Cable Broadband

Cable Broadband uses the same basic principal as DSL in that the bandwidth needed to accomplish the primary function is only a fraction of the available bandwidth on the wire, or in this case, cable. Cable access is slightly different, however, in the way it divvies up the available frequencies.

The cable spectrum was already divided into several hundred 6 MHz blocks to account for the various cable channels. Your cable-ready TV simply tunes its receiver to the frequency that corresponds to the channel you choose. For Internet capabilities, each user is assigned one or more blocks for downstream data. (Each 6 MHz block is good for about 30 Mbps of data.) For the upstream piece, the lower end of the spectrum is divided into 2 MHz blocks because most people pull down more information than they upload. Each subscriber is assigned one or more 2 MHz blocks.

Cable Equipment
Cable requires that each home user have a cable modem because cable-ready televisions provide their own filtering. Interference between data and TV signals is not an issue.

The cable company must have a cable modem termination system (CMTS) to aggregate upstream and downstream.

Limitations and Advantages
A CMTS has a fixed number of slots (1000) meaning that there is a limit to the number of subscribers who can access the service. With relatively few subscribers, each user enjoys greater throughput than when the number of subscribers reaches its maximum and each user is assigned a single pair of "channels." Cable is not distance sensitive, however, and the signal can be amplified if needed.