Let me preface this with the fact I am not a Docsis Engineer, I just stayed in a Holiday Inn Express last night.
We provide internet services to apartment complexes, with fairly small cable plants (our largest is 150 modems). We continue to have issues at all of our sites, which I blame on plant configuration due to nobody knowing how to configure a Docsis plant. One very common issue we are facing is high levels of noise.
Here are our current settings:
Downstream:
QAM 256
Interleaver depth: 32
frequency: 747 MHz
Upstream:
QPSK - 2560K
Channel 1 freq: 40 MHz
Channel 2 freq: 30 MHz (if used)
The upconverter is configured to mix with the video at around 30 dB, which translates into it hitting the modems a few dB above 0.
These settings are what were in use when I started here. If they are completely out in left field, feel free to say so.
Like I mentioned earlier, we are having issues with noise, especially in the return path. I'm looking at our worst system right now, with a modem transmitting at 6.5 dB, reporting a downstream SNR of 29 dB, downstream Microreflections of 36 dBc and an upstream SNR bouncing between 21 and 26 dB on both channels. Those numbers are actually quite high right now. In the evenings it's not uncommon for the upstream SNR's to drop below 20. We are getting a lot of packet loss leading to slow speeds at these sites.
So I guess what I'm asking is, does this configuration look okay? What specifically should I be looking for as I diagnose the noise issue? When mixing with the video, should the Docsis signal be at the same level, above or below it? How much? Any thing else I should know?
Unfortunately, I am not a good RF guy, but here is my two cents anyways. :D
for Qam256 we like to have an SNR of 30 or better on the downstream on the upstream we strive for 25db+
We serve Apartment complexes too, and we also experience return path probs from time to time. The SNR usually does not affect speeds unless it is in low teens, and usually indicates a more serious problem. Do you have a spectrum analyzer? Sounds like some common path probs to me. When looking at a spectrum analyzer common path is parallel spikes through the carrier, and is usually caused by dissimilar metals touching i,e, bad fitting, loose corroded etc... You could also have Ingress which on a spectrum would be rolling waves through your carrier. Similar causes.
Things to do to troubleshoot, Check fittings for bad or corroded fittings. If you live where temperature fluctuates a lot in the evenings may also want to check AGCs. Try and isolate the trouble area by turning off sections of the node one at a time until you find troublesome area.
You use a higher upstream frequency then us( we use 19-26Mhz) but other causes could also be HAM radios or something similar. We have had other exotic cases of AC on the lines causing troubles, But if i had to guess i would say you probably have Either common path distortion or ingress.
Usually our Upstream Probs have to be insane to really affect customers speeds especially in Apartment complexes, we usually have problems with customers aggressively P2Ping or Viruses that use up all our available upstream bandwidth.
I am a cmts / rf guy, your best bet would be to look at your return going into your receivers.
see if your cmts has spectrum config. if so, if there's a return problem on let's say 36 Mhz it will move the modem to 29 Mhz.
Make sure they have the latest firmware, older firmware will have the modems reset. current release will stay online and just change to the new freq.. if you can't look at the return, does your cmts look at it for you? here's an example of mine with return spectrum.
Cmts2#sho int cable 0/0 upstream 3 spectrum
Frequency (Hz) Voltage (microvolt) Power (dBmV)
5000000 54 -25.4
5200000 52 -25.7
5400000 47 -26.6
5600000 71 -23.0
5800000 55 -25.2
6000000 56 -25.0
6200000 3 -50.5
6400000 38 -28.4
6600000 56 -25.0
6800000 56 -25.0
7000000 44 -27.1
7200000 42 -27.5
7400000 34 -29.4
7600000 28 -31.1
7800000 29 -30.8
8000000 31 -30.2
8200000 35 -29.1
8400000 36 -28.9
8600000 41 -27.7
8800000 53 -25.5
9000000 52 -25.7
9200000 49 -26.2
9400000 57 -24.9
9600000 59 -24.6
9800000 52 -25.7
10000000 51 -25.8
10200000 54 -25.4
10400000 54 -25.4
10600000 46 -26.7
10800000 47 -26.6
11000000 47 -26.6
11200000 43 -27.3
11400000 35 -29.1
11600000 52 -25.7
11800000 47 -26.6
12000000 31 -30.2
12200000 34 -29.4
12400000 32 -29.9
12600000 29 -30.8
12800000 37 -28.6
13000000 36 -28.9
13200000 36 -28.9
13400000 29 -30.8
13600000 31 -30.2
13800000 39 -28.2
14000000 39 -28.2
14200000 48 -26.4
14400000 29 -30.8
14600000 30 -30.5
14800000 28 -31.1
15000000 23 -32.8
15200000 20 -34.0
15400000 21 -33.6
15600000 20 -34.0
15800000 21 -33.6
16000000 20 -34.0
16200000 19 -34.4
16400000 17 -35.4
16600000 18 -34.9
16800000 18 -34.9
17000000 58 -24.7
17200000 18 -34.9
17400000 19 -34.4
17600000 19 -34.4
17800000 19 -34.4
18000000 21 -33.6
18200000 22 -33.2
18400000 20 -34.0
18600000 16 -35.9
18800000 17 -35.4
19000000 17 -35.4
19200000 16 -35.9
19400000 17 -35.4
19600000 16 -35.9
19800000 16 -35.9
20000000 15 -36.5
20200000 18 -34.9
20400000 16 -35.9
20600000 15 -36.5
20800000 16 -35.9
21000000 16 -35.9
21200000 15 -36.5
21400000 16 -35.9
21600000 18 -34.9
21800000 16 -35.9
22000000 28 -31.1
22200000 14 -37.1
22400000 18 -34.9
22600000 16 -35.9
22800000 19 -34.4
23000000 23 -32.8
23200000 17 -35.4
23400000 18 -34.9
23600000 19 -34.4
23800000 76 -22.4
24000000 19 -34.4
24200000 18 -34.9
24400000 17 -35.4
24600000 15 -36.5
24800000 20 -34.0
25000000 14 -37.1
25200000 14 -37.1
25400000 16 -35.9
25600000 15 -36.5
25800000 15 -36.5
26000000 16 -35.9
26200000 16 -35.9
26400000 19 -34.4
26600000 16 -35.9
26800000 15 -36.5
27000000 18 -34.9
27200000 16 -35.9
27400000 14 -37.1
27600000 33 -29.6
27800000 16 -35.9
28000000 14 -37.1
28200000 16 -35.9
28400000 15 -36.5
28600000 23 -32.8
28800000 16 -35.9
29000000 17 -35.4
29200000 16 -35.9
29400000 18 -34.9
29600000 25 -32.0
29800000 14 -37.1
30000000 15 -36.5
30200000 57 -24.9
30400000 15 -36.5
30600000 15 -36.5
30800000 16 -35.9
31000000 14 -37.1
31200000 26 -31.7
31400000 15 -36.5
31600000 20 -34.0
31800000 17 -35.4
32000000 15 -36.5
32200000 16 -35.9
32400000 15 -36.5
32600000 16 -35.9
32800000 15 -36.5
33000000 19 -34.4
33200000 15 -36.5
33400000 15 -36.5
33600000 14 -37.1
33800000 13 -37.7
34000000 15 -36.5
34200000 16 -35.9
34400000 17 -35.4
34600000 13 -37.7
34800000 13 -37.7
35000000 19 -34.4
35200000 14 -37.1
35400000 18 -34.9
35600000 17 -35.4
35800000 15 -36.5
36000000 16 -35.9
36200000 15 -36.5
36400000 14 -37.1
36600000 14 -37.1
36800000 29 -30.8
37000000 15 -36.5
37200000 16 -35.9
37400000 15 -36.5
37600000 14 -37.1
37800000 13 -37.7
38000000 14 -37.1
38200000 14 -37.1
38400000 12 -38.4
38600000 34 -29.4
38800000 14 -37.1
39000000 14 -37.1
39200000 15 -36.5
39400000 16 -35.9
39600000 16 -35.9
39800000 16 -35.9
40000000 17 -35.4
40200000 15 -36.5
40400000 35 -29.1
40600000 15 -36.5
40800000 17 -35.4
41000000 17 -35.4
41200000 15 -36.5
41400000 15 -36.5
41600000 15 -36.5
41800000 17 -35.4
everything got shifted over to the left when posted. but shows the power level at different freq.'s good tool to see what's good or bad ;-)
Everybody's right as far as your return being too close to the end of the spectrum. 5-42Mhz is the spec. but if there's a diplex filter that's cutting it off too soon you will have issues. and with the center freq. of 40Mhz , carrier is 3.2Mhz wide
Anyone know if its possible get this information from C3 without buying the Arris Spectrum Analyser ?
Thanks!
I haven't worked with the C3, but know that on the Motorola cmts's you can run a command to check the return, how I got the output posted earlier. if it's a coax or hfc network, you'll want the modems to transmit around 45 and receive around 0. rather than use 40 Mhz for the return, try a little lower, like 38 Mhz. wireless would be better around 51 dbmv return transmit levels. The head end tech or field tech should have a meter or analyzer to check the return. when running 64Qam down and QPSK on the return does the system run ok?
I am a different guy, with noise problems too ...
will wait for vortmax or open another thread.
Thanks
Demian
Hi cmcaldas, I was looking something else but I ended watching this post so I run the same command on my cmts and saw that everything was on -60 dBmV. It is bad? I'm not an RF guy but the output is very different from yours so I would like your advice.
Frequency (Hz) Voltage (microvolt) Power (dBmV)
5000000 0 -60.0
5200000 0 -60.0
5400000 0 -60.0
5600000 0 -60.0
5800000 0 -60.0
6000000 0 -60.0
6200000 0 -60.0
Thanks in advance.
Your upstream frequency is kinda on the high side and I would worry about roll-off. feel free to drop me an e-mail to chat about maybe looking over your configs on the CMTS to make sure you are good to go.
slimjim100(at)gmail.com
Regards,
Brian Wilson
www.Slimjim100.com
Slimjim100(at)gmial.com
CISSP, CCAI, CCNA, CCSE, JNCIA, Security+, Network+, MCP
I will also consider myself somewhat of a noob on the full specs of an HFC network but I do know that if we see anything under 35db for SNR we can start to worry. Our SNR is quite constantly 40db. If we run under 30 then we definitly have to do a truck roll to either the house or go out and fix an LE or something repair the issue. If you setup some monitoring on the whole plant as I've discussed in some other threads you can quickly discover where issues are coming up by browsing a few graphs. Microreflections is an entirely different story, I read documentation that it's important but that's all the more they ever say. Hopefully i'll have time to read more on that soon!
Hi Again Vortmax,
Here are a few comments from one of our NOC Admins [i]"Brent. S"[/i] who took a look at your situation.
Your DS SNR may be too low to be using QAM 256, unless you needs that extra data-bandwidth you'd see a 3+ improvement in DS SNR by switching back to QAM 64
Also double check that the up converter is really on-channel.
Some have built-in offsets and if you're off by 1/4Meg or so it often still works but the performance suffers.
Given the small number of customers involved, you can likely get away with a less aggressive setting (that 2560K is the symbol rate, we normally discuss it by bandwidth ie 200KHz wide to 3.2MHz wide.) So dialing that down would likely provide a small amount of relief.
Then too, 40MHz is right near the upper edge of the available spectrum, 32MHz might serve better (a spectrum analyzer would give a better indication of the sweet spot in the plant)
[i]RE: The up converter is configured to mix with the video at around 30 dB, which translates into it hitting the modems a few dB above 0.[/i]
What the modems are seeing sounds like a reasonable value but a combining network with 30dB inputs? I'm used to DS numbers quite a bit higher (like video injected in the mid to high 40's and data ~6 dB higher) So it may be worth revisiting that (more DS signal == better noise immunity of course)
[i]RE: Like I mentioned earlier, we are having issues with noise, especially in the return path. I'm looking at our worst system right now, with a modem transmitting at 6.5 dB, reporting a downstream SNR of 29 dB,
downstream Microreflections of 36 dBc and an upstream SNR bouncing between 21 and 26 dB on both channels. Those numbers are actually quite high right now. In the evenings it's not uncommon for the
upstream SNR's to drop below 20. We are getting a lot of packet loss leading to slow speeds at these sites.[/i]
There are lots of factors that can cause that kind of behavior. Generally its something fairly obvious like a cracked casing allowing ingress or water in an amp... at this level treat it like it was a video channel and diagnose what other factors might be interfering. Otherwise making some of the adjustments above may reduce some of the poor performance you're seeing here.
[i]RE: So I guess what I'm asking is, does this configuration look okay? What specifically should I be looking for as I diagnose the noise issue? When mixing with the video, should the Docsis signal be at the same level, above or below it? How much? Any thing else I should know?[/i]
Its not out in left field but does sound like it could use a little tweaking. If money is available moving to a more sophisticated solution (ie Cisco) will give you more tools to improve some of your issues. ie Cisco's allow the assignment of upstream _spectrum_ and the CMTS will have the modems walk through that available spectrum (in divisions based on your UC settings ie. every 1.6MHz) and stop when the signal characteristics are within specified tolerances and go back to walking if something strays (ie SNR drops to 20dBmV at 25MHz, the CMTS will tell modems to try 26.6MHz, see how that looks and repeat until it either finds clean spectrum or hits the top of the configured spectrum and starts over at the bottom)
Hope this helps,
Please email me directly, if we can assist you further.
Scott
Parasun Technologies Inc.
scoleman@parasun.com
Sorry for the long hiatus guys. I got tied up in some provisioning stuff. Anyway, thanks for all the advice. I did use a lot of it and it did help out a lot. We managed to get the major problems under control (for the most part), but they aren't ideal. The big issue we were having was related to return path balancing in the amps. Modems were not transmitting high enough to get above the noise floor. After adjusting the padding and knocking the center frequencies down below the roll-off, the plant conditions seemed to improve. I am still facing the issue of lower frequencies being really noisy though. It seems that on most sites, I can safely set my lowest center frequency around 20 MHz, but anything lower is unusable. However, I do notice that return path SNR increases as I lower the center frequency up until a limit, then craps out completely.
I am also experimenting with dropping my upstream symbol timing, but have some questions about it. By default, we ran it at 2560K, which (from what the documentation says) is good for 5.12 Mbit. Now this is faster then the main connection feeding the site and MUCH faster then any observed outbound traffic. If I never see outbound traffic rates higher then 1 Mbit, could I safely scale the symbol timing back to 640K ? Would that by advantageous? Also, on the properties where we have two return paths, the load is balanced between them. Would I be able to scale back the timing even further and again, would I notice any difference? The biggest complaint right now is slow speeds, which I understand a poor return path can cause.
I would post some configs, but the C9 CMTS isn't like cisco. I have an image of the ROM chip, so I have all the config files, but would have to piece them together to get a single config.