A recent question from a reader provided an interesting response - the answer: maximum transmit power is 52.2 dBmV for DOCSIS 3.0 cable modems under certain conditions. So why are DOCSIS 3.0 cable modems transmitting 52.2 dBmV?
This will seem surprising if you have spent the past ten years working in a DOCSIS 1.x or 2.0 nerwork. Even if you have been working in a DOCSIS 3.0 environment with an unbonded upstream for sometime, you would still be seeing maximum upstream transmit powers well in excess of +58 dBmV. For DOCSIS 3.0 cablem modems, they are quite happy blasting out a hot 58.2 dBmV in 16-QAM mode.
Without getting into all of the details of how the CMTS makes the power calculations, the CMTS must maintain a relatively constant total output power from a cable modem whether it is transmitting with one channel or four channels. The transmit power will also vary depending upon the cable modem's modulation between QPSK up to 64-QAM. So there are multiple combinations, but the DOCSIS 3.0 RFI specifications has a few tables to simplify these. Here a couple of those as examples. First the single channel table:
So this looks fairly similar to a typical legacy DOCSIS cable modem. Although notice the QPSK Pmax transmit power which is as high as 61 dBmV. That's 3 dB higher than the 58 dBmV of a legacy DOCSIS cable modem. This shouldn't normally be a problem unless your network is configured for QPSK station maintenance and you have a cable modem that is at maximum transmit for its data burst descriptors, which are set to 16-QAM. The 16-QAM will transmit at 58 dBmV and QPSK will transmit at 61 dBmV, which is a potential environment for return path laser clipping. But hey, we only have one channel turned on now. Let's look at the four channel condition:
Now look at the significant decrease in power from the first table to the second table. For a one channel QPSK to a four channel 64-QAM, the delta is as much as 10 dB. This could represent two DOCSIS 3.0 cable modems operating on the same upstream channel, one running in DOCSIS 2.0 mode with QPSK station maintenance and one with four bonded upstreams at 64-QAM.
Now, back to the original title of my post and the question that was posed to me, why did a reader see so many DOCSIS 3.0 cable modems transmitting at 52.2 dBmV? The answer is very simple. They were transmitting at their maximum transmit level. That being in a 16-QAM modulation with four bonded upstream channels. Since 52.2 dBmV is such an uncommon level for most people to see it is understandable that this level would not come to mind as being a maximum transmit level. But with DOCSIS 3.0 we all have to learn that the old rules do not hold true and learn the new rules.