DFWMAC-DCF CSMA/CA with ACK Station has to wait for DIFS before sending data Receiver ACKs immediately(after waiting for SIFS DIF S)if the packet was received correctly (CRC)) Receiver transmits acK without sensing the medium If ACK is lost, retransmission is performed Automatic retransmission of data packets in case of transmission errors CS5602: Principles and Techniques for Sensors and Information Perception
DFWMAC-DCF CSMA/CA with ACK ▪ Station has to wait for DIFS before sending data ▪ Receiver ACKs immediately (after waiting for SIFS < DIFS) if the packet was received correctly (CRC)) ▪ Receiver transmits ACK without sensing the medium. ▪ If ACK is lost, retransmission is performed. ▪ Automatic retransmission of data packets in case of transmission errors CS5602: Principles and Techniques for Sensors and Information Perception 26
DFWMAC-DCF CSMA/CA With AcK DIFS Sender Data SIFS Receiver ACK DIFS Other Contention w Stations Waiting Time Contention CS5602: Principles and Techniques for Sensors window and Information Perception
DFWMAC-DCF CSMA/CA with ACK CS5602: Principles and Techniques for Sensors and Information Perception 27 t SIFS DIFS Contention W ACK Waiting Time Other Stations Receiver Sender Data DIFS Contention Window
Problems with CSMaca -Hidden terminal problem Exposed terminal problem S5602scihepdesdSErushamigatstukSensors and Information Perception
CS5602: Principles and Techniques for Sensors CSCE 496/896: Sensor Networks 28 and Information Perception Problems with CSMA/CA ▪Hidden terminal problem ▪Exposed terminal problem
Hidden Terminal problem B DATA DATA A senses the channel free and sends data C cannot hear a and senses the channel free DatA packet collides at b S5602scihepdesdSErushamigatstukSensors and Information Perception
CS5602: Principles and Techniques for Sensors CSCE 496/896: Sensor Networks 29 and Information Perception Hidden Terminal Problem ▪ A senses the channel free and sends DATA ▪ C cannot hear A and senses the channel free ▪ DATA packet collides at B A B C DATA DATA
Exposed Terminal problem DATA B DATA Tx inhibited B sends daTa to A(overheard by c) C inhibits its transmission to D since channel is busy A cannot hear c c-D transmission can actually take place without collisions S5602SGihelpde%udSErshmgNatstukSensors and Information Perception
CS5602: Principles and Techniques for Sensors CSCE 496/896: Sensor Networks 30 and Information Perception Exposed Terminal Problem ▪ B sends DATA to A (overheard by C) ▪ C inhibits its transmission to D since channel is busy ▪ A cannot hear C ▪ C-D transmission can actually take place without collisions A B C D DATA DATA Tx inhibited