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Registered: ‎16-11-2015
Message 1 of 18 (9,928 Views)

Acceptable Power Level (dBmV) and RxMER (dB)

As a new customer are the below stats seen as acceptable? 

 

In particular the Power Level (dBmV) and RxMER (dB) 

 

Thanks for any advice

 

Downstream        
 DS-1DS-2DS-3DS-4DS-5DS-6DS-7DS-8
Frequency (Hz)203000000211000000219000000227000000235000000243000000251000000259000000
Lock Status(QAM Lock/FEC Sync/MPEG Lock)LockedLockedLockedLockedLockedLockedLockedLocked
Channel ID12345678
Modulation256QAM256QAM256QAM256QAM256QAM256QAM256QAM256QAM
Symbol Rate (Msym/sec)6.9526.9526.9526.9526.9526.9526.9526.952
Interleave DepthI=12I=12I=12I=12I=12I=12I=12I=12
J=17J=17J=17J=17J=17J=17J=17J=17
Power Level (dBmV)8.288.288.78.88.598.929.69.55
RxMER (dB)34.2134.2134.0834.2134.2134.0834.2134.21
Pre RS Errors90738106656142321125100129020170351137680146842
Post RS Errors282299304283367917941301
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Message 2 of 18 (19,791 Views)
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confirmed by alainchateau
‎17-11-2015 08:26

Re: Acceptable Power Level (dBmV) and RxMER (dB)

The levels are in VM spec.

The downstream power levels are above optimal but still within VM tolerances.

The SNR levels are on the low side of VM's acceptable levels but still within tolerance, but ideally should be higher.

There seems to be quite a few pre RS errors which if not a historical accumulation may suggest the downstream is not quite as clean as it should be. However, it looks like the RS algorithm is correcting these errors so they  should not significantly affect performance.

 

Are you experiencing any issues?

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Message 4 of 18 (19,764 Views)
Helpful Answer
confirmed by alainchateau
‎17-11-2015 08:26

Re: Acceptable Power Level (dBmV) and RxMER (dB)

[ Edited ]

They're borderline acceptable.

 

Your downstream levels are within the acceptable range of -6 to +10 dBmV.

 

When they go outside that range during cold weather then you will likely have problems with reliability and/or low speeds, particularly as your RxMER is low already. The higher the RxMER the greater range of power that can be tolerated (the above quoted range is for people with good RxMER levels).

 

Btw, there is no "optimal" power levels anymore, for at least a year now since VM changed their system.

 

Although I also have my downstream levels on the high side during winter (to allow for the drop during summer), you should allow a few dBmV for when it gets colder, as the downstream levels will rise even more. Don't be surprised if your downstream levels go out of range at some point in the next couple of months.

 

The threshold for low RxMER is 34.5 dB. As you can see you are below that. I would want to see the RxMER around 37-38 dB for a good signal, or at least above 35. There's not usually much you can do about that though, depending on where the additional noise is coming from (e.g. more users modems on the same cable).

 

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Message 2 of 18 (19,792 Views)
Helpful Answer
confirmed by alainchateau
‎17-11-2015 08:26

Re: Acceptable Power Level (dBmV) and RxMER (dB)

The levels are in VM spec.

The downstream power levels are above optimal but still within VM tolerances.

The SNR levels are on the low side of VM's acceptable levels but still within tolerance, but ideally should be higher.

There seems to be quite a few pre RS errors which if not a historical accumulation may suggest the downstream is not quite as clean as it should be. However, it looks like the RS algorithm is correcting these errors so they  should not significantly affect performance.

 

Are you experiencing any issues?

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Message 3 of 18 (9,912 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)

Apart from maintenance work which is known to be affecting the whole Bristol area currently and reducing dl speeds to 4 meg, no.
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Message 4 of 18 (19,765 Views)
Helpful Answer
confirmed by alainchateau
‎17-11-2015 08:26

Re: Acceptable Power Level (dBmV) and RxMER (dB)

[ Edited ]

They're borderline acceptable.

 

Your downstream levels are within the acceptable range of -6 to +10 dBmV.

 

When they go outside that range during cold weather then you will likely have problems with reliability and/or low speeds, particularly as your RxMER is low already. The higher the RxMER the greater range of power that can be tolerated (the above quoted range is for people with good RxMER levels).

 

Btw, there is no "optimal" power levels anymore, for at least a year now since VM changed their system.

 

Although I also have my downstream levels on the high side during winter (to allow for the drop during summer), you should allow a few dBmV for when it gets colder, as the downstream levels will rise even more. Don't be surprised if your downstream levels go out of range at some point in the next couple of months.

 

The threshold for low RxMER is 34.5 dB. As you can see you are below that. I would want to see the RxMER around 37-38 dB for a good signal, or at least above 35. There's not usually much you can do about that though, depending on where the additional noise is coming from (e.g. more users modems on the same cable).

 

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Message 5 of 18 (9,868 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)

Out of interest, why isn't there an "optimal" range anymore. I always thought optimal was within acceptable range with a 3dBmv headroom as recommended by CISCO.

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Message 6 of 18 (9,861 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)

You might both be interested in my conversation with Virgin online support today. 

 

ME:

Hi, I have some concerns over the Power Levels (dBmV) and RxMER (dB) on my line after seeking advice from the community forums.

They seem to be on the outer limits of what is deemed acceptable.

[You're chatting with Robert.]

Robert:

Hi Alain. I'll take a look.

Robert:

Thanks. Testing now.

ME:

my results were as follows: Power Level 8.28 , 8.28, 8.7, 8.8, 8.59, 8.92, 9.6, 9.55

ME:

rxMER (db) 34.21, 34.08, 34.21, 34.21, 34.08, 34.21, 34,21

ME:

Pre RS Errors 90738, 106656, 142321,125100, 125100, 129020, 170351, 137680, 10:12:21

Robert:

Your power levels are fine Alain.

You're on 256 qam, and the power range is -13 to 17dBmv

Your Snr readings are also fine.

ME:

Right I see, excellent, how about  the RXmer. I've been told 35 should be an ideal minimum

Robert:

RXmer, or modem signal to noise is fine. around 30 would be the point we'd look at a tech booking.

ME:

Thanks for confirming Robert.

one last thing

Robert:

Upstream power fine at around the 40 mark.

Robert:

Fire away.

ME:

Our internet speed dropped to 3meg last night and latency was around 200ms. Speaking to other members on the community forums suggested I wasn’t the only person suffering this drop in speeds in my area

One member stated that all customers had been diverted onto one line due to the current upgrades happening in the BS postcode area. Can you confirm if this is the case?

Robert:

I can check your cable and UBR history. Hold on a few moments here please Alain. I'll need to log into another tool.

ME:

Thank you

Robert:

It does indeed appear you have been moved. To a new cmts entirely. from aztw-cmts-26 to aztw-cmts-30. Carried out at some point between the 13th and today. That in itself isn't a bad thing. In fact, this should be a lot better. Even during the peak hours at its highest load, the capacity on that CMTS only ever hits around 40%.

I suspect any reduction in speed, or brief disconnection, will have been due to the change. How is your connection currently?

ME:

It was fine as I left home this morning. sitting at 55 meg and a 20ms ping

Robert:

Bang on. I think you've nothing to worry about here Alain. Your current CMTS is in great shape. 256 Qam, plenty bandwidth. Wouldn't mind being on that myself. Smiley Surprised)

ME:

Great news. Really appreciate your confirmation on all these matters.

ME:

I'll see how the line performs this evening during peak hours

Robert:

No trouble Alain. Is there anything else sir?

ME:

All good for now. Thank you for all your help

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Message 7 of 18 (9,852 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)

[ Edited ]

The power levels quoted are the DOCSIS 3 specified allowable power range. This does not mean Broadband hardware can operate in all of the range. IIRC Comcast in the USA use between -10dBmV to +15dBmV for example

 

Looks like something going on in Bristol. I used to be on the Aztec West core 30 server, but just did a tracert and I am bypassing Aztec West all together. I am now going from the CMTS in Hawkfield straight to Bromley now.

 

Looks like VM are playing regimentation roulette again. Hopefully they have relieved the pressure on your local segment and you might get some decent speeds this evening.

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Message 8 of 18 (9,845 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)


alainchateau wrote:

 

Robert:

Your power levels are fine Alain.

You're on 256 qam, and the power range is -13 to 17dBmv

 

While your power levels are fine at the moment, no allowance is made for colder weather. VM are usually minimalist on giving out useful info. He also shouldn't be quoting Docsis spec power levels as they're useless in this case, but the actual network operating range I gave.

 

For more info see: http://community.virginmedia.com/t5/QuickStart-set-up-and/POWER-LEVELS-amp-SNR-TECHNICAL-PRIMER-Apr-...

 

Your Snr readings are also fine.

 

That's debateable. Fine as in you're not getting significant post-rs errors at the moment.

 

ME:

Right I see, excellent, how about  the RXmer. I've been told 35 should be an ideal minimum

Robert:

RXmer, or modem signal to noise is fine. around 30 would be the point we'd look at a tech booking.

 

VM's RxMER threshold is much lower than what would be acceptable for a user. That doesn't mean you should follow it. Each dB is an order of magnitude more noise than the last. By 33 dB you'd probably be seeing significant post-rs errors which are undesirable (other than the boot-time errors you're seeing, which are fine). By 32 dB you're toast.

 

Neither the power levels or RxMER have anything to do with your speed problems though.

 

Robert:

Bang on. I think you've nothing to worry about here Alain. Your current CMTS is in great shape. 256 Qam, plenty bandwidth. Wouldn't mind being on that myself. Smiley Surprised)

 

Btw, all CMTS have the same bandwidth across 8 channels as other CMTS. They're all 256 QAM. A technical advisor should know the difference between bandwidth, utilisation and throughput. Each channel has 50Mb of useable bandwidth x 8 = 400Mb, which is then divided by all the users on that node/CMTS.

 

How much bandwidth a CMTS has no relation to how much speed its users get. What he should be saying is "your CMTS and NODE has low utilisation".

 

 

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Message 9 of 18 (9,834 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)

Low and behold the Bristol issue persists. Back down to 5 meg and 120ms+ pings this evening.

Was absolutely fine for the first two weeks of the new service.
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Message 10 of 18 (9,816 Views)

Re: Acceptable Power Level (dBmV) and RxMER (dB)

Hi alainchateau,

 

Welcome to the community and thanks for posting. I am very sorry you're experiencing a slowdown in speed and an increase in latency.

 

I've managed to locate your account to check this. I can see the CMTS upgrade on 13/11 to aztw-cmts-30. The area usage is very low so this isn't the cause of the problem. I agree the power and SNR levels are borderline which could be a contributing factor.

 

I see you've posted on this thread where others in Bristol have reported issues. Looking at the notes it appears there's a configuration issue with aztw-cmts-30 which is in hand with the networks team. The current review date has been given for 20/11 but updates are coming in daily.

 

Seeing as everyone started having these issues on 16/11 I assume they're all connected so sending an engineer will not do anything about that although it may be worth keeping an eye on your levels as one may be needed once this is all over.

 

Are you able to set up a Broadband Quality Monitor? This will allow you to keep an eye on things and you'll be able to share that with us so we can see.

 

Hope to hear from you soon.

 

Thanks


Rich
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