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A couple of DOCSIS newbie questions if I may?

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collangm
A couple of DOCSIS newbie questions if I may?

Hi,
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,

Gary

collangm
I guess I should have also

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.

Thanks,

Gary

cmcaldas
If's And's or But's

Gary,
what system is your cable plant? domestic or Euro docsis? depends how clean your plant is and if the cmts your using supports scdma for noisy cable plant.
as for what problems the wrong channel or frequency you use causing problems for the cpe... if the modem has problems staying online, so will the customer.
I would recommend 555Mhz down and 32Mhz upstream frequencies. default examples. modem receive level of 0 and transmit of 45.
snr around 30 and you will be doing great.

~Carl

collangm
Thanks Carl.

Thanks Carl.
The intent is to build a model network in the lab to replicate that in the field. We will be employing a Cisco uBR10012 (I-CMTS) platform with 20-25 Cisco EPC3010 (EuroDOCSIS 3.0) CPE modems. We are primarily concerned with DOCSIS Data (i.e Broadband Internet) transmission only and will therefore not be providing any cable TV services (Analogue or Digital (DVB-C)). In the real-world, does this mean we can effectively use that part of the spectrum normally reserved for TV broadcast services or are these still out-of-bounds due to possible ingress from other external RF sources (i.e. Freeview (DVB-T) TV services - Digital TV in the U.K.)? Are there strict guidelines on what or not to use within the allotted spectrum band (5-65MHz + 112-862MHz) for DOCSIS transmissions over the cable plant?

Cheers,

Gary

cmcaldas
offair channels

as long as the field engineers keep the cable plant tight or clean, those channels can be used. since going with Cisco, I would ask them what they have seen with the Euro customers they have. I haven't worked much with Cisco Euro and could not give an honest answer. 50Mhz on the return should be good for the upstream, downstream, I would leave that to others what should be used. Being 8Mhz wide downstream, if using a system with a downstream pilot, I would put the carrier close to the pilot and use it as reference, it would be a more stable channel compared to one at the end of the forward spectrum

~Carl

collangm
I think perhaps my initial

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?

Thanks,

Gary

cmcaldas
Yes

it can be used, would recommend it be monitored. but is used with other cmts Euro systems. the cable system can use the same frequencies as off-air channels. ingress and egress, where those signals can leak into the cable plant would be the possible problem. example, the cable shield is cracked or broken and signal leaks out causing interference with channel 4 off air. ham radio, down around the 9Mhz can ingress in the same point and travel all the way back to the head end.
those would be the kind of problems seen

~Carl

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