I have a friend who recently got a cable broadband connection. The internet man told him, if you get slower than X mbps speed, just call me, and we will increase the speed.

How does that work really? Is it like a water supply where you increase the amount of water and voila problem solved? And why does the speed decrease below specified amount anyway?


just call me, and we will increase the speed.

In the world of cable modems, a single channel carries ~39Mbps down and ~10Mbps up. That is the physical signaling rate -- much like ethernet commonly supports 10, 100, and 1000. The DOCSIS configuration sent to the modem during startup tells it, among other things, the user rate for both up an down. The modem can only really police upstream, the network handles downstream. As such, the quickest, simplest, and most common method for "increas[ing] the speed" is to change the numbers in the config file.

The other, less commonly used, path is to move the modem to different upstream and/or downstream channels. This is usually an automatic traffic engineering process on the CMTS ("headend") to balance load across channels (i.e. minimize congestion), but it is still a configurable item. Changing channels can also move you to a less noisy channel; cable plants are dynamic things and some channels will have more/less noise (or signal) than others.

** DOCSIS 3.0 introduces channel bonding. As such, the modem uses all the available channels at once -- there's no channel to move to/from as it will already attempt to use all of them. That said, may US providers have not started bonding upstream, yet, so that can still be manipulated.

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  • when they increase rate, does it increase equally for all users? or can it be targeted at specific user? – user13107 Jun 28 '14 at 23:20
  • It can be targeted to a single modem, but almost no one does that. ISPs have multiple tiers -- standard, shared configs -- and it's simple to move to the next higher tier. – Ricky Beam Jun 29 '14 at 3:20

The speed is determined by the raw data rate, the number of connections and the number of errors. If you have few errors, the data speed is close to the speed of the equipment. If you have many errors, then the data speed is significantly slower. For most household connections, the raw data speed (that is paid for) is slower than what the equipment can handle. So if your friend happens to have a lot of neighbors on at the same time he is (since cable shares the line with neighborhood) and he has a noisy connection, they would increase the data rate so he gets what he pays for.

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    they would increase the data rate so he gets what he pays for. yes, how does that work? Is there a switch which increases data rate? – user13107 Jun 28 '14 at 3:41
  • @user13107 Yes, there is an electronic switch (more like a register) that holds the data transfer rate, which the hardware uses to determine the frequency. – LDC3 Jun 28 '14 at 3:53
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    Two ways: 1) by changing the profile rate in the config sent to the modem, 2) move the modem to a different channel. (DOCSIS 3 channel bonding makes option 2 moot.) – Ricky Beam Jun 28 '14 at 6:04
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    @RickyBeam Please post that (with a little more explanation) as an answer. – slhck Jun 28 '14 at 7:25

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