![]() ![]() Receiver: Listen for and receive Data,Wait SIFS, Send ACK, Wait DIFS ![]() Sender: Wait DIFS, Send Data, Wait SIFS, Listen for and receive ACK (until maximum ACK timeout), Repeat Normal Transmission flow without RTS/CTS handshaking Default CTS and ACK timeouts vary between manufacturers. ![]() Note: DIFS/SIFS/PIFS are used for physical layer carrier sensing while the MAC layer performs the collision detection using CTS and ACK timeouts. In 802.11b, the constants are mandated by IEEE as follows: The times are defined such that the RTS, CTS and ACK frames are given a higher priority (ie once a packet transmission sequence has begun, the station holds onto the channel until it is finished) PIFS is the DIFS for the access point in a special access method known as Point Coordination Function. SIFS is the amount of time a station must wait before sending or beginning to receive a RTS, CTS or ACK frame. Other constants that may restrain the maximum link distance are SIFS (Short Inter Frame Spacing), DIFS (Distributed IFS) and PIFS (Point Coordination IFS.) DIFS is the amount of time a station must sense a clear radio before beginning a new transmission sequence. Slottime is more important for collision avoidance when multiple stations are connected, or when one is trying to simulate full duplex communication, while ACK timeouts are more important for point to point communication. The important constants to consider are Slottime, CTS timeout and ACK timeout. All 802.11 a/b/g wireless devices use a number of time constants, defined by the IEEE specifications, to sense other carriers using the wireless media and avoid collisions (compared to 802.3 which only senses collisions) as well as for retransmission of lost frames. ![]()
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