IoT Access Technology IEEE 802.15.4: Physical Layer

The 802.15.4 standard accommodates a wide set of PHY alternatives, spanning from 2.4 GHz to sub-GHz frequencies within ISM bands. The initial IEEE 802.15.4-2003 standard delineated merely three PHY alternatives utilizing direct sequence spread spectrum (DSSS) modulation. DSSS is a modulation technique that deliberately expands a signal in the frequency domain, leading to increased bandwidth.

The original physical layer transmission options were as follows: 

• 2.4 GHz, 16 channels, with a data rate of 250 kbps 
• 915 MHz, 10 channels, with a data rate of 40 kbps 
• 868 MHz, 1 channel, with a data rate of 20 kbps

The physical layer frame for 802.15.4 is shown in the following Figure. The Preamble and the Start of Frame Delimiter fields make up the synchronization header for this frame. The Preamble field, which marks the beginning of the frame and synchronizes data transmission, is a 32-bit 4-byte pattern. The receiver is informed that the frame contents begin immediately after this byte via the Start of Frame Delimiter field.

The PHY Header portion of the PHY frame constitutes merely a frame length value. It informs the receiver of the total data anticipated in the PHY service data unit (PSDU) segment of the 802.4.15 PHY. The PSDU constitutes the data field or payload. The numerous iterations and amendments of 802.15.4 over the years by different working groups can render it fairly challenging to comprehend. Consequently, it is imperative to note the specific versions of 802.15.4 that different devices support. Products and solutions must adhere to the appropriate IEEE 802.15.4 specification, frequency range, modulation, and data rate when detailing their physical layer implementation.

The maximum size of the IEEE 802.15.4 PSDU is 127 bytes. This value is considerably smaller than the minimum MTU setting of other upper-layer protocols, such as IPv6, which has a minimum MTU of 1280 bytes. Consequently, fragmentation of the IPv6 packet must transpire at the data link layer to accommodate larger IPv6 packets within IEEE 802.15.4 frames.

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