LOW levels on upstream indicates the opposite of what folks here are saying.
Yes, the level from the modem is controlled by the far-end equipment. There is an ideal point where modems would work best, that level is +45dBm.
The return path (upstream) has many variables not the least of which is cable loss and splitter losses. Since the return must include signals coming from ALL modems in a given area(node), and they're all transmitting in-turn on the same frequency(ies), the levels arriving at the far end must be controlled. The levels all need to arrive both in-sequence AND at very similar levels so that they all can be understood without having to resort to instantaneous AGC or other impractical wackiness.
Modems are only able to generate signals within a certain range, due to fundamental RF electronics reasons. In order to be certified as whatever DOCSIS compliant, they need to be able to output within a certain range. This range ends up being anywhere from +38 to about +54, which can also depend on how many upstream channels and which modulation are used.
If a modem is connected to the coax network with a very low customer-premesis loss(single device), and happens to be at the far end of a very long outdoor system(long cable and low value splitter), you can end up with a scenario that the downstream signal is kind of strong, while the upstream level is beyond minimum(less than +38). The outside system is configured for consistent downstream signal levels, but they don't take upstream levels into account, so since cable loss is a function of frequency(higher frequency = much higher loss), and upstream frequencies are very low compared to downstream signals, you end up with very low loss in the upstream, and controlled loss in the downstream. the result is that your upstream signal cannot be understood by the far end because it's too loud, so packets are lost.
in this relatively rare scenario, the service person has three options:
1: install an attenuator to get the downstream signal as weak as possible while still being good enough for no errors(about -10db), while hopefully introducing enough loss to get the modem transmit levels out of the dump and into the +38 or better.
2:install an "equalizer" that attenuates low frequencies more than it does higher frequencies, when option#1 fails to achieve the goal. The equalizers might be unobtainium depending on the company.
3: ask the network design/maintenance team to alter the network and reduce the amount of amplification of the upstream side of the inline amplifiers in order to make it possible to get this equipment working correctly, and without causing customers closer to the active equipment from having the opposite problem.
In my days at a tech, I encountered a LOT of resistance to option #3, as it was always slow, and usually they disagreed with my assessment... as we're delving into territory that many techs in those systems fail to completely understand. I usually had problems with high transmit levels in high--rises because of lack of amplification in the upstream, as the designers failed to anticipate that there would never be any low-value taps on that leg of the system, and all customers in a highrise would usually be fed by -26 to -29dB of taps before ever getting to the customers, where normal outdoor suburb taps values are -23 coming right out of the amp.
I hope this helps to clear up some of the non-intuitive-ness that comes with such high tech...