Data Flow Examples

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.

Case 1: Communication Inside BSS

AP knows which stations are registered with it so it knows when it can send frame directly to the destination.

Case 2: Ad Hoc

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.

Case 3: To the Internet

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.

Case 4: From Internet to Station

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.