Hi there,
I'm a newby in this DOCSIS and Cable world, so don't hesitate to correct any of my flaws. I was wondering what's the best frequency you could/should use in your upstream? I want to scale for two stacked frequencies afterwards, but for now, let me know which is the best for one channel (my upper limit is 42 MHz because of physical limitations).
It depends on each node condition or you could use one generic frequency for all your US?
Thanks in advance.
I prefer to use spectrum group. This is where you tell the CMTS which range of frequencies it can use.
eg cable spectrum-group 1 band 20000000 42000000
This gives the upstream the ability to automatically move if it gets hit by noise
Thanks for your quick response Michael. I didn't know anything about that command! Pretty interesting! But, what if a need to stack two US channels into one US connector? What if you need to set alarms in a guardian? In addition to this, does a CM come offline if the CMTS detects some noise or the adjustment is executed just once?
On my D3 systems we put this in the global
cable spectrum-group 1 band 20000000 4200000
And on each Interface Cable x/y/z we put this
cable spectrum-group 1
The CMTS is smart enough to know how many US are in each grouping (due to the fiber-node statements)
Eg if you have 2 US, it puts both into that same block of spectrum, and they will be non-overlapping.
If there is noise the CMTS can automatically move ("hop") the US,
and/or shrink its width (if you have set up eg " cable upstream x channel-width 6400000 3200000" or similar.
Modems dont go offline when either of these happen.
You can also do similar stuff on D2 systems
No need at all to hardcode the freq, that is just taking away a big method of automatic noise defence !
I understood! :D :D :D Thanks a lot for your help!!! I'm telling you how is it going with these new commands I'm gonna put on our CMTSs.
One last question: do you know if this feature is available for UBR10-MC5X20U-D and UBR10-MC5X20H-D? I have read that "S" line cards also have an integrated Spectrum Analyzer, but I'm not pretty sure this is the same for "H" and "U" cards.
How do you use a return sweep with a setup like that?
Do you mean how it is possible using cable spectrum-groups? Because I don't know either. Maybe mbowe could help us.
You are right, this would cause problems for return sweep. You would have to block out the whole spectrum-group range from your upstream channel map.
Hi,
You should avoid such frequency like 21, 24, 27-29 MHZ ; also you have to use HP filters to cut ingresses from TV sets - 39 MHz
and from poor quality power supply's.
The best idea to increase SNR because of the noise is to narrow the bandwidth or change modulation profile to QPSK.
Regards
Darek
Thanks darekc! I'll be taking these advices into consideration!!!