WLAN MAC Protocol Inside
This article describes control frame, data frame and management frame of the WLAN MAC layer as specified in IEEE 802.11 MAC layer specifications.
The sub-fields of frame control field of control frames are described below.
The control frames control the exchange of data frames between the stations(STAs).
Some very useful control frames are as follows:
ACK or Acknowledgement frame: The receiving station sends an acknowledgement frame to the sending station after receiving the data without any error. If the ack frame is not received within the pre-determined time period by the transmitter then the transmitter re-sends the data frames.
RTS/CTS frame: The RTS(Request to Send) and CTS(Clear to Send) frames provide an optional collision reduction scheme for APs with hidden stations(STAs). If any WLAN node has any frame to be sent it will send out RTS frame. This serves two funtions viz. help sending station to reserve the RF link and also indicates all the other stations not to transmit. If any target station receives RTS, it will respond with the CTS. Like the RTS frame, CTS will inform to nearby stations to be silent i.e. do not transmit any frame over RF.
The data frame format is described below in the following figure.
The content of the Address fields of the data frame is dependent upon the values of "To DS" and "From DS" bits. Where content of a field is shown as NA(not applicable); the respective field is omitted.
Note 1: The address-1 always holds the RA(Receiver Address) of the intended receiver or receivers(e.g. in multicast operation). Note 2 : The address-2 always holds the address of STA which is transmitting the MAC frame.
The WLAN management frame is depicted below in the figure.
A station utilizes contents of address-1 field for address matching to perform receive decisions. In cases wheere address-1 field contains a group address and also if the frame type is other than beacon frame type; in these cases BSSID is validated to ensure that the broadcast/multicast originated is in the same BSS.
Authentication frame: WLAN authentication begins with the WNIC(i.e. wireless network interface card) by sending an authentication frame to the AP containing its identity.
Association Request frame: This is sent by STATION. It basically enables AP to allocate resources and also synchronize.The frame carries information about the WNIC. In addition it carries supported data rates and SSID of the network with which station wishes to associate. If the request is accepted , then the AP reserves memory and it also establishes association ID for WNIC.
Association response frame: This is transmitted by an AP to a STA. It tells acceptance or rejection verdict of the Association Request Frame. If the verdict is acceptance then the information field of the frame contains association ID and supported data rates.
Beacon frame: It is sent periodically from an AP to announce its presence. It provides SSID and other information parameters for WNICs within the coverage range.
De-authentication frame: This is sent from a STA within to terminate connection from the another STATION.
Disassociation frame: This is sent from a station wishing to terminate connection.This is the best method to allow the AP to de-allocate memory and remove WNIC details from the association table.
Probe request frame: It is sent from Station when it requires information from the other station.
Probe response frame: It is sent from an AP in response to probe request frame. It contains capability information and data rate supported.
Re-association request frame: WNIC sends a re-association request when it drops from the range of currently associated AP and finds another AP with stronger signal. The new AP co-ordinates forwarding of any information that lies in the buffer of previous AP.
Re-association response frame: It is sent by AP. It indicates acceptance ot rejection of re-association request frame transmitted by WNIC. Frame includes association ID and supported data rates.
WLAN MAC frames Links
WLAN Association Request and response frame
WLAN authentication Request and deauthentication frame
WLAN beacon frame
WLAN passive vs active scanning frame
WLAN Probe Request and response frame
WLAN reassociation Request and response frame
WLAN RTS and CTS frame