IoT Access Technology IEEE 802.15.4: MAC Layer

The IEEE 802.15.4 MAC layer regulates access to the PHY channel by specifying the method by which devices within the same vicinity will share the designated frequencies. This layer also coordinates the scheduling and routing of data frames. 

 

The 802.15.4 MAC layer executes the subsequent functions: 

• Network beaconing for devices functioning as coordinators (New devices utilize beacons to connect to an 802.15.4 network) 
• Association and disassociation of a device with a PAN 
• Device security 
• Dependable connection communications between two peer MAC entities 

The MAC layer accomplishes these responsibilities through the utilization of multiple preset frame types. Specifically, the 802.15.4 standard delineates four categories of MAC frames. 

• Data frame: Manages all data transfers 
• Beacon frame: Utilized in the dissemination of beacons from a PAN coordinator 
• Acknowledgment frame: Validates the successful receipt of a frame. 
• MAC command frame: Accountable for regulating communication among devices 

All four 802.15.4 MAC frame types adhere to the frame format illustrated in the following Figure. Observe that the MAC frame is transmitted as the PHY payload. The 802.15.4 MAC frame comprises the MAC Header, MAC Payload, and MAC Footer fields.

The MAC Header field consists of the Frame Control, Sequence Number, and Addressing fields. The Frame Control field delineates properties like frame type, addressing modes, and other control flags. The Sequence Number field indicates the identifier for the frame's sequence. The Addressing field delineates the Source and Destination PAN Identifier fields, along with the Source and Destination Address fields. 

 

The MAC Payload field differs according on the specific frame type. For instance, beacon frames contain distinct fields and payloads pertinent to beacons, but MAC command frames possess various data. The MAC Footer field is merely a frame check sequence (FCS). An FCS is a computation derived from the data within the frame, utilized by the receiving party to verify the integrity of the data included in the frame. 

 

IEEE 802.15.4 mandates that all devices must accommodate a distinct 64-bit extended MAC address, derived from EUI-64. Due to the limit payload of 127 bytes, 802.15.4 specifies the assignment of a 16-bit "short address" to devices. This abbreviated address is specific to the PAN and significantly decreases the frame overhead in comparison to a 64-bit extended MAC address. Nonetheless, it is important to recognize that the utilization of this abbreviated address may be confined to particular upper-layer protocol stacks.

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