There are some QoS features in WiFi itself. http://en.m.wikipedia.org/wiki/Wireless_Multimedia_Extensions
I do not think they can be leveraged to artificially rate-limit though, as they seem to only be able to set priorities, not bitrates.
I would keep it simple and do all my traffic-shaping at the gateway. That seems to be standard practice.
I'm rate limiting guests on an open network currently via bandwidth qos on the router. The other option is to rate limit downlink and uplink at the WiFi access point? Any right way to do this?
Apparently someone (me) did not read your post accurately enough. Lemme try again... :\
The only reason I would shape at the AP would be if my LAN bandwidth was becoming saturated and I wanted to stop client->AP->LAN->AP->client traffic, or perhaps if the client is attempting a DoS, it would be best to stop the traffic at the closest node (AP), so your LAN would not get as much DoS traffic. Otherwise, I would employ QoS at the gateway router because it is easier and I can think of no reason not to.
Pick whichever is easiest. Traffic-shaping rarely has a "right" or a "wrong". It is usually "better" or "best" type of thing, since traffic-shaping is largely a request rather than a demand. Like shaping egress vs shaping ingress. Only egress is under your control, ingress traffic is under the control of the sender who may or may not obey your requests.
More details could help if you've got some especially troubling situation.
Is shaping at the gateway router not working? Are you trying to control download or upload? Download, like I partly explained above, cannot be fully controlled, or in the case of the UDP protocol, you cannot control download/incoming traffic at all.
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