A cable modem is a type of Network Bridge and modem that provides bi-directional data communication via radio frequency channels on a hybrid fibred-coaxial (HFC) and radio frequency over glass (RFOG) infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high bandwidth of a HFC and RFOG network. They are commonly deployed in Australia, Europe, Asia and the Americas.

Hybrid networks

Hybrid Networks developed, demonstrated and patented the first high-speed, asymmetrical cable modem system in 1990. A key Hybrid Networks insight was that in the nascent days of the Internet, data downloading constitutes the majority of the data traffic, and this can be served adequately with a highly asymmetrical data network (i.e. a large downstream data pipe and many small upstream data pipes). This allowed CATV operators to offer high speed data services immediately without first requiring an expensive system upgrade. Also key was that it saw that the upstream and downstream communications could be on the same or different communications media using different protocols working in each direction to establish a closed loop communications system. The speeds and protocols used in each direction would be very different. The earliest systems used the Public switched telephone network (PSTN) for the return path since very few cable systems were bi-directional. Later systems used CATV for the upstream as well as the downstream path. Hybrid’s system architecture is used for most cable modem systems today.

cable modem flap

Cable modem can have a problem known in industry jargon as “flap” or “flapping” A modem flap is when the connection by the modem to the head-end has been dropped (gone offline) and then comes back online. The time offline or rate of flap is not typically recorded, only the incidence. While this is a common occurrence and usually unnoticed, if a modem’s flap is extremely high, these disconnects can cause service to be disrupted. If there are usability problems due to flap the typical cause is a defective modem or very high amounts of traffic on the service provider’s network (upstream utilization too high). Types of flap include reinsertions, hits and misses, and power adjustments.

What are cable modem hannels?

Cable modems will advertise that they support 8 downstream and 4 upstream channels.  A channel in this context is almost synonymous with channels you use to watch television shows, but instead of being used to broadcast TV, they are used to transmit data. Each channel uses a different frequency (just like normal television channels) and therefore, more channels means more data can be sent or received. Bottom Line: More channels means, more data can be sent or received, but only if your ISP supports the latest DOCSIS standard and also supports that many channels (read below on “Channel Bonding” to know why). For example, if you have a cable modem that supports 8 downstream channels, but your ISP only uses 4 channels for downstream, then you’ll only get the 4. You’ll notice that this technology is backwards compatible.

Traditionally analog television transmits each channel in 6 MHz frequency intervals between the 85MHz and 850 MHz range, as illustrated below.

What’s left over is unused bandwidth in the 5-85 MHz and 850-1000 MHz range. In DOCSIS 3.0 and earlier, this unused bandwidth is reclaimed to create additional 6 MHz channels and they are used as downstream/upstream channels for transmitting internet data. Understand that this analogy has been greatly oversimplified in order help someone who is unfamiliar with these concepts. In reality, the upstream frequencies are different than the downstream frequencies and as of DOCSIS 3.1, the 6Mhz frequency band is no longer used and is replaced by 20kHz to 50,kHz wide channels that are bonded (channel bonding is described below).  Having that said, the concept is the same.


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