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QoS & wireless


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Had a VERY strange problem with a cs410 which may affect windows/linux installs also, but I thought I would share to perhaps save people some of the pain I've gone through over the past few days (bear with me, it's long!).


We needed to put some wireless phones on a building site, so got polycom 8002's - quite rugged and wireless. Put a 3com 7760 wireless access point in with a 12db antenna and hooked it all up to a cs410 through a switch. Phones worked, registered etc. but after 1-2 seconds the audio would die, and often the access point would seem to have a fit and would need a reboot. This would happen when using X-lite on my laptop over the wireless, but not when plugged straight into the switch. Further testing revealed audio was going out, but not back. The PBX was sending it, but the AP didn't look like it was broadcasting it.


Strange thing was, if I turned off the WMM (wireless Qos apparently) setting on the access point it would all work ok, but the polycoms required this to before they would associate with an AP, so turning it off wasn't an option. I assumed the AP must have been dodgy, so took it home and rigged it up to get the phones to register to our office PBX. Everything worked flawlessly. By now, I was really confused. I'd ruled out the cs410, as that worked fine wired, I'd ruled out the polycoms as I'd had them working fine in the office, and I'd ruled out the access point as it worked at home. So there wasn't much else to try.


However... I was struck by a thought that the difference between the cs410 and our PBX is operating system, and I'm fairly certain I didn't do the reg hack to enable dscp on our windows box whereas it's enabled as standard on linux. So this evening I've changed the <tos_rtp> section of the pbx.xml file to 0, and lo and behold, it all starts to work. So it looks like the access point isn't handling the DSCP header correctly or can't allocate enough bandwidth and giving up somewhere along the way. Just setting it to a slightly lower priority (104) also seems to work.


So all in all, it seems 3com 7760 access points don't do QoS properly, which is a shame because they're been good in every other respect. Someone with more experience with wireless Qos may be able to shed some light on this and get it working without hacking PBXnSIP, but it's all new to me - I didn't even know there was such a thing up until a few days ago!

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