Special Topics in Computer Engineering Wireless Networks

1. Special Topics in Computer Engineering Wireless Networks By: Mohammad Nassiri Bu-Ali Sina University, Hamedan

2. Access method in Wireless Ad-hoc Networks

3. Ad-hoc mode in 802.11 • Ad Hoc • Simplest • Rapid deployment • Peer-to-peer • No administration • Basically, ad-hoc mode in 802.11 does not support multi-hop transmission. However, there are a lot of mechanisms to provide the multi-hop transmission with the help of Layer-3, namely, IP layer.

4. Multi-hop Ad-hoc Networks • An Ad-hoc network • Direct transmission with neighboring nodes • Each node can be router and so it can relay traffic. • B relays packet from A to C • Self-configuration, Self-healing • In this lecture, MAC issues in Wireless Ad-hoc Networks

5. Recall • Rx = Reception Range • CS = Carrier Sensing Range • A can communicate to B • C can only sense a transmission emitted from A • D cannot overhear A

6. RTS/CTS for hidden problem • D and C are hidden to A • D is within CSR of B • A sends to B, D sends to B, collision is possible.  RTS/CTS fails to resolve hidden terminal in this case

7. RTS/CTS for exposed nodes ? • RTS/CTS cannot handle exposed node problem • The left-hand scenario

8. Masked node • C cannot decode CTS from B • It’s NAV is not up to date. • Later it can collide the transmission of A to B by sending an RTS. • C is masked by B and D

9. Blocked nodes in 3 pairs • We consider blocked nodes in the scenario of three parallel pairs • node in the middle has almost no possibility to access the channel • Studied by Chaudet et al. 2005 • e.g. each pair in a room • A, C and E are emitters • Emitter C is starved by transmissions of A and E.

10. A DATA C E DATA Three pairs • How does legacy DCF work in this scenario when A and E are transmitting ? Busy Channel EIFS Backoff DIFS DATA DATA DATA DATA DATA DATA DATA DATA C is starved by A and E

11. A C E Three pairs • How does legacy DCF work in this scenario when C is transmitting ? Busy Channel EIFS Backoff DIFS DATA DATA DATA DATA DATA DATA DATA Long term Unfairness

12. DCF evaluation in a chain Throughput for chain with different length Claude Chaudet: IEEE com. Magazine 2005

13. Next 5 slides from Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks? Shugong Xu Tark Saadawi June, 2001 IEEE Communications Magazine (Adapted from mnet.cs.nthu.edu.tw/paper/jbb/010704.pps)

14. 1 2 3 4 5 6 Destination Source Destination Source Serious Unfairness – (1) • 2 TCP Connections • First session starts at 10.0s ( 6  4 ) • Second session starts 20.0s later ( 2  3 )

15. Serious Unfairness – (2) First session start Second session start

16. Serious Unfairness – (3) • The throughput of the first session is zero in most of its lifetime after the second session starts. • There is not even a chance for it to restart. • The loser session is completely shutdown even if it starts much earlier.

17. 1 2 3 4 5 6 Destination Source Destination Source Serious Unfairness – (6) • Discussion: • Node5 cannot reach node4 when • Node2 is sending (collision) • Node3 is sending ACK (defer)

18. Conclusion • The hidden terminal problem still exists in multihop networks. • The exposed terminal problem will be more harmful in a multihop network and there is no scheme in IEEE 802.11 standard to deal with this problem. • The binary exponential backoff scheme always favors the latest successful node. It will cause unfairness.

19. Multiple Channels for Wireless Networks

20. Each device has 1 radio. All radios are tuned to the same channel. Traditional Ad Hoc Network: Single Channel

21. 1 1 2 defer Motivation • Exploit multiple channels to improve network throughput’ … why ? • Greater parallel communication is possible

22. t=1 Sender 2 frequency t=2 Sender 3 frequency Typical Wireless Networks Each network uses 1 channel only. Power Density t=0 Sender 1 frequency Can we do better? : : Channel 2 Channel 3 Channel 1

23. t=1 Sender 2 Sender 1 Sender 4 frequency t=2 Sender 3 Sender 4 Sender 2 frequency : : Can we do better? Simultaneous sending on different channels? Power Density t=0 Sender 1 Sender 4 Sender 3 frequency Channel 2 Channel 3 Channel 1

24. Goal • Given a wireless network where: • M(>1) channels are available • each node has 1 tunable radio • each node has many neighbors • Design a Multi-Channel MAC protocol: • increases total network throughput • achieves low average delay • robust, practical

25. t=0 Sender 1 Sender 4 Sender 3 frequency t=1 Sender 2 Sender 1 Sender 4 frequency Why Multi-Channel MAC? Multi-Channel MAC Single “Super” Channel t=0 Sender 1 frequency t=1 Sender 2 frequency

26. M-Channel Schedule example

27. M-Channel Schedule example

28. Core Design Issues Q1: Which channel is receiver Y listening on? Q2: Is channel i free? time=t ? ? ? frequency receiver Y time=t Free ? frequency Chan i

29. Multi-channel Hidden Terminals

30. Multi-channel Hidden Terminals • Observations • Nodes may listen to different channels • Virtual Carrier Sensing becomes difficult • The problem was absent for single channel

31. Multi-Channel MAC Protocols • (1) Dedicated Control Channel (2 radios) • Dedicated control radio & channel for all control messages • DCA [Wu2000], DCA-PC [Tseng2001], DPC [Hung2002]. • (2) Split Phase • Time divided into alternate (i) channel negotiation phase on default channel & (ii) data transfer phase on all channels • MMAC [J.So2003], MAP [Chen et al.] • (3) Common Hopping Sequence • All idle nodes follow the same channel hopping sequence • HRMA [Tang98], CHMA, CHAT [Tzamaloukas2000] • (4) Parallel Rendezvous • Each node follows its own channel hopping sequence • SSCH [Bahl04], McMAC ()

32. Ack Data Data Ack ... Data Ack RTS(2,3) CTS(2) RTS(3) CTS(3) Protocol (1): Dedicated Control Channel Channel Keys: 2 Radios/Node; Rendezvous on 1 channel; No time sync Ch3 (data) Ch2 (data) Ch1(Ctrl) Time Legend: Node 1Node 2Node 3Node 4

33. ... Rts Cts Data Ack Rts Cts Data Ack Hello(2,3) Hello(1,2,3) Ack (1) Ack (2) Protocol (2): Split-Phase Keys: 1 Radio; Rendezvous on a common channel; Coarse time sync Channel Ch3 ... Unused Ch2 ... Ch1 ... Time Data TransferPhase Control Phase

34. Data/Ack ... RTS+CTS Protocol (3): Common Hopping Key: 1 radio; Non-busy nodes hop together; Tight time sync Channel Ch4 Ch3 Ch2 Ch1 1 2 3 4 5 6 7 8 9 10 11 Time Enough for RTS/CTS

35. A MAC protocol based on Split Phase

36. Beacon Time A B C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode) • Doze mode – less energy consumption but no communication • ATIM – Ad hoc Traffic Indication Message

37. Beacon Time ATIM A B C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode)

38. Beacon Time ATIM A B ATIM-ACK C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode)

39. Beacon Time ATIM ATIM-RES A B ATIM-ACK C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode)

40. Beacon Time ATIM ATIM-RES DATA A B ATIM-ACK Doze Mode C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode)

41. Beacon Time ATIM ATIM-RES DATA A B ATIM-ACK ACK Doze Mode C ATIM Window Beacon Interval 802.11 PSM (Power Saving Mode)

42. 802.11 PSM (Power Saving Mode) Summary • All nodes wake up at the beginning of a beacon interval for a fixed duration of time (ATIM window) • Exchange ATIM during ATIM window • Nodes that receive ATIM message stay up during for the whole beacon interval • Nodes that do not receive ATIM message may go into doze mode after ATIM window

43. MMAC : Assumptions • All channels have same BW and none of them are overlapping channels • Nodes have only one transceiver • Transceivers are capable of switching channels but they are half-duplex • Channel switching delay is approx 250 us, avoid per packet switching

44. MMAC : Steps • Divide time into beacon intervals • At the beginning, nodes listen to a pre-defined channel for ATIM window duration • Channel negotiation starts using ATIM messages • Nodes switch to the agreed upon channel after the ATIM window duration

45. MMAC • Preferred Channel List (PCL) • For a node, PCL records usage of channels inside Tx range • HIGH preference – always selected • MID preference – others in the vicinity did not select the channel • LOW preference – others in the vicinity selected the channel

46. MMAC • Channel Negotiation • Sender transmits ATIM to the receiver and includes its PCL in the ATIM packet • Receiver selects a channel based on sender’s PCL and its own PCL • Receiver sends ATIM-ACK to sender including the selected channel • Sender sends ATIM-RES to notify its neighbors of the selected channel

47. Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval MMAC

48. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) C D Time ATIM Window

49. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) ATIM- ACK(2) C D ATIM Time ATIM- RES(2) ATIM Window

50. MMAC Common Channel Selected Channel ATIM- RES(1) ATIM RTS DATA Channel 1 A Beacon Channel 1 B CTS ACK ATIM- ACK(1) ATIM- ACK(2) CTS Channel 2 ACK C Channel 2 D DATA ATIM Time ATIM- RES(2) RTS ATIM Window Beacon Interval