802.11 Frame Format

802.11 Frame

Field	Length (Bytes)
Frame control	2
Duration, id.	2
Address 1	6
Address 2	6
Address 3	6
Sequence control	0, or 2
Address 4	6
QoS control	0, or 2
HT control	0, or 4
Frame body	Variable
Frame check sequence	4

• Frame Control
  • type of packet (control, management, data)
  • subtype (association, RTS, CTS, Ack)
  • indication of to/from distribution system (internet rather than intranet), the DS bits
• Duration
  • specifies on-air time of full packet in $\mu s$.
  • there is no actual length field because we could have a very large packet
  • See NAV.
• Sequence control (4-bit fragment number, 12-bit sequence number)
• Quality of Service Control (Identifies traffic category)
• High Throughput Control (configurations for selecting best data rate)
• Frame Body
  • max size depends on PHY
• Frame check sequence (32-bit CRC)

The meaning of Address $\{1,2,3,4\}$ fields depends on what Frame Control’s DS to and from bits are. In particular,

Case	To DS	From DS	Address 1	Address 2	Address 3	Address 4	Use Case
1	0	0	Destination Addr	Source Addr	BSS ID	-	Direct communication
2	0	1	Destination Addr	BSS ID	Source Addr	-	Traffic from Internet
3	1	0	BSS ID	Source Addr	Destination Addr	-	Traffic to Internet
4	1	1	Receiver Addr	Transmitter Addr	Destination Addr	Source Addr	Repeater / Mesh

Sending Frames in WiFi

• Frame bursting: transmit multiple frames in a row
• Frame fragmentation: split service data over multiple frames
• Frame aggregation: multiple service data packets in a single frame

Calculating Packet Durations

Depends on what is being processed

• 6 Mbps for header
• 24 Mbps for payload
• Add $10\mu s$ for SIFS, $34\mu s$ for Ack.