1 “Open” Problems in Mobile Ad Hoc Networking Nitin Vaidya University of Illinois at Urbana-Champaign Keynote talk presented at the Workshop on Wireless Local Networks (in conjunction with 26 th Conference on Local Computer Networks), Tampa, Florida, November 14, 2001 © 2001 Nitin Vaidya

2 Mobile Ad Hoc Networks  Formed by wireless hosts which may be mobile  Without necessarily using a pre-existing infrastructure  Routes between nodes may potentially contain multiple hops

3 Mobile Ad Hoc Networks  May need to traverse multiple links to reach a destination

4 Mobile Ad Hoc Networks (MANET)  Mobility causes route changes

5 Why Ad Hoc Networks ?  Potential ease of deployment  Decreased dependence on infrastructure

6 Many Applications  Personal area networking  cell phone, laptop, ear phone, wrist watch  Military environments  soldiers, tanks, planes  Civilian environments  taxi cab network  meeting rooms  sports stadiums  boats, small aircraft  Emergency operations  search-and-rescue  policing and fire fighting

7 Many Variations  Fully Symmetric Environment  all nodes have identical capabilities and responsibilities  Asymmetric Capabilities  transmission ranges and radios may differ  battery life at different nodes may differ  processing capacity may be different at different nodes  speed of movement  Asymmetric Responsibilities  only some nodes may route packets  some nodes may act as leaders of nearby nodes (e.g., cluster head)

8 Many Variations  Traffic characteristics may differ in different ad hoc networks  bit rate  timeliness constraints  reliability requirements  unicast / multicast / geocast  host-based addressing / content-based addressing / capability-based addressing  May co-exist (and co-operate) with an infrastructure- based network

9 Many Variations  Mobility patterns may be different  people sitting at an airport lounge  New York taxi cabs  kids playing  military movements  personal area network  Mobility characteristics  speed  predictability direction of movement pattern of movement  uniformity (or lack thereof) of mobility characteristics among different nodes

10 Challenges  Limited wireless transmission range  Broadcast nature of the wireless medium –Hidden terminal problem  Packet losses due to transmission errors  Mobility-induced route changes  Mobility-induced packet losses  Battery constraints  Potentially frequent network partitions  Ease of snooping on wireless transmissions (security hazard)

11 Research on Mobile Ad Hoc Networks Variations in capabilities & responsibilities X Variations in traffic characteristics, mobility models, etc. X Performance criteria (e.g., optimize throughput, reduce energy consumption) + Increased research funding = Significant research activity

12 Hidden Terminals & RTS/CTS Handshake

13 ABC 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, collision will occur at node B

14 RTS/CTS Handshake  Sender sends Ready-to-Send (RTS)  Receiver responds with Clear-to-Send (CTS)  RTS and CTS announce the duration of the transfer  Nodes overhearing RTS/CTS keep quiet for that duration  RTS/CTS used in IEEE D C BA CTS (10) RTS (10) 10

15 Problems in Ad Hoc Networking

16 Problem Space  Practical considerations  Consumer demand or lack thereof  Standardization  Government regulations  Technical issues

17 Problem Space Link Network Transport Physical Upper layers

18 Physical Layer

19 Physical Layer  Traditionally, not much interaction between physical layer and upper layers  Many physical layer mechanisms not beneficial without help from upper layers  Example: Adaptive modulation

20 Adaptive Modulation  Channel conditions are time-varying AB

21  Choose modulation scheme as a function of channel conditions

22 Adaptive Modulation  If physical layer chooses the modulation scheme transparent to MAC  MAC cannot know the time duration required for the transfer  Must involve MAC protocol in deciding the modulation scheme  Some compliant implementations use a sender- based scheme for this purpose  Receiver-based schemes can perform better

23 Sender-Based “Autorate Fallback” MAC Protocol D C BA 1Mbps 2Mbps  Sender decreases rate after N consecutive ACKS are not received  Sender increases rate after Y consecutive ACKS are received DATA 2Mbps

24 Performance of Sender-Based “Autorate Fallback” Expected ARF CCK (11Mbps) CCK (5.5Mbps) QPSK (2Mbps) BPSK (1Mbps)

25 1Mbps 2Mbps  Sender sends RTS containing its best rate estimate  Receiver chooses best rate for the conditions and sends it in the CTS  Sender transmits DATA packet at new rate  Information in data packet header implicitly updates nodes that heard old rate Receiver-Based Autorate MAC Protocol D C BA CTS (1) RTS (2) 2 1

26 Physical Layer  Several other physical layer capabilities call for changes to upper layers of protocol stack  Example: Power control

27 Power Control  Transmit power determines  “Range” of a transmission  Interference caused at other nodes BCDA

28 Power Control  Transmit power determines  “Range” of a transmission  Interference caused at other nodes BCDA

29 Benefits of Power Control  Transmit a packet with least transmit power necessary to deliver to the receiver  Save energy: Important benefit to battery-powered hosts  Reduce interference  Can allow greater spatial reuse

30 Power Control  Power control introduces asymmetry  D transmits to C at low power, but B uses high transmit power to transmit to A  B may not about D-to-C transmission, but can interfere with it BCDA

31 Power Control  Proposals for medium access control and routing with power control exist  Do not solve the problem satisfactorily  Ideal solution will  Reduce energy consumption, and  Maximize spatial reuse

32 Directional / Smart Antennas  Various capabilities  Sectored antennas (fixed beam positions)  Beam steering  Tracking a transmitter  MAC and routing protocols for ad hoc networks using such antennas  How to take into account antenna capabilities? Network may be heterogeneous

33 Physical Layer  Are ad hoc networks benefiting from the progress made at physical layer ?  Other interesting areas  Efficient coding schemes  Various diversity techniques

34 Physical Layer: Simulation Models  Insufficient accuracy in commonly used physical layer models  Physical link state is not binary as often assumed  Reliable packet reception does not depend just on distance  Transmit power  Interference level  Fading  Need to use realistic models  Modulation scheme  Coding

35 Link Layer

36 Interesting Link Layer Issues  Medium access control  Retransmission mechanisms  Transmission scheduling  Which pending packet should a node attempt to transmit?  Adaptive parameter selection  Frame size  Retransmission limit

37 QoS in Medium Access Control  Many proposals for achieving fairness  Fair scheduling schemes attempt to provide equitable sharing of channel  Unpredictable nature of transmission errors makes it difficult to make hard guarantees  Need to develop a probabilistic framework

38 QoS in MAC  Easier in a centralized protocol (such as point coordination function), than in a distributed protocol  Distributed MAC appears more suitable for ad hoc networks, however  Perhaps a hybrid protocol will be best  How to design such a protocol ?

39 Transmission Scheduling  When multiple packets pending transmission, which packet to transmit next?  Choice should depend on  Receiver status (blocked by some other transmission?)  Congestion at receivers  Noise level at receivers  Tolerable delay for pending packets –Need interaction between upper layers and MAC

40 MAC for Multiple Channels  How to split bandwidth into channels?  How to use the multiple channels ? Dedicated channel for control ?

41 Network Layer

42 Reactive versus Proactive Routing  Reactive protocols  Maintain routes between nodes that need to communicate  Proactive protocols  Maintain routes between all node-pairs  Lot of activity on routing protocol design

43 Routing  Reactive and proactive protocols are quite well-understood  Designing reactive protocols: “Solved” problem  Designing proactive protocols: “Solved” problem  At least, when using common assumptions about the network  Interesting problems exist when other issues are considered (such as QoS or physical layer properties)

44 Reactive versus Proactive  Choice of protocol depends on  Mobility characteristics of the nodes  Traffic characteristics  How to design adaptive protocols ?  Existing proposals use a straightforward combination of reactive and proactive  Proactive within “radius” K  Reactive outside K  Choose K somehow

45 Reactive versus Proactive  Need a more flexible way to manage protocol behavior  Assign proactive/reactive tag to each route (A,B) ?  How to determine when proactive behavior is better than reactive ?

46 Address Assignment  How to assign addresses to nodes in an ad hoc network ?  Static assignment  Easier to guarantee unique address  Dynamic assignment  How to guarantee unique addresses when partitions merge?  Do we need to guarantee unique addresses ?

47 Transport Protocols

48 TCP  TCP performance degrades in presence of route failures  TCP cannot distinguish between packet losses due to route change and due to congestion  Reduces congestion window in response Unnecessary degradation in throughput

49 TCP  Several solutions have been proposed to fix this  These techniques somehow inform TCP sender that the packet losses are due to route failure  TCP does not decrease congestion window in response

50 TCP  New route may differ significantly from old route  Proposals for TCP-over-ad-hoc tend to use old timeout and congestion window after a route change  Does not seem like a good idea  How to choose appropriate timeout and congestion window after detecting a route change ?

51 Other Issues

52 Algorithms

53 Distributed Algorithms  Rich body of work on distributed algorithms in traditional distributed environments  Shared memory  Message ordering  Clock synchronization  Leader election

54 Distributed Algorithms  Existing algorithms can usually be used on ad hoc networks without affecting correctness  Performance on ad hoc networks may not be good  Existing algorithm treat link repairs/failures as random events  With mobility, link failure/repairs are correlated with host movement

55 Distributed Algorithms  How to design distributed algorithm exploiting the correlation between mobility and link failure/repair ?

56 Distributed Algorithms  Traditionally, complexity is measured as a function of problem “size”  Number of nodes  Number of failures  How to analyze algorithm complexity as a function of mobility ?  What measure of mobility is amenable to such an analysis ?  Need to capture the correlation without making the measure too complex

57 Security Issues

58 What’s New ?  Wireless medium easy to snoop on  With ad hoc networking, hard to guarantee connectivity  Easier for intruders to insert themselves into network

59 Authentication  How to authenticate a node ?  May not have access to a certification authority

60 Resource Depletion Attack  Intruders may send data with the objective of congesting a network or depleting batteries A CB D T intruder U Bogus traffic

61 Routing Attacks  Intruders may mis-route the data  not delivering it to the destination at all, or  delaying it significantly  How to detect such attacks ?  How to tolerate such attacks ?

62 Traffic Analysis  Despite encryption, an eavesdropper can identify traffic patterns  Traffic patterns can divulge information about the operation mode  Traffic analysis can be prevented by presenting “constant” traffic pattern –Insert dummy traffic  How to make this cheaper ?

63 Other Issues

64 Incentives for Ad Hoc Routing  Why should I forward packets for some other nodes ?  Need some incentive mechanism  Policies to determine reward for performing each operation

65 Applications  New applications for ad hoc networks ?

66 Hybrid Environments  Use infrastructure when convenient  Use ad hoc connectivity when necessary or superior E A BS1BS2 X Z infrastructure Ad hoc connectivity

67 Summary

68 Summary  Plenty of interesting research problems  Research community disproportionately obsessed with routing protocols

69 Summary  Interesting problems elsewhere at the two ends of the protocol stack  How to design algorithms and applications ?  How to exploit physical layer techniques ? Increase interaction between physical layer and upper layers Link Network Transport Physical Upper layers

70 Summary  Hybrid environments require revisiting protocol design decisions

71 Tutorials  Visit for my tutorials onhttp://  Mobile ad hoc networking  TCP over Wireless

72 Thank you !! Comments/questions to © 2001 Nitin Vaidya