It turns out, even in WiFi, there’s sometimes too much of a good thing. Take Data Rates for example. Wireless design best practices, the CWDP included, generally recommend disabling the lowest data rates in a wireless network. We do this to prevent slow clients that can only support (for various reasons) the bottom data rates from slowing down all the other clients that are connected at higher performing rates – shared medium remember? It’s become common for people to disable all the DSSS rates in 2.4 GHz (1, 2, 5.5 and 11 Mbps) to keep the ultra-damaging legacy 802.11b devices from crippling the network. So generally speaking, we design and implement modern wireless networks with data rates pruned somewhat to improve performance.
To set the stage a little, I have a customer that is a retailer, with stores all across Canada. Their set-up on the wireless is pretty simple; Cisco hardware that’s a little long in the tooth, but serviceable. They have Guest WiFi exclusively on 2.4 GHz and the Corporate and VoWiFi on 5 GHz only for simplicity. I got contacted recently because their Cisco 7925 wireless VOIP phones are “cutting out all the time and unusable.” Lucky for me this is reported in the local store, so no airplane-time. The contact asks me to go to site, because he suspects there are holes in the coverage, and he’d like me to walk around and survey the store for signal strength. Since the store isn’t all that large, I do a quick walk around, gathering survey results in ESS and take a peek:
No real gaping holes in the coverage, maybe a little hot (we lowered the RRM Tx Power Max Threshold), but there isn’t anything that jumps out and says “hey that back stockroom has no signal.” Time to look at the WLC I guess.
The customer had taken my advice a while back and had created separate RF Profiles for each store for flexibility. Take a look at what I found when I went into the 802.11a profile specific to that store under the 802.11 tab:
I’m all for pruning data rates, but this seemed a little….extreme. The disabled MCS rates aside, having Basic Data Rates disabled all the way to 48 Mbps can’t be a good thing. Not for a client device that’s portable in the extreme, walking around a store with displays and shelving, and doors to go through, and all with a real-time VOIP call going on. There’s bound to be a time that phone feels the need to rate shift because maybe the SNR just isn’t enough because a canoe or a human body got in the way. Without some lower data rates available, the phone just dropped its association and the call.
Sure enough, we enabled the data rates down to 24 Mbps, making it Mandatory, and the report back from the store a couple of weeks later was “working a million times better, no dropped calls.” Lesson learned; most everything in moderation, and a little bit of thought before enabling or disabling something.
angrywirelessguy? 