High Priority Channel / Maxing Upload Interaction Question


I'm a bit confused comparing the cambium manual and my own test results. We have an SM configured with 1500 kbits of total CIR and a high priority channel of 500 Kbits CIR. Diffserv over 802.11p is set.

I ran a test trying to upload a 3GB file from two computers, one where (and confirmed by pcaps) all traffic flagged via EF46 DSCP and another where data was flagged 0x02. Per the manual my expected result was seeing the upload on EF46 being able to send atleast 500 kbits/sec of guarenteed bandwidth but the result was that connection consistently dropping below 500, at times as low as 30KB (240 kbits)

What is the expected behavior in this situation?

A) Should the SM decide on ingress into the buffer the ethernet traffic it accepts for the next frame to send based on DSCP flag first? IE: first 500 kbits of the 1500kbits reserved for high priority traffic.


B) Is this high priority channel only relevant when using less then total uplink(IE: when maxing out the upload connection it just scans the next 1500kbits pending delivery and seperates them per queue) without regard to which 1500kbits came in last?

It doesn't seem like there is any control inspecting high priority packets into the SM's ethernet interface causing,  when a few devices are competing for the uplink bandwidth, for the high priority packets not making it into the transmit causing dropped packets or TCP retransmits for devices that are latency intolerant. Does the traffic need to be not only flagged for DSCP but also shaped to limit it to the maximum uplink before hitting the SM for this to work correctly?



Hi Tim,

May i have you Email ID, Company name and the contact number, so that we can open a case for you in our Cambium Support and work on this.

Thank You.

Yogesh Giriappa
Cambium Networks – Technical Support Team

We were able to do some extensive testing and locate the problem. I don't belive it's mentioned anywhere in cambium documentation.

If you have the Max Burst Uplink Data Rate is set to the sustained rate of the SM the high priority channel does not work correctly. You get packet loss and jitter on high priorty flagged traffic.

If you set the Max Burst Uplink Data Rate to be ~ 2x the sustained data rate it works perfectly.


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