What Ken said. Also, the upper band isn't a flat 36 dBm; the SM gets to transmit at up to 53 dBm under the PTP rules.
On Thu, Nov 21, 2019 at 11:55 AM Ken Hohhof <[email protected]> wrote: > Yes, that’s true, but a higher gain antenna at the SM end helps rcv but > not xmt. And SMàAP is the direction you may actually need a better > signal because the AP likely has a sector antenna and is mounted higher so > it sees more interference. > > > > It would not be unusual to have a 16 dBi antenna at the AP but a 25 dBi > antenna at the SM. The antenna gain would help the rcv signal at the SM, > but it would probably have to lower its conducted power by 9 dB to stay > within the regulatory EIRP limit. > > > > In contrast, in U-NII-3 the CPE end is treated as point-to-point and can > use antenna gain to exceed the AP limit of 36 dBm EIRP (subject to OOBE > limits). > > > > *From:* AF <[email protected]> *On Behalf Of *Adam Moffett > *Sent:* Thursday, November 21, 2019 11:27 AM > *To:* [email protected] > *Subject:* Re: [AFMUG] dumb DFS questions > > > > There might be something I don't understand, but I thought you had flat > EIRP limit of +30dbm whether it's an SM or an AP. > > On 11/21/2019 12:11 PM, castarritt . wrote: > > 6 dBm loss for the AP transmit isn't the end of the world. It's the up to > 23 dBm loss on the SM transmit power that destroys the usefulness of DFS > for PTMP past a couple miles. The ~16 dBi gain 90° sectors 2-300' up in > the air just can't hear those SMs over all the noise they are picking up. > What we need is the ability to run downlink on DFS and uplink on 5.2 or 5.8. > > > > > > On Thu, Nov 21, 2019 at 10:56 AM Adam Moffett <[email protected]> wrote: > > Yeah I think on most equipment you can set alternate channels that are > just shifted over 5mhz from where you were. And yeah I think the channel > needs to be clear for a few minutes before you can go back to it. > > Assuming you don't really have a TDWR near you, I don't think DFS events > are that big of a deal. My understanding is that DFS events are more > likely if you lie to the software about antenna gain to cheat the EIRP > limit. False detects happen, but I don't think it's a daily event. > Disclaimer: I've mostly used it on Point to point with dishes. I'm not > sure if you'd pick up more anomolies on a sector antenna. > > The biggest bummer is the EIRP limit. When you're trying to get that 32 > SNR for the 256QAM then losing 6db kind of hurts. Or when you've already > got someone hooked up 10 miles away and lowering the power ruins them. > > Where you really want to use DFS (In my opinion) is at a site where you > have a bunch of customers within 1-2 miles. Unfortunately I don't have > sites like that. > > -Adam > > > > > > On 11/21/2019 11:31 AM, Ken Hohhof wrote: > > We mostly avoid DFS frequencies on APs because of the impact if we get > false radar detects. Also we are mostly a Cambium shop. So I’m a bit > confused about DFS on other vendor equipment like Ubiquiti as well as home > routers. > > > > Question 1 – what happens when there’s a DFS detection? On the Cambium > gear, we have to select 1 or 2 alternate frequencies. But on other gear, I > don’t see this. When there’s a DFS hit, does it jump to another random > frequency? Does it rescan the current frequency until it tests clear and > only then resume transmission? Is the answer right in front of me and I’m > being stupid? Maybe in the case of routers they are exempt because of low > EIRP? > > > > Question 2 – what about 40/80/160 MHz channels? We have a competitor > using Ubiquiti gear and advertising residential subscriber speed plans up > to 100x100. Clearly they must be using at least 40 MHz channels if not 80 > MHz, or else their marketing people have burning pants and long noses. And > I don’t see how a WISP, especially one surrounded by other WISPs, could use > wide channels other than in DFS bands. We have some PTP links using 40 MHz > but only 10 and 20 MHz channels on our APs. So assuming you are using 40 > or 80 MHz in DFS, what happens when there’s a DFS detect? Does the whole > 40 or 80 MHz have to find a new home? Can it slide over 2.5 or 5 MHz and > substantially overlap the previous occupied spectrum? DFS bands come with > enough spectrum to use wide channels, but is there enough to jump around > when you take a DFS hit? > > > > -- > AF mailing list > [email protected] > http://af.afmug.com/mailman/listinfo/af_af.afmug.com > > > > -- > AF mailing list > [email protected] > http://af.afmug.com/mailman/listinfo/af_af.afmug.com >
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