This is sometimes knows as "power save mode" and is the highest supported on the 7920 phone. It prevents the AP from forwarding arp requests to clients if it alreeady knows the client, thus saving the wake up time required on the client.
U-APSD is the highest level of power save on the 7921. Although dot11 arp cache should also be enabled.
Use the dot11 arp-cache global configuration command to enable client ARP caching on the access point. ARP caching on the access point reduces the traffic on your wireless LAN and increases client battery life by stopping ARP requests for client devices at the access point. Instead of forwarding ARP requests to client devices, the access point responds to requests on behalf of associated client devices and drops ARP requests that are not directed to clients associated to the access point. When ARP caching is optional, the access point responds on behalf of clients with IP addresses known to the access point but forwards through its radio port any ARP requests addressed to unknown clients. When the access point knows all the IP addresses for associated clients, it drops any ARP requests not directed to its clients. In its beacon, the access point includes an information element to alert client devices that they can safely ignore broadcast messages to increase battery life.
[no] dot11 arp-cache [optional]
ARP caching is disabled by default.
This example shows how to enable ARP caching:
AP(config)# dot11 arp-cache