What's behind Comcast's signal specs?
January 13, 2010 8:02 AM Subscribe
Electrical engineers and others with a pretty in-depth understanding of communications tech: why does Comcast have a specified minimum for transmission signal strength from cable modems on its network? Or a maximum for reception signal strength?
posted by kavasa to computers & internet (5 answers total)
I'm a new-ish Comcast tech, and one of the things I do on an internet/voip installation is make sure that the equipment's signal strength is in spec. There are three required categories:
-Upstream power in dBmV
-Downstream power in dBmV
-Signal to Noise Ratio
SNR has a minimum but no maximum, which makes sense to me. Why does downstream have a max, though? It has nothing to do with speed, the system checks your account for the speed you're paying for and hands the modem a bootfile that throttles accordingly. And it's not that too much signal creates noise, either - often the modems with too much downstream also have a very high SNR. Similarly, why is there a minimum for transmit power? I understand the maximum: if the modem is right at its transmit power limit, it's going to intermittently lose the ability to reach all the way back up the line and drop connection. But if it's at a low transmit power and line conditions change, it can always just add a little more juice.
Anyway, thanks in advance!