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| Downstream Power
You generally want between -12db and +12db. Most modems are rated from -15 to +15. Anything less or more than that and you may have quality issues.
I personally prefer to not have less than -7db. If you want to raise your signal level a bit, check my troubleshooting and splitter section.Downstream SNR
This number is best over 30, but you may not have any problems with down to 25. Anything less and you will probably have slow transfers, dropped connections, etc.
See my "Downstream SNR" definition for more information on this.Upstream Power
The lower this number is, the better. If it is above 55, you may want to see if you can reconfigure your splitters. Anything above 57 is not good and should be fixed ASAP. (This is getting pretty close to not being able to connect.)Upstream SNR
Anything above 29 is considered good. The higher this number is, the better. If this number is below 25 and 29, you have a minute amount of noise leaking in somewhere. If it's anything less than 25, you want to get it fixed as you may have a lot of packet loss or slow transfer rates.
See my "Upstream SNR" definition for more info.
Feedback received on this FAQ entry:
- This really needs to be bought current.
2012-07-06 00:05:10 (EG )
- Upstream SNR:
If a tech gets another lower, he got work to do
2012-01-19 14:03:01 (drcable )
- Whenever I setup a nodes RevTx I oscillate @ 12 & 28Mhz into a -20Db testpoint w/ 37 out of the oscillator. My goal is reach the CMTS @ 0Db If it is to low then padding may be added to the rdr if it is to high padding is taken away. blah blah blah (there is much more to it then that) therefore in our systems we will never see a modem TX lower then 37..... Unless someone that doesn't know what they are doing throws a return amp in someones house, lowers a tap value, or tunes an amps return up far 2 high. FYI most of the time a 26value tap is first in line therefore (26+17= 41) If the modem was connected straight to the tap its UPTX would be 41. Oh yeah not every system sets up there nodes the same way which is why there are so many different answers. I do have to say that by keeping the UPTX a little higher it keeps the modems talking louder and keeps the noise lower in the system.... Think about it its a win win... Unless you have 20 outlets and want them all active all the time. Then I would say "Your SOL buddy"
- You never want your modem t be running off an amplifier! It will almost always result in problems. I was a cable instaler for 5 years. The first thing I would look fo when diagnosing HSI issues was whether ornot there was an amp in the mix. If there was, I would allways reconnect the HSI line before the amp via a splitter. For HSI, an amp will only amplify the bad signals when it comes to the internet. Don't assume that an amp will give you better levels because there is more signal. It absolutely does NOT work that way.
- Motorola SB 6120 (DOCSIS 3) on COX Cable (Phoenix) -- Premier Service (20/2)
Modem Signal Readings:
Downstream S.N.R.= 37-37-38-38 db
Downstream Power = 0-0-0-0 dBmV
Upstream Power = 47 dBmV
Internet Connection Speeds: 25-Test Average =
Download = 37.0 Mbps.
Upload = 3.8 Mbps.
Pings from Phoenix to Los Angeles & return time = 23ms; Packet Loss = 0; Jitter = 2ms. -- M.O.S. @ 4.39
All day long.
- a link for the definitions for:
would be a very good addition.
- How about an Upstream power in the lows 30? Mine is at 34 db and I have read in several sites that the recommended values are in the lows 40 up to 48 db and below 35 db can cause issues. I donīt understand that completely because I know the upstream power is the power the modem has to use to get the head end to get a satisfactory signal so the lower the better.
So why a 34 db transmit power is considered "worse" than a 42 db one?
if you can please answer me to firstname.lastname@example.org
- Upstream SNR shouldn't really be here, it's not customer visible, no-one will have this value on their modem so it could lead to confusion.
2010-06-30 04:12:11 (Ignite )
- Most of the comments here are on the money. There's also a great set of numbers for Adelphia at: http://www.dslreports.com/faq/5862 and I believe those numbers are applicable for any DOCSIS 2 system.
Downstream, you want a signal as close to 0dBmV as you can get - +/- 3dB is fine; +/- 6dB is probably OK, more than that you should be looking for bad connections before calling out the cable folks. Even more important is the received SNR reported by the modem - for higher speeds, anything below 33dB is worrisome, and 35+ is better.
The upstream power reported by your modem is a measure of how loud it has to "shout" to be heard by the "head end" - like everything else, if you "crank it to 11", you get distortion, the upstream signal quality degrades, and the speed may step down. Lowest upstream level should be around 35, highest should be 52, mid 40s preferred. If your cable modem reports more than 52 upstream power, you need to look at your cables and splitters.
2010-04-10 08:58:44 (mprogers )
- technically low transmit levels (upstream power) can cause connectivity problems too. One major cable company spec is 31 min 53 max. Believe it or not low transmit levels do occur just not nearly as frequently as high transmit levels.
- EG is correct. The statement, "The lower this number is, the better." is misleading. You will find a lot of different opinions on what threshold is best and each cable system will differ when it comes to what works in the real world vs. DOCSIS spec. In a DOCSIS 1 or 2 system with QAM16 modulation for upstream, you could get away with levels as low as mid-20's, but it would not be optimal.
This is even more crucial when you use an eMTA for voice and internet because low upstream levels can bring call quality symptoms to the front that internet-only use doesn't necessarily present.
In a DOCSIS 3 system, however, I suggest that the upstream levels be in the 40s range. The QAM64/256 upstream frequencies are much more sensitive to noise than the QAM16's, especially in the lower range (5-28mhz). Therefore, the modem needs to work harder to be "heard" by the CMTS. Additionally, if you are sitting at the end of the system (referred to as a "end of line" or "low value" tap), your modem really should optimally be between 45-49, since the modems have to talk further upstream through the entire leg of a system to be "heard".
Many others may dispute that last statement as lower levels may work flawlessly in various other systems. I am simply speaking from my personal experience in the few systems where I have worked and stating what I have found to be optimal over the last 10 years.
2009-11-15 17:15:30 (MadMANN3 )
- good faq thanks for you
- specs are ranged from a -9db to +9db for best results. trying to be as close to 0 as you can.
- My modem reports power levels in dBmv. What's the difference between this and db?
- Will moving the cable modem to the most upstream splitter improve bandwidth and speeds?
- After reading some information in this page, I made some 'reordering' on my cabling. I eliminated a two way splitter from the main line (in:cable service - Out:Modem and 8 port splitter).
Here are the main differences:(I didn't check SARA status yet, only the cable modem readings)
Downstream RCV Power Level: from -2.9dBmV to 3.9 dBmV (increase)
Upstream TX Power Level: from 44.0dBmV to 50.0 dBmV (decrease)
So even though I dropped the splitter, the passive return signal of the amp is worse than the two way splitter. I think the circuitry of the amp is causing the signal degradation.
- >>"Upstream Power:
The lower this number is, the better."
Yes, but to a point. With the implementation of QAM 16 upstream modulation, anything lower than around +30 dB may cause connectivity issues. The range should now be +30 dB to +55 dB
This number is best over 30, but you may not have any problems with down to 25."
With the implementation of QAM 265 downstream modulation, the minimum figure should now be 33 dB or higher.
25 dB is now considered way out of spec.
2008-03-15 12:35:23 (EG )