REMINDER BASIC KNOWLEDGE BASIC CSMA/CA(FLOWCHART) Distributed foundation wireless medium Access control- Distributed coordinated Function CSMA/CA (DFWMAC-DCF) for IEEE 802.11) Wait for frame Wmit until current wait IFS backoff Transmit frame CS5602: Principles and Techniques for Sensors and Information Perception
REMINDER BASIC KNOWLEDGE BASIC CSMA/CA (FLOWCHART) (Distributed Foundation Wireless Medium Access Control - Distributed Coordinated Function CSMA/CA (DFWMAC-DCF) for IEEE 802.11) CS5602: Principles and Techniques for Sensors and Information Perception 16
BASIC CSMA/CA Contention window 工Fs 工Fs (Randomized Back-off Mechanism) Medium Busy Next Frame Station senses Direct access if Slot Time the channel medium is free≥TFs andⅰ t is idle CS5602: Principles and Techniques for Sensors and Information Perception
BASIC CSMA/CA CS5602: Principles and Techniques for Sensors and Information Perception 17 Direct access if Slot Time medium is free IFS t Medium Busy IFS IFS Next Frame Contention Window (Randomized Back-off Mechanism) Station senses the channel and it is idle
BASIC CSMA/CA a station with a frame to transmit senses the medium (channel) IF IDLE waits to see if the channel remains idle for a time equal to IFs (Inter-frame spacing). If so, the station may transmit immediately. IF BUSY >(either because the station initially finds the channel busy or because the channel becomes busy during the IFS idle time), the station defers transmission and continues to monitor the channel until the current transmission is over5602: Priciples and Techniques for Sensors
BASIC CSMA/CA ▪ A station with a frame to transmit senses the medium (channel). ▪ IF IDLE → waits to see if the channel remains idle for a time equal to IFS (Inter-frame spacing). If so, the station may transmit immediately. ▪ IF BUSY → (either because the station initially finds the channel busy or because the channel becomes busy during the IFS idle time), the station defers transmission and continues to monitor the channel until the current transmission is over. CS5602: Principles and Techniques for Sensors and Information Perception 18
BASIC CSMACA Once the current transmission is over, the station delays another IFS If the medium remains idle for this period, the station backs off using a binary exponential backoff scheme and again keeps sensing the medium. Backoff scheme The station picks up a random number of slots(the initial value of backoff counter) within a maximum contention window to wait before transmitting its frame CS5602: Principles and Techniques for Sensors and Information Perception
BASIC CSMA/CA ▪ Once the current transmission is over, the station delays another IFS. ▪ If the medium remains idle for this period, the station backs off using a binary exponential backoff scheme and again keeps sensing the medium. ▪ Backoff scheme ▪ The station picks up a random number of slots (the initial value of backoff counter) within a maximum contention window to wait before transmitting its frame. CS5602: Principles and Techniques for Sensors and Information Perception 19
BASIC CSMA/CA MAc runs a random number generator to set a BACKOFF CLoCK for every contending station Then the coNTENTIoN WINDoW starts in which all stations with packets for transmission run down their BACKOFF clocks The first station with its clock expiring starts transmission Other terminals sense the new transmission and freeze their clocks to be restarted after the completion of the current transmission in the next contention period. CS5602: Principles and Techniques for Sensors and Information Perception
BASIC CSMA/CA ▪ MAC runs a random number generator to set a BACKOFF CLOCK for every contending station. ▪ Then the CONTENTION WINDOW starts in which all stations with packets for transmission run down their BACKOFF clocks. ▪ The first station with its clock expiring starts transmission. ▪ Other terminals sense the new transmission and freeze their clocks to be restarted after the completion of the current transmission in the next contention period. CS5602: Principles and Techniques for Sensors and Information Perception 20