I've read all the whitepapers and watched the videos, but I can't figure out what is actually happening when front or back is selected. I understand the re-use parners should have the opposite setting (front v back), but why? What is actually happening and why is this setting necessary?
I have a feeling both frequency re-use partners don't transmit at exactly the same time as is advertised with a slight offset based on the front/back setting, but that is only a guess.
Please explain what ths setting is actually doing.
I've read all the whitepapers and watched the videos, but I can't figure out what is actually happening when front or back is selected. I understand the re-use parners should have the opposite setting (front v back), but why? What is actually happening and why is this setting necessary?
I have a feeling both frequency re-use partners don't transmit at exactly the same time as is advertised with a slight offset based on the front/back setting, but that is only a guess.
Please explain what ths setting is actually doing.
Thanks!
Hi maglito,
The explanation you are asking for is confidential information. That setting is one of the reasons why GPS sync/frequency reuse works on ePMP, which is unique for 802.11 products. Without revealing too much, the frames are not offset, they are exactly in sync and transmit and receive at the same time. But with this setting each AP is aware that there is another AP trying to reuse the same frequency and "adjusts" its mechanism to not interfer with the SMs on the other AP.
Frankly, I've been wondering the same thing. I want to know how to configure the front/back settings for use in a ABABAB environment. Of the three radios on the same channel, which one(s) should be configured as front and which one(s) should be configured as back? Your document titled "The Transitionfrom Unsynchronized to GPS Synchronized Networks - Finally Explained" clearly shows an ePMP tower configured with six sectors and two channels on the very last page. How do we configure the radios in this scenario?
Thanks for pointing out the document. With ePMP, we do not support a ABABAB six sector deployment where only two frequencies are reused. I think the document meant to say ABCABC. I will check into this and get back to you.
As for which APs should be set to "Front Sector" and "Back Sector", it is on the APs using the same frequency. In other words, in an ABAB deployment, the AP using frequency "A" should have it set to "Front Sector" and the other AP also using frequency "A" should have it set to "Back Sector". Similarly for the APs using frequency "B". This setting is discussed in detail in the ePMP User Guide available here: https://support.cambiumnetworks.com/files/epmp/. It lists out a few possible scenarios and provides guidance on how this setting should be used.
Yes, I do understand that when using two radios on a single channel one must be set to front and the other back. I don't think that ABABAB was a mistake in the document. I think the author very clearly intended to illustrate a 6-sector 2-channel deployment as the diagram shows two channels each being used three times, and there is a footnote mentioning that antennas with greater than -30 dB of loss at 90-degrees are needed. From what I have been able to find, none of Cambium's sector antennas meet that side lobe specification, so its mention can't be an accident. Such a side lobe requirement would not be necessary in an ABCABC scenario where the two sectors sharing the same channel would be oriented 180 degrees away from each other.
Yes, I do understand that when using two radios on a single channel one must be set to front and the other back. I don't think that ABABAB was a mistake in the document. I think the author very clearly intended to illustrate a 6-sector 2-channel deployment as the diagram shows two channels each being used three times, and there is a footnote mentioning that antennas with greater than -30 dB of loss at 90-degrees are needed. From what I have been able to find, none of Cambium's sector antennas meet that side lobe specification, so its mention can't be an accident. Such a side lobe requirement would not be necessary in an ABCABC scenario where the two sectors sharing the same channel would be oriented 180 degrees away from each other.
Kevin
Hey Kevin, I assume you're talking about this image on the last page?
As you surmised, there's no way to specify a front or back sector in this ABABAB configuration. I'm almost 100% certain the author made a mistake and intended to use an 'ABCABC' configuration, which would make much more sense and allow for the front or back setting to be applied properly. Additionally, in this ABCABC configuration, one would need 3rd party antennas (Cambium doesn't have an ePMP OEM sector antenna less than 90deg, but one could possibly use a PMP450 60deg antenna with RF adapters) with excellent F/B ratios and a reduction in horizontal beam width to 60deg or even 45deg to help decrease side-lobe gain. Here's what I believe the author inteded to publish:
Yep, that is the page. I hear what you're saying, but the footnote on that page reinforces my belief that the author did intend to write what he/she wrote. There are 3rd-party 60 and 45-degree antennas that do meet the footnote's side lobe spec, and the OEM 90-degree sector would work fine in an ABCABC setup with no footnote needed.
My challenge is that we are trying to use ePMP in an extremely noisy urban RF environment. The OEM 90-degree sector is just too wide and sees to much. If we use an antenna with a narrower beamwidth, we get acceptable performance. Even with the narrower antennas, it's hard to find two 20 MHz channels, yet alone three, on which to operate. Being able to use an ABABAB or even an ABABABAB configuration would give us the added flexibility we really need.
ababab isn't going to happen, ABAB can. you need 32 DB CIR, i've never seen any sector antennas drop 20 db in 60 degrees let alone 32. if you are urban, 5ghz will likely suite you better. if your more rural, ABC in 10mhz or 20 mhz channels on 2.4 work well with the recommended antennas (i actually just posted photos of this arrangement in the "your stores" segment.
also, the EPMP system only supports front and back antennas (it has a special mechanism that needs enable for ABAB or ABCABC to work. so 3 antennas using the same signal won't align properly. it may work enough to kinda go, but won't be right.
If you do manage to find antennas that drop 32 DB in just 60 degrees and has a 60 degree wide face side, I'm sure we'd all love to have them, me especially.
if you can't find them, good sector tilt will go along way, we limit our outer edges to 3 or 3.5 miles with tilt, it significantly reduces the noise they hear, it doesn't completely eliminate the noise, but helps a lot in town.
Maybe the new RF Elements Symmetrical Sector antenna (scalar horn antennas) may be something to consider and look at. They even did a webinar on it earlier this year. So it can give you a pretty good idea on how their new antenna technology works.
Here is a question I have... If all sectors have internal GPS and are firing at th same time why are the front / back colo options necessary any way? I do not see the difference having front / back enabled for two sectors versus just having them synced without utilizing the front / back settings. We usually have ABCD to work with any way, but I would still like to understand the differece front/back make in the sync process vs. just having them gps timed for TX between the APs. Thanks
carullos wrote: ...but I would still like to understand the differece front/back make in the sync process vs. just having them gps timed for TX between the APs...
Hi Carullos. I think Cambium considers this as part of their ''secret magic sauce'' and I don't think they are going to reveal too many details about the actual how/why of this particular setting. They have previsouly said that this is the way that one AP knows that another AP is trying to share the frequency. But they've also said that the exact technique of what the mechanism tweaked is ''why ePMP works'' and why others who have tried to do that have failed. So - Im guessing that they are not going to reveal much more of their 'intellectual property' other than - that's how each AP knows another AP is there. :)
A higher level conversation on this list is not going to lead another manufacturer to fix their products or steal the secret sauce. I simply want to know it I set app ePMP sectors on different freqs on one tower to the same uplink and downlink settings, distance etc without using the ront and back settings (read - like PMP450) do they all TX att eh same time? A simple yes or no would be fine. I assume yes because thats how everything else works and why the frame settings have to be the same between APs. Assuming this is the case, I can't imagine what front / back setttings do. Don't really care so long as they all fire at the same time... but if they are not firing at the same time and require front / back settings to do this even when on different frequencies then I believe there is a gross misunderstanding and expectation about how sync benefits between towers and sectors. Thanks
A higher level conversation on this list is not going to lead another manufacturer to fix their products or steal the secret sauce. I simply want to know it I set app ePMP sectors on different freqs on one tower to the same uplink and downlink settings, distance etc without using the ront and back settings (read - like PMP450) do they all TX att eh same time? A simple yes or no would be fine. I assume yes because thats how everything else works and why the frame settings have to be the same between APs. Assuming this is the case, I can't imagine what front / back setttings do. Don't really care so long as they all fire at the same time... but if they are not firing at the same time and require front / back settings to do this even when on different frequencies then I believe there is a gross misunderstanding and expectation about how sync benefits between towers and sectors. Thanks
Hi carullos,
If the APs have the same DL/UL ratio and have the frame size, the will start and stop TXing at the same time. In ePMP, unlike PMP 450, the Max range (distance) settings between the APs do not matter and does not impact frame start times.
The "Front Sector" and "Back Sector" setting is only required if you plan to reuse the same frequency on back-to-back sector on a tower or sectors on two nearby towers that can hear each other.
A higher level conversation on this list is not going to lead another manufacturer to fix their products or steal the secret sauce. I simply want to know it I set app ePMP sectors on different freqs on one tower to the same uplink and downlink settings, distance etc without using the ront and back settings (read - like PMP450) do they all TX att eh same time? A simple yes or no would be fine. I assume yes because thats how everything else works and why the frame settings have to be the same between APs. Assuming this is the case, I can't imagine what front / back setttings do. Don't really care so long as they all fire at the same time... but if they are not firing at the same time and require front / back settings to do this even when on different frequencies then I believe there is a gross misunderstanding and expectation about how sync benefits between towers and sectors. Thanks
but why give anyone else a clue? they've done what essentially everyone in the field said can't be done with a low cost PHY.... I wouldn't even begin to open my mouth about what mechanisms have been put into play. what it does is everything that has been over looked by everyone else. The cambium team has shared more than i ever thought they would about under the hood of the EPMP radio, i completely understand why they wont explain the flux capacitor to anyone, even at a high level. ;)
I assume you would know the answer but...I have absolutely seen the distance setting affect performace on synced APs, also from the optimization guide... This parameter is set based on the farthest SM being deployed. Calculate the distance of farthest subscriber module using Link Planner or any other suitable tools. Set this value 1 to 2 miles higher than the calculated number. Do not set this parameter to the max value as it will waste time allocated for data transmission when it is not needed
it will waste time for data transmision...this would indicate to me that changing that value would affect the timing frame, if I am wrong just disregard and I will go on doing it as I have always been since it works. If I am right I just wanted to make sure people are not shooting themselves in the foot.
I assume you would know the answer but...I have absolutely seen the distance setting affect performace on synced APs, also from the optimization guide... This parameter is set based on the farthest SM being deployed. Calculate the distance of farthest subscriber module using Link Planner or any other suitable tools. Set this value 1 to 2 miles higher than the calculated number. Do not set this parameter to the max value as it will waste time allocated for data transmission when it is not needed
it will waste time for data transmision...this would indicate to me that changing that value would affect the timing frame, if I am wrong just disregard and I will go on doing it as I have always been since it works. If I am right I just wanted to make sure people are not shooting themselves in the foot.
-Ben
Hi Ben,
You are correct in that the distance settings affects performance but it does not affect the ability to sync two APs. This is because the calculations required to determine downlink and uplink framing are designed such that transmissions will not overlap. In other words, once you set the frame size (2.5 ms or 5 ms) and the Duty Cycle on two APs the same, the frame timing (Tx portion and RX portion) are the same between them. The distance settings govern how much time needs to be "wasted" to accomodate the propogation delay from AP to the SM and vice versa WITHIN the Tx portion of the frame or WITHIN the RX portion of the frame. The actual Tx start and stop does not change within the full TDD frame. Therefore, uneccessarilly setting the Max Range too high will result in increased "wasted" time (to accomodate calculated progpogation delay) which means less time to transfer real data but it does not impact the ability to sync two APs. This is why we recommend setting the Max Range just above the distance at which the farthest SM is deployed in the sector.
Is there a table out there that relates expected bandwidth vs duty cycle and distance? It would be nice to have a rough idea for site planning. We have settled on 9miles as it seems to provide the best bandiwdth and distrance from our emperical evidence, but if we can increase our outbound by x amount by building more sites to cover the same area it would be nice to know.
Is there a table out there that relates expected bandwidth vs duty cycle and distance? It would be nice to have a rough idea for site planning. We have settled on 9miles as it seems to provide the best bandiwdth and distrance from our emperical evidence, but if we can increase our outbound by x amount by building more sites to cover the same area it would be nice to know.
Hi Ben,
There isnt a table that relates the two parameters since there are more parameters that impact the bandwidth you can expect. Also each deployment is unique in terms of number of SMs, distance of each SM, MCS levels achieved on downlink and uplink per SM, terrain etc. I recommend plugging in values into the ePMP Capacity Planner or even better use LINKPlanner to predict the performance. In fact, impact of Max Range to sector performance is negligible compared to the other parameters I listed above.
the speed penalty from propagation delay is related to the speed of light, at 40 miles, your looking at a little less than 3% penalty, if I did my math correctly. as Sriram pointed out, the propagation delay is the least of your worries at a range were it will become on an issue. speed of light is 186 miles per millisecond, 186,000 miles per second.
at 40 miles, your looking at a little less than 3%
Thats what I was wondering we installed pretty much everything at double 15, it sounds like worrying about keeping your distance setting fine tuned would be a waste of time.
