View Full Version : What is QAM64?
I have the NTL 10 meg service and recently my faulty NTL 200 modem was changed to a blue NTL 250. When I interrogate it I get the following information:-
Downstream Lock : Locked
Downstream Channel Id : 50
Downstream Frequency : 402750000 Hz
Downstream Modulation : QAM64
Downstream Symbol Rate : 5056.941 Ksym/sec
Downstream Interleave Depth : taps32Increment4
Downstream Receive Power Level : -2.8 dBmV
Downstream SNR : 32.5 dB
Can anyone please tell me what QAM64 means?
QAM 64 is simply the method used for encoding data.
QAM 64 is simply the method used for encoding data.
Thanks for the prompt response. I have seen elsewhere that NTL also uses 256QAM but can you please explain how this differs from QAM64?:confused:
---------- Post added at 11:51 ---------- Previous post was at 11:45 ----------
With little research I have found an explanation here:-
http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
It suggests that as ISP's demand for more efficient bandwidth utilisation and building on its experience with 64 QAM transmission over cable, then NTL has begun to implement a 256 QAM transmission system that provides far more efficient use of cable system bandwidth and expands channel capacity. It would seem that this expanded channel capacity results in a 44 percent increase in information rate and a 50 percent increase in video content as compared to 64 QAM. With it, NTL will be able to carry two HDTV channels instead of just one in a 6-MHz space. The added capacity enables expanded video, modem, telephony and business data services. 256 QAM transmission also makes it possible to substantially increase the number of cable services on bandwidth-limited networks designed for analog video performance. This capability might allow deferral of costly upgrades/rebuilds.
Based upon http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation .
QAM is a complicated subject, but put simply, QAM256 offers more bandwidth (so, theoretically, higher speeds), but is more susceptible to interference and faults.
Based up http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation .
QAM is a complicated subject, but put simply, QAM256 offers more bandwidth (so, theoretically, higher speeds), but is more susceptible to interference and faults.
It certainly is complicated and I am grateful for helping me try to decipher it :nworthy:
To send data, the transmitter varies the amplitude and phase of a carrier signal. In QAM64 there are 64 possible combinations of amplitude and phase for each period of time, or symbol, of the carrier. With QAM256 there are 256 possible combinations in the same period - thus increasing the data capacity four fold but making it more difficult for the receiver to discriminate between each signal.
It's easiest to visulalize by looking at the constellation diagrams of each:
QAM4 (=quadrature phase shift keying):
Download Failed (1)
QAM16:
Download Failed (1)
QAM64:
https://www.cableforum.co.uk/images/local/2006/11/9.jpg
QAM256:
Download Failed (1)
...and if you're flumoxed by what I and Q mean - well, I is the 'in phase' part of the carrier and Q is the 'in quadrature' part. In this picture the I is in blue and Q is in green.
https://www.cableforum.co.uk/images/local/2006/11/4.gif
Creative
17-11-2006, 19:07
To send data, the transmitter varies the amplitude and phase of a carrier signal. In QAM64 there are 64 possible combinations of amplitude and phase for each period of time, or symbol, of the carrier. With QAM256 there are 256 possible combinations in the same period - thus increasing the data capacity four fold but making it more difficult for the receiver to discriminate between each signal.
It's easiest to visulalize by looking at the constellation diagrams of each:
QAM4 (=quadrature phase shift keying):
http://www.sat-steve.de/files/signale/qpsk.jpg
QAM16:
http://www.sat-steve.de/files/signale/qam.jpg
QAM64:
https://www.cableforum.co.uk/images/local/2006/11/9.jpg
QAM256:
http://www.syrus.ru/files/devices/control/at2500hm/5.jpg
...and if you're flumoxed by what I and Q mean - well, I is the 'in phase' part of the carrier and Q is the 'in quadrature' part. In this picture the I is in blue and Q is in green.
http://www.wikirecording.org/images/3/37/Phase_90deg.gif
Excellent diagrams and explanation Altis, as always!
Only one thing I'd say, and thats that theres not a fourfold increase in data capacity in QAM64 to 256 migration. The data capacity increases by a third. It goes from 6 bits per symbol to 8 bits per symbol. In terms of cable TV for a symbol rate of 6.887Msymbols/s the data rate goes from 41.322Mb/s to 55.096Mb/s. The RF channel has to have improved SNR for QAM256. If memory serves its an extra 3dB's required over QAM64. Escapee?
Only one thing I'd say, and thats that theres not a fourfold increase in data capacity in QAM64 to 256 migration. The data capacity increases by a third. It goes from 6 bits per symbol to 8 bits per symbol.Of course it does. What a ninny I am!
Mukesh kumar jha
02-12-2008, 05:13
Q. What is QAM-64?What is its process of working?
Q. What is stream ?
Mukesh kumar jha
14-02-2009, 07:24
Q. What is Qam-64 ?
Q. What is the working of harmonic BNSG 9000 ?
No need to post the question twice.
As for what is QAM64, look at http://en.wikipedia.org/wiki/QAM
Alternatively, read the article I linked to in an earlier post.
Ignitionnet
14-02-2009, 17:36
Q. What is Qam-64 ?
Q. What is the working of harmonic BNSG 9000 ?
The BNSG 9000 is an edge/external QAM.
If you don't know what 64QAM / an edge/external QAM are I'm really not going to take the time to explain the workings of one to you, as they matter little more than how a nuclear power plant generates your electricity for you :)
I'm actually trying to figure out if you're seriously bored and involved in the cable industry or just randomly asking questions. Asking what a BNSG 9000 is is kinda specific for my taste, so I'll run with option 1 ;)
I'm actually trying to figure out if you're seriously bored and involved in the cable industry or just randomly asking questions.It's just noise ;)
Ignitionnet
15-02-2009, 09:32
I did notice when reading it that I was struggled to keep the points within constellation boundaries, all that noise ruining my MER.
In all of this, remember that the higher the QAM, the greater the throughput but the more sensitive it is to noise. What you *really* need is QAM settings that adjust automatically. Funnily enough, that's exactly what the boffins thought, too.
BB (who's spent the last couple of months writing software to analyse RF parameters automagically, taking into account the QAM settings).
Ignitionnet
15-02-2009, 18:22
In all of this, remember that the higher the QAM, the greater the throughput but the more sensitive it is to noise. What you *really* need is QAM settings that adjust automatically. Funnily enough, that's exactly what the boffins thought, too.
BB (who's spent the last couple of months writing software to analyse RF parameters automagically, taking into account the QAM settings).
Ah you mean some goodness like the MC28S and MC5X20S that can automagically adjust upstream parameters width, frequency and modulation to account for different conditions on the RF goodness?
Cisco had a few masterplans on this and have had those line cards about for quite a while. Think ntl / TW / VM had some resistance to them as it would complicate troubleshooting, I guess that's part of what your software eliminates!
vBulletin® v3.8.11, Copyright ©2000-2024, vBulletin Solutions Inc.