Preambles | Wi-Fi's Approach to Wireless

Because 802.11 allows transmitters to choose from among multiple data rates, a receiver has to have a way of knowing what the data rate a given frame is being transmitted at. This information is conveyed within the preamble (see Figure 1).

Figure 1: 802.11 Preambles Illustrated
The preamble is sent in the first few microseconds of transmission for 802.11, and announces to all receivers that a valid 802.11 transmission is under way. The preamble depends on the radio type, but generally follows the principle of having a fixed, well-known pattern, followed by frame-specific information, then followed by the actual frame. The fixed pattern at the beginning lets the receiver train its radio to the incoming transmission. Without it, the radio might not be able to be trained to the signal until it is too late, thus missing the beginning of the frame. The training is required to allow the receiver to know where the divisions between bits are, as well as to adjust its filters to get the best version of the signal, with minimum distortion. The frame-specific information that is included with the preamble (or literally, the Physical Layer Convergence Procedure (PLCP) following the preamble, although the distinction is unnecessary for our purposes) names two very important properties of the frame: the data rate the frame will be sent at, and how long the frame will be.
All preambles are sent at the lowest rate the radio type supports. This ensures that no matter what the data rate of the packet, every radio that would be interfered with by the transmission will know a transmission is coming and how long the transmission will last. It also tells the receiver what data rate it should be looking for when the actual frame begins. All devices within range of the transmitter will hear the preamble, the length field, and the data rate. This range is fixed—because the preamble is sent at the lowest data rate in every case, the range is fixed to be that of the lowest data rate. Note that there is no way to change the data rate at which the preamble is sent. The standard intentionally defines it to be a fixed value—1Mbps for 802.11b, and 6Mbps for everything else.
When a radio hears a preamble with a given data rate mentioned, it will attempt to enable its modem to listen for that data rate only, until the length of the frame, as mentioned in the preamble, has concluded. If the receiver is in range of the transmitter, the modem will be able to properly detect the frame. If, however, the receiver is out of range, the receiver will hear garbage. The garbage will not pass the checksum (also garbage), and so will be discarded.
To prevent radios from interpreting noise as a preamble, and locking to the wrong data rate for a possibly very long length, the frame-specific information has its own checksum bit or bits, depending on the radio type. Only on rare occasions will the checksum bit fail and cause a false reception; thus, there is no concern for real deployments.
In summary, a receiver then works by first setting its radio to the lowest common denominator: the lowest data rate for the radio. If the fixed sequence of a preamble comes in, followed by the data rate and length, then the radio moves its modem up to the data rate of the frame and tries to gather the number of bits it calculates will be sent, from the length given. Once the amount of time necessary for the length of the frame has concluded, the radio then resets back to the lowest data rate and starts attempting to receive again.

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