POWER LEVELS & SNR: A TECHNICAL PRIMER (updated October 2016)
POWER LEVELS & SNR: TECHNICAL PRIMER (Updated October 2016) PART A - APPLICABLE DOCSIS STANDARDS INTRODUCTION There are no precise numbers for power levels although you can be sure that the numbers offered as optimal here will not be the cause of any problems. This article partially explains how power levels are applied across the different network components. It also suggests that if your power level and connexion are stable, then power levels won't be your problem. Conversely, the article explains what the effects of out-of-range power could be if speeds are poor or the connexion is unstable. The article acknowledges the 50/100/200/300 mbps tiers. There are four parts to this article: PART A: Deals with the EuroDOCSIS standards applicable to VM's Upstream & Downstream service components. PART B: Deals with Upstream transmission and what happens under various circumstances. PART C: Deals with the Downstream and what happens under various circumstances. PART D: Provides a bibliography of learned sources for the data tables in this Primer. Please note – there is no Glossary of Terms. These can and should be looked up on Google! You will find two useful tutorials covering the terms “DOCSIS” and “CMTS” here and here Acknowledgement is given for the contributions made on the VM Forum (in no particular order) by Horseman, Mud_Wizard, James_W (Forum Team), and previously, Ignition, Cabsandy, Apcyberax, Canveyboy & Paultechy. CABLE MODEM NOMINAL SPECS DOCSIS 3 POWER LEVEL STANDARDS (CM-SP-PHYv3.0-I12-150305) VM's DOCSIS 3 network is actually branded EuroDOCSIS (8 MHz downstream bandwidth) and differs from the North American implementation which uses 6 MHz downstream bandwidth. The US system can squeeze 96 channels into the available downstream spectrum whereas EuroDOCSIS supports 72 channels and each transmission unit can carry 33% more payload than DOCSIS. The available spectrum is shared between broadband and TV. This article is concerned only with broadband. You will see the term QAM used in this article. It means Quadrature Amplitude Modulation and is a method of modulating digital signals onto a radio-frequency carrier signal. Further discussion is out of scope. Without making this too complicated EuroDOCSIS sets a power range for a single channel & for a bonded group. The CableLabs specification does not venture beyond 256QAM. For the Downstream, VM uses 256QAM modulation having moved away from 64QAM. The modem MUST be able to accept RF modulated input signals with the characteristics defined in the tables below (though that doesn’t mean that data should be intelligible with levels at the extremes). Downstream Power: Taken from Table B-16 in the DOCSIS 3 spec: dBmv @ 6952 Ksyms/sec Per DS Channel Bonded total power ** DOWNSTREAM 64QAM -17 to +13 dBmv < 33 dBmv DOWNSTREAM 256QAM -13 to +17 dBmv < 33 dBmv ** On the matter of aggregated/bonded downstream power. It's not a case of just adding up the dBmv listed on your 8, 12, 16 or 24 channels. The (logarithmic) dBmv scale is relative to 0dBmv and a +3 dBmv increase in power level doubles the power arriving at the HUB (you can Google all that). Let's say that the power level on a single channel is 0 dBmv. Now let's double that to 2 channels so your aggregate power in dBmv terms is +3 dBmv. Make that 4 channels and it is +6 dBmv. The SuperHub 1 & 2/ac range see a spread of 16 DS channels (each 8 MHz wide) in your set from which you can actually use 8 according to the valid BPI+ key associated with your lease. But all 16 channels hit your HUB. 16= 2^4 so the aggregate downstream power is at least 4 x 3 dBmv = 12 dBmv if the average power is 0 dBmv. A good read on this, by Ron Hranac of Cisco, is to be found here. The Hub 3, depending on how it is configured, tunes 8, 12, 16, 20 or 24 channels from a 24 channel set. On the Superhub range, you will bust the EuroDOCSIS 3.0 downstream limit of 33 dBmv if the average power across all downstream channels reaches 22 dBmv. A clue to this would be that what you see averages c. 22 dBmv. (It's a bit more complicated than that because higher frequencies, if not subject to power equalisation /slope, bring down the average power - but that's not happening now to an extent that affects this article). On the Hub 3, you will bust the EuroDOCSIS 3.0 downstream limit of 33 dBmv if the average power across all downstream channels reaches 20 dBmv. The "busting" scenarios are very rare in practice and would only occur if the local street box amplifier was incorrectly calibrated and you were right next to it. Upstream Power: Taken from Table B-15 in the DOCSIS 3 spec (DOCSIS 2 is used for Upstream) - VM supplies 2 or 3 or 4 x upstream channels according to your contracted upstream speed. VM are also moving to 10:1 ratio so that upstream would be 10% of the downstream headline speed once implemented.: DOCSIS 2 @ 5120 Ksyms/sec Single Channel Locked 2 Channels Locked 3 or 4 Channels Locked UPSTREAM 16QAM +23 to +58 dBmv +23 to 55 dBmv +23 to 52 dBmv UPSTREAM 64QAM +23 to +57 dBmv +23 to 54 dBmv +23 to 51 dBmv DOCSIS 3 ERROR STANDARDS A quick word about “symbols”, the unit into which data is packed. The higher the bit density, the more efficient the data transport is but the more prone it becomes to errors (corruption). The table below shows the packing density currently used in VM’s cable network. Modulation Bits/symbol Notes QPSK (Legacy) 2 Sometimes used for modem registration 16QAM (US) 4 VM is improving their plant so that 64QAM can be the norm 32QAM (US) 5 A noisy 64QAM channel will regress to 32QAM then 16QAM 64QAM (US) 6 This is the VM targeted norm for Upstream modulation 256QAM (DS) 8 This is the highest DS modulation allowed in DOCSIS 3.0 Error correction is a complicated subject including “interleaving” & “codewords”. Suffice it to say that error correction operates on interleaved codewords; each codeword comprises a number of symbols. Commonly known as SNR (Signal to Noise Ratio), CMs MUST perform according to the table below by achieving a downstream Codeword Error Rate (CER) ≤ 9 x 10e-7. Modulation CM Input Power SNR + 3dB 64QAM -17 to + 17 dBmv ≥ 25.5 dB ≥ 28.5 dB 256QAM -13 to -6 dBmv ≥ 34.5 dB ≥ 37.5 dB 256QAM -6 to +17 dBmv ≥ 31.5 dB ≥ 34.5 dB The above SNR values are minima laid down by the EuroDOCSIS 3.0 spec, §B.6.3.3.2.1 based on what leaves the CMTS & arrives at a CM. Headroom MUST be added of at least 3 dB for impairments en route. For VM's DOCSIS 3 modems, which are customised and branded to VM, no formal specs are obtainable. So falling back on the DOCSIS 3 spec is the only available guidance against which problem reports can be judged. .... PART B is in the next post.149KViews41likes3CommentsGetting a better Wireless Network Signal
Seeing so many posts about Good, Bad & ugly Wifi, I thought I would post some suggestions for people out there. There are definitely good, midrange & poor routers with quite variable performance wifi. Never expect a mass market product to be the best performer. If you want that, then pay for it. Number one problem is simply so many households getting online in our wonderfully small country ! Therefore we all see loads of neighbours' wifi all the time. Number two problem is many people try to find some "less used channel" and manually config their wifi in an attempt to compete for RF space ! Consequently we end up with wifi channels all over the place. 802.11 works on the principle that nearby access points should be using channels 1,6 & 11, ideally with Auto-Mode for channel selection. This way each device intelligently finds bandwidth for your data. To avoid collisions, 802.11 wireless devices use a listen before speaking approach when accessing the wireless medium. Specifically, devices perform a Clear Channel Assessment (CCA) by listening to see if another device is actively transmitting on the channel before attempting to send its own frames. When a device detects another transmission in progress, it will perform a random back-off for a short period of time after which it would perform another check before attempting to transmit again. If the channel is clear after a check, the device can access the channel and send some data. When neighbouring households start doing this, the "in between" channels get used, BUT your access point cannot see these transmissions as data, but see them as RF interference. . . . so your Wifi no longer negotiates correctly and you lose performance. Example of Bad Wifi channel usage: Good Wifi channel usage: Same arguments apply to 5Ghz wifi, but at least up there we have more spectrum available, and access points don't transmit as far !! Always try to put your main AP in the centre of your building. It's ok to add repeaters, but stick to the same RF channels, so they can negotiate properly. At the end of the day, if you get all your neighbours together and agree to stick with 1, 6, & 11 channel usage, then everybody's wifi will improve. Lastly, I know some people will disagree with all this, but that's life !! Search the inter web and learn.79KViews68likes0Comments