ePMP 1000 latency for GPS Sync on 2.5 firmware


I read on the support website that 2.5 Software release include a reduction of latency for GPS by 40%.

Could you provide some example and real numbers?

The ePMP Capacity Planner Tool R2.4.3 isn't updated to the last version and doesn't include Force 180 and Force 200 SM, isn't it?

Thank you

2.5 Software Release 

New features inlcude: 

  • Deployments can now be synchronized with PMP 100 deployments
  • Graceful migration of PMP 100 to ePMP in a spectrally efficient manner
  • Reduction of latency on the ePMP system for GPS Sync Operating mode by 40%

Hi Giuseppe4;

Here are three real world screen shots.  This is from my desktop at work, to my ePMP SM at home. So, this is through our wired network in the store, out through our AirFiber Wireless backbone a couple hops, and then through the ePMP AP to a connected ePMP SM.  There is about 1ms from my desktop to the ePMP Access Point

This is 5 ms Fixed Frame (GPS Synced / TDD / 75%-25%) - 15ms average.


This is  2.5 ms Fixed Frame (GPS Synced  / TDD / 75%-25%) - 8ms average.


This is 5 ms Flexible Frame (TDD / Flexible %) - 8ms average.


Of course, the upside of 'Flexible' is that give the max throughput up or down depending on your needs, and the latency is low - but it (obviously) can't really be Syncronized across multiple sectors/towers, since one sector/tower may be uploading while another one is still downloading.  GPS Synched on fixed frame lengths and the same U/D Ratio will add in a few ms of latency obviously, but then all the towers/sectors will all be transmitting at the same instant and will be receiving at the same time and greatly increasing the ability for the secotrs to hear the SM's and greatly decreasing self interference.


Great answer! Thank you!
May Inask you the throughput difference between 2.5 and 5ms and Synced/Not Synced?
Are you the only user on AP?
What firmware version do you have?


Yes, on this Access Point, I am the only client. This was our first test AP when we were first evaluating ePMP vs some other brands.  To describe this setup, this is right in downtown, the AP is on the top of an 18 story appartment building, and shooting right across residential town to my house about 4 KM away. I previously had a different brand of 2.4Ghz link to my house, and it would struggle to get 1Mbit sometimes.  At other times I could get 3mbit or 5mbit if I waited till 3 AM to use it - but it was suffering from interference pretty badly. I can hear a bazillion routers in 2.4Ghz in the site survey, so I wasn't too surprised. 

So, I fgured it would be a good test for ePMP - I honestly figured it would fail about as badly, but figured then we'd at least know the worst possible case. So, I switched out the AP to a ePMP Access Point (same mount, same sector, same everything) and I switched out my SM from the other brand to a ePMP (same mount, same dual pole grid, same everything) so that this was JUST a comparison of Brand X to Brand Cambium.

My performance went from ~1 or 5 Mbit, to about 100 Mbit throuhput.  I get ~70 Mbit down and ~30 Mbit up when testing to our local SpeedTest server at the shop.


As for your other questions, I see very little real-world throughput difference from one to the other as it varies a bit from test to test and day to day anyway. However, 5ms is probably 10% faster overall I'd guess, and it just seems to work better for me. I think the 2.5ms was primarly designed to allow better syncronization with the older Motrorolla PMP100 radios, and no so much for lowering latency. Your mileage may vary though...

If lowest latency is the most important thing, then you can choose to NOT syncronize and you can lower the latency the most, but then you defeat much of the magic that is ePMP.  If you want it to be scalable and if you want to try to mitigate as much self-interference as possible, then you're going to want to add in that extra few ms of latency and you're going to want to GPS Sync everything with Fixed U/D Ratios.

For me, I'm normally on 5ms frame size. For me, an extra 7 ms of latency plus or minus is nothing - we are probably 50 ms from Google.com through the internet anyway, so to me it doesn't much matter if that's 50 + 8 ms, or if it's 50 +15 ms - there's not much difference.

I'm running FIrmware 2.6 and I haven't really had any issues - it just works and works and works, despite the cold, despite the interference.


Thank you very much, your answer are very helpful!

I'm waiting for my first ePMP shipping. I own a WISP and I would like to switch to Cambium ePMP. Your results seems to be very promising!

I always considered only 5GHz because 2.4GHz is crowded by home routers and so on, but your experience seems to prove that you can have good results using Cambium and GPS Sync!

I'm wondering if with a big number of customers the latency grows up using GPS sync... 

Well, I can't actually give you 'real world with ePMP' on our network, since we're still deploying and don'thave any large client count sectors yet. However, I can tell you that we've used Canopy 900Mhz stuff for years, and with only 3-4 Mbit on the AP for everyone to share, it'll do a remarkable job of scaling everyone and keeping latency predicable.  I can also tell you that when I went to the ePMP Training Session in Edmonton last fall, there were a couple WISP's there that had changed out from Ubiquiti to ePMP and they had  60+ SM's on each Access Point, and they were thrilled.

Of course, they were 5Ghz (not 2.4) and they were doing relatively low plas of 3 and 5 Mbit plans only, and their install criteria insisted on every install being -60 or better, and if not they left the low signal guys on thier other technology and so on. But they felt that 60 was quite doable as a client count as long as the install criteria was strict. But wiith other gear, 20 or 25 is starting to stretch things.

And yes, latency can potentially certainly increase depend on a whole bunch of things.  If you understand how the scheduler works, the Access Point basically looks at everyone who needs to receive or transmit data, and the AP give them a time slot for their data.  So, it's basically saying ''SM#13, you can transmit in 320ns, SM#17 you can transmit in 480ns" and so on. So, if you have 40 clients and they all need to transmit data at the same time, and if you have them all set on a plan with 10 Mbit download and 2 Mbit upload, and if you've got your ratios set to 75%/25% and if you have 5ms frame size.....    well then there is only certain math that works - there is only 25% of that 5ms frame that is the upload slot, and at some point there is no math that makes it all work out. There isn't going to be 40x10 Mbit download and 40x2 Mbit uplaod all a the same time on that AP - so something is going to give - that's just physics.

But, what the Cambium ePMP scheduler does is an 'air fairness' so that everyone get's a fairly fair split of the available air time, and it'll try to slow everyone down fairly and not let someone close and loud monopolize the air time, and it does it in a way to minimize ontention or colissions or ACK's (which waste air time for everyone on other gear). The AP and the SM's know when they should listen and when they should talk, and they don't need to do a 'clear channel sense' to see if the channel is clear or not, and then maybe transmit and maybe still bump into another customer who's transmitting at the same time and so on.. The Cambium ePMP scheduler does a much much more organized job of all this.

Also, with almost all other makes/brands, every packet that is transmitted is 'ACKed' from the far side, so as distance from the AP to the SM's start to increase, performance decreases - and it decreases for everyone, even the close guys. So it's CPE#1 talks and waits for it's ACK. CPE#7 talks and waits forit's ACK and so on. While each CPE is waiting for it's ACK's, nothing else can happen for any other CPE either.  With Cambium ePMP, the AP knows how far away the farthest SM is, and when the AP transmits it's data to each SM, it also tells them when to send back their ACK's - so the Cambium Access Point basically does SM#1 here is your data (and wait X ns), SM#7 here is your data (and wait Y ns), SM#24 here is your data (and wait Z ns) and then all in sequence, all the SM's can then send back an ACK in the 'ACK schedule'.  Much much much more efficient.

So, people are often talking about 802.11AC an dhow much faster that could be - and that's partly true.  However, there are tons of 802.11AC gear out there that wastes 1/3 of the potential air time with collisions, senses, ACKs and so on.  802.11n gear that wisely uses 100% of the potential of the chipset can end up being much faster in real world, much lower latency in real world, much more scalable in real world.

Oh...  I'm talking a lot....  I think I had too much Coffee today...  ;)


No problem, it's great to share :-D

I hope I'll have good experience with ePMP 1000. Currently, using MikroTik, I'm limited to about 25 users per sector, I can't do frequency reuse, and I'm giving 10/1Mbps plans. I hope I can have 60-80 users per sector, or at least have a good frequency reuse so I can have multiple sectors with the same frequency!
Who knows, only experience will tell if Cambium performs good!

Thank you for your time


As you have indicated, the ePMP Link Capacity Planner Tool has not been updated for Force 180 and Force 200 subscriber modules yet. From the tool perspective, the only thing that changes with the Force 180 and Force 200 is the antenna gain, so you could work-around this limitation selecting the "Third-Party Connectorized SM" option and entering the appropriate antenna gain that corresponds to the Force 180 or Force 200 for the planning.


Thank you for your answer!
I can’t set 2.5ms, it doesn’t accept the value.

Is the latency intended “per client”?
It seems excessive to me with more than 40 clients and 5ms

Hi.  If you are asking me, I'm not sure I understand your question?

Are you thinking that the 2.5ms frame a 'latency per client'?  Or can you restate your question so that I understand? :)

I'm asking Luis about the latency you have with 60 users on a single sector using 2.5ms frame.

If you look at "ePMP Capacity Planner Tool R2.4.3" here:


You can see that "Frame duration" is set 5ms as default.

Selecting 2.5ms it can't calculate capacity and latency.

Selecting 5ms and 75/25 and for example 50 users on a sectors I have "Scheduling latency (ms)" 25.8ms DL and 88.3ms UL.

Is that the latency that each user will have using 5ms frame or not?

On the Guide I read:

The DL/UL/Total scheduling latency (in ms) shows the time needed at the AP to schedule all the SMs connected in the sector, and it depends on the number of SMs set in the SYSTEM CONFIGURATION section. This is less than the total latency the system experiences, which depends on additional factors.

But what does it mean exactly?


I don't know what are the specific assumptions behind the estimation of latency in the Planner Tool but I would guess that it is possibly assuming that all SMs in the system have data waiting to be transmitted to them and data waiting to be transmitted by them (no SM is in idle mode). With 50 SMs in the sector, and under that assumption, the AP cannot service or schedule all SM in one frame, so it will need several frames to do so.


Sure, but what is the expected latency per user with 50 users? :smiley:

@giuseppe4 wrote:
Sure, but what is the expected latency per user with 50 users? :-D

We have a sector that has 31 clients on it using 20MHz channel, 75/25 split, GPS sync, and 2.5ms frame and we're seeing an avgerage of 20ms during peak usage times when the AP is busiest. During those peak times the AP is pushing close to 40mbps down, and 3mbps up.

We did have an AP that had 55 clients on it, but we split it up. It was quite awhile ago and before the 2.5ms frame option. I remember that we were seeing around 25-30ms using flexible frames.

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Hi.  For my 2c, that number is 'maximum scheduled latency' IF all 50 clients are uploading and downloading at that maximum capacity.  So, everyone that needs to receive or transmit data will get some air time - so if EVERYONE had their hand up to transmit data, then they'd each get a schedule of when to do that, and if you have 50 clients all trying to transmit in 25% of a 5ms frame, there simply isn't enough time to get all 50 clients done in a single frame, and it'd be scheduled across more than one frame.  So, that number (AFAIK) is the total theoretical schedule time, so YES that could be the latency.  However, in real-world (which you asked for) examples, there are several other factors to consider.

First - of course it would be very unusual to have all 50 clients trying to transmit at full capacity all the time.  That would be a rare case, and if it was comon for you to have 50 people all trying to get 10 Mbit download and 2 Mbit upload all at the same time - then you don't have 500/100 Mbit of capacity anyway - so if you wanted to deliver that, you'd have to have multiple AP's sectors anyway.

Secondly - with other non-GPS-Synced products, they wouldn't schedule in latency.  BUT, that latency would still be there. It wouldn't be scheduled in, so it would instead be chaotic latency.  AP Sectors would be transmitting whenever they figured they needed to - and deafening another sector that was trying to listen to a client.  Clients would be talking whenever they figured they needed to, and their data would be coliding with other client data and they'd both (or all) need to retransmit.  Clients would be talking whenever they figured they needed to, and their data woudn't be heard by the AP's sector, because on of your other AP's would be talking to it's clients and deafening your sector.  The latency would still be there - but in a chaotic disorganized manner.

Also - with Cambium ePMP - if you don't want to use a fixed percentage (50/50 or 75/25 or whatever) you can choose Flexible Frame and then there is a dynamically allocated upload or download capacity, depending on demand.  That can also lower latency quite a bit, but at the expense of the benefits of scheduling syncronization. The scheduler is a very, very good thing - even if putting all the clients into allocated time slots unavoidably adds some latency in the process.  However, be aware that even if you choose to use a Flexible Frame mode - or even with a product with a non-scheduled MAC like most other brands - if you have 50 clients all uploading and downloading at maximum capacity, the latency is going to go way up just simply because it's going to take the AP X ms to get to everyone.  With a non-scheduled MAC, that latency will be chaotic, and it HAS to be higher when packets are going to collide, when every SM has to do a Carrier Sense first, when evey SM has to do an individual ACK for it's packets (which will sometimes themselves not get through) and so on.  A scheduled time slot to talk is the most efficient way to organize all that traffic.



Almost everything ninedd posted above is correct. The latency predicted in the Planner is when all SMs want to transmit data at the same time. 

One minor correction in what ninedd said above regarding Flexible mode. Flexible mode is still a TDD frame and the scheduler is exactly the same as the fixed ratios. Each SM is scheduled and told by the AP when they can transmit. Flexible mode retains all the benefits of a TDD scheduler. The only difference is that in Fixed ratio the frame size and duty cycle is fixed, allowing synchronization with another AP. Whereas in Flexible mode, the frame size and duty cycle dynamically changes every frame based on traffic demand in each direction. Hence it is near impossible to synchronize an AP running Flexible mode with another AP. 



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Thank you for your answers!

It's all very clear now!

Eric Ozrelic, may I ask you why did you split up a sector with 55 users?

What profiles did you have on it?

So the 2.5ms frame option is to reduce latency at the cost of about 10% throughput?

One minor correction in what ninedd said above regarding Flexible mode. Flexible mode is still a TDD frame and the scheduler is exactly the same as the fixed ratios. Each SM is scheduled and told by the AP when they can transmit. Flexible mode retains all the benefits of a TDD scheduler.

Argh - I did know that, honest. :)  I misspoke. :)  I meant to say 'without the benefits of syncronization', not 'without the benefits of scheduling'.  Honest.  :)

What I meant about potentially benefits of Flexible Frame was that the download % and upload % are dynamically allocated - so if your users just happen to be all uploading lots of data, then that upload data won't need to be limited to just 25% of the time of that 5ms frame. In Flexible Frame, that upload data can dynamically get 70% or 80% of the 5ms frame if it needs to, and that can get more data serviced per frame, and potentially increase performance and potentially decrease latency.

BUT - for me - GPS Syncing with idential frame sized and ratios and timings is WELL worth a couple/few extra ms of latency though. Being able to reuse frequencies is so valuable., and not having one of your AP's broadcasting while deafen the other AP's is so valuable.