802.11 FRAME TYPES
In order for a device to successfully connect to a wireless network, several different frame exchanges must take place. Various frame types allow for specific functions to occur. They include the authentication and association process, reserving the medium, exchanging data, and power save functions.
IEEE 802.11 Frame Types:
Generelly devices communicate by sending RF waves to each other through the air.
These RF waves carry the digital data from one device to another.
At this stage, the information traveling through the air is organized into frames.
These frame types play various roles depending on the information being sent WLANs use three different frame types:
Management Frames :
Management frames play a very important role in assisting wireless devices in searching and connecting to the wireless network using the authentication and association processes.
These frames are exchanged between an access point (AP) and the client devices desiring to join the wireless network.
✓Beacon
Contains all the information that the client devices should know before joining any basic service set (BSS).
✓Probe Request
Sent by client devices containing the SSID of the network to which they desire to connect.
✓Probe Response
Contains the same information that the beacon does with the exception of the traffic indication message (TIM)
✓Authentication
Creates an initial connection between the AP and the client device.
✓Deauthentication
Notification sent by either of the devices, AP or client device, to deauthenticate from each other.
✓Association Request
Sent by client devices to the AP to become a part of a basic service set.
✓Association Response
Sent by the AP to the client device allowing or disallowing BSS joining.
✓Diassociation
Notification that is sent by either of the devices, AP or client device, to disassociate from each other.
✓Reassociation Request
Sent by the client device to the new AP while roaming
✓Reassociation Response
Sent by the new AP to the roaming client device.
✓Announcement traffic indication message (ATIM)
Used by ad-hoc client devices for power management.
✓Action
Triggers an action that is to be performed.
Control Frames:
Control frames reserve the wireless medium for providing contention free environments and sending data acknowledgments so that the transmission happens with as few collisions as possible.
Some control frames help client devices to fetch data from the AP when the client devices return from a power save state mode
✓Acknowledgment (ACK)
Notifies the transmitting client device that the unicast frame has been delivered successfully.
✓Request to Send (RTS)
It is used by the transmitting client device that sets the network allocation vector (NAV) for reserving the radio frequency (RF) medium to avoid collision
✓Clear to Send (CTS)
Resets the NAV of the listening devices to reserve the RF medium
✓Power Save Poll (PS-Poll)
Uses to request data frames in the Power Save mode
✓Block ACK Request (BlockAckReq)
Requests acknowledgment of correctly received frames.
✓Block ACK (BlockAck)
Notifies the multiple frames have been delivered successfully
✓Control Wrapper
Carriers any other control frame together with the High Throughput Control (HTC) field.
Data Frames:
As their name implies, data frames are used to carry data payload or Layer 3 information between wireless devices. A special type of data frame is the null data, or null function, frame, which helps implement power save features and is not used to carry any data payload.
There is also a variant of the null frame called the QoS null frame which is used with quality of service power save mode.
✓Data (simple data frame)
Indicates the MSDU upper layer information encapsulated in the frame body.
✓Null Function (Null data)
Notifies the AP to change the power save status by updating the Power Management bit. This frame is used by the client station.
✓QoS Data
Indicates the QoS frame that carries data.
✓QoS Null
Indicates the QoS frame that does not carry any data.
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