Join 3,551 readers in helping fund MetaFilter (Hide)


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?

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!
posted by kavasa to Computers & Internet (5 answers total)
 
And it's not that too much signal creates noise, either
IANAEE, but I spent 5 years, off and on, as a (CATV) Splicer.

Too much signal creates what I always heard described as "crossmod". (mention of crossmod and harmonics here)

Crossmod was always easily demonstrated by looking at a television - if the incoming signal is too hot, the channels will bleed into each other, with lines and noise that, once you'[ve seen it once, are forever instantly recognizeable as crossmod. (As opposed to too little signal, which leads to snowy pictures (or pixelated, choppy pictures when digital)

IIRC, during wreckout/rebuild we used to aim for 12/10 as measured by our Waveteks at the house block (on 72 and 3 respectively - analog channels). No clue what the sweeps tuned things to, but I'm guessing something close.
posted by namewithoutwords at 8:28 AM on January 13, 2010


also keep in mind that you cannot forever reamplify a signal - every time you amplify the signal, you also amplify the noise (which is where SNR comes in.) Thus, I would expect the minimum transmit rating to be the minimum level at which, when tied to an acceptable SNR, the message can be amp'd/retransmitted successfully over all the actives back to the CO. If the initial transmission is too weak, and SNR is high (that is, there is a lot of noise) then by the time it makes it past the LE, through the Amp, over the BTD, out of the Node, and back to grandmother's house the CO, it has been amp'd/degraded into unintelligable garble.
posted by namewithoutwords at 8:35 AM on January 13, 2010


Right. Too much power means some amplifier up the line is turned up too high. Signals bleed through the line, or even bleed out of the line which the FCC doesn't like, and probably overloads the receiver circuit in the cable modem. Certainly, the receiver has a limit as to how much power it can auto adjust for, and probably gets noisy when it approaches that limit.

Also, I believe the upstream transmit from the cable modem is a time slice divided protocol. Say, 10 modems share the upstream channel. If 9 of them are at normal levels and one is too "loud", the receiver up the stream probably has trouble adjusting itself constantly and probably has to drop down to a lower rate in order to maintain clean data.

Too little upstream transmit power may have a similar issue besides that namewithoutwords said. The tiny transmitter in the cable modem probably gets noisy at too low transmit levels. A too low transmit level probably means something is misconfigured up the line- the Widget that receives the upstream signal might be sending out bad information back to the cable modem as to what power to transmit at.

(Background, for the uninitiated: the cable tv wire that comes to the house is analog. For digital TV, what they did was take those analog channels and instead of transmitting television pictures, they transmit a digital stream that the receiver can reconvert into pictures. Same idea as taking a voice phone line and using a modem to transmit digital data.

The cable modem does the same thing, but in two directions. For downstream data, it uses the same structure as the digital video. The ISP sends a shitload of packets down one or more of those channels, and your cable modem just passively listens to them all and passes the ones destined for you into your computer.

For upstream, it is harder. The cable modem fires up, sends out a "here I am!" message and the Widget at the other end of the line sends back a "recieving you loud and clear" message. Or it sends a "too loud!!" message. Or it sends nothing. The cable modem turns up its upstream power level and tries again until it gets a response. Once they get all that squared away, the Widget tells the cable modem "hey, when you need to send something, use this frequency and this time-slice." When that modem has something to send, it transmits a signal on the wire going back up the line. The power that it transmits at has to be strong enough to get back to the widget, but low enough to not cause interference on all the downstream stuff.)
posted by gjc at 9:17 AM on January 13, 2010


Not an engineer, but I did headend work at a cable company for a few years.

Too much downstream will overdrive the receiver, like overdriving speakers with audio. The reason your high downstream installs are also high SNR is probably because your downstream is within the "tolerance" they include when they give you the max downstream - if your max is, say, 50 dBmV, the modem will probably handle up to, say, 80. (Sorry, I don't remember specific numbers anymore.) They don't want to add an amp somewhere one day and have a bunch of customers drop off, so they leave some headroom in there. Also bear in mind that they probably have a couple dozen different modems and set-tops that they support, and they want your install to be able to work with a set-top that might be more sensitive to a hot downstream, even if you aren't installing one at the time.

The upstream minimum is more about the SNR at the headend, which you can't measure - as namewithoutwords describes, if your modem is putting out a low signal, it can get lost in the noise on the way back to the headend. Cable modems are unintuitive in that adding an attenuator can improve the signal - you attenuate both the signal and the noise by, say, 6 dB, and the modem increases its transmit power by 6 dB, so you've dropped the noise but maintained the signal.
posted by pocams at 9:23 AM on January 13, 2010


Thanks everyone! These were some really great answers posted very quickly. :D
posted by kavasa at 11:44 AM on January 13, 2010


« Older I have an invitation to visit ...   |  Is there a way to seamlessly a... Newer »
This thread is closed to new comments.