900Mhz AP dropping SM's

We have two 900Mhz AP’s which are 20 miles apart both covering the same valley. One AP has 33 SM’s, the other has 53 SM’s. For about 12 months both AP’s have been rock solid and the links to the SM’s have been rock solid.

About five weeks ago the AP with 33 SM’s started randomly dropping SM’s for about five minutes and then they all re-reged and stayed up. We first thought the problem was interference so we setup SM’s as Spectrum Analyzers and waited for the problem to occur. When it occurred there was not interference. To backup that the problem was not interference, if we rebooted the AP right when the problem started withing 30 seconds of the AP coming back up all the SM’s were registered.

Next we upgraded from 7.3.6 to 8.1.4 on the all our 900Mhz AP’s and SM’s and this did not solve the problem. I disabled 2x mode and this did not help. Then one day the problem with the AP with 33 SM’s quit and the same problem began with the AP with 53SM’s.

I have tried to downgrade the AP still having the problem to 7.3.6 and this did not help so it it back at 8.1.4.

I have tried control slot settings of 2, 3, 4. No difference with the problem.

I have adjusted the power level on the SM’s so that on the AP the Power Level of the SM’s is between -74 to -86. We have a low noise floor.

Each AP is setup the same except the frequency, Max range 28 miles, 75% download, 2 control slots. One AP is on 906Mhz, the other is on 915Mhz.

Any suggestions?

Just a try, you might try reducing your max range to the furthest sm for each ap.

How are the AP’s getting their sync?

I’d increase the low limit to -80 and high limit to -70. The Rx threshold of the radio is -89 so you are not leaving much room.

I adjusted the max range to be just over the furthest SM for both AP’s and the problems persists.

The units are synced through 5.7 backhauls and have not been losing sync.

I tried turning off all of the SM’s which had a signal worse than -80 and the problem still persisted with the AP that had 53 SM’s. Adjusting RF levels I was able to change the range between -70 and -80. This did not fix the problem. Niether was or is their a consistency with SM’s with lower signals being the SM’s that drop.

The site which the problem is worse (that had 53 SM’s) is also on the top of a mountian, it is a 360 sector, and there also is not a consistency with whcih side of the mountain SM’s are dropping off from.

I have now installed prizm and see that on occasion some of the SM’s create 6MB of data / second on both the RF side and the Ethernet side. Each of these offending SM’s are limited to (up/down 128/384), (burst up/down 2560/7680). This traffic does not propogate past the AP. This data is not coming from the Ethernet side.

Any ideas?

You said that both AP’s are getting sync from BH’s. Where are the BH’s getting sync from? How many hopps are their between the BH’s and the sync source? If the BH’s are getting sync at the other end the 900’s should still be in sync. If not they could be out of sync .

I assume you are using omni’s on these sites correct? When you do a install do you record the RSSI, Jitter, Rx signal, Uplink and downlink effic’s of the radios for future use? If you do and you compare those with the radio’s now do you see any degridation in the links? If the antennas don’t have internal grounding and you dont use a surge suppressor between the AP and the antenna, the omni can act like a giant capacitor, build up a charge and release it into the AP damaging the Rx.

Another option is I have noticed on 900 sites using omni’s. Is that when you start to get around 50 SM’s on the AP you start to see more re-reg’s. The AP has to listen to SM’s in all directions. It is possable that transmitting SM’s that are on the back side of the AP from the ones dropping out are cousing a broblem. I can say that at the sites that we have changed from omni’s to 3, 4, or 6 sector’s have improved in performance.

Purchasing SyncPipe’s for both sites seems to have fixed the problem. I was just syncing the timing through the backhauls. In the future I will probably just use SyncPipe’s or CMM at every site with a AP.