Frame Formats
The IEEE 802.11 MAC accepts MSDUs (MAC Service Data Unit) from higher layers in the protocol stack for the purpose of reliably sending those MSDUs to the equivalent layer of the protocol stack in another station.
MAC adds information to the MSDU in the form of headers & trailers to create an MPDU (MAC Protocol Data Unit).
MPDU is then passed on to the physical layer to be sent over the wireless medium to the other station.
MAC may fragment MSDUs into several frames.
The general IEEE 802.11 frame format is as shown below-
The details of the fields are as under-
Function | To DS | From DS | Address1 | Address 2 | Address 3 | Address 4 |
IBSS | 0 | 0 | RA=DA | SA | BSSID | N/A |
From AP | 0 | 1 | RA=DA | BSSID | SA | N/A |
To AP | 1 | 0 | RA=BSSID | SA | DA | N/A |
Wireless DS | 1 | 1 | RA (Receiver Addrss ) | TA (Transmitter's Address) | DA (Destination Address) | SA (Sender's Address) |