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tt0180

join:2010-12-16
reply to gar187er

Re: Zoom 5341G (4x4) vs Zoom 5341J (8x4)

I know that the LED's on the front of most D3 modems do change when a bonded connection has occurred but in this instance, I was only comparing it to the older 5341G which does not. On the 5341G, it shows a solid green regardless and the only way to know is through the web interface.

As far as the SNR is concerned... What is the REAL SNR? I mean, you can take a precisely calibrated piece of lab equipment and measure it but that will not mean anything to the actual device using the signal.

As far as the modem is concerned, all that matters to it is the SNR that it is able to perceive. It is essentially demodulating a signal by accurately measuring a voltage above a noise floor and the greater the difference, more success it will have.

The 5341G, from what I have seen so far, racks up more correctable/uncorrectable words than the 5341J. From an initial perspective, it looks as if the 5341J is able to measure these voltages from the noise floor more accurately.

As far as the 8 other modems are concerned, I probably should have been a bit clearer.

Yes, nodes do have 500+ devices on them but all 500+ aren't Tx/Rx at the same time.

I am not that versed when it comes to the cable side of things but I do know quite a bit when it comes to basic commo theory. I believe that when the CMTS has, lets say, a 10 mb file to send to device number 231, the CMTS looks at all the other devices communicating on the channels between it and device 231 and assigns it a time slot appropriately.

If I remember correctly, the bottom line is that ONE 256 QAM DS channel is only capable of 40ish Megabits per second... Period.

If all 500 devices were each capable of 40 Mbits at the same time, you're looking at 20,000 Mbits on one channel.

Yes, the time is shared.

If 8 of your neighbors connected to the same 4 channels as yourself, were actively maxing out their DS connection at the same time as yourself, you would have an issue.

You would have 4 channels with 40 Mbits a piece. With 9 people actively transmitting, you would each be assigned 1/9th of the available time.

At least, that is my understanding of it.

Tony


gar187er
I do this for a living

join:2006-06-24
Dover, DE
kudos:4
real snr/MER/BER? you mean measuresd by a calibrated SLM? yea thats what it is.....your moed is calibrated to read it....its too busy doing other things.....hell modems can show bad snr yet the channel is perfectly fine.....

whats perceived should matter at all....what the actual modulation error ratio should....

also measuring the rf from the noise floor has NOTHING to do with codewords....codewords are the actual data in the modulated CW...doesnt matter is your SNR is 32, or 39.....yes under 32 you have issues, but your codewords SHOULD not change inside of the range under the same conditions....if they do, something else is goin on....

and yes thats a good overview of TDMA...and waiting 1/10th of a second isnt that much of a deviation.....its just inherent in the docsis system....people should knwo if they want steady ping/low jitter, go to dsl
--
I'm better than you!

tt0180

join:2010-12-16
Well, I suppose what I am trying to say is this.

Lets say that I have horrible hearing and that my friend Jake has immaculate hearing. (You could even say that I have average hearing and that my friend has superhuman hearing. The point that I am trying to make is that not all Receivers perceive the world in the same way.)

Anyhow, lets move forward with my hearing is bad (I have this constant ringing at times) but I have no problem making out a conversation in a room with a few other people talking.

When it turns into a party however, I cannot make out a thing.

So, you have the real noise floor and then you have the perceived noise floor which is created by the less than perfect receiver.

When it comes to QAM, the modem needs to accurately measure voltages (amplitude) at very specific points in the carrier wave.

Yes, for the most part, once you are above a certain threshold you are good. The precision of the channel tuner will be one of the main factors when it comes to SNR.

Having a higher SNR will help you out when the noise floor jumps due to random outside interference.

Again, however, I do not have a lot of experience with the commercial cable end of things. All of my experience is in SATCOM and general commo.