802.11 MAC Series – Basics of MAC Architecture – Part 2 of 3By CWNP On 02/03/2016 - 160 Comments
Welcome back to the next blog of the 802.11 series. In this blog, we will discuss different types of frames.
As you know, a frame is a unit of data used in the Data-Link layer of the OSI model. Different frames are exchanged between devices to successfully connect to the wireless network. The IEEE 802.11 standard specifies the following three frame types which are further subdivided into subtypes.
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. Management frames do not go beyond the Data Link layer of the OSI model and do not carry any upper-layer information. They are also known as Management MAC Protocol Data Unit (MMPDU), and do not contain MSDUs. The following subtypes of the management frame are:
- 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.
- Disassociation: 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.
Beacon Management Frame
The Beacon subtype is one of the most important management frames. It is normally referred to simply as a beacon. Beacons are the core of the wireless network because they contain all the information that the client devices should know before joining any basic service set created by the AP.
In the infrastructure mode of the basic service set, the AP sends the beacon frames whereas the client devices listen them in order to select the appropriate AP for connection. Beacons are transmitted in the interval of roughly 10 times per second.
In the independent mode of the basic service set or ad hoc, client devices send beacon frames. The following parameters comprise the body of the beacon frame:
- Time stamp: Helps client devices to keep their clock synchronized with the AP.
- Spread Spectrum Parameter Sets: Specifies the type of spread spectrum technology used by the AP. The value of this parameter can be FHSS, DSSS, HR-DSSS, ERP, OFDM, HT, or VHT.
- Channel Information: Specifies the channel used by the AP or an ad hoc network.
- Data Rates: Specifies the basic and supported data rates by the AP.
- Service Set Capabilities: Helps to identify the service set from where the beacon frames are generated. The service set can be an infrastructure mode or an ad hoc mode.
- SSID: Specifies the logical name of the wireless network.
- Traffic Indication Map (TIM): Contains the AID of the associated client devices whose data is buffered on the AP that need to be delivered.
- QoS Capabilities: Contains information related to Quality of service and Enhanced Distributed Channel Access (EDDCA).
- Robust Security Network (RSN) Capabilities: Specifies the information related to cipher and the authentication methods, such as TKIP and CCMP.
- Vendor Proprietary Information: Contains the vendor specific information.
Control frames reserve the wireless medium for providing contention free environment 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. The following are the subtypes of control frames:
- Acknowledgment (ACK): Notifies the transmitting client device that the unicast frame has been delivered successfully.
- Request to Send (RTS): 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.
- CTS to Self: Is used by the AP to ensure that all other client devices must reserve the medium until the transmission of Data and ACK.
- 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 carry the data payload or Layer 3 information between wireless devices. The layer 3 – 7 MSDU data payload is encrypted to implement data privacy over the wireless network. The Data subtype is one of the most important data frames, which is also known as simple data frame that contains the MSDU upper-layer information. Another special subtype of data frame is the Null function, which is also known as Null Data that does not carry any data payload. The Null Function subtype is also used for implementing power save feature by which client devices inform the AP about their status of power save. The following are the subtypes of data frames:
- 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.
- QoS Data + CF-ACK: Indicates the QoS frame that carries data.
- QoS Data + CF-Poll: Indicates the QoS frame that carries data.
- QoS Data + CF-ACK + CF-Poll: Indicates the QoS frame that carries data.
- QoS CF-Poll: Indicates the QoS frame that does not carry any data.
- QoS CF-ACK + CF-Poll: Indicates the QoS frame that does not carry any data.
You will learn more about the above frames in the subsequent blogs of the 802.11 series. In the next blog, we will take a look at the phases of wireless network discovery that help client devices to choose the best wireless network. Til then happy reading!!