1. 1 CS 6910 – Pervasive Computing Spring 2007 Section 6 (Ch.6): Multiple Radio Access Prof. Leszek Lilien Department of Computer Science Western Michigan University Slides based on publisher’s slides for 1 st and 2 nd edition of: Introduction to Wireless and Mobile Systems by Agrawal & Zeng © 2003, 2006, Dharma P. Agrawal and Qing-An Zeng. All rights reserved. Some original slides were modified by L. Lilien, who strived to make such modifications clearly visible. Some slides were added by L. Lilien, and are © 2006-2007 by Leszek T. Lilien. Requests to use L. Lilien’s slides for non-profit purposes will be gladly granted upon a written request.

2. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 2 Chapter 6 Multiple Radio Access

3. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 3 Outline Introduction Contention Protocols ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) CSMA/CD (CSMA with Collision Detection) CSMA/CA (CSMA with Collision Avoidance)

4. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 4 6.1. Introduction © 2007 by Leszek T. Lilien Recall Large # of traffic channels on each BS Bec. traffic channels used by 1 MS exclusively for call duration Small # of control channels on each BS Bec. control channels shared by many MSs for short periods Too expensive/inefficient to assign control channnel for call duration MSs compete for these few shared control channels For call setup, etc. BS Forward (downlink) control channel MS Reverse (uplink) control channel Forward (downlink) traffic channel Reverse (uplink) traffic channel

5. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 5 Introduction – cont. 1 Multiple access control channels Each MS attached to a transmitter/receiver Communicates with its BS via a control channel (CC) CC shared by other MSs => “multiple access” CC Transmission from any MS is received by all MSs within its radio range Shared Multiple Access Medium MS 4 MS 3 MS 2 MS 1 … MS n …

6. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 6 Multiple access issue If more than one MS transmits at a time on a CC to BS, a collision occurs on this CC How to determine which MS can transmit to BS? Multiple (radio) access protocols (a.k.a. MAC sublayer protocols) Solve multiple access issues Specify how shared control channels are to be accessed by M Introduction – cont. 2 (Modified by LTL) Shared Multiple Access Medium MS 4 MS 3 MS 2 MS 1 … MS n …

7. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 7 Medium (Channel) Sharing Techniques Medium- sharing Techniques Static Channelization Dynamic Medium Access Control Scheduled Random Access Static channelization (static medium access control) – channel assigned in a pre-specified way and not changed Dynamic channelization (dynamic medium access control) - channel allocated as needed and not changes with time (Modified by LTL)

8. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 8 Recall ISO Open Systems Interconnection Reference Model for networks Communication subnetwork = 3 lowest layers of OSI Network layer (NET) Data link layer (DLL) Physical layer (PHY) 6.2. Multiple (Radio) Access Protocols = MAC Sublayer Protocols © 2007 by Leszek T. Lilien

9. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 9 Comm subnet in wired LANs, MANs, PRNs, PANs LAN = local area network / MAN = metropolitan area network PAN = personal area network / PRN = paceket radio network Classic (wire!): point-to-point full-duplex channels Comm subnet in wireless systems Broadcast half-duplex channels Broadcast over airwaves - not point-to-point Radio transmission goes from xmitter to rcvr - not full-duplex Can emulate point-to-point xmission in wireless systems In order to use solutions (e.g., protocols) from wired netwroks over wireless Note: broadcast possible in some wired net topologies E.g. bus, ring,... 6.2. Multiple (Radio) Access Protocols = MAC Sublayer Protocols – cont. 1 © 2007 by Leszek T. Lilien

10. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 10 Q: How to emulate point-to-point xmission in wireless systems? A: Modify OSI for point-to-point xmission in wireless systems Specifically, split DLL into 2 sublayers: Logical link control (LLC) sublayer (upper sublayer) Medium access control (MAC) sublayer (lower sublayer) MAC sublayer protocols = multiple access protocols = multiple radio access protocols (see above) (Modified by LTL) © 2007 by Leszek T. Lilien 6.2. Multiple (Radio) Access Protocols = MAC Sublayer Protocols – cont. 1 DLL NET

11. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 11 Types of multiple access protocols (cf. next slide) : Contention-based protocols - resolve a collision after it occurs Execute a collision resolution protocol after each collision Collision-free protocols (a.k.a. conflict free p.) - ensure that a collision never occurs E.g., a bit-map protocol and binary countdown protocol Types of Multiple Access Protocols (Modified by LTL)

12. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 12 Classification of Multiple Access Protocols Random access protocols – upon collision, retrans- mit after a ran- dom delay Collision resolution protocols - upon collision, retransmit according to a more sophisti- cated method Multiple-access protocols Contention-basedCollision-free Random access Collision resolution FDMA TDMA CDMA Token Bus DQDB etc. ALOHA CSMA BTMA, ISMA etc. TREE WINDOW etc. DQDB: Distributed Queue Dual Bus BTMA: Busy Tone Multiple Access ISMA: Idle Signal Multiple Access ALOHA and CSMA protocols will be discussed in this section.

13. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 13 6.3. Contention-Based Protocols Major categories of contention-based protocols 1) ALOHA (a.k.a. Pure ALOHA) 2) Slotted ALOHA 3) CSMA (Carrier Sense Multiple Access)... discussed in turn below... © 2007 by Leszek T. Lilien

14. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 14 6.3.1. ALOHA (a.k.a. Pure ALOHA) Developed in the 1970s for a packet radio network by University of Hawaii To interconnect islands’ campuses / Single-hop network Principles: Sender S may xmit a packet at any time (hoping no collision will occur) S finds out whether transmission was successful or experienced a collision by listening for an ACK broadcast from the destination station Successful if an ACK arrrives within a time-out period T If no ACK within T, S assumes that there was a collision => packet was lost after colliding with pckt of another station If there was a collision, S retransmits after some random time Analogy: 2 people entering a doorway simultaneously Collision can occur (Modified by LTL)

15. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 15 6.3.1. ALOHA – cont. Collision in Pure ALOHA (can last up to 2T) 12332 Time Collision Retransmission MS 1 packet Wait for a random time MS 2 packet MS 3 packet t - T tt + T T Recall ALOHA principles: Sender S may xmit a packet at any time S waits for an ACK broadcast from the destination station If there was a collision, S retransmits after some random time Example: Packet 2 from MS2 & Packet 3 from MS3 collide MS3 retransmits Packet 3 pretty soon (random wait) MS2 retransmits Packet 2 much later (random wait) Note: Collision can block channel for period up to 2T (Modified by LTL)

16. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 16 ** OPTIONAL ** Throughput of ALOHA n  !n (2G) nP e 2G   T – packet length (duration) Probability P(n) that n packets arrive in time 2T -- If 2 packets of length T arrive in time > 2T, they didn’t collide where G is normalized offerred traffic load for the channel Probability P(0) that a packet is successfully received (without collision) - calculated by letting n=0 in P(n) Throughput S Maximum throughput for ALOHA (occurs for G = ½ - cf. p. 129) Analogy: two people sufficiently distant from each other do not collide in a doorway (Modified by LTL)

17. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 17 Improvements over Pure ALOHA: Time is slotted Slot length = packet length (duration) = T Packet transmission can start only at the beginning of a slot This reduces collision duration Pure ALOHA: If Packet 1 is almost finished when it collides with just-starting Packet 2, collision can lasts for nearly T+T = 2T (details: Slide +2) Slotted ALOHA: 2 Packets can collide only when both are just starting => collision can last at most T (details: Slide +4) (Modified by LTL) 6.3.2. Slotted ALOHA

18. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 18 6.3.2. Slotted ALOHA – cont. Collision in Slotted ALOHA (can last max. T) 12&32 Time Collision Retransmission 3 Slots MS 1 packet MSs 2 & 3 packets Wait for a random time Recall Slotted ALOHA improvements: Time is slotted / Packet can only start at the beginning of a slot This reduces collision duration Collision can block channel for max. T Example: Packet 2 from MS2 & Packet 3 from MS3 collide MS2 retransmits Packet 2 pretty soon (random wait) MS3 retransmits Packet 3 much later (random wait) Note: Shows again that collision can block channel for period T (Modified by LTL)

19. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 19 ** OPTIONAL ** Throughput of Slotted ALOHA Probability of no collision Throughput S Maximum throughput for slotted ALOHA (occurs for G = 1 - cf. p. 130)

20. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 20 Comparison of Throughputs for ALOHA and Slotted ALOHA Slotted Aloha Aloha 0.368 0.184 Traffic load G Throughput S th (Modified by LTL)  Max. throughput for ALOHA (A) S th = 0.184 at G = ½  Max. throughput for Slotted ALOHA (SA) S th = 0.368 at G = 1  Notice that S th for SA is exactly 2 x bigger than S th for A  Hypothesis: bec. max. duration of a collision for SA (=2T) is 2 x smaller than for A (=T)

21. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 21 6.3.2-B. CSMA (Carrier Sense Multiple Access) Group of Protocols Max. throughputs for Pure & Slotted ALOHA are 0.184 & 0.368 CSMA protocols give better throughput than both Aloha protocols By avoiding collisions Basic improvement in all CSMA protocols over ALOHA protocols: Listen to the channel (“sense” for the presence of the “carrier”) before transmitting a packet Don’t transmit if channel busy Avoids some collisions - but not all See Fig - next slide

22. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 22 Perspective on MAC & Access Methods Medium Access Control (MAC) Different nodes must gain access to the shared medium in a controlled fashion (otherwise there will be collisions) Access methods FDMA - assigning channels in frequency domain TDMA - assigning time slots in time domain CDMA - assigning code sequences in code domain CSMA - assigning transmission opportunities in time domain on a statistical basis © 2006, Michael Hall, Helsinki Univ. of Technology DLL NET

23. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 23 Basic Collision Mechanism in CSMA Protocols Some collisions avoided Packet 4 was not sent so it did not collide with Packet 2 Packet 5 was not sent so it did not collide with Packet 3 Collisions can still occur if sensing occurs nearly simulta- neously Packet 6 collided with Packet 7 7 Time 123 Collision 6 MS 4 senses, avoids collision Delay for MS 4 MS 5 senses, avoids collision MS 5 Delay MS 1 senses, sends packet MS 2 senses, sends packet MS 3 senses, sends packet MS 6 senses, sends packet MS 7 senses, sends packet (Modified by LTL)

24. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 24 1) Basic CSMA, often just CSMA (Carrier Sense Multiple Access) 2) CSMA/CD (CSMA with Collision Detection) 3) CSMA/CA (CSMA with Collision Avoidance) Types of CSMA Protocols © 2007 by Leszek T. Lilien

25. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 25 (Basic) CSMA improvement over ALOHAs: Sender S starts transmission only if no transmission is ongoing 6.3.3 (Basic) CSMA Protocol © 2007 by Leszek T. Lilien

26. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 26 Types of CSMA Protocols CSMA Nonpersistent CSMA Persistent CSMA Unslotted nonpersistent CSMA Unslotted persistent CSMA Slotted nonpersistent CSMA Slotted persistent CSMA 1-persistent CSMA p-persistent CSMA

27. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 27 Nonpersistent/x-persistent CSMA Protocols Nonpersistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately Step 2: If the medium is busy, wait a random amount of time and then go to Step 1 Note: Random backoff reduces probability of collisions Wasted idle time if the backoff time is too long 1-persistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately Step 2: If the medium is busy, continue to listen until medium becomes idle, and then transmit immediately Note: There will always be a collision if two or more nodes wait for idle medium Usually senders stop transmission attempts after a few unsuccessful tries

28. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 28 Nonpersistent/x-persistent CSMA Protocols – cont. p-persistent CSMA Protocol: Slotted time required. Step 1: If the medium is idle, transmit with probability p, or delay till next time slot with probability (1-p) Step 2: If transmission was delayed by one time slot, go to Step 1 Step 3: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1 Note: P-persistent CSMA is a good tradeoff between nonpersistent and 1-persistent CSMA

29. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 29 How to Select Probability p ? Assume: N - # nodes that can be active at a time (i.e., ready to send a packet) => Np - the expected number of nodes that will attempt to transmit once the medium becomes idle Np ≥ 1 => a collision is expected to occur => to avoid collisions, network must make sure that Np < 1 (Modified by LTL)

30. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 30 Throughput for Different ALOHA and CSMA Protocols with  =0.01 0 1 2 3 4 5 6 7 8 9 Traffic Load G 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Throughput S th Aloha Slotted Aloha 1-persistent CSMA 0.5-persistent CSMA 0.1-persistent CSMA 0.01-persistent CSMA Nonpersistent CSMA  =  / T (p. 132)  - propagation delay T – packet transmission time Recall: p-persistent: transmit with probability p if medium idle Observe: p = 0.01 is best in this Fig. (others suffer more collisions reducing thruput => “don’t hurry too much”) (Modified by LTL)

31. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 31 6.3.4. CSMA/CD (CSMA with Collision Detection) In Basic CSMA: If 2 terminals begin sending packets at the same time, each will transmit its complete packet Even if collision is taking place Problem: Wasting medium for an entire packet duration Solution principle (main CSMA/CD idea): Backoff immediately after a collision Modified by LTL

32. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 32 6.3.4. CSMA/CD (CSMA with Collision Detection) – cont. Solution: CSMA/CD (CD = collision detection) Step 1:If the medium is idle, then transmit Step 2:If the medium is busy, continue to listen until the channel is idle, then transmit Step 3:When a collision is detected during transmission, cease xmitting immediately Step 4:Wait a random amount of time & go to Step 1 Improvement over Basic CSMA: Able to terminate xmission as soon as collision detected Prevents wasting whole time slot (slot = packet duration) Modified by LTL

33. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 33 Use of Jamming Signal in CSMA/CD Collision Mechanisms in CSMA/CD 1) Without a jamming signal 2) With a jamming signal Jamming signal facilitates collision detection by other terminals Note: Different timing assumptions in the textbook (pp. 135-136) © 2007 by Leszek T. Lilien

34. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 34 1) Collision Mechanism in CSMA/CD Without Jamming Signal AB τ – propa- gation delay  cd – time to detect collis.  ≈ 0 T0T0 A begins transmission at T 0 when channel is idle [will reach B at T 0 +  ] AB B begins transmission just  before A’s trans- mission reaches B Time T 0 + τ -  AB T 0 + τ + τ cd AB B detects collision  cd after A’s transmission reaches B & stops its xmission immediately (Modified by LTL) A detects collision  cd after B’s xmission reaches A [at (T 0 +  +  cd ) + + (  - (  +  cd )) +  cd ≈ T 0 +2  +  cd for  ≈0 ≈ T 0 +2τ +  cd => Transmission period:  = 2  +  cd gap ~  distance ~  distance ~  cd length ~  +  cd distance ~  cd distance ~  - (  +  cd )

35. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 35 AB T0T0 A begins transmission at T 0 when channel is idle (will reach B at T 0 +  ) AB B begins transmission just  bef. A’s trans- mission reaches B Time T 0 + τ -  AB T 0 + τ + τ cd A B T 0 +2τ+ 2τ cd B detects collision  cd after A’s transmission reaches B & starts sen- ding jamming signal (not shown yet) to warn other terminals about collision (Modified by LTL) B finishes sending jam. sig. (red) that lasted  cr A detects jam. sig.  cd after it reaches A [at (T 0 +  +  cd +  cr ) + (  -  cr +  cd ) = T 0 +2  +2  cd ] T 0 +τ+τ cd +τ cr AB => Transmission period:  = 2  + 2  cd 2) Collision Mechanism in CSMA/CD With Jamming Signal τ – prop. delay  cd – time to detect collis.  ≈ 0  cr – duration of jamming signal – long enough to ensure that all terminals detect collision (p.136/1) gap ~  distance ~  distance ~  cd length ~  cr length ~  +  cd distance ~  -  cr +  cd length ~  cd distance ~  -  cr

36. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 36 T – packet xmission time G = gT – normalized offered load  - propagation delay  =  / T - normalized time unit  ’ =  / T, where  is propagation path loss slope

37. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 37 Time MS A’s frame MSs B & C sense the medium MS B resenses the medium and transmits its frame MS B’s frame Backoff - delay for B Backoff - delay for C MS C resenses the medium but defers to MS B 6.3.5. CSMA/CA (CSMA with Collision Avoidance) A basic collision avoidance (CA) scheme CSMA/CA rule: Backoff before collision (to avoid collisions) (Modified by LTL)

38. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 38 When medium idle for a period ≥ DIFS => can xmit immediately DIFS = Distributed InterFrame Space In 802.11b networks, DIFS = 50 μs © 2006, Michael Hall, Helsinki Univ. of Technology 6.3.5. CSMA/CA (CSMA with Collision Avoidance) – cont. 1 DIFS -> (Modified by LTL)

39. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 39 Types of CSMA/CA 1) Basic CSMA/CA 2) CSMA/CA with ACK 3) CSMA/CA with RTS and CTS... discussed in turn... © 2007 by Leszek T. Lilien

40. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 40 1) Basic CSMA/CA Terminal T senses the medium before xmission 1) If medium is busy, T defers access until the end of current xmission - (b1) in Fig. 2) If medium is idle or once it becomes idle: T defers access for time period DIFS - (a) and (b2) in Fig. T picks a random backoff period - (c) in Fig. As long as medium idle during this backoff period, T keeps counting down Whenever medium busy during this backoff period, T freezes its counter After medium becomes idle again: T defers access for the DIFS period T resumes countdown T can start transmission when backoff counter = 0 (Modified by LTL) ≤ DIFS Medium Busy DIFS Contention window (cf. next slide) Deferred access (b) - deferred till idle (b1) + deferred for DIFS after became idle (b2) Rand. backoff period (c) (after deferments) Slot (cf. next slide) Time Could not start xmission (a) since medium became busy in ≤ DIFS b1 b2

41. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 41 1) Basic CSMA/CA – cont. 1 All except red text repeated (in larger font) Terminal T senses the medium before xmission 1) If medium is busy, T defers access until the end of current xmission 2) If medium is idle or once it becomes idle: T defers access for time period DIFS T picks a random backoff period Backoff counter - initialized with the value of backoff period As long as medium idle during this backoff period, T keeps counting down = decreases its backoff counter I.e., count down as long as medium idle, stop countdown when medium busy again Whenever medium busy during this backoff period, T freezes its counter After medium becomes idle again: T defers access for the DIFS period T resumes countdown T can start transmission when backoff counter = 0 (Modified by LTL)

42. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 42 1) Basic CSMA/CA – cont. 2 Fragment of the CSMA/CA protocol: As long as medium idle during this backoff period, T keeps counting down = decreases its backoff counter I.e., count down as long as medium idle, stop countdown when medium busy again (Modified by LTL) ≤ DIFS Medium Busy DIFS Contention window Deferred access (b) - deferred till idle (b1) + deferred for DIFS after became idle (b2) Rand. backoff period (c) (after deferments) Slot Time Could not start xmission (a) since medium became busy in ≤ DIFS b1 b2 Idle – starts countdown Busy – stops countdown Use of slots: Backoff counter is decreased by 1 each time medium is idle for 1 time slot interval

43. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 43 Contention Window (CW) Recall: Backoff counter is decreased by 1 each time medium is detected to be idle for an interval of one time slot => backoff period = backoff number (bn) = # of idle slots within CW to wait before being allowed to xmit a frame Random bn value is chosen based on CW size CW_size: 31 - 1023 slots (=> bn can be from 31 to 1032) bn value: uniform distribution over 0 … CW_size If transmission of a frame unsuccessful & frame to be re- transmitted => CW_size is doubled before retran. (up to 1023) © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology (sl.19) (Modified by LTL) 1) Basic CSMA/CA – cont. 3

44. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 44 Selection of Random Backoff Value Recall: If transmission of a frame unsuccessful & frame allowed to be retransmitted => => before each retransm. CW_size is doubled (up to 1023) => Pr{ bn value is larger } is higher Since it is unlikely that several stations will choose the same value of bn, collisions are avoided © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 20) 1) Basic CSMA/CA – cont. 4 (Modified by LTL)

45. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 45 2) CSMA/CA with ACK Positive Acknowledgement (ACK) frame from receiver upon reception of each data frame ACK sent after receipt + after time interval SIFS SIFS = Short Interframe Space Receiver transmits ACK without sensing the medium (cf. next) If ACK lost, retransmission of ACK DIFS Medium Busy ACK Data Other MS Source MS Destination MS DIFS SIFS Contention window Defer accessBackoff after defer Time SIFS < DIFS E.g., in 802.11b networks: SIFS = 10 μs / DIFS = 50 μs (Modified by LTL)

46. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 46 When a frame is received without bit errors, receiving station B sends ACK back to vmitting station A © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology CSMA/CA with ACK – cont. 1 ->

47. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 47 CSMA/CA with ACK – cont. 2 Receiver transmits ACK without sensing the medium It works well because medium (radio channel) is “reserved” for ACK Medium “reserved” during the transmission sequence: Transmitted frame + SIFS + ACK Next frame (from any station) can be transmitted at earliest after ACK and after next DIFS period © 2006, Michael Hall, Helsinki Univ. of Technology -> (Modified by LTL)

48. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 48 Recal: SIFS < DIFS -- E.g., in 802.11b: SIFS = 10 μs / DIFS = 50 μs When 2 stations try to access the medium at the same time, the one that has to wait for the shorter SIFS period “wins over” the one that has to wait for the longer DIFS period “Wins over” = waits for shorter time Just bec. SIFS < DIFS In other words, ACK frame waiting for SIFS has higher priority over Data frame waiting for DIFS © 2007 by Leszek T. Lilien Figure based on © 2006, Michael Hall, Helsinki Univ. of Technology CSMA/CA with ACK – cont. 3 Now you see why medium “reserved” in quotes - not real reservation, just given an advantage to guarantee it always wins a race (Modified by LTL) C -> D Data SIFS DIFS

49. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 49 When Station S wants to send a frame & channel busy => S must wait a backoff time before it may to xmit frame Reason? Next two slides… © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 16) CSMA/CA with ACK – cont. 4

50. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 50 No backoff => collision is certain Suppose that stations B and C waiting to access channel When channel becomes idle, B & C start sending their packets at the same time => collision! © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 17) CSMA/CA with ACK – cont. 5

51. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 51 Random backoff => Pr { collision } is reduced Contending stations generate random backoff numbers bn Backoff counters count down, starting from bn When backoff counter reaches zero, station allowed to send its frame All other counters stop counting until the channel becomes idle again Example below (fig): B & C back off after deferring for DIFS C has small bn – xmits sooner B has large bn – waits longer © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 18) CSMA/CA with ACK – cont. 6 (Modified by LTL)

52. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 52 Example for CSMA/CA with ACK Four stations: A, B, C, D © 2007 by Leszek T. Lilien © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 21) Observe: C attempts to access medium before B does Coincidentally, C’s backoff number is smaller (Modified by LTL)

53. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 53 © 2006, Michael Hall, Helsinki Univ. of Technology (sl.22) Example for CSMA/CA with ACK – cont. 1 Observe backoff counter for B: Frozen when medium becomes busy When C starts xmission (when C’s backoff counter becomes 0) B resumes countdown when medium becomes idle After C completes its xmission (Modified by LTL)

54. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 54 © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 23) Example for CSMA/CA with ACK – cont. 2 Observe backoff counter for D: Frozen when medium becomes busy When B starts xmission (when B’s backoff counter becomes 0) D resumes countdown when medium becomes idle After B completes its xmission (Modified by LTL)

55. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 55 3) CSMA/CA with RTS and CTS Transmitter sends an RTS (Request To Send) after medium has been idle for time interval > DIFS Receiver responds with CTS (Clear To Send) after medium has been idle for SIFS Data can be xmitted (after SIFS) ACK sent after SIFS DIFS Medium Busy CTS RTS Other MS Source MS Destination MS SIFS Contention window Defer access Backoff SIFS Data SIFS ACK Time DIFS (Modified by LTL)

56. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 56 Ladder Diagram for CSMA/CA with RTS/CTS Compare this diagram to the previous one RTS CTS Data ACK Source MS Destination MS Propagation delay SIFS Propagation delay SIFS DIFS Other MS 3) CSMA/CA with RTS and CTS – cont. 1a

57. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 57 3) CSMA/CA with RTS and CTS – cont. 1b RTS/CTS performs channel reservation for data xmission Collision can occur at worst during sending RTS control message Bec. SIFS (preceding CTS, Data, ACK) < DIFS (preceding other MS’s RTS) DIFS Medium Busy CTS RTS Other MS Source MS Destination MS SIFS Contention window Defer access Backoff SIFS Data SIFS ACK Time DIFS (Modified by LTL)

58. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 58 3) CSMA/CA with RTS/CTS – cont. 1c RTS = Request To Send / CTS = Clear To Send RTS/CTS scheme - used as a countermeasure against the “hidden node” problem Hidden node problem WS 1 & WS 2 can hear AP but not each other => If WS 1 sends a packet, WS 2 does not notice this (and vice versa) => collision! © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 25)

59. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 59 RTS/CTS scheme makes reservation of the medium to avoid collisions RTS reserves medium for time needed for xmission of: CTS + Data + ACK + 3 * SIFS = SIFS + CTS + SIFS + Data + SIFS + ACK CTS reserves medium for time needed for xmission of: Data + ACK + 2 * SIFS (original: “3 * SIFS [?]) = SIFS + Data + SIFS + ACK © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 26) 3) CSMA/CA with RTS/CTS – cont. 1b

60. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 60 © 2006, Michael Hall, Helsinki Univ. of Technology 3) CSMA/CA with RTS/CTS – cont. 2 Danger of collision only during RTS + SIFS (see Fig. below), because: WS 2 does not hear the RTS frame from WS 1 to AP => WS 2 misses reservation for CTS + Data + ACK + 3 * SIFS WS 2 can hear the CTS frame from AP to WS 1 => WS 2 knows about reservation for Data + ACK + 2 * SIFS

61. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 61 Advantages of RTS & CTS Advantage #1 of RTS/CTS: Reduces collision probability if the data frame is very long compared to the RTS frame Does not prevent collisions during very short RTS frames Not a big problem - they are not very likely Prevents more likely collisions during long data frame © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 28)

62. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 62 Advantage of RTS & CTS – cont. Advantage #2 of RTS/CTS Avoids having long intervals with high collision danger (same Fig) Good! Long interval w/ high collision danger should be avoided Should be for the following reasons: Larger probability of collision Greater waste of capacity if a collision occurs and the frame has to be retransmitted Frames shorter than a certain threshold value can be transmitted without using RTS/CTS Because small waste of medium if collision occurs for short frames => low risk Threshold parameter (dot11RTSThreshold) set in a xmitting station © 2006, Michael Hall, Helsinki Univ. of Technology (sl. 29)

63. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 63 1)(Basic) CSMA 1a) Nonpersistent CSMA – Slotted – Unslotted 1b) Persistent CSMA Subcategories # 1 of Persistent CSMA: – Slotted – Unslotted Subcategories # 2 (orthogonal) of Persistent CSMA: –1-persistent CSMA – p-persistent CSMA 2) CSMA/CD (CSMA with Collision Detection) 3) CSMA/CA (CSMA with Collision Avoidance) 3a) Basic CSMA/CA 3b) CSMA/CA with ACK 3c) CSMA/CA with RTS/CTS Summary: Full Taxonomy of CSMA (Carrier Sense Multiple Access) Protocols © 2007 by Leszek T. Lilien

64. Copyright © 2003, Dharma P. Agrawal and Qing-An Zeng. All rights reserved 64 The End of Section 6