Chapter 7 Wireless Ad Hoc Networks 7: Wireless Ad Hoc Networks 7-1
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 7-2
Mobile Ad Hoc Networks May need to traverse multiple links to reach a destination 7: Wireless Ad Hoc Networks 7-3
Mobile Ad Hoc Networks (MANET) Mobility causes route changes 7: Wireless Ad Hoc Networks 7-4
Why Ad Hoc Networks? Ease of deployment Speed of deployment Decreased dependence on infrastructure 7: Wireless Ad Hoc Networks 7-5
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 7-6
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 7-7
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 7-8
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 7-9
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 7-10
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 adhoc? Can several nodes together act as an antenna array? Practical issues? Software Radio GPS The ability to quickly switch the operating frequency may provide opportunities, but also challenging Location information may help 7: Wireless Ad Hoc Networks 7-11
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 7-12
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 7-13
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 7-14
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 7-15
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 7-16
Overall Picture MAC Protocols for Ad Hoc Contention-based Contention-based with reservation Contention-based with scheduling Other Protocols Sender initiated Receiver initiated RI-BTMA MACA-BI MARCH synchronous DPS DWOP DLPS asynchronous MMAC MCSMA PCM RBAR Single channel MACAW FAMA Multiple channel BTMA DBTMA ICSMA D-PRMA CATA HRMA SRMA/PA FPRP MACA/PR RTMAC 7: Wireless Ad Hoc Networks 7-17
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 7-18
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 7-19
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 7-20
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 7-21
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) BEB BEB copy S1 AP S2 S1-AP 48.5 23.82 S2-AP 0 23.82 7: Wireless Ad Hoc Networks 7-22
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) F inc (x) = MIN[l.5x, CW max ] Upon success it is decreased by 1 F dec (x) = MAX[x-1, CW min ] 7: Wireless Ad Hoc Networks 7-23
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 A B C D 7: Wireless Ad Hoc Networks 7-24
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 A B C D 7: Wireless Ad Hoc Networks 7-25
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 7-26
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 7-27
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 7-28
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 7-29
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 7-30
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 7-31
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 7-32
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 7-33
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 7-34
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 7-35
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 7-36
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 7-37
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 7-38
Route Discovery in DSR Y Z S E A B H C I G F K J D M N L Represents a node that has received RREQ for D from S 7: Wireless Ad Hoc Networks 7-39
Route Discovery in DSR Y Broadcast transmission [S] Z S E A B H C I G F K J D M N L Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ 7: Wireless Ad Hoc Networks 7-40
Route Discovery in DSR Y S E [S,E] Z A B H C [S,C] I G F K J D M N L Node H receives packet RREQ from two neighbors: potential for collision 7: Wireless Ad Hoc Networks 7-41
Route Discovery in DSR Y Z S E A B H C I G F [S,C,G] K [S,E,F] J D M N L 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 7-42
Route Discovery in DSR Y Z A B H S C I E G F K [S,E,F,J] M J D [S,C,G,K] N L 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 7-43
Route Discovery in DSR Y Z A B H S C I E G F K J [S,E,F,J,M] M L D N Node D does not forward RREQ, because node D is the intended target of the route discovery 7: Wireless Ad Hoc Networks 7-44
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 7-45
Route Reply in DSR Y S E RREP [S,E,F,J,D] Z A B H C I F M J G D K RREP [S,C,G,K,D] N L Represents RREP control message 7: Wireless Ad Hoc Networks 7-46
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 7-47
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 7-48
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 7-49
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 7-50
Transmission Control Protocol (TCP) Reliable ordered delivery Implements congestion avoidance and control Reliability achieved by means of retransmissions if necessary End-to-end semantics Acknowledgements sent to TCP sender confirm delivery of data received by TCP receiver Ack for data sent only after data has reached receiver 7: Wireless Ad Hoc Networks 7-51
Challenges Throughput unfairness Unfairness at MAC layer Transport layer should take this into account Resource constraints Power and bandwidth constraints Separation of congestion control and reliability control Completely decoupled transport layer Wired network: transport protocol completely separated from underlying layer Ad hoc network: interaction with network and MAC layer is expected for adaptability 7: Wireless Ad Hoc Networks 7-52
Challenges (Cont.) Misinterpretation of congestion Traditional mechanism: packet loss, timeout Ad hoc loss/delay due to High bit error rate due to varying link condition Packet collisions due to contention and hidden terminal Path breaks due to node mobility Dynamically changing topology Frequent path breaks Partitioning and merging of networks High delay in reestablishment of path 7: Wireless Ad Hoc Networks 7-53
Performance of TCP Several factors affect TCP performance in MANET Wireless transmission errors Multi-hop routes on shared wireless medium For instance, adjacent hops typically cannot transmit simultaneously Route failures/changes due to mobility 7: Wireless Ad Hoc Networks 7-54
Throughput over Multi-Hop Wireless Paths Connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop TCP Throughput using 2 Mbps 802.11 MAC 1600 1400 1200 1000 800 600 400 200 0 1 2 3 4 5 6 7 8 9 10 Number of hops TCP Throughtput (Kbps) 7: Wireless Ad Hoc Networks 7-55
Impact of Caching Route caching has been suggested as a mechanism to reduce route discovery overhead Each node may cache one or more routes to a given destination When a route from S to D is detected as broken, node S may: Use another cached route from local cache, or Obtain a new route using cached route at another node 7: Wireless Ad Hoc Networks 7-56
Why Performance Degrades With Caching When a route is broken, route discovery returns a cached route from local cache or from a nearby node After a time-out, TCP sender transmits a packet on the new route However, the cached route has also broken after it was cached timeout due to route failure timeout, cached route is broken timeout, second cached route also broken Another route discovery, and TCP time-out interval Process repeats until a good route is found 7: Wireless Ad Hoc Networks 7-57
To Cache or Not to Cache Caching can result in faster route repair Faster does not necessarily mean correct If incorrect repairs occur often enough, caching performs poorly Need mechanisms for determining when cached routes are stale 7: Wireless Ad Hoc Networks 7-58
Caching and TCP performance Caching can reduce overhead of route discovery even if cache accuracy is not very high But if cache accuracy is not high enough, gains in routing overhead may be offset by loss of TCP performance due to multiple time-outs 7: Wireless Ad Hoc Networks 7-59
How to Improve Throughput (Bring Closer to Ideal) Network feedback Inform TCP of route failure by explicit message Let TCP know when route is repaired Probing Explicit notification Reduces repeated TCP timeouts and backoff 7: Wireless Ad Hoc Networks 7-60
TCP with ELFN Explicit Link Failure Notification Not totally new, e.g., ECN bits in TCP To provide the TCP sender with information about link and route failures, so that it can avoid responding to the failures as if congestion has occurred How does it work? When a TCP sender receives an ELFN: disables its retransmission timers and enters a stand-by mode While on standby: A packet is sent at periodic intervals to probe the network to see if a route has been established If an acknowledgment is received: leaves stand-by mode and restores the retransmission timers 7: Wireless Ad Hoc Networks 7-61
Performance with Explicit Notification throughput as a fraction of ideal 1 0.8 0.6 0.4 0.2 0 2 10 20 30 mean speed (m/s) Base TCP With explicit notification 7: Wireless Ad Hoc Networks 7-62
Issues: Network Feedback Network knows best (why packets are lost) Network feedback beneficial Need to modify transport & network layer to receive/send feedback Need mechanisms for information exchange between layers [Holland99] discusses alternatives for providing feedback (when routes break and repair) [Chandran98] also presents a feedback scheme 7: Wireless Ad Hoc Networks 7-63
TCP Performance Two factors result in degraded throughput in presence of mobility Loss of throughput that occurs while waiting for TCP sender to timeout This factor can be mitigated by using explicit notifications and better route caching mechanisms Poor choice of congestion window and RTO values after a new route has been found How to choose cwnd and RTO after a route change? 7: Wireless Ad Hoc Networks 7-64
Issues: Window Size After Route Repair Same as before route break: may be too optimistic Same as startup: may be too conservative Better be conservative than overly optimistic Reset window to small value after route repair Let TCP figure out the suitable window size Impact low on paths with small delay-bw product 7: Wireless Ad Hoc Networks 7-65
Issues: RTO After Route Repair Same as before route break If new route is long, this RTO may be too small, leading to timeouts Same as TCP start-up (6 second) May be too large May result in slow response to next packet loss Another plausible approach RTO new = f(rto old, route-length old, route-length new ) E.g.: RTO new = RTO old * route-length new /route-length old Not evaluated yet Pitfall: RTT is not just a function of route length 7: Wireless Ad Hoc Networks 7-66
Summary Still many remained topics related to wireless ad hoc networks More research opportunities in Wireless mesh networks With fixed infrastructure as wireless infrastructure Multi-radio multi-channel architecture Wireless sensor networks Energy consumption is one of the key challenges Application specific demands, including localization, coverage, event detection/collection, etc. 7: Wireless Ad Hoc Networks 7-67