Be aware: I did not go too deep into this problem. Any further observation is welcome.
A colleague had an ADSL internet access from Telecom Italia attached to a Cisco 857 router, which did work fine. He connected to an Alcatel Lucent AP-103 access point (manufactured by Aruba Networks) to the router. WiFi clients were able to connect to other clients in the same LAN, wired or wireless, and had access to the router. They were not able to access the Internet. IP config was OK (it was delivered to both wired and wireless clients by the same DHCP service on the Cisco router).
Throwing debug ip packet on the router showed that packets were regularly going out the ATM0 interface, just like the packets from the wired clients, but nothing was returning back.
It turned out that the only difference in traffic going through the router was that the Alcatel AP was marking traffic with DSCP value CS3 (0x18) and for some reason, traffic marked this way was dropped somewhere by the provider.
I found two possible solutions. One is resetting the DSCP field on the router. The 857 does not support class-map and policy-map like the 877 and above, so I had to use a route-map:
route-map setDSCP permit 10 set ip precedence routine interface Vlan1 ip policy route-map setDSCP
The other solution was to reset the DSCP value on the access point: on the web page go to the SSID – Edit – Advanced . On the right there is a little table with 4 lines for 4 traffic type categories, background, best effort, voice, and video. You can set the DSCP value for these. I simply (maybe too simply) set 0 for it on all the 4 lines. This shows in the text confguration as this:
wlan ssid-profile "MySsid" wmm-background-dscp "0" wmm-best-effort-dscp "0" wmm-video-dscp "0" wmm-voice-dscp "0"
As stated before, there is for sure a reason for setting the DSCP value that way and maybe I was just too simplistic to reset it to 0 for all the traffic types. But the solution did just work, the colleague was satisfied, and I hope this can help others, who maybe can give some further toughts about the topic.