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Tiểu luận môn mạng máy tính Carrier Sense Multiple Access with Collision Avoidance CSMACA 1. Basic CSMACA operation 2. Wireless medium access example 3. Usage of RTS CTS 4. Fragmentation CSMACD (Collision Detection) is the MAC method used in a wired LAN (Ethernet). Wired LAN stations can (whereas wireless stations cannot) detect collisions.
Contents Contents 1. Basic CSMA/CA operation 1. Basic CSMA/CA operation 2. Wireless medium access example 2. Wireless medium access example 3. Usage of RTS / CTS 3. Usage of RTS / CTS 4. Fragmentation 4. Fragmentation !" ## $ %# & '#() * +#,-. "!-. # 1. Basic CSMA/CA operation $ -, )) !" ## $ ,-. & %# * '#( / 01#012 " !-.,# 1. Basic CSMA/CA operation ! . Here is Infrastructure basic service set (BSS). . All stations and AP have equal priority transmission in downlink (from the AP) and uplink (from a station) is similar. " 3- 4 1. Basic CSMA/CA operation '5 '5 2 !01 % #01! #,(6! -62#,-6 '## (2, )+)- +# 1. Basic CSMA/CA operation $ '5 '5 '5 2 !01 % #01! 6758'582 %9#, #9'5 9# #) # 9# 1. Basic CSMA/CA operation & '5 '5 '5 2 !01 % #01! %#6" 4)6 :66-:: 9# 1. Basic CSMA/CA operation * '5 '5 '5 2 !01 % #01! 4)6 '))4;<#) #,6 #6 9# '#, 6## 4;< 1. Basic CSMA/CA operation / %#6'5'5" '5'#0=µ>µ '55'#0/=µ&*µ )( #'5# '5'('56)'5 ?=$, ?=$, ?=$6 ?=$6 1. Basic CSMA/CA operation [...]... slides… 1 Basic CSMA/CA operation No backoff => collision is certain Suppose that several stations (B and C in the figure) are waiting to access the wireless medium When the channel becomes idle, these stations start sending their packets at the same time => collision! Station A ACK Station B Collision! Station C DIFS 1 Basic CSMA/CA operation Backoff => collision probability is reduced Contending stations... unlikely that several stations will choose the same value of bn, collisions are rare The next slides show wireless medium access in action The example involves four stations: A, B, C and D ”Sending a packet” means ”Data+SIFS+ACK” sequence Note how the backoff time may be split into several parts 2 Wireless medium access example Wireless medium access (1) Station A Contention Window ACK Defer Station B Backoff... (backoff time expires first) and starts sending packet 2 Wireless medium access example Wireless medium access (2) 3) Station D also wishes to send a packet Station A Station B 4 Station C ACK 3 Defer Station D DIFS DIFS 4) However, station B is ”winner” and starts sending packet 2 Wireless medium access example Wireless medium access (3) 5) Station D starts sending packet Now there is no competition... Advantage of RTS & CTS (2) A long collision danger” interval (previous slide) should be avoided for the following reasons: Larger probability of collision Greater waste of capacity if a collision occurs and the frame has to be retransmitted A threshold parameter (dot11RTSThreshold) can be set in the wireless station Frames shorter than this value will be transmitted without using RTS/CTS 4 Fragmentation... ACK Danger of collision NAV in RTS NAV in CTS NAV = CTS + Data + ACK + 3xSIFS NAV = Data + ACK + 3xSIFS 3 Usage of RTS & CTS Advantage of RTS & CTS (1) Usage of RTS/CTS offers an advantage if the data frame is very long compared to the RTS frame: WS 1 AP WS 1 AP RTS Data frame CTS (RTS/CTS used) ACK Short interval: collision not likely Data frame (RTS/CTS not used) ACK Long interval: collision likely... operation Wireless medium access (6) Transmission is not allowed as long as NAV is non-zero Transmitted frame ACK NAV value is given here DIFS NAV SIFS Next frame DIFS Virtual carrier sensing It means that a NAV value is set in all stations that were able to receive a transmitted frame and were able to read the NAV value in this frame 1 Basic CSMA/CA operation Wireless medium access (7) Transmitted... operation Wireless medium access (8) Channel was idle at least DIFS seconds Transmitted frame (A=>B) ACK (B=>A) DIFS SIFS t > DIFS Next frame (from any station) When a station wants to send a frame and the channel has been idle for a time > DIFS (counted from the moment the station first probed the channel) => can send immediately 1 Basic CSMA/CA operation Wireless medium access (9) Channel was busy... other => WS 1 If WS 1 sends a packet, WS 2 does not notice this (and vice versa) => collision! AP WS 2 3 Usage of RTS & CTS Reservation of medium using NAV SIFS WS 1 RTS Data frame CTS AP SIFS NAV in RTS NAV in CTS DIFS ACK SIFS NAV = CTS + Data + ACK + 3xSIFS NAV = Data + ACK + 3xSIFS 3 Usage of RTS & CTS Danger of collision only during RTS WS 2 does not hear the RTS frame (and associated NAV), but... packet 2 Wireless medium access example Wireless medium access (3) 5) Station D starts sending packet Now there is no competition Station A Station B ACK Station C 5 Station D DIFS DIFS 2 Wireless medium access example No shortcuts for any station… Transmitted frame (A=>B) Backoff ACK (B=>A) DIFS SIFS Next frame (A=>B) DIFS When a station wants to send more than one frame, it has to use the backoff mechanism... receive a transmitted frame and were able to read the NAV value in this frame 1 Basic CSMA/CA operation Wireless medium access (7) Transmitted frame Long transaction ACK ACK NAV DIFS SIFS DIFS Virtual carrier sensing using NAV is important in situations where the channel should be reserved for a ”longer time” (RTS/CTS usage, fragmentation, etc.) 1 Basic CSMA/CA operation NAV value is carried in MAC . Contents Contents 1. Basic CSMA/CA operation 1. Basic CSMA/CA operation 2. Wireless medium access example 2. Wireless medium access example 3. Usage of RTS / CTS 3. Usage of RTS / CTS 4. Fragmentation 4.