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50. Cable Modems

The cable modem technology is a competitive technology to bridge the last mile. Cable television companies are battling head to head with the telephone companies to provide high speed bandwidth to the homes. The telephone companies have the digital equipment backbone starting at the central office but are crippled by the existing local loop cable.

The cable television companies have the high speed bandwidth to the homes but don't have the digital equipment backbones at the head end (the head end is where all the television signals in a cable TV line originate from). Cable modems use the existing cable TV line to provide the high speed bandwidth.

It is an asymmetrical transfer rates with the upstream data transfer rate at 2 Mbps. The downstream data transfer rate is a maximum of 30 Mbps. Most users connect the cable modem to their 10 Mbps ethernet NIC and don't utilize the cable modems full bandwidth.. Switching to a 100 Mbps ethernet NIC would give them full bandwidth.

The actual transfer speed depends on number of users that are on-line. The cable line is shared with the other subscribers in the local neighborhood. Most cable companies use dynamic IP addressing, each time the user connects, the user is assigned a new IP address. For a fee, permanent static IP addresses can be assigned.

Most cable TV companies are placing high performance web proxy servers at the head end. These servers store the most commonly accessed webpages and files locally at the head end. The user's web browser first checks the proxy server to see if the file has been downloaded there. If it hasn't then it goes out on the Internet to download it. The storing of the webpages and files on the local proxy server reduces the load on the communication servers to the Internet and gives the impression of extremely fast Internet access.

Cable Modems Standards

There are three competing standards for cable modems at the time of this writing:

the European standard DVB/DAVIC
the American standard MCNS
the Geneva standard IEEE 802.14.

Only the Geneva standard guarantees the transfer rate.

Cable Modems Premise Equipment

Cable Modems Premise Equipment

The cable modem is connected to the existing cable TV RG59 coax line using a standard RF connector. The output of the cable modem is a 10BaseT or 100BaseT ethernet connection to your NIC.

Cable Modems Security Issues

Cable modems have some security issues. Users can see others on network neighborhood in Windows. Some systems have each cable modem connection is encrypted.

The assignment of IP addresses is based on the MAC address of the ethernet card. Hackers can access the network if they know another users MAC address.

Cable Modem Advantages

Fast data transfers, up to 30 Mbps if using a 100BaseT NIC
Competitive pricing against competing technologies.
Easy to install - home prewired.

Cable Modem Disadvantages

The available bandwidth depends on the number of users on the local cable TV line segment.
There is an asymmetrical transfer rate. Upstream is slower than downstream.
There can be a bottleneck at the communication server at the head end.

Introduction to Data Communications
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