I needed to know the answer to this question for an upcoming Marketwatch column. So I asked superstar engineer Milo Medin what he thought since he was the chief architect for the @Home cable modem rollout and a cable modem pioneer. The barebones answer is about 100 Mbps down. But the real answer is more complicated and interesting enough to post on the blog.

Question: What is the fastest practical Internet access speed that can be delivered over the cable Tv/Cable modem network?

Answer: Well, doing something straight forward would be taking your best plant, 860 Mhz, which has 135 6 mhz channels in the forward direction and digitizing all of it with a 256 QAM modulation scheme. That would yield 135*42 Mbps = 5.670 Gbps if you digitized all the forward on a modern cable plant.

Now, that is a shared network, but you can reduce the sharing quite a bit without getting too expensive by using DWDM wavelengths over fiber. A couple test markets in the US went down to 50 homes passed per optical node so they could eliminate line amps (all the homes were driven by the fiber node). If you then took that entire digitized payload and fed 50 homes by that, each home could be running more than 100 Mbps downstream continuously. So there is a ton of bandwidth available there.

Now, there are some issues:

1) You aren’t going to be able to remove all analog from the plant. Basic cable will almost always be analog, and while that’s inefficient, it’s not likely to change. Still, that’s maybe 15 channels, so there is still a good chunk left.

2) The upstream will be a problem. You have to have enough upstream data capacity to at least service TCP acks for the downstream data, at least if you are using TCP for transport. Given you only have 5-42 in the reverse, as oppose to 50-860 in the forward, that’s a larger limiter. You can subdivide that down so fewer and fewer users are sharing that, but to the extent that is also down in the forward, it doesn’t give you much leverage. On top of the fact it’s only 37 Mhz, it’s a noisy environment, so only about half that is useful. So call that 18 Mhz. Now, that also cannot be coded at the same rate, because of the noise, so you can get maybe a max of 16 QAM in the reverse. That means about a max of 57 Mbps in the reverse, or about 1/100th that of the downstream. That limits you, as you need probably a minimum of 1/10th of the download speed in the reverse to effectively use the downstream.

3) Cable modems typically don’t tune across a large swath of channels like that. That is, each 6 Mhz channel does about 42 Mbps of data rate in the downstream (256QAM). To be able to go faster than that, an individual model must be able to stripe data across multiple channels, and that means being able to tune multiple channels and decode them. That affects cost of the device.

Milo

Now you know.