On Sun, 18 Oct 2020 19:32:40 -0700, Johann Beretta
>On 10/18/20 1:25 PM, Jeff Liebermann wrote:
>>> My gear (Ubiquiti) supports 5, 8, 10, 20, 30, 40, 50, 80 Mhz wide channels.
>> In what country? See:
>The United States. I'm using official firmware on a US radio. I had
>heard that newer radios were limited to 10MHz as the smallest slice, but
>older gear is grandfathered in.
I'll assume a Ubiquiti M5 radio. I have some really old M5-Bullet
radios, with firmware that can't be upgraded to the latest greatest.
However, my house is a mess resulting from my office move, and I'm not
inclined to dig one out and check what it can do. I did some Googling
and found that the Rocket-M5 does support 5 and 10 MHz channel
bandwidth, so I'll assume that your unspecified M5 version also does
The article conveniently explains part of the logic behind using wider
channels and mostly answers my question from my previous rant, which
you deleted and/or ignored. Basically, the approximate math is
simple. If your WISP configures their access point for a 40 MHz
bandwidth channel and the ISP has 10 full time connected users, the
system can deliver no more than 4 Mbits/sec to each user. If the WISP
reduces the occupied bandwidth to 5 MHz, and still has 10 full time
users, each one will only get 0.5 MBits/sec, which is inadequate. If
your WISP doesn't have much of a user load, or doesn't overload the
channel with too many wireless users, 5 MHz occupied bandwidth will
work just fine. Note that this simplistic channel loading estimate
ignores various factors that will either increase or decrease the
channel loading. For example, I'm assuming that the channel usage is
sustained at the maximum available rate, which is sometimes a bad
assumption. This becomes really messy if the streaming media provider
adjusts their deliver rate based upon error rate levels returns from
the viewers computer or media player.
Also, there is a problem. This assumes that the WISP has exclusive
use of the channel and that there are no other users on the same
channel. Any co-channel users will appear as interference causing the
WISP access point to lower the data rate to a level where the BER (bit
error rate) is high enough to produce usable throughput. In many
cases, this throughput reduction can be drastic, but for this
discussion, I'll assume it reduces throughput to half. That means
that delivering a given amount of data will double the air time (how
long the transmitter occupies the channel) and delivery will therefore
take twice as long. Actually, it's longer because the packet size is
also reduced, but to keep things simple, I'll ignore that. The result
of slowing down due to interference is that every users connection
slows down, and data takes twice as long to deliver. Instead of ten
happy Netflix viewers, the ISP support phone will have 10 irate
customers complaining of buffering.
So, what can an WISP do? Well, it could not load the channel to the
maximum capacity for a given occupied bandwidth. It could add another
radio on a different channel and move some of the customers there. Or,
it could just size the occupied bandwidth setting to match the actual
channel loading with some overhead left for interference and high
So, why did your WISP use 5 MHz. None of the advanced 5GHz mode
beyond 802.11a are going to work well crammed into a 5 MHz occupied
bandwidth channel. I'm not sure if 802.11a will work in a 5 MHz
channel. 802.11ac requires an 80 MHz channel. It would be
interesting to sniff the traffic between your Ubiquiti M5-something
radio and the WISP access point with a Wi-Fi Analyzer (Android) or
something similar. My guess(tm) is you're running 802.11a.
So, what kind of performance can one expect in a 5 MHz wide channel
compared to a 20 MHz channel? That would 1/4th the speed *OR* double
the range due to increase in power density (dBm/Hz). That's why it
was attractive to your WISP. Cut the data rate in half yields a range
increase of sqrt(2) or 1.414.
That's also why the FCC and other regulators seem to have purged 5 and
10 MHz occupied bandwidth from the rules-n-regs. It's much too close
to narrow band modulation and carries some of the detrimental effects
of narrow band modulation. It was fine when the typical 5 Ghz signal
used 20 MHz modulation. However, with 40, 80, and 160 MHz now
available, the narrower occupied bandwidths had to go.
>> 10 MHz is
>> available but I don't know any situation where it might be useful.
>As for the usefulness of 10Mhz, well... Seriously?
>I can think of all sorts of things.. Namely anything where you need
>70mbps or less.
>Or, in a really crowded area, you might be able to find 10Mhz of clean
>My own link to my WISP is 10Mhz (I have my own dedicated AP). Delivers
>me everything I need without having another 10Mhz just polluting the area.
>I could probably get away with 5Mhz, but I've upgraded to the AC line
>and that is no longer an option. It is, however, still an option in the