DOCSIS is the acronym for Data Over Cable Service Interface Specification, the standard for cable modems. In order for cable broadband to work on a systems designed for analog television signals, DOCSIS uses one or more standard TV channels on the download side. The upload channels have to do something different than normal TV reception requires. They have to send information in the reverse direction, from customer premises modem back to the cable head end. To avoid a collision with all those signals coming down, the upstream connection uses frequencies below the TV band.
Cable systems have been running DOCSIS version 1.0, 1.1 or 2.0. They are all based on using a single 6 MHz standard TV channel to carry about 40 Mbps in the downlink direction. Now, 40 Mbps sounds like pretty decent bandwidth if you could get it. But you can't. That 40 Mbps has to be shared between you and all your neighbors. Up to 1,000 of them all getting the same feed on the same channel. Across town, other users are sharing a different channel.
This level of oversubscription explains why your connection speed varies so much. What you experience at any moment in time depends on how many other users are online and whether they are casually surfing the web or downloading videos.
Network performance can be improved by increasing the number of channels available to serve cable modems and assigning fewer users per channel. But to really goose up the bandwidth, you need wider channels. Wider channels don't play with the TV system, but you can bond channels together to combine their bandwidths. It's philosophically similar to bonding channels to increase bandwidth on T1 business lines. That's what the DOCSIS 3.0 specification does. It provides for bonding up to 10 channels of 40 Mbps each for a total of around 400 Mbps. There is a possibility that this could be expanded to 1 Gbps or more in the future.
So, there's no fiber involved? This is done all over standard coax cable? Actually, there is fiber involved behind the scenes. Most cable systems are designed around a combination of fiber optic cable distribution to neighborhood nodes and then copper coaxial cable the premises. This is a HFC or Hybrid Fiber/Coax system. From the customer's point of view it doesn't matter. the customer connection is a F connector on a RG-6/U cable.
The network performance, however is very fiber-like. If your computer says you're connecting at 40 to 50 Mbps it's hard to tell without looking if there is a fiber optic cable plugged into the back of that modem or not. Some Comcast cable customers may have DOCSIS 3.0 by the end of this year.
There are now three competing systems designed to increase broadband Internet access speeds for residential users. FTTP or Fiber to the Premises is the system chosen by Verizon. It's a pure fiber optic network from central office to the user. AT&T is taking a different approach of FTTC or Fiber to the Curb. From there a high speed DSL connection carries the signal into the premises over telephone line. Cable operators use the HFC or Hybrid Fiber/Coax with fiber to neighborhood nodes and then conventional TV cable into the premises.
It seems logical that all-fiber networks are going to have the highest ultimate bandwidths eventually. But for now, the hybrid networks of fiber & telephone wiring or fiber & coaxial cable are doing a good job keeping up with the steady advance of premium service bandwidth.
