Monday, April 18, 2005

How DOCSIS Mixes Broadband With Cable TV

Cable modem service is hard to beat for residential and home office Internet users. It's reasonably priced at $45 a month or so. That's about what you'd pay for DSL and far less than the least expensive T1 lines. Of course, you can't run a server or provide Internet service beyond members of your household. But for most of us who just want to enjoy always-on Internet at broadband speeds up to 3 Mbps for personal use, it's a good price point. It can be even cheaper than dial-up when you want to have 2 or 3 computers online at the same time. When you factor in the cost of two dial-up services plus an extra phone line, cable Internet looks pretty reasonable. So, how does it work and what performance can you expect?

Like WiFi, broadband over cable is standardized. In this case, the organization is known as CableLabs or Cable Television Laboratories, Inc.. It was founded in 1988 as a nonprofit R&D consortium for the cable television industry. The major contribution of CableLabs is the DOCSIS specification. DOCSIS stands for Data Over Cable Service Interface Specifications. Your cable system will use one of the DOCSIS specifications and your cable modem will be certified to that level.

The original DOCSIS 1.0 specification became available in 1995. It's since been updated to DOCSIS 1.1 to include additional security and QoS Quality of Service features to support VoIP and interactive gaming. DOCSIS 2.0, the current specification, increased the upstream bandwidth. That's the bandwidth from your computer to the Internet. It is generally much less than the downstream bandwidth in both cable and DSL broadband services. The reason is that the pages and software you download are much larger than the keystrokes, email and occasional file transfers that you upload.

That asymmetrical bandwidth works well for mixing data with TV signals on cable systems. The upload channels are placed in the otherwise unused portion of the cable spectrum between 5 MHz and 42 MHz. Channel 2, the lowest broadcast frequency, uses 54 to 60 MHz. Upload bandwidth was originally 500 Kbps to 10 Mbps. DOCSIS 2.0 expanded that to 30 Mbps by using a more efficient modulation scheme and wider channels.

Now don't go getting too excited about the 30 Mbps. You don't get to use it all yourself. In fact, most cable companies limit upstream bandwidth to 128 or 256 Kbps. That's because that limited upstream bandwidth has to be shared with all the users on the system.

The same is true of the downstream bandwidth. Data is sent from the Internet through the cable system to your computer by making it look like any other TV channel. The cable system allocates one or more 6 MHz channels in the TV bands for broadband use. Each channel is capable of up to 36 Mbps of bandwidth. Once again, that bandwidth must be shared between all users. The data is framed using MPEG-2, just like satellite or digital cable television. A special program identifier or PID tells the cable modems that the signal on a particular channel is data not television. Of the 36 Mbps available, you'll likely get 600 Kbps to 3 Mbps at any given time.

At the cable head end, the CMTS or Cable Modem Termination System is responsible for slicing and dicing the Internet backbone connection among the cable modems on the system. There might be as many as a hundred to a thousand other broadband users sharing the backbone connection. A DS3 backbone can provide 45 Mbps, which is adequate for a single channel cable modem system. For larger systems, an OC3 connection running at 155 Mbps can handle several channels.

Cable modem technology is still evolving. CableLabs is working on the next upgrade, DOCSIS 3.0, to keep up with impending competition from fiber to the home and WiMax. DOCSIS 3.0 is likely to include channel bonding that would combine channels to increase bandwidth per customer from 10 Mbps to perhaps as high as 100 Mbps.

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