802.11 Frame

Structure of a WiFi frame (well, 802.11 specifically):

Address Fields

Address fields are interpreted differently based on the ToDS and FromDS bits in the frame’s Control field (above, it’s the very first field)

• In ad-hoc mode (one node is sending directly to another), both DS bits are 0:
  1. Address 1 is target node
  2. Address 2 is source node
  3. BSS ID
• When AP is sending to host in infrastructure mode, ToDS is 0 and FromDS is 1:
  1. Destination
  2. BSS ID
  3. Sender address
• When host is sending to AP in infrastructure mode, ToDS is 1 and
• When both DS bits are 1
  1. Ultimate destination
  2. Immediate sender
  3. Immediate destination
  4. Original source

TODO what's in the book doesn't match the slides!

Case 1: Router R1 sends data to host H1

1. R1 knows H1’s IP address, runs ARP to get MAC address
2. R1 encapsulates the data in Ethernet frame
   • Source address: R1’s MAC address
   • Destination address: H1’s MAC address
3. AP converts the Ethernet frame to WiFi frame
   • Address 1: H1’s MAC address
   • Address 2: AP’s MAC address
   • Address 3: R1’s MAC address
4. H1 can determine that the frame has come from R1 by checking the Address 3 field

Case 2: Host H1 responds by sending packet to router R1

1. H1 creates a WiFi frame
   • Address 1: AP’s MAC address
   • Address 2: H1’s MAC address
   • Address 3: R1’s MAC address
2. AP can determine that the frame should be forwarded to R1 by checking Address 3 field
3. AP converts the WiFi frame to Ethernet frame
   • Source address: H1’s MAC address
   • Destination address: R1’s MAC address