I'm setting up a Docsis 1.1 uBR7223 on a small rural catv plant. We're using 25 channels for TV (2-25) and our bandwidth is about 230Mhz. Without losing any TV channels, which frequency can I use for Downstream? Which is known to work best and do any of these require FCC or FAA permits/reporting? What do I need to look out for?
The docsis 1.1 spec found here (page 9) specifies you can use any frequency greater than 88mhz and less than 860 mhz for the downstream or forward frequency.
using a catv frequency chart available here there are only 5 available video channels greater than 88 mhz and less than 230 mhz that are not assigned to CATV channels 2-25 they are A-5,A-4,A-3,A-2,A-1 more commonly known as channels 95-99. Keep in mind these video channels are in the FM band so you need to be careful when choosing your frequency, cuz it may be possible when there are probs with the cable plant that FM stations will bleed thru onto your cable plant and interfere with your signal. A spectrum will be invaluable here.
if you are transmitting/receiving over cable plant no additional FCC permits over what you need to be a cable operator are required to use these frequencies. As for which works best, hard to say gonna have to look at a spectrum analyzer and see which center frequency looks cleanest, I have used A-3 (center frequency 105mhz) on a 7246 cmts without too much trouble.
hope this gives you some direction.
Quick answer with a question. First off stay away from any channel that you have off-air, especially the fm band. ingress on older cable plants is always an issue. do you have a pilot on your downstream for agc? if so, I would recommend putting your docsis carrier close to it. the tech's in the field could use the pilot for reference as far as signal level goes and the carrier would have less change in level. up here in New England for example, summer's in the 80's and winter in the 30's or less. that cause levels to change allot. but that's my two cents' worth
~Carl
The person who set this plant up said he used channel 13 (ESPN) as a pilot. We don't have a specific pilot generator. We also don't have much for test equipment (just a portable db meter). There is a pretty strong FM station around 107Mhz near here, so maybe A-1 and A-2 (from chart) might be good choices for the modems?
Our system is located in western Oregon in a coastal temperate zone where it rarely goes higher than 80 or lower than 25. Avg temp is 62.
A1 and A2 would be off limits because of aircraft. if there's no airports in the area that might be ok, make sure you do your cli reports though.
Some more things to consider:
1) The higher the frequency, the higher the attenuation of the cables. Even if using AGC-enabled amplifiers with a pilot, there's always one part between the last one and the client. There been quite a few situations where choosing a lower frequency has saved us from installing an amplifier to reach just one more remote client.
2) Placing the DOCSIS carriers below *all* analogue TV channels gives you the ability to serve internet-only customers without giving them the chance to watch TV without paying for it by using a low-pass filter, which passes everything up to & including the DOCSIS carriers.
3) In some countries (e.g. Bulgaria) there are some frequency ranges forbidden by the local FCC-like authorities to be used even in cable plants for analogue transmissions. They do allow for digital transmissions at that frequencies, so this might be a good way to optimize the use of the cable plant.
4) If you operate several DS channels, place them one after another or as close as possible. This gives you the ability to balance/move modems between them without having to worry about varying with the change DS signal levels.
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Lancey, can you elaborate on item #2? All our amps and filters use a bandpass of 50-230 for downstream and 5-48 for upstream. Our upconverter is 50-1ghz. I haven't found any over the counter products that would allow downstream traffic below 50Mhz. What products do you use to do this?
No, you have not understood me. DOCSIS downstream channels can't go below 55 MHz I believe, and it all depends on the capabilities of the modems you have. According to the standards, downstreams should be above 88 MHz.
I'm speaking about regular downstream frequencies. The point is to use the first available channels (according to your channel plan) for DOCSIS downstreams, and place all the analogue TV channels above them. E.g. let's say you have 2 DOCSIS downstreams, you place them at PAL/D channels 2 & 3. If you have a customer which wants to pay for internet only, but does not want to pay for TV, you put a ~85 MHz low-pass filter in front of their households, so they get the DOCSIS downstreams only.
Of course, this is an example only, and this advice is only relevant if you want to be able to service internet-only customers and are concerned about TV service theft from them.
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Oh, we're US, channel 2 (55.25) is a much lower freq. than PAL. We're going to have a lot of subscribers wanting Internet, but not wanting to get TV. Using a High-pass filter, it's easy enough to block the upstream channels for non-internet subscribers, but I just don't see an easy way of blocking TV for those who only want to pay for internet. Especially if we use channel A-2 (109.275) for downstream. As soon as word gets out, all our internet subs would cancel the TV subscription and then hook'em up for free.
Airport or no airport, I'd strongly suggest A-2 downstream in a 220Mhz system. It's in the middle of your spectrum and will stay more consistent level-wise relative to AGC fluctuations and certainly be less affected rolloff-wise at end of line situations than trying to sneak your data somewhere up on top
of your spectrum. Furthermore, if off-air ingress of any kind is affecting your forward path, your reverse will not even be unusable. I'd advise getting
someone to do a thorough leakage ride-out and system alignment/sweep in both directions before even trying to deploy data services beyond a lab
setting! If all ends up looking good there, 21MHz upstream is usually a fairly nice spot in a decently maintained system -- unless you're using telco return, of course. Good luck.
Regards,
psmit
Oops. I obviously meant that if your forward path (downstream) is so full of holes that aircraft frequencies (or any other frequencies) are getting in, your return path (upstream) would NOT be usable for the same reasons,
but not necessarily because of the same sources of ingress. That's why a leakage ride-out is so important prior to two-way services deployment and why a system alignment and sweep are absolutely mandatory prior to any
reasonable chance for success at deploying same -- provided all the RF issues are appropriately identified and addressed in BOTH directions of your proposed 'network' (existing 220MHz cable plant). It all boils down to the
integrity of your physical medium of service delivery - whether it is for my old school LANcity modem platform, or the latest greatest version of whatever else is coming down the pike tomorrow on GigE Fiber to the home. If your plant is junk, none of it will work right, if at all.
Again, spend the cash to get your system analyzed for the services you anticipate deploying. And spend more
cash to acquire the necessary testing and monitoring gear to keep things running smoothly -- presumably with
competent resources to handle the ongoing monitoring and maintenance tasks required for a two-way environment. And there are significant ongoing maintenance and monitoring tasks associated with any two-way cable system be it DOCSIS or a simple RF return from a modulator at your town hall meeting place every other Monday evening. If you don't have the resources to t-shoot the problems when they happen, it's gonna be a long, long, day for the poor sap assigned to the duty with just an old Sadelco meter and no clue.
On the other hand, if you are a small rural system in Oregon just wanting to integrate cable modem services into a small system, DOCSIS is hardly mandatory for doing so and you may want to consider other ways of
providing those services such as the last greatest Gen3 cable modem platform offered by what is now Arris
that can be provisioned and run on Windows 95 if you can believe that.
Regardless of your choices, the first one you need to make is whether or not your cable system is healthy enough to support two-way services -- DOCSIS, Gen3 modems, or Town Hall video. Period. Ya need the test
and maintenance gear to set the network up and keep it tuned with the personnel who know how to do it.
And as for transmitting your downstream data below analog TV (sub-diplex filter?), it's not possible.
Best of luck with your endeavors.
psmit