabit service, which David Reed buys and reports gives him the promised 20 megabits most of the time. In practice, most nodes run with 5-10 megabits typical load, as only a fraction of the time multiple users on one node simultaneously download megabits and fill the pipe. Statistical multiplexing (sharing) works much better than people expect on cable modem networks, especially on well managed nets that split nodes and otherwise expand capacity when needed because a node gets frequently congested.

Comcast and cable suppliers are working on several techniques to allow customers to get more than the 6 or 8 meg typically allocated, while not causing undue congestion. "Speed burst" technology tests the network load, and if uncongested temporarily raises the speed of an individual modem making a fast download for a brief period. While that's marketed as "doubling" speeds to 12 and 16 megabits, bursts to 20 and 25 megabits are also practical.

The new technologies require upgraded equipment and are typically being tested first and then rolled out market by market. So it would be no surprise if a subscriber in Philadelphia (Comcast's home town) is benefiting from a test or early deployment of faster speeds than Comcast customers elsewhere.

100 megabit+ (shared) cable modems are being deployed in Japan, Quebec, and France, bonding 3 or 4 35 megabit channels for higher speeds. These are early "DOCSIS 3.0" products, unlikely to be widely deployed in the U.S. until 2009. Comcast's CEO, Brian Roberts, demonstrated 100 megabits at the cable show in Las Vegas this spring, and will probably test widely in 2008 and go into deployment (especially where Verizon is building FIOS) the following year. DOCSIS 3.0 requires a new cable modem unit, however, so this customer is unlikely to be an early tester.

That doesn't explain why the test to San Francisco only ran at 4 megabits, which could be explained by node congestion a few minutes later, inferior Comcast backbone connections to Speakeasy's host in San Francisco, or other circumstances. For more details on coming faster cable modems, google DOCSIS 3.0.

Dave Burstein

Editor, DSL Prime