Exploring Wireless Ad-Hoc Networks: Principles, Challenges, and Solutions

1. Chapter 7Wireless Ad Hoc Networks 7: Wireless Ad Hoc Networks

2. What is an Ad Hoc Network? • Definitions: • An ad-hoc network is one that comes together as needed, not necessarily with any assistance from the existing Internet infrastructure • Instant infrastructure • A MANET is a collection of mobile platforms or nodes where each node is free to move about arbitrarily • A MANET: distributed, possibly mobile, wireless, multihop network that operates without the benefit of any existing infrastructure (infrastructure-less), except the nodes themselves 7: Wireless Ad Hoc Networks

3. Mobile Ad Hoc Networks • May need to traverse multiple links to reach a destination 7: Wireless Ad Hoc Networks

4. Mobile Ad Hoc Networks (MANET) • Mobility causes route changes 7: Wireless Ad Hoc Networks

5. Why Ad Hoc Networks ? • Ease of deployment • Speed of deployment • Decreased dependence on infrastructure 7: Wireless Ad Hoc Networks

6. Fundamental Challenges It is better to know some of the questions than all of the answers. — James Thurber (1835-1910) 7: Wireless Ad Hoc Networks

7. 1. Energy Efficiency • No infrastructure means must rely on batteries (or, in general, limited energy resources) • Possible solutions • Selectively sending nodes into a sleep mode • Using transmitters with variable power (the Power Control problem) • Using energy-efficient paths • Using cooperative techniques (still relatively new) 7: Wireless Ad Hoc Networks

8. 2. Mobility • Mobility-induced route changes • Mobility-induced packet losses • Mobility patterns may be different • Controlled e.g. robots • Offers opportunities for improving the network functions e.g. connectivity, coverage • Uncontrolled e.g. nomadic users • Offers challenges to network design • But also offers opportunities for improvement, e.g. • Users “carry” delay-tolerant data closer to destination • Delay Tolerant Network (Challenge Networks) 7: Wireless Ad Hoc Networks

9. 3. QoS • Providing QoS even in wired networks (e.g. the Internet) is a challenging problem • Wireless RF channels further complicate the problem • Unpredictability • Medium access: broadcast medium with hidden terminal problem • Possible solutions: • New MAC design • Cross-layer integration: allow different layers to adapt depending on available information at other layers 7: Wireless Ad Hoc Networks

10. 4. Scalability • Limited wireless transmission range • Whether the network is able to maintain an acceptable level of service even as the number of nodes is increased • How fast the network protocol control overhead increases as N increases • Possible solutions: • Introducing hierarchy • Utilizing location information • Limiting reactions to changes • Fixing things (e.g. paths) locally 7: Wireless Ad Hoc Networks

11. 5. Utilizing New Technologies • What are the gains that could be achieved by using newly available technologies such as • Smart directional (beamforming) antennas • Increases the spatial reuse in cellular, but how about ad-hoc? • Can several nodes together act as an antenna array? Practical issues? • Software Radio • The ability to quickly switch the operating frequency may provide opportunities, but also challenging • GPS • Location information may help 7: Wireless Ad Hoc Networks

12. 6. Security • Ease of snooping on wireless transmissions • From crypto point of view, lack of a trusted authority is one of the main challenges • How to generate/share keys reliably • Harder to track or even detect attackers in a wireless environment, given that: • Network relies on in-situ connections to other nodes which may be malicious • Malicious nodes may be especially harmful by injecting bogus control packets • DoS attacks that deplete a node’s battery 7: Wireless Ad Hoc Networks

13. 7. Lack of Reference • Lack of sufficient experimental data to confirm models • What does a multi-hop path really mean? • What is a link? • Simplistic models that do not capture the complexities, or complex models that do not lead to insights? • Are the protocols good enough, have they reached closed to the best possible? • Good balance between mathematical and experimental work 7: Wireless Ad Hoc Networks

14. Multiple-Layer Problem • PHY • Adapt to rapid changes in link characteristics • MAC • Minimize collision, allow fair access, and semi-reliably transport under rapid change and hidden/exposed terminals • Network • Determine efficient transmission paths when links change often and bandwidth is at a premium • Transport • Handle delay and packet loss statistics that are very different than wired networks • Application • Handle frequent disconnection and reconnection as well as varying delay and packet loss characteristics 7: Wireless Ad Hoc Networks

15. Several Major Issues • MAC protocols for ad hoc networks • Routing in ad hoc networks • Transport protocols for ad hoc networks 7: Wireless Ad Hoc Networks

16. Design Goals for MAC Protocols • Allow fair access to the shared radio medium • Distributed protocol • Available bandwidth must be utilized efficiently • Control overhead should be minimized • Ensure fair bandwidth allocation to competing nodes • Reduce the effect of hidden/exposed terminals • Effectively manage the power consumption • Provide QoS support for real-time traffic • Protocol should be scalable 7: Wireless Ad Hoc Networks

17. Overall Picture MAC Protocols for Ad Hoc Contention-based Contention-based with reservation Contention-based with scheduling Other Protocols • DPS • DWOP • DLPS • MMAC • MCSMA • PCM • RBAR Sender initiated Receiver initiated • RI-BTMA • MACA-BI • MARCH synchronous asynchronous Single channel Multiple channel • D-PRMA • CATA • HRMA • SRMA/PA • FPRP • MACA/PR • RTMAC • MACAW • FAMA • BTMA • DBTMA • ICSMA 7: Wireless Ad Hoc Networks

18. Contention-based Protocols with Reservations • Use a bandwidth reservation technique • Contention occurs only at resource reservation phase • Node gets an exclusive access to the media once bandwidth is reserved • D-PRMA • Distributed packet reservation multiple access protocol • SRMA/PA • Soft reservation multiple access with priority assignment • RTMAC • Real-time medium access control protocol 7: Wireless Ad Hoc Networks

19. Contention-based Protocols with Scheduling • Focus on packet scheduling at the nodes and transmission scheduling of the nodes • DPS • Distributed priority scheduling • DWOP • Distributed wireless ordering protocol • DLPS • Distributed laxity-based priority scheduling 7: Wireless Ad Hoc Networks

20. Contention-based Protocols w/o Reservation/Scheduling • MACA • Multiple access collision avoidance protocol • MACAW • Media Access Protocol for Wireless LAN • BTMA • Busy tone multiple access protocol • MARCH • Media access with reduced handshake 7: Wireless Ad Hoc Networks

21. MACA: Multiple Access Collision Avoidance • Proposed as an alternative to CSMA/CA • Handle hidden and exposed terminal issues using RTS-CTS • RTS and CTS packets carry the expected duration of the data transmission • A node near the sender that hearing RTS do not transmit for a time to receive CTS • A node near the receiver after hearing CTS differs its transmission • If the neighbor hears the RTS only, it is free to transmit once the waiting interval is over neighbor sender receiver neighbor RTS RTS CTS CTS Data Data 7: Wireless Ad Hoc Networks

22. MACAW: Enhancement of MACA • Issue 1: potential flow starvation due to BEB • Both S1 and S2 have the high volume of traffic, S1 seizes the channel first • Packets transmitted by S2 get collided and it doubles CW • The probability that S2 seizes the channel decreasing • Solution in MACAW • Packet header contains the field set to the current back-off value of the transmitting node • Node receiving this packet copies this value to its back-off counter • If all the nodes can hear each other, eventually they will have the same back-off counter (fairness) S1 S2 × AP 7: Wireless Ad Hoc Networks

23. MACAW (Cont.) • Issue 2: backoff calculation adjusts too rapidly • After every successful transmission, return to the case where all stations have a minimal backoff counter, and then must repeat a period of contention to increase the backoffs • Solution in MACAW • Gentler adjustment • Upon a collision, the backoff interval is increased by a multiplicative factor (1.5) Finc(x) = MIN[l.5x, CWmax] • Upon success it is decreased by 1 Fdec(x) = MAX[x-1, CWmin] 7: Wireless Ad Hoc Networks

24. MACAW (Cont.) • Issue 3: Neighbor receivers problem • When node A sends an RTS to B, while node C is receiving from D, node B cannot reply with a CTS, since B knows that D is sending to C • When the transfer from C to D is complete, node B can send a Request-to-send-RTS (RRTS) to node A • Node A may then immediately send RTS to node B D C B A 7: Wireless Ad Hoc Networks

25. MACAW (Cont.) • This approach, however, does not work in the scenario below • Node B may not receive the RTS from A at all, due to interference with transmission from C D C B A 7: Wireless Ad Hoc Networks

26. BTMA: Busy Tone Multiple Access • One of the earliest solutions for hidden terminal problem • Multi-channel protocol • Control channel: used for busy tone transmission • Data channel: used for data transmission • Three variants: • BTMA (Busy Tone Multiple Access) • DBTMA (Dual Busy Tone Multiple Access) • RI-BTMA (Receiver-Initiated BTMA) 7: Wireless Ad Hoc Networks

27. BTMA (Cont.) • Basic idea • Node senses the control channel to check whether the busy tone is active • If not, turns on busy tone signal and starts data transmission • If yes, waits for a random period of time and repeats • Any node that senses the carrier on the incoming data channel also transmits a busy tone • Pros and Cons • Simple with extremely low collision probability • Bandwidth utilization is low (blocked in two-hop neighbor) • Multiple channels 7: Wireless Ad Hoc Networks

28. Several Major Issues • MAC protocols for ad hoc networks • Routing in ad hoc networks • Transport protocols for ad hoc networks 7: Wireless Ad Hoc Networks

29. Why is Routing in MANET Different? • No specific nodes dedicated for control • Host mobility • Link failure/repair due to mobility may have different characteristics than those due to other causes • Rate of link failure/repair may be high when nodes move fast • Different node characteristics • E.g. power constraints, multiple access issues • New performance criteria may be used • Route stability despite mobility • Energy consumption 7: Wireless Ad Hoc Networks

30. Unicast Routing Protocols • Many protocols have been proposed • Some have been invented specifically for MANET • Others are adapted from previously proposed protocols for wired networks • No single protocol works well in all environments • Some attempts made to develop adaptive protocols 7: Wireless Ad Hoc Networks

31. MANET Protocol Zoo • Topology based routing • Proactive approach, e.g., DSDV. • Reactive approach, e.g., DSR, AODV, TORA. • Hybrid approach, e.g., Cluster, ZRP. • Position based routing • Location Services: • DREAM, Quorum-based, GLS, Home zone etc. • Forwarding Strategy: • Greedy, GPSR, RDF, Hierarchical, etc. 7: Wireless Ad Hoc Networks

32. Routing Protocols • Proactive protocols • Determine routes independent of traffic pattern • Traditional link-state and distance-vector routing protocols are proactive • Reactive (on-demand) protocols • Discover/maintain routes only when needed • Source-initiated route discovery • Hybrid protocols 7: Wireless Ad Hoc Networks

33. Trade-Off • Latency of route discovery • Proactive protocols may have lower latency since routes are maintained at all times • Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y • Overhead of route discovery/maintenance • Reactive protocols may have lower overhead since routes are determined only if needed • Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating 7: Wireless Ad Hoc Networks

34. Tradeoff (Cont.) • Which approach achieves a better trade-off depends on the traffic and mobility patterns • Reactive protocols may yield lower routing overhead than proactive protocols when communication density is low • Reactive protocols tend to loose more packets (assuming that network layer drops packets if a route is not known) • Proactive protocols perform better with high mobility and dense communication graph 7: Wireless Ad Hoc Networks

35. Single Path vs. Multipath • Single path • Use one path from source to destination • Similar to wired routes • Advantages: • Simple to implement • Disadvantages: • Source must find a new route to destination if old one fails • Multipath • Use more than one path from source to destination • Advantages: • Load balancing can occur • Higher tolerance to link failures • Disadvantages: • Adds complexity to receiver and sender 7: Wireless Ad Hoc Networks

36. Short Hops vs. Long Hops • Research to date suggests short-hop • Provides lower energy consumption • Lower transmission power needed due to shorter distance between nodes • Provides higher link capacity • Higher received signal strength due to shorter distance between nodes • Long-hop intuitively should have less total delay due to • Less total hops • Smaller total processing delay 7: Wireless Ad Hoc Networks

37. Some Existing Wireless Routing Protocols • DSDV • WRP • CGSR • STAR • OLSR • FSR • HSR • GSR • DSR • AODV • ABR • SSA • FORP • PLBR • CEDAR • ZRP • ZHLS • RABR • LBR • COSR • PAR • LAR • OLSB 7: Wireless Ad Hoc Networks

38. Dynamic Source Routing (DSR) • Reactive, source-based • When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery • Source node S floods Route Request (RREQ) • Each node appends own identifier when forwarding RREQ 7: Wireless Ad Hoc Networks

39. Route Discovery in DSR Y Z S E F B C M L J A G H D K I N Represents a node that has received RREQ for D from S 7: Wireless Ad Hoc Networks

40. Route Discovery in DSR Y Broadcast transmission Z [S] S E F B C M L J A G H D K I N Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ 7: Wireless Ad Hoc Networks

41. Route Discovery in DSR Y Z S [S,E] E F B C M L J A G [S,C] H D K I N • Node H receives packet RREQ from two neighbors: • potential for collision 7: Wireless Ad Hoc Networks

42. Route Discovery in DSR Y Z S E F [S,E,F] B C M L J A G H D K [S,C,G] I N • Node C receives RREQ from G and H, but does not forward • it again, because node C has already forwarded RREQ once 7: Wireless Ad Hoc Networks

43. Route Discovery in DSR Y Z S E F [S,E,F,J] B C M L J A G H D K I N [S,C,G,K] • Nodes J and K both broadcast RREQ to node D • Since nodes J and K are hidden from each other, their • transmissions may collide 7: Wireless Ad Hoc Networks

44. Route Discovery in DSR Y Z S E [S,E,F,J,M] F B C M L J A G H D K I N • Node D does not forward RREQ, because node D is the intended targetof the route discovery 7: Wireless Ad Hoc Networks

45. Route Discovery in DSR • Destination D on receiving the first RREQ, sends a Route Reply (RREP) • RREP is sent on a route obtained by reversing the route appended to received RREQ • RREP includes the route from S to D on which RREQ was received by node D 7: Wireless Ad Hoc Networks

46. Route Reply in DSR Y Z S RREP [S,E,F,J,D] E F B C M L J A G H D K I N RREP [S,C,G,K,D] Represents RREP control message 7: Wireless Ad Hoc Networks

47. Dynamic Source Routing (DSR) • Node S on receiving RREP, caches the route included in the RREP • When node S sends a data packet to D, the entire route is included in the packet header • Hence the name source routing • Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded 7: Wireless Ad Hoc Networks

48. DSR Optimization: Route Caching • Each node caches a new route it learns by any means • When node S learns that a route to node D is broken • Uses another route from its local cache, if such a route to D exists in its cache • Otherwise, node S initiates route discovery by sending a route request • Intermediate node X on receiving a Route Request for some node D can send a Route Reply • If node X knows a route to node D • Use of route cache • Can speed up route discovery • Can reduce propagation of route requests 7: Wireless Ad Hoc Networks

49. DSR Pros and Cons • Advantages: • Less memory storage needed at each node since full routing table is not needed • Lower overhead needed because no periodic update message are necessary • Nodes do not need to continually inform neighbors they are still operational • Disadvantages: • Possible transmission latency due to reactive approach • Stale routes can occur if links change frequently • Message size increases as path length increases • Collisions between route requests propagated by neighboring nodes • Route Reply Storm due to nodes replying using their local cache 7: Wireless Ad Hoc Networks

50. Several Major Issues • MAC protocols for ad hoc networks • Routing in ad hoc networks • Transport protocols for ad hoc networks 7: Wireless Ad Hoc Networks