Wireless Medium Access Control (MAC) (Refer Section 7.3.1 and 7.3.2 in textbook) Slides Adopted from: Romit Roy Choudhury Wireless Networking Lectures University of Illinois at Urbana Champaign 1 Wired Vs Wireless Media Access Both are on shared media. Then, whats really the problem ? 2 The Channel Access Problem
Multiple nodes share a channel A A B B C C Pairwise communication desired Simultaneous communication not possible
MAC Protocols Suggests a scheme to schedule communication Maximize number of communications Ensure fairness among all transmitters 3 The Trivial Solution A A B B C C collision
Transmit and pray Plenty of collisions --> poor throughput at high load 4 The Simple Fix A A Dont Dont transmit transmit B B
Transmit and pray C C Can Can collisions collisions still still occur? occur? Plenty of collisions --> poor throughput at high load Listen before you talk Carrier sense multiple access (CSMA)
Defer transmission when signal on channel 5 Collisions in CSMA (Carrier Sense Multiple Access) Collisions can still occur: spatial layout of nodes Propagation delay non-zero between transmitters When collision: Entire packet transmission time wasted
note: Role of distance & propagation delay in determining collision probability 6 CSMA/CD (Collision Detection) Keep listening to channel While transmitting If (Transmitted_Signal != Sensed_Signal) Sender knows its a Collision ABORT 7 2 Observations on CSMA/CD
Transmitter can send/listen concurrently If (Transmitted - Sensed = null)? Then success The signal is identical at Tx and Rx Non-dispersive The TRANSMITTER can detect if and when collision occurs 8 Unfortunately Both observations do not hold for wireless Because 9 Wireless Medium Access Control
C A D B Signal power = is Received Power at B
is Transmit Power at A is Distance between A and B Distance 10 Wireless Medium Access Control C A D B Signal power Decoding threshold
Sensing threshold Distance 11 Wireless Media Disperse Energy A cannot send and listen in parallel C A D B Signal power
Distance 12 Collision Detection Difficult A B D C Signal reception based on SINR Transmitter can only hear itself Cannot determine signal quality at receiver
13 Calculating SINR A D B C 14 Red Blue = collision Red signal >> Blue signal X
C A D B Signal power Distance 15 No Collisions As signal at C is above sensing threshold, hence, C does not transmit X
C A D B Signal power Distance 16 C cannot sense A, assumes channel is free, transmits and collides at B C is the hidden terminal to A X
C A D B Signal power Distance 17 C cannot sense A, assumes channel is free, transmits and collides at B C is the hidden terminal to A X C
A D B Signal power Decrease sensing threshold C will not transmit No collisions Distance 18 Exposed terminal problem 19
Exposed terminal problem X C A D B Signal power Distance 20 Exposed terminal problem
X C A D B Signal power Distance 21 Any Questions at this point?
22 So, how do we cope with Hidden/Exposed Terminals? 23 The Emergence of MACA, MACAW, & 802.11 Wireless MAC proved to be non-trivial 1992 - research by Karn (MACA) 1994 - research by Bhargavan (MACAW)
Led to IEEE 802.11 committee The standard was ratified in 1999 24 So, how do we cope with Hidden/Exposed Terminals? 25 The Emergence of MACA, MACAW, & 802.11
Wireless MAC proved to be non-trivial 1992 - research by Karn (MACA) 1994 - research by Bhargavan (MACAW) Led to IEEE 802.11 committee The standard was ratified in 1999 26 IEEE 802.11 RTS = Request To Send CTS = Clear To Send M Y S
RTS D CTS X K 27 IEEE 802.11 silenced M Y S Data
D silenced ACK X silenced K silenced 28 IEEE 802.11 M
silenced M Y S Data D silenced ACK X silenced K silenced
29 802.11 Steps All backlogged nodes choose a random number R = rand (0, CW_min) Each node counts down R Continue carrier sensing while counting down Once carrier busy, freeze countdown
Whoever reaches ZERO transmits RTS Neighbors freeze countdown, decode RTS RTS contains (CTS + DATA + ACK) duration = T_comm Neighbors set NAV = T_comm Remains silent for NAV time 30 802.11 Steps Receiver replies with CTS Also contains (DATA + ACK) duration. Neighbors update NAV again Tx sends DATA, Rx acknowledges with ACK
After ACK, everyone initiates remaining countdown Tx chooses new R = rand (0, CW_min) If RTS or DATA collides (i.e., no CTS/ACK returns) Indicates collision RTS chooses new random no. R1 = rand (0, 2*CW_min) Note Exponential Backoff Ri = rand (0, 2^i * CW_min) Once successful transmission, reset to rand(0, CW_min) 31 But is that enough?
32 RTS/CTS Does it solve hidden terminals ? Assuming carrier sensing zone = communication zone E RTS F CTS A
B C CTS D EEdoes doesnot notreceive receiveCTS CTSsuccessfully successfullyCan Canlater laterinitiate initiatetransmission transmissionto
toD. D. Hidden terminal problem remains. Hidden terminal problem remains. 33 Hidden Terminal Problem How about increasing carrier sense range ?? E will defer on sensing carrier no collision !!! E RTS
F CTS A B C Data D 34 Hidden Terminal Problem But what if barriers/obstructions ??
E doesnt hear C Carrier sensing does not help E RTS F CTS A B C Data
D 35 Exposed Terminal E should be able to transmit to F Carrier sensing makes the situation worse F E RTS A B C
D 36 Thoughts ! 802.11 does not solve HT/ET completely Only alleviates the problem through RTS/CTS and recommends larger CS zone Large CS zone aggravates exposed terminals Spatial reuse reduces A tradeoff RTS/CTS packets also consume bandwidth Moreover, backing off mechanism is also
wasteful The search for the best MAC protocol is still on. However, 802.11 is being optimized 37 Takes on 802.11 Role of RTS/CTS Useful? No? Is it a one-fit-all? Where does it not fit? Is ACK necessary? MACA said no ACKs. Let TCP recover from losses Should Carrier Sensing replace RTS/CTS? New opportunities may not need RTS/CTS
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