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 frame body contains the MSDU from the higher layers.

The details of the fields are as under-

• Frame Control:    16 bit field. Contains information that is used to interpret all the subsequent sub-fields of the MAC header.
• Duration/ID:        16 bit field. Alternately contains duration information for updating NAV or a short ID, called the association ID (AID), used by a mobile station to retrive frames that are buffered for it at the AP.
• Address1, 2, 3, & 4: These field contain information as indicated in the table below-

• 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)

• Sequence Control Field: 16 bit field. Sub fields are 4 bit fragment number & a 12 bit sequence number. It is used to allow a receiving station to eliminate duplicate received frames. Sequence number subfield contains a 12 bit number assigned by the sending station to each MSDU.
• Frame body     Contains MSDU from higher layers. Variable in length. May be as long as 2304 bytes without WEP or 2312 bytes with WEP.
• Frame Check Sequence:   32 bit field. Contains the result of applying CRC-32 polynomial ot MAC header & frame body.