ePMP Force 300 downlink slow, uplink ok

I setup a PTP wireless bridge between 2 x ePMP 300's about 1km apart for a clients Internet.  The AP sits on a pole with an upstream router and out to the Internet.  The SM sits on the clients building and hooks up to the back of a firewall. It's basically at an airfield, so I chose not to use DFS because I kept getting interference and it would constantly swap channels and create issues.  Anyways, long story short I installed this about 3 months ago, set it up and optimised it and got around 150/150Mbps download and upload speeds via www.speedtest.net.  It ran fine for several months, with the exception of the odd drop out in very heavy winds or poor weather, but all in all it maintained good consistant performance. 4.1.2 firmware was run at both ends.

After the recent Christmas break, the client rang to complain about very poor Internet performance when they had got back to work.  We tested and noticed the performance dropped to ~5Mbps down and ~30Mbps up. The weather was a little on the windy side over the break so figured something had moved, however checking eAlign shows RSSI is still good, around -57dbm constantly. I altered the operating frequency of the AP because the spectrum analyzer showed some other heavy use around the same frequency which did improve performance.  However since, I have been unable to get back to the 150/150 speeds we were getting prior.  I went around in circles so much I tried upgrading firmware to 4.1.4 which they're currently on.  All settings are basically back to what we had prior to Xmas.

My issue right now, is I'm currently at about 50Mbps download and 140Mbps upload, so the uplink is came back close to normal but download is still slow.  Checking performance there are a large number of retransmits for the downlink.  The MCS for uplink shows 97.5% in the MCS9/256QAM which is great, however the downlink shws about 38% of packets in this modulation, with another 32% in MCS8/256QAM, then it drip feeds in %'s in other MCS' below that.  Prior to Xmas, the downlink MCS was similar to the uplink, where we had around the 97-99% in MCS9/256QAM.  

So my question is, what would cause the downlink to run slow, yet the uplink to run fine?  I'm running a 50/50 ratio, and nothing has changed here (we have also tested performance from the upstream router and it's most defintely the wireless bridge at fault)


You have pretty much narrowed down the problem. The information you gave seems to indicate some other transmitter occupying the spectrum which is why changing channels seemed to have helped a little bit. With the downlink only being less than desired, it appears to be interference at the SM/Slave side, from the fact that there are a ton of retransmission and low MCS rates. Have you run a spectrum analysis on the Slave side? It would give you a good idea of the interference/noise level at the Slave side. What are you serving on the Slave side? Is it a home or Enterprise business? Do you know if they fired up any WiFi routers that may be causing this interference? By default WiFi routers hop channels and you could ask them to lock it to a channel so it doesn't interfere with the ePMP Slave.


Thanks for the reply.

Yes I did spectrum analysis on the slave side and hence why I shifted frequencies. As mentioned it's for an enterprise business (a private aeroclub) so a lot of wireless transmissions floating around in the air.  I've asked the customer if they have put any Wifi in recently or any other wireless transmitting device nearby that may have caused this, awaiting their reply.  There are 4 x internal Cambium AP's (for staff/guest wireless) in their main building however these are a good 50 metres or more away from the slave dish.  The slave dish points across the airfield to a remote location that is well away from the internal wifi and has a very clear line of sight.  Planes do fly within the vicinity of the PTP link however they don't come in the direct path.  I will check their internal APs anyways and see if locking to a different channel will help.


Ugh, yes client mentioned that another wireless device was attached to the same pole as the SM dish was on, they turned it off, we re-ran tests and back to 150/150 speeds now.  Thanks for the (in hindsight obvious) response - I've learnt a fair bit in the past 24 hours!

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Excellent. Glad you were able to solve the problem!

Don’t ya love it when that happens? Install a customer, working perfectly, they then go hang something new which breaks things… and then call for you to fix it. LOL, awesome.

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