Downstream QAM

[gcollins]

I noticed recently that the downstream QAM on my cable modem had altered from QAM256 down to QAM64.

The upstream seems to be the same at QAM16.

There doesn't seem to be any degredation in service I still get 20meg when using multi threaded applications and streaming works fine from iplayer etc.

Why would the QAM change back down?

I'm not expert in such things but wondered why this would happen as surely QAM256 is better?

[Welshchris]

Post ur model power levels.

Could be a network issue eg they changed it or a PL issue.

[Ignitionnet]

I noticed recently that the downstream QAM on my cable modem had altered from QAM256 down to QAM64.

The upstream seems to be the same at QAM16.

There doesn't seem to be any degredation in service I still get 20meg when using multi threaded applications and streaming works fine from iplayer etc.

Why would the QAM change back down?

I'm not expert in such things but wondered why this would happen as surely QAM256 is better?

You may have moved to another downstream. In some areas there are multiple downstreams some on 256QAM some on 64QAM.

Which area of the country are you in, and what downstream frequency are you on?

[gcollins]

I'm in the Lichfield area (31) but currently at work so can't accurately post modem levels until later.

However, from memory my downstream was 0dbv and SNR was around 35.

I did have an engineer visit around a month ago and he placed a 6dbv forward path attenuator onto the back of the modem to bring the levels down (when I was on QAM256).

I'll post definitive later.

[Ignitionnet]

OK - you may find that you were on 402.75MHz and have moved to either 331MHz or 330.75MHz. You may even be on 298.75, 306.75, 314.75 or 322.75.

Usually lower QAM is used because either the network has some power loading issues on the downstream path and using 256 will cause problems, this applies to 298.75 - 322.75, or it's the default on some builds when they add another downstream to an area at 331 / 330.75MHz.

Cheers.

[Sephiroth]

OK - you may find that you were on 402.75MHz and have moved to either 331MHz or 330.75MHz. You may even be on 298.75, 306.75, 314.75 or 322.75.

Usually lower QAM is used because either the network has some power loading issues on the downstream path and using 256 will cause problems, this applies to 298.75 - 322.75, or it's the default on some builds when they add another downstream to an area at 331 / 330.75MHz.

Cheers.

Another pearl of wisdom dropped casually into the proceedings. To place this onto a real world level (you know what I'm like, my friend), can you please elaborate further:

1. Where would the power loading issues occur? The Hub? The CMTS?

2. What is the definition of an area? Everything that goes into a hub?

3. In the real world, what would the addition of a 100 house estate at 50% take-up potentially do to the QAM frequency?

4. What is the real-world effect seen by the user of a rduced frequency?

Cheers to you too.

[Ignitionnet]

1) Downstream optics - the laser at the headend / hubsite feeding an optical node or group of nodes. These take an RF feed in which is used to modulate the downstream laser. As with upstream lasers if you hit one of these with too much RF it will overdrive it causing distortion and lower signal quality.

2) In this instance 'area' indicates a node or group of nodes sharing the same downstream bandwidth. Can be used interchangably with 'MAC Domain' when referring to DOCSIS downstreams as every modem on a downstream is in the same layer 2 / MAC layer domain.

3) Nothing.

4) So long as area isn't oversubscribed nothing.

[gcollins]

Post ur model power levels.

Could be a network issue eg they changed it or a PL issue.

OK - you may find that you were on 402.75MHz and have moved to either 331MHz or 330.75MHz. You may even be on 298.75, 306.75, 314.75 or 322.75.

As promised, but doesn't appear to match what Broadbandings said above?

Cable Modem Downstream
Downstream Lock : Locked
Downstream Channel Id : 50
Downstream Frequency : 458250000 Hz
Downstream Modulation : QAM64
Downstream Symbol Rate : 6952 Ksym/sec
Downstream Interleave Depth : taps12Increment17
Downstream Receive Power Level : 0.6 dBmV
Downstream SNR : 36.5 dB

[Ignitionnet]

Ah ok, I forgot that the very odd area has its' extra downstreams in different places.

You've been moved to where most areas are on 298.75 or whatever except in your case it's up at higher levels is all. You'll be on QAM64 / 64QAM due most likely to laser loading issues as I mentioned above.

Just your area is an exception rather than the rule but you've been moved to the 50Mbps network, and it happens to be on different frequencies in your area to the 'normal' builds.

Cheers.

[gcollins]

Thanks for the quick reply.

I've also posted the question on the Virgin newsgroups so it'll be interesting to see if there's any additional information as to 'why'.

[Ignitionnet]

Just taxed your IP address from the newsgroups ;)

Yes you have been moved to the 50M kit.