PSDU

A PSDU is the chunk of data that the physical layer of a wireless transmitter converts into radio waves. The term comes from the OSI model, where the physical layer is the lowest level, dealing directly with the medium (air, copper, fiber) rather than logic or protocol. When a higher layer hands down a message to the physical layer saying "transmit this", what the physical layer receives is a PSDU. It then adds its own framing, preamble, and modulation to turn those bits into electromagnetic pulses.

In Wi-Fi (802.11), for example, a PSDU might be 1 to 11454 octets long, depending on the standard and data rate. The MAC layer above creates a frame and passes it down; the physical layer wraps it with a preamble (used for synchronization), a header (containing transmission rate and length information), and error-correction codes. The whole assembled packet, including all the physical-layer overhead, becomes what actually travels over the air. Different wireless standards, from Bluetooth to 5G NR, all use PSDUs, though the sizes and structure vary.

Breaking down the term: service data unit (SDU) is the information passed from one layer to the next in the OSI stack. When the MAC layer hands data down, it becomes an SDU from the MAC layer's perspective. When the physical layer receives it, that same data is now called a PSDU, emphasizing that it is the physical layer's unit of work. This naming convention is standardized in IEEE specifications and helps engineers communicate exactly which layer of abstraction they are discussing.

PSDU duration is critical in real systems. At 6 Mbps in 802.11b, a 1500-octet PSDU takes about 2.4 milliseconds to transmit. At 54 Mbps in 802.11g, the same data takes roughly 275 microseconds. Longer PSDUs are more efficient (less overhead per bit) but riskier in noisy channels: a single bit error corrupts the entire unit and forces retransmission. Engineers tune PSDU length against channel quality and latency requirements.

In practice, the PSDU is where modulation happens. The bits are encoded using BPSK, QPSK, QAM, or OFDM depending on the standard and desired data rate. At this stage, burst errors, phase noise, and multipath fading affect whether the receiver correctly decodes the bits. The physical layer's FEC (forward error correction) code is applied to the PSDU before modulation, allowing the receiver to fix some errors without retransmission. The PSDU is thus the boundary between the world of bits and the world of waves.