Wireless Medium Access Control Protocols

1. Wireless Medium Access Control Protocols CS 851 Seminar University of Virginia www.cs.virginia.edu/~cs851-2/course.html

2. What Are We Going to Learn? • Understand and appreciate fundamental principles in Wireless Medium Access Control Protocols • Basic theory • A real MAC protocol: IEEE 802.11b • Open problems • Survey the state-of-the-art research and develop new ideas

3. A C Throughput B D 2 4 6 8 10 12 14 16 Number of Nodes Why Need MAC ? • Wireless medium is a broadcast medium • Multiple nodes may access the medium at the same time • Medium Access Control Protocol: • Define rules to allow distributed nodes to communicate with each other in an orderly and efficiently manner

4. Ideal MAC Protocol • Limited Delay • High Throughput • Fairness • Stability • Robustness against channel fading • Low power consumption • Support for multimedia

5. Wireless MAC Issues • Half-Duplex Operation • Time Varying Channel • Burst Channel Errors • Location Dependent Carrier Sensing • Hidden Terminal • Exposed Terminal • Capture

6. Hidden Terminal Problem • Node B can communicate with A and C both • A and C cannot hear each other • When A transmits to B, C cannot detect the transmission using the carrier sense mechanism • If C transmits to D, collision will occur at B A B C D

7. X Exposed Terminal Problem • Node C can communicate with B and D both • Node B can communicate with A and C • Node A cannot hear C • Node D can nor hear B • When C transmits to D, B detect the transmission using the carrier sense mechanism and postpone to transmit to A, even though such transmission will nor cause collision A B C D

8. Power Difference Of A and D signals D B A C Capture Problem • A and D transmit simultaneously to B, the signal strength received from D is much higher than that from A, and D’s transmission can be decoded without errors. This will result unfair sharing of bandwidth.

9. Wireless MAC Protocols Distributed Mac Protocols Centralized MAC Protocols Random access Guaranteed access Classification of Wireless MAC Protocols

10. Distributed MAC Protocols • Collision avoidance mechanisms • Collision avoidance with out-of-band signaling • Collision avoidance with control handshaking • Distributed Random access protocols • DFWMAC (used in IEEE 802.11) • EY-NPMA (used in HyperLan)

11. Centralized MAC Protocols • Work for centralized wireless networks • Base station has explicit control for who and when to access the medium • All nodes can hear from and talk to base station • All communications must go through the base station • The arbitration and complexity are in base station • Base station decides who and when to access the medium

12. MACA:A New Channel Access Method for Packet RadioPhil Karn 1990 • Main Contribution: • Insights: • Carrier sense (send side) approach is inappropriate for wireless networks • Contention/collision will occur at receiver side • Proposed a three-way handshake MAC protocol : MACA CSMA/CA  MA/CA  MACA

13. Fundamental Assumptions • Symmetry • A can hear from B  B can hear from A • No capture • No channel fading

14. CTS (10) RTS (10) RTS: Request To Send CTS: Clear To Send DATA Three-Way Handshake • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • Sender sends DATA PACKET • RTS and CTS announce the duration of the transfer • Nodes overhearing RTS keep quiet for some time to allow A to receive CTS • Nodes overhearing CTS keep quiet for some time to allow B to receive data packet D B A C E

15. More Details for MACA • A sends out RTS and set a timer • If A receives CTS before timer go to zero, OK • Otherwise, A assumes there is a collision at B • Double the backoff counter interval • Randomly pick up a timer from backoff counter interval • Send next RTS after time go to zero • B sends out CTS, then set a timer • If data packet arrives before timer go to zero, OK • Otherwise, B can do other things • C overhears A’s RTS, set a timer which is long enough to allow A to receive CTS. After the timer goes to zero, C can do other things • D overhears B’s CTS, set a timer which is long enough to allow B to receive data packet. • E overhears A’s RTS and B’s CTS, set a timer which is long enough to allow B to receive data packet. • RTS and CTS can also contain info to allow sender A to adjust power to reduce interference

16. CTS DATA RTS RTS RTS B A C Hidden Terminal Problem Still Exists • Data packet still might suffer collision Four-way handshake (add ACK message) Sender will redo RTS/CTS/DATA/ACK if no ACK is received If ACK is sent out, but not received by sender, after receiving new RTS, receiver returns ACK instead of CTS for new RTS

17. RTS CTS RTS CTS DATA B A C Exposed Terminal Problem Still Exists • Node C can not receive CTS

18. MACAW: A Media Access Protocol for Wireless Lan’sV. Bharghavan, A. Demers, S. Shenker, and L. Zhang (Sigcomm 1994) This paper refined and extended MACA • Fixed RTS/CTS packet size as 30 bytes • New back-off algorithms • Multiplicative increase and linear decrease (MILD) • Synchronize back-off counter using piggyback message • Multiple stream model (V-MAC) • Modified control messages • Four-way handshake • Five-way handshake • RRTS

19. CTS RTS RTS: Request To Send CTS: Clear To Send DATA ACK Four-Way Handshake • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • Sender sends DATA PACKET • Receiver acknowledge with ACK • RTS and CTS announce the duration of the transfer • Nodes overhearing RTS/CTS keep quiet for that duration destination source

20. Exposed Terminal Problem Still Exists • When B transmits, C is unable to hear CTS • To relieve exposed terminal problem, let C know B does transmit DATA • Extra message DS

21. CTS RTS DS RTS: Request To Send CTS: Clear To Send DS: Data Sending DATA ACK Five-Way Handshake • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • Sender sends DATA SENDING (DS) • Sender sends DATA PACKET • Receiver acknowledge with ACK • RTS and CTS announce the duration of the transfer • Nodes overhearing RTS/CTS keep quiet for that duration destination source

22. CTS ACK DATA RTS RTS RTS B A Unfairness • Using RTS/CTS/DATA/ACK or RTS/CTS/DS/DATA/ACK might cause unfairness • A sends data to B; D sends data to C • A and D have enough data to send • C can hears from B and D, but not A • B can hear from A and C, but not D • A is in luck and gets the channel • D sends RTS and times out • Backoff window for D repeatedly doubles • For the next transmission: • A picks a random number from a smaller window • Unequal probability of channel access • Throughput for flow A B > 90 % • Throughput for flow D  C ~ 0% C D

23. CTS ACK RRTS: Request for RTS DATA RTS RRTS RTS RTS B A Request for RTS (RRTS) • Try to solve unfairness by having C do the contending for D C D

24. CTS RTS RTS: Request To Send CTS: Clear To Send DATA Request for RTS (RRTS) • CSMA/CA with binary exponential backoff • IEEE 802.11 data transmission is accomplished via a four-way handshake destination source

25. Backoff Algorithms • When collision occurs, node A pick up a random number T from [1,Bo], then retransmits RTS after T time unit • How to determine Bo • Upper bound Bo_max • Low bound Bo_Min • After each collision Bo_new = Fun_inc(Bo_old) • After each successful transmission Bo_new = Fun_dec(Bo_old) • Binary exponential backoff (BEB) algorithm • Fun_inc(Bo_old)=min{2*Bo_old, Bo_max} • Fun_dec(B_old)=Bo_min • Multiplicative increase linear decease (MILD) • Fun_inc(Bo_old)=min{1.5*Bo_old, Bo_max} • Fun_dec(B_old)=max{Bo_old -1, Bo_min}

26. MAC Multiple Stream MAC Single Stream MAC MAC MAC Node MAC Node Multiple Stream Model (V-MAC) • Single stream model merge traffic from different flows into a mixed stream and uses a single MAC • Multiple stream model uses multiple MAC (one flow one MAC) to achieve fairness • This idea was used by Intersil Company to proposed a new MAC for IEEE 802.11e

27. Open Problems • Adaptive MAC to achieve fairness in ad-hoc networks • Does upper layer operation need to depend on MAC?

28. Dual Busy Tone Multiple Access (DBTMA) : A Multiple Access Control Scheme for Ad Hoc Networks Z. Haas and J. Deng IEEE Trans. on Communications June, 2002 This paper completely solves hidden and exposed terminal problems

29. DATA RTS RTS B A DBTMA • Two narrow-bandwidth tones • BTt • Send out by the node which has data to send • Increase the probability of successful RTS reception • Stop after sensing BTr • BTr • Send out by the node which received RTS • Acknowlesdge RTS reception • Provides continuous protection for the transmitted data packet • Stop when completely receives the data packet • All nodes sensing any busy tone are not allowed to send RTS • Any node no sensing any busy tone is allowed to transmit

30. Finite State Machine of DBTMA

31. Related Works • BTMA :ok • RI-BTMA ?? • MACA:done • MACAW : done • FAMA : FAMA-NPS, FAMA-NCS :ok • IEEE 802.11 MAC:done • WCD: ok

32. Example • When A has data to send • Senses BTt and BTr • If both are clear • Turns on BTt • Sends RTS • Sets a timer for BTr and enters WF_BTR state • If BTr is sensed, waites for tmw, then sends data packet • Otherwise, timer goes to zero, A goes to IDLE state • Otherwise • Sets a random timer and goes to CONTENT state • If BTt or BTr is still sensed when timer goes to zero, A goes to idle state • Otherwise, A turns on BTt if no any busy tone signal is sen • When B receives RTS, B turns on BTr and sets a timer and enters WF_DATA state • If B has not received data packet before timer goes to zero B turns off BTr and goes to idle

33. BTt of A tmw A DATA RTS B RTS DATA BTr of B Time Diagram of DBTMA

34. Performance Analysis • Assumptions: • A lot of nodes and all nodes are in the same broadcast domain • No channel fading, capture effect • Packet collisions are the only reason for packet errors • Data processing time and transmit/receive turn around time are negligible • Bandwidth consumption of busy tones is negligible compared with data channel

35. Performance Analysis (cont)

36. Channel Throughputs of DBTMA

37. Channel Throughput

38. Comparisons of Channel Throughput

39. Performance of Different Length of Control Packet

40. Network Utilization of DBTMA in Multi-Hop Networks