Currently, DOCSIS 3 uses 5-42 MHz for the upstream.

For example, that means my cable modem is transmitting a signal down the copper at 5 MHz.

The thing about coaxial networks is there's a single copper conductor carrying everything.

So what about my neighbors. They have to transmit on 5 MHz too.

What if me and all my neighbors are uploading something at the same time. How does DOCSIS work without interference?

The reason I post this question only regarding upstream and not downstream is because I can see how downstream would work. The cable headend can send everyone's data to everyone on the node. And only the right person can decrypt it. Everyone else discards what they cant decrypt.

The upstream is different because everyone's modem would act like a signal jammer to everyone else, wouldn't it? Or does everyone on the same node use a different frequency. If that's true, then that makes me wonder what exactly nodes do.

  • 5-85MHz actually. Read the specs if you really want to know how it works: cablelabs.com/specification/…
    – Ricky
    Commented Jan 9, 2015 at 1:58
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 11, 2017 at 3:18

1 Answer 1


6.2 Upstream

6.2.1 Overview

The upstream Physical Media Dependent (PMD) sublayer uses an FDMA/TDMA (herein called TDMA mode) or FDMA/TDMA/S-CDMA (herein called S-CDMA mode) burst type format, which provides six modulation rates and multiple modulation formats. The use of TDMA or S-CDMA mode is configured by the CMTS via MAC messaging.

FDMA (frequency division multiple access) indicates that multiple RF channels are assigned in the upstream band. A CM transmits on one or more RF channels and may be reconfigured to change channels. ...

TDMA (time division multiple access) indicates that upstream transmissions have a burst nature. A given RF channel is shared by multiple CMs via the dynamic assignment of time slots. S-CDMA (synchronous code division multiple access) indicates that multiple CMs can transmit simultaneously on the same RF channel and during the same TDMA time slot, while being separated by different orthogonal codes.

CableLabs spec (CM-SP-PHYv3.0-I11-130808)

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