Cable Modems

Cable television networks, outside of the public switched telephone network, are likely the most common networks used by home users. This marketing aspect combined with the infrastructure to create broadband cable networks is an excellent combination for data networking applications.

The cable modem is the hardware used to connect to the Internet through the cable provider’s network. Traditional cable networks were uni-directional by nature, and data networking is bi-directional; however, cable providers have long needed the capability to transmit upstream information to provide additional services based upon conventional cable technology, such as pay-per-view. The bandwidth and technology designed to provide this upstream channel is what allows cable networks to provide an upstream data path. (The fact that the technology to pump data upstream is not always there in every cable system should not be considered trivial—many cable networks have required upgrades to provide cable network services.) Figure 12.5 shows a cable modem-based network.

The cable modem has a LAN connector that is typically an RJ-45 10baseT connector and a “WAN” connector that is connected with a local splitter that feeds the cable modem and the cable–ready television or the set–top box for cable television. The splitter is responsible for feeding a 6Mhz downstream and 2Mhz upstream signal between the cable modem and the cable modem termination system (CMTS).

The cable 6Mhz downstream channel can provide up to 56Mbps. It should be noted that although there is a shared medium to the CMTS, the cable modems do not communicate with each other—traffic must first go through the CMTS. Also note that the CMTS does not provide IP routing capability.The 2Mhz upstream provides up to 3Mbps bandwidth.

Although cable modems use a shared bus, the bandwidth available, consumer acceptance of cable television, and (lack of) availability of other technologies such as DSL will ensure that cable modems are here to stay.