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DOCSIS/RF Newbie Question

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DOCSIS/RF Newbie Question

PostAuthor: collangm » Mon Nov 15, 2010 8:52 am

I'm fairly new to the DOCSIS arena and am having trouble understanding a few, what I consider to be, key points relating to the DOCSIS RF spectrum and availability to services over an HFC Cable network.

1. What is (or is there any) separation between each adjacent Downstream or Upstream channel in the DOCSIS/EuroDOCSIS spectrum?

(DOCSIS uses 6MHz and EuroDOCSIS 8MHz channel widths for downstream as a example. What is the spacing between each channel and is this spacing (if present) the same for all Upstream channel widths? i.e. 200KHz-6400KHz).

2. The DOCSIS/EuroDOCSIS specified operating frequeny bandwidth range is between 5MHz and 862MHz. This is split into Upstream (5MHz-42 or 65MHz) and Downstream (55MHz (108MHz for data) to 862MHz). Within this frequency band sits FM radio, Analogue TV, Digital TV and a host of other potential invasive services (such as mobile phones). With this in-mind, what are the actual realistic and usable frequencies that can be used by DOCSIS in both U/S and D/S that do not interfere with or operate in the same frequency bands? (I.e. TV (analogue and digital) both have strict frequency bands).

3. Are any of the above documented anywhere as the specifications to not make mention of this and simply indicate the entire spectrum available to 'Cable-based' delivery systems?

Many Thanks,



I guess I should have also add - 'or am I looking at this in completely the wrong way and being a bit of a thickie?!!'

I assume that as FM Radio, Analogue TV and Digital TV use specific frequencies within the same RF band as DOCSIS/EuroDOCSIS, then these would not be usable due to cross-interference. Of course this depends on who is operating the cable network and what services they are providing? But, what is the likelyhood of ingress from external sources (named above) at the CPE end (i.e. the weakest RF point in the network), hence my original question.



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Re: DOCSIS/RF Newbie Question

PostAuthor: collangm » Wed Nov 17, 2010 6:56 am

Could someone please verify my logic, below;

I think perhaps my initial question is now incorrect in it's assumption. Looking further into the way in which each 8 MHz channel is utilised, the key point seems to be that multiple services can be modulated onto the same channel (i.e. SD-TV, HD-TV, Data) and so therefore the restrictions imposed by traditional terrestrial broadcast services does not apply in the cable network. Is this assumption correct so that in-effect the entire frequency band (5MHz-862MHz) is available to EuroDOCSIS data tansmission irrespective of whether other services are also using that frequency/channel?



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Re: DOCSIS/RF Newbie Question

PostAuthor: mbowe » Thu Jan 06, 2011 12:20 am

Each channel sits adjacent to the next

So for North America (6MHz wide channels), you could setup your CMTS to be using downstreams eg
The downstreams could be carrying data, such as Internet and or mutlicast/IPTV

And then you might be putting out analog TV channels on your network as well, using frequencies like eg
As long as no two services overlap, all is good
In terms of choosing which frequencies to use on your network, you do need to consider what signals might leak into your coax cables. So best to avoid any frequencies currently used by strong local UHF TV etc

For the upstreams you choose what width you want to use eg
or any mix of above. You again can stack these side by side.
In terms of picking which frequencies to use, between 20MHz and 40MHz is pretty safe, and going below 20 is risky you can start to run into problems with interference

Hope that helps

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