30 Jan 2020

Poor Wifi Performance with Meraki and Zebra

Had Customer getting poor performance between Meraki AP and Zebra Scanner.   The new custom app would freeze and lockup.

I checked the Wifi installation and apart from some AP mounting issues the wireless was good.

Speed tests from the device to the WLAN PI  were showing 400 Mbps  and using Wifi Scanners there were very few retransmission's.

Next was a WIFI Capture using Air-check G2.  Make sure you capture the connection to the network (4 ways handshake)  so you decrypt the packets later in wireshark.

Analysis in wireshark showed a lot of TCP retransmission's  but why ?

Pings from the network to the wireless device were rock solid and were < 1ms   and this is where I should have realised  something was wrong.   It was too good for a wireless network., but I did not pick this up.

Meraki are great  you can do packet captures on the wireless and wired side of the AP.

A capture on the wireless side showed the same as the Air-check G2 capture   a lot of TCP re transmission and resets.

I do not know why but I did a capture on the WIRED side and I was glad I did.

This show another MAC using the same IP address  (A Printer !)

Was the issue as simple as a duplicate IP.  After checking the DHCP scope the printers static IP was exactly in the DHCP range !

Changing the printer to be a DHCP reservation  and rebooting the wireless device   the device got a new IP and all was working fine....

This was good new as the customer was going to get another scanner to test with and it would have worked perfectly and then might have wrongly assumed that the other scanner was faulty.

After talking to the IT support dept they had received calls with issues with the printer but they thought was resolved as the scanner was not used over the last 2 weeks (till I came in !)

Contributing to this was the LONG DHCP least time so the scanner kept getting the same IP address from DHCP.

So a wireless issue was a simple duplicate IP.  

I should have picked up at the PING stage,    I was pining the printer on the LAN not the wireless device. !!!

But did resolve AP install issues  and optimised the wireless for these devices.

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