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Case 1: Packet from a station under one AP to another in same AP’s coverage area.
Case 2: Packet between stations in an IBSS.
Case 3: Packet from an 802.11 station to a wired server on the Internet.
Case 4: Packet from an Internet server to an 802.11 station.
AP knows which stations are registered with it so it knows when it can send frame directly to the destination.
Direct transmit only in IBSS (Independent BSS), i.e., without AP.
Note:
in infrastructure mode (i.e., when AP is present), even if B can hear A, A sends the frame to the AP, and AP relays it to B.
MAC A determines IP address of the server (using DNS).
From the IP address, it determines that server is in a different subnet.
Hence it sets MAC R as DA.
Address 1: BSSID Address 2: MAC A Address 3: DA
AP will look at the DA address and send it on the ethernet.
AP is an 802.11 to ethernet bridge.
Router R will relay it to server.
Packet arrives at router R – uses ARP to resolve destination IP address.
AP knows nothing about IP addresses, so it will simply broadcast ARP on its wireless link.
DA = all ones – broadcast address on the ARP.
MAC A host replies with its MAC address (ARP reply)
AP passes on reply to router.
Router sends data packet, which the AP simply forwards because it knows that MAC A is registered.
