What is a Cable Modem?

Cable modems are very popular in VoIP settings as many Cable providers supply a VoIP service of ready-made connections to one. VoIP is so popular with cable because of the high bandwidth available and ease with which a user can get a service configured and started. Most cable modems have connectors that a phone can simple plug into others may require and adapter but more importantly they all can handle quality of service.

Quality of service (QOS) is used to tag the voice (VoIP) packets as they traverse the cable network ensuring that they are given priority in times of network congestion. Most Cable operators offer their own VoIP service for this reason.

A cable modem can come in several forms but the most common now are fully featured supplying bridged Ethernet ports to connect a home network, a router, with DHCP for auto IP addressing and firewalls and Network address translation. In order to separate the home configurable items from the cable operator’s demarcation there is usually a virtual Ethernet port separating the two maintenance domains.

One problem that cable modems can have is when the connection to the local area is a bridged LAN configuration then everyone on that local loop shares the same wire and therefore the same bandwidth. VLans can mitigate any privacy security concern but not upstream bandwidth contention issues. Cable suppliers therefore use contention and sharing algorithms to try to smooth out the mechanism issues when attempting to send traffic upstream. The basic problem is several cable modems share the same single upstream channel, therefore the principle is to split the channel into many mini-channels and assign each modem a time slot on which to transmit. The modem learns its time slot allocation from a broadcast MAP which tells it all it needs to know to be able to synchronize its upstream communication streams.

Alasdair Gilchrist

Alasdair is a technical writer with interest in business practice, operational strategy, start up philosophy and affordable technology. He lives in Nonthaburi, Thailand with his wife and daughter, and writes terrible novels as a hobby.

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