Reply
Superuser
Sephiroth
Posts: 11,965
Registered: ‎01-09-2009

POWER LEVELS & SNR: A TECHNICAL PRIMER

INTRODUCTION

 

There are no precise numbers for power levels although you can be sure that the numbers offered in this thread will not be the cause of any problems.  This thread partially explains how power levels are applied across the different network components.  It also suggests that if your power level is stable and the connexion is stable, then power levels per se won't be your problem.  Conversely, the thread explains what the effects of out-of-range power could be if speeds are poor or the connexion is unstable.

 

Acknowledgement is given for the contributions to this material by Horseman, Ignition, Lee_G (Team Member), Apcyberax, Canveyboy and Paultechy.

 

 

CABLE MODEM NOMINAL SPECS

 

The CMs are advertised with a wider power range than is considered healthy for a stable connection.  For example, the Ambit 256 modem is spec'd for downstream in the range -15 to +15 dBmv (slight variations according to 64QAM or 256QAM but these are ignored for the purpose of this article.  There are also variations attributable to the local implementation of DOCSIS or EURODOCSIS which employ different channel bandwidth and frequency plans; these variations are also ignored for the puroses of this article. 

 

The upstream is spec'd in the range 8 dBmv to 58 dBmv (16QAM) and 61 dBmv (QPSK).  16QAM should not go above 58 dBmv.

 

 

UPSTREAM TRANSMISSION

 

The so called "fibre-optic broadband" is more accurately referred to as "cable broadband" because at both ends (home and head end), termination is by means of coaxial copper cable.  It is a DOCSIS requirement that each active node (e.g. Optical Node), on the upstream path receives a minimum of 15 dBmv. This 15 dBmv is not rigidly applied because it is obviously impossible for all modems to hit the node at the same power.  There is intermediate upstream amplification.

 

So let's look at a theoretical (but practically founded) scenario.  We will use the nominal 15 dBmv for the purposes of this illustration. In the diagram below, the path to the optical node might comprise the following elements, each contributing to attenuation:

 

 

 Upload power calculation

 

In the above example, the upstream power would be 48.5 dBmv.  If the coax cable goes to a 10 dB tap point instead of 23 dB, then the upstream power would be 35.5 dBmv, making 15 dBmv at the optical node.  The coax attenuation varies according to upstream frequency (the higher it is, the greater the attenuation but for the small upstream range that isn't a huge problem).  Doubling the coax length doubles the attenuation.

 

Any impairments outside of these "known" attenuations requires extra power to assure 15 dBmv received at the optical node.  So the CMTS manages this by simply deciding whether or not the CM has been able to communicate properly with the CMTS.  If not, during initial ranging, the CMTS commands a 0.25 dBmv increment until communication is established.  During a session, periodic maintenance opportunities ("keep-alives") occur. If impairments have arisen that prevent the keep-alive communication from succeeding, the CMTS offers a series of further opportunities, each failure resulting in a T3 message. After 16 such T3 messages, the CMTS removes the modem from its polling list and a T4 timeout occurs with a subsequent modem reset and possible power ramping as per initial ranging.

 

Modems to a single street cabinet are arranged (through differently attenuated tap points) to have as similar a transmit power as possible so as to preserve SNR; otherwise there would be wide SNR variations between modems close to the street cabinet and those further away.  That said, the CMTS will command upstream power according to the attenuation (and SNR) seen for a given modem.  So you can see why a ramping upstream power number is potentially something to worry about, whereas a stable, but high number without problems is nothing to worry about.

 

THE EFFECTS OF LOW UPSTREAM POWER

 

This can be a tricky one.  A house that is close to the street cabinet needs less power than one that is further away, subject to the tap point used at the street cabinet.  This is why upstream values like 29 dBmv can work OK (though there can then be SNR issues) - but it could mean that the house is connected to a low attenuation tap point and thus the downstream power might be too high.

 

If you are only 20m from the street cabinet instead of 100, you immediately gain 3 dB and if you are on the 10 dB tap, you only need 32.5 dBmv to make 15 dBmv at the optical node.

 

So, if you are some distance from the street cabinet AND your upstream power is low (with possible attendant SNR issues) AND you're having problems, then you could have a faulty modem.

 


DOWNSTREAM TRANSMISSION

 

The CMTS at the other end puts out upwards of 42 dBmv. The level arriving at the first downstream node is variable depending on plant condition and so the output of a particular line card matches those dependencies.  This approach ensures (in a correctly maintained system) that the signal reaches the optical node at 15 dBmv. When the signal emerges at the optical node in the street cabinet, it has to overcome the 33.5 dB built in attenuation (see above diagram) on the 23 dB tap point, or 20.5 dB attenuation if connected to the 10 dB tap point.  PLUS another 10 dB due to the much higher downstream frequency;  so that's 43.5 dB or 30.5 dB attenuation for downstream in the two tap point cases.  But 100m further down the road, the coax component of attenuation doubles adding 14 dB (more for higher frequencies), making the example values 57.5 dB and 44.5 dB.

 

So now you can see why downstream power levels can go negative when the target output of the optical node is 46 dBmv.

 

THE EFFECTS OF HIGH DOWNSTREAM POWER

 

High downstream power (say in double figures), occurs if:

 

  • The CM is near to the street cabinet and the illustrated attenuation figures are much lower AND the installation engineer has neither put the cable onto a higher attenuation tap nor has he added a forward path attenuator at the CM;

  • A faulty street amplifier is "overdriving" the power level.

 

High downstream power amplifies noise (but if the CM is close to the street cabinet, SNR wiill be high and noise will be low).  High downstream power also overdrives the CM amplifier, but not all CMs are as sensitive as each other.  So this is a moving target.

 

At this point, more needs to be said about the downstream. As most of you know, the downstream RF signal is is QAM modulated (which you can read about here). High power levels can cause QAMs around the tuned one to break through the tuner's isolation and interfere with the intended signal.

DOCSIS 3 modems work differently from DOCSIS 1.x (standard) modems.  The standard modem tunes to an RF channel 6 to 8 MHz wide (DOCSIS/EURODOCSIS) whereas the DOCSIS 3 modem digitally samples 100 MHz of the RF.  This additional analogue to digital conversion is reflected in the less high SNRs (RxMER) evident in the DOCSIS 3 modem stats.  There is a good technical read on some of this here.

 

 

RECOMMENDED POWER LEVELS

 

Bearing in mind the above explanations, it should be clear why the following levels are recommended.  They take into account

 

  • the modem's specifications
  • the technical explanations provided above
  • the anecdote on the forums as to where problems arise
  • the values based on geography (i.e. network build) recommended by Virgin Media staff

 

DOWNSTREAM POWER (Optimal)

 

Rule of thumb:    -3 dBmv to +6 dBmv

 

The above advice compares with the VM stated optimum values by region (according to implemented infrastructure standards) and modulation:

QAM64 (Knowsley networks)  -7 dBmv to +3 dBmv

QAM64 (Langley/Bromley)      -10 dBmv to 0 dBmv

QAM256                                     -3 dBmv to +7 dBmv

 

This is not cast in stone.  A good modem (Cisco 2100 comes to mind) can handle nearer the DOCSIS spec if the level is steady and the experience good.  If the experience is bad, then look for high downstream power, particularly above the 9 dBmv mark.

 

 

UPSTREAM POWER (Optimal)

 

35 dBmv to 50 dBmv

 

This compares with the VM stated optimum values:

40 dBmv to 50 dBmv

 

This too is not cast in stone as the technical explanation implies.  The CMTS commands the output power according to the target receive power seen against actual receive power.  Local settings, e.g. cabinet tap point positioning, deal with hitting the first node at the right power level.  If the CM is close to the street cabinet, then you wouldn't expect to see a high upstream power level if your neighbour had a lower level unless you had been placed on a different tap point.  

 

DOCSIS 3 line cards in the CMTS have additional attenuation built in so as to force an increase in upstream power from the cable modem in order to improve the SNR. 

 

Furthermore, DOCSIS 3 has additional power demands when driving higher bandwidth (e.g. 6.4 MHz or bonded channels).  Further reading can be found here.

 

In the general case, and particularly DOCSIS 3, if the power is steady in the 55 - 58 dBmv range and you have no problems, then DOCSIS is doing its job.  If the power is fluctuating during initial ranging (including after a T4 event), then the 58 dBmv is not enough to establish communication with the CMTS.  The problem could lie on the CMTS side of the network, although most cases are local with a faulty amplifier in the street cabinet or some attenuation on the line to the street cabinet.

 

 

SNR (RxMER) - DOWNSTREAM

 

64QAM     29 dB minimum;  32 dB or above desired

256QAM   32 dB minimum;  35 dB or above is desired

 

Tech support would raise their eyebrows at the minimum for 256QAM, especially on the 50 Mbps tier.  An engineer visit is not always necessary if the reported values are slightly outside the quoted range; tech support would always check for network issues before concluding that the matter needs to be resolved locally.

 

64 QAM has a density of 6 bits per symbol and has a greater tolerance for noise than 256QAM which has 8 bits per symbol.  There are variances that can be attributed to DOCSIS and EURODOCSIS plant resulting in a higher SNR requirement for EURODOCSIS with its wider channel.

 

The formal lower threshold for 64QAM/256QAM is respectively 24 dB/30 dB.  On top of that must be added at least 3 dB headroom for variations one normally finds (like due to temperature, humidity).  Ideally for 64QAM and definitely for 256QAM, additional headroom should be given for other factors like RF ingress, fridges and so on.

 

There is no relationship between SNR and downstream power levels other than the peril of amplifying noise if power is too high.

 

 

SNR/MER - UPSTREAM (measured at the CMTS)

 

Not quoted as this is measured at the CMTS.

 

One might be tempted to argue that QPSK/16QAM are an opportunity for allowing cheap plant to be installed.  But actually this is not the case bearing in mind the increase in upstream speed being rolled out in 2010/2011. 

 

The Virgin cable network is a collection of different network builds by different companies originating in the mid-1990s. These builds vary in quality and hence setting at the CMTS.  Some are on DOCSIS (5 - 42 MHz); others (later) on EURODOCSIS (5 - 65 MHz, soon going up to 85 MHz).  The upstream frequencies are a noisy part of the spectrum, particularly the lower frequencies which thus tend to be QPSK modulated.

 

But, if plant is improved, then better than 16QAM can be achieved with consequently higher data rates. 

 

END.

--------------------------------------------------------------------------------
Seph.

My advice is at your risk. (Please don't PM me for help)
Modem Mode is for the enlightened
Please use plain text.