Timing and Range Questions

Sorry if these are noob questions, but I have heard mixed answers recently on topics regarding timing and range.

1.  Do all access points running 5GHz and co-located on a tower have to run the same timing regardless of frequency seperation?  I have always understood that they did, but I heard someone suggest that two un-timed APs seperated by 20MHz or more will not interfere with each other.

2.  5ms timing provides better throughput, but what does it do to the latency?  Obviously you jump from a minimum round trip time of 5ms to 10ms, but does the latency get drastically worse for inefficient connections?

3.  Can 5ms timing help solve interference in noisy situations?

4.  Which hurts the overall throughput on an access point the most? the "Max Range" set under the AP radio tab, or the furthest connected SM?

Thank you

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To answer your questions...

1. No, not all radios have to be sync'd. The size of the guard bands between un-synced radios are heavily dependent on the TX power (and quality) of the radios collocated, the type (and quality) of antennas used, and the physical separation between all of them. Even if you do choose to sync all the radios at the site, they don't necessarily need to use the same timing settings. Cambium has several frame calculators both in the support area and in the radios themselves where you can play with settings to see if the frame start/stop times will conflict. You have to be careful, but you can run different timing settings on various collocated radios and still be in sync.

2. 5ms gives you roughly 10% better throughput at roughly 2x latency. From experience the latency does not get drastically worse using 5ms in poor conditions and I believe that smaller channels widths benefit more from a 5ms frames.

3. I don't believe so. I think the main reason why the 2.5 and 5ms frame times exist is for compatibility between ePMP, PMP100, PMP4XX, and PMP320 radios.

4. From what I understand back in the PMP100 days the max range setting had more of an impact... today with ePMP and PMP450, the max range setting has little impact. Subscribers that are at the edge, with poor modulation are going to be more detrimental to the overall health of the AP then the max range setting.


Thank you for the response.  Especially the answers on the 5ms timing is helpful.  I have been thinking about 5ms timing as a solution to a very noisy mountain.  Seems like it might not be the answer if it doesn't help improve efficiencies or gain much throughput.

Try using a PMP450i AP or an ePMP 2000 AP w/BSA for sites with lots of noise, and use a smaller channel width, along with 5ms frames to squeeze a bit more throughput out of the connections.

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As Eric mentions, in smaller channel bandwidths such as 5 or 10 MHz, a 5ms frame can have very significant increases in total throughput.

As you acknowledge, however, this may not solve an interference or weak signal issue.  It more efficiently uses the frame for data symbols, but doesn't make the link any more robust.

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Thank you for the info.  That helps