To sync or not to sync... That is the question....

Other than frequency re-use / self interference avoidance, am I gaining anything performance wise with sync? Less noise from non-syncronized sources on the same channel?  We have ~10 APs deployed around town and have enough geographic separation that we can re-use frequencies without worrying about our own APs or SMs hearing other APs. Our APs are mostly on mini-pops so they are not sending RF very far anyway. We are 100% syncronized right now but don't really need to be from a self interference perspective.  I am trying to decide if we want to stay syncronized (and why) or switch to flexible to gain throughput (we sell some very high speed packages).

Obviously if we need to get into re-use with closer APs, we can turn it back on.

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I would also be curious to know if there is any advantage to using Sync beyond working with other synced AP's (your own or others).  Does turning on GPS sync do anything at all as far as mitigating interference from non-synced sources of noise or have any advantage over running flexible in this scenario ?

Sync does not help with battling external unsync'd interference sources. So if you are not interested or need to frequency reuse, be spectrally efficient, reduce self interence and reduce the overall noise generated by your network, you can turn off sync and use flexible mode to take advantage of the more efficient use of the frame via dynamic duty cycle and frame size.

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I've shut off sync and gone flexible on a couple APs to test. I fell like DL Utilization is much worse with sync off - or am I imagining that?  A 40 Mhz AP with 75/25 hits 100% DL Utilization around 60 Mbps throughput with 10 SMs - on average (I know lots of things contribute to Utilization).  However, in flexible mode, I am hitting 100 % DL utilization around 35 Mbps total throughput.

If I were to guess I would assume you're hitting 100% download utilization because the total availalble download is reduced due to interference caused by the lack of synced AP's.

Good point by Chris. 

Did the Downlink MCS change when you changed from Sync/Fixed to Flexible? Lower MCS will use more of the frame for the same amount of data. This could be the reason why the throughput is lower when the frame goes to 100% utlization. 

Another reason could be is that there is more uplink data now increasing the UL portion of the frame in Flexible mode vs. a fixed 25% duty cycle on the UL. Are you able to see more uplink traffic after switching to flexible? 

If Chris' point is accurate, than running with sync on helps avoid interference that is not generated by my network. The APs where I have turned off sync are either not sharing channels with any of my other APs or they are geographically protected and can't hear my other AP on the same channel.  Where I am seeing this the most is APs that are in the non-DFS bands - and thus hear a bunch of home routers.

I have not noticed a large increase in upload throughput - it remains around 2 - 4 Mbps fairly constant on all my APs during busy times.

MCS for uplink and downlink stayed the same when we went to flexible.

Hello guys,

Here also I noticed this same behavior that when the ap is in flexible mode the use of DL frame reaches 100% around 30 Mbps, and when in 75/25 reaches 100% around 65-70 mbps with the same 64 sm connected To the AP. The uplink troughput is even in the two conditions as it is limited to the client's ppoe authentication. This ap operates on a 20Mhz channel.

Flexible frame uses as much downlink frame time and as much uplink frame time as needed based on data available to send. So it is possible that the frame util reaches 100% a lot more than you see with fixed frame. Flexible frame is dynamic and changes in length as well as duty cycle split, dynamically every frame. If there is 30 MB to send, the DL portion of the frame may be smaller than when there is 60 MB to send. 

My question to you is that are you seeing just the frame util reach 100% or are you seeing lower peak sector throughput in general with Flexible compared to Fixed? Your sector throughput should be similar or better than Fixed if there is data to send in any given direction. 

I see both, 100% frame used and sector does not exceed 30 mbps, when I changed to 75/25 immediately after clients reconnect the increased troughput to 40-45 mbps with frame time in averages of 50-60%.


@AlexTrevisol wrote:

I see both, 100% frame used and sector does not exceed 30 mbps, when I changed to 75/25 immediately after clients reconnect the increased troughput to 40-45 mbps with frame time in averages of 50-60%.


Ok, in your case I need more information. Did you have sync on this AP to sync with other APs? How does the DL MCS compare with Fixed vs. Flexible?

It is not synchronized with other APs, in this particular tower I only have one more ap that is about 15 meters away from this one in question and in another channel very distant.

Levels of mcs did not come to look in detail, but I believe that in flexible or 75/25 should not change since the other parameters have not changed.

If the demand for data is the same and there are no interferers or self interferers and you believe the MCS is the same before and after and there is no heavy UL data, then there is no technical reason why your sector throughout is lower. Are you receiving complaints from end users that thier throughout is lower when in Flexible mode? I must be missing a key piece of information. Please feel free to contact support so they can take a closer look at your AP.

Thanks,
Sriram

I see very similar behavior. Here are some screen shots from SNMP tracking of DL Utilization and Throughput of an AP.  AP is now in flexible mode on a 40 MHz channel. It has 8 SMs attached to it. One of them is maxing out his 25 Mbps package. We use MIR on the radio to control customer speed packages.  This AP used to be 75/25 synced and never reached 100% DL Utilization. After moving it to flexible, it get up to 100% all the time.  SMs are all under 500 meters from the AP and all are 15/15 MCS.

Here is DL Utilization graph and then Throughput from the same time period:


@Au Wireless wrote:

I see very similar behavior. Here are some screen shots from SNMP tracking of DL Utilization and Throughput of an AP.  AP is now in flexible mode on a 40 MHz channel. It has 8 SMs attached to it. One of them is maxing out his 25 Mbps package. We use MIR on the radio to control customer speed packages.  This AP used to be 75/25 synced and never reached 100% DL Utilization. After moving it to flexible, it get up to 100% all the time.  SMs are all under 500 meters from the AP and all are 15/15 MCS.

Here is DL Utilization graph and then Throughput from the same time period:

...


Hi AU, 

Like I mentioned in my previous post, you will see utilization at 100% a lot more with Flexible because it shrinks and expands the frame duration and adjusts the duty cycle based on data demand. With Fixed, you will see less utilization because the frame is fixed and does not shrink to accomodate just what's in the data buffer. With Fixed, the frame duration is either 5ms or 2.5ms that you configure. With Flexible it can be anywhere between 2ms and 10ms and that changes dynamically (possibly every frame) depending on data demand.

And if I'm to take a guess on your next question, it is not straightforward to determine if your AP is at capacity in Flexible mode. 

Thanks,

Sriram

OK. That makes sense I suppose.

Is there a negative impact to customers with the DL utiliztion hitting 100% or will it "self adjust" as more users grab data?  I'm looking at this now strictly from a performance impact to the end user. Are they getting better performance in a PtMP world with 75/25 or flexible - assuming MCS remains the same and there is no interference issues.


@Au Wireless wrote:

OK. That makes sense I suppose.

Is there a negative impact to customers with the DL utiliztion hitting 100% or will it "self adjust" as more users grab data?  I'm looking at this now strictly from a performance impact to the end user. Are they getting better performance in a PtMP world with 75/25 or flexible - assuming MCS remains the same and there is no interference issues.


Assuming MCS is the same and there are no interference issues and you don't need to sync, Flexible is the way to go. Flexible is more efficient in this scenario. Sure, between 75% DL (Fixed) and perhaps say, 85% DL (Flexible) may not happen all the time as the demand for data may not be constant. On the other hand, if Flexible shrinks your frame, your latency goes down and that will help with TCP performance. 

Thanks

Sriram