RFZigbee Physical Layer Frame Format (PPDU)
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This page describes the Zigbee physical layer (PHY) frame format, focusing on the components of the PPDU (PLCP Protocol Data Unit). As depicted in the figure for a generic packet format, the PPDU comprises the preamble, Start Frame Delimiter (SFD), frame length, and PSDU (PHY Service Data Unit).
Generic Packet Format
The following table outlines the structure of a generic packet format:
| Field | Octets | Description |
|---|---|---|
| Preamble | 4 | 32 zeros used for synchronization. |
| SFD | 1 | Start Frame Delimiter, marks the beginning of the data. |
| Frame Length | 1 | 7 bits for frame length, 1 reserved bit. |
| PSDU | Variable | PHY Service Data Unit; carries the actual payload. |
| SHR | Consists of Preamble and SFD | |
| PHR | Includes Frame length field | |
| PHY Payload | Contains the data being transmitted. |
Start Frame Delimiter (SFD)
The SFD is an 8-bit (1 octet) field that acts as a delimiter between the preamble and the actual packet data. Its specific bit pattern is:
| Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|---|
| Value | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| ----- | 1 | 1 | 1 | 0 | 0 | 1 | 0 | 1 |
The combination of the Preamble and the SFD is collectively referred to as the SHR (Synchronization Header). The Frame length is part of PHR (Physical Header) as shown in above table.
Frame Length
The frame length field, as the name suggests, indicates the total number of octets contained within the PSDU (physical layer frame payload). It’s a 7-bit field within the PHR.
| Frame Length Values | Payload |
|---|---|
| 0-4 | Reserved |
| 5 | MPDU (ACK) |
| 6-7 | Reserved |
| 8 to aMaxPHYPacketSize | MPDU |
PSDU (PHY Service Data Unit)
The PSDU carries the actual physical layer information. It can also contain MAC sublayer information (MPDU - MAC Protocol Data Unit) when the packet length is 5 octets or greater. The variable-length payload within the PSDU ultimately carries the MAC sublayer information, which contains the data being transmitted.
