wireless & mobile networking cs 752/852 - spring 2011
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Wireless & Mobile Networking CS 752/852 - Spring 2011. Ad Hoc Routing Layer - I. Tamer Nadeem Dept. of Computer Science. The OSI Communication Model. The OSI Communication Model. MANET – Mobile Ad Hoc Networks. - PowerPoint PPT PresentationTRANSCRIPT
Wireless & Mobile Networking
CS 752/852 - Spring 2011
Tamer NadeemDept. of Computer Science
Ad Hoc Routing Layer - I
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The OSI Communication Model
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The OSI Communication Model
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MANET – Mobile Ad Hoc Networks
Collection of mobile nodes connected by wireless links, forming an autonomous network
Nodes run on batteries – power consumption is an issue
Thus routing is multi-hop each node also acts as a router
Scalability is an issue
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MANET
• No infrastructure, lack of centralized control
• Frequent topology changes due to node mobility
• All communications over wireless medium
Cellular Communications
Fixed, pre-located base stations
Static backbone network topology
Reliable backbone links; only last link is wireless
MANET vs Cellular
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Applications
• Civilian environments
• meeting rooms
• smart homes
• multimedia classrooms
• Emergency operations
• disaster relief
• search and rescue
• law enforcement
• Military applications
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Challenges
• Limited wireless transmission range
• Broadcast nature of the wireless medium
• 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)
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Network connectivity defined by node proximity and wireless medium characteristics
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Topology Challenges
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Due to mobility the network topology undergoes rapid changes
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MAC Challenges
• Hidden terminal problem
• Unreliable physical layer
• multi-path fading
• high Bit Error Rate (BER)
• other impairments
• Mobility
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Routing Schemes
Broadcast
Multicast
Unicast
Anycast
Geocast
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Unicast Routing inMobile Ad Hoc Networks
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Routing in MANET
• Why is routing in MANET challenging?
• Without (necessarily) using a pre-existing infrastructure
• Main culprit: mobility
• network topology is changing rapidly (link failure/repair)
• Rate of link failure/repair may be high when nodes move fast
• Main challenge: maintain and distribute up-to-date routing information without saturating the network
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Routing in MANET
• Numerous protocols have been proposed
• Flavors:
• adapted from protocols previously proposed for wired networks
• invented specifically for MANET
• Major approaches
• Proactive: routing information is maintained proactively regardless of communication requests
• e.g. traditional link-state and distance-vector routing protocols are proactive
• Reactive: a route to a destination is found and maintained only when the route is needed
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Proactive vs Reactive
• 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
• Which approach is better depends on traffic and mobility patterns
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Sample Routing Protocols
• Pure flooding
• DSR
• AODV
• LAR
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Flooding (1/5)
• Sender S broadcasts data packet P to all its neighbors
• Each node receiving P forwards P to its neighbors
• Sequence numbers are used to avoid forwarding the same packet more than once
• The destination D does not forward the packet
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Represents transmission of packet P
Represents a node that receives packet P forthe first time
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Flooding (2/5)
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Node H receives packet P from two neighbors: potential for collision
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Flooding (3/5)
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Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P
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Flooding (4/5)
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• Flooding completed• Nodes unreachable from S do not receive packet P (e.g., node Z)• Nodes for which all paths from S go through the destination D also do not receive packet P (example: node N)
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Flooding (5/5)
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• Advantage: simplicity
• Potentially, very high overhead
• data packets may be delivered to too many nodes who do not need to receive them
• Potentially lower reliability of data delivery
• flooding uses MAC broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead
• Broadcasting in IEEE 802.11 MAC is unreliable
• Need end-to-end reliability
Flooding: Summary
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Flooding Control Packets
• Many protocols perform limited flooding of control packets, instead of data packets
• Control packets are used to discover routes or to send link-state updates
• Discovered routes are subsequently used to send data packet(s)
• Overhead of control packet flooding is amortized over data packets transmitted between consecutive control packet floods
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Dynamic Source Routing (DSR) [Johnson96]
• When source S wants to send a packet to node D, but does not know a route to D, it initiates a route discovery
• Source node S floods a Route Request (RREQ)
• Each node appends own identifier when forwarding the RREQ
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DSR: Route Discovery (1/3)
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Represents transmission of RREQ
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[S]
[X,Y] Represents list of identifiers appended to RREQ
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[S,E]
[S,C]
DSR: Route Discovery (2/3)
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Nodes J and K both broadcast RREQ to node DSince nodes J and K are hidden from each other, their transmissions may collide
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[S,C,G,K]
[S,E,F,J]
DSR: Route Discovery (3/3)
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• Destination D upon receiving the first RREQ, sends back to S a Route Reply (RREP)
• RREP is sent on the route obtained by reversing the route appended to the received RREQ
• RREP includes the route from S to D on which RREQ was received by node D
DSR: Route Reply (1/2)
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RREP [S,E,F,J,D]
Represents RREP control message
DSR: Route Reply (2/2)
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• S upon receiving RREP, caches the route included in the RREP
• When 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 the packet should be forwarded
DSR: Data Delivery (1/2)
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DATA [S,E,F,J,D]
Packet header size grows with route length
DSR: Data Delivery (2/2)
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DSR Optimization: Route Caching
• Each node caches a new route it learns by some means
• When S finds route [S,E,F,J,D] to node D,
S also learns route [S,E,F] to node F
• When node K receives the Route Request [S,C,G],
K learns route [K,G,C,S] to S
• When node F forwards Route Reply RREP [S,E,F,J,D],
F learns route [F,J,D] to D
• When node E forwards Data [S,E,F,J,D],
it learns route [E,F,J,D] to node D
• A node may also learn a route when it overhears data packets
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• Advantages:
• can speed up route discovery
• can reduce propagation of route requests (RREQ)
DSR: Route Caching (1/3)
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[S,E,F,J,D][E,F,J,D]
[C,S]
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[F,J,D],[F,E,S]
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RREQ
Route Reply (RREP) from node K limits flooding of RREQIn general, the reduction may be less dramatic
[K,G,C,S]RREP
DSR: Route Caching (2/3)
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• Stale caches can adversely affect performance
• With passage of time and host mobility,
cached routes may become invalid
• In some cases, a sender may try several stale routes (obtained from its local cache, or from cache of some other node), before finding a good route
DSR: Route Caching (3/3)
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DSR: Route Maintenance - Route Error (RERR)
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RERR [J-D]
When J’s attempt to forward a data packet (with route S-E-F-J-D) on link (J-D) fails, J sends a route error (RERR) to S along route J-F-E-SNodes hearing RERR update their route cache to remove link (J-D)When node S receives the RERR, it initiates a new RREQ
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Summary of DSR
• Routes maintained only between nodes who need to communicate
• Route caching can further reduce route discovery overhead
• A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches
• Complicated Cache Management mechanisms
• Packet header size grows with route length due to source routing
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Ad-hoc On-Demand Distance Vector Routing (AODV)
• Route Requests (RREQ) are forwarded in a manner similar to DSR
• When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source
• When the intended destination receives a Route Request, it replies by sending a Route Reply
• Route Reply travels along the reverse path set-up when Route Request is forwarded
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AODV: Route Request (1/5)
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Represents transmission of RREQ
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Represents links on Reverse Path
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AODV: Route Request (2/5)
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F sets up a reverse path entry in its routing table:
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Destination Next Hop
S Ereverse path entry
Represents links on Reverse Path
AODV: Route Request (3/5)
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AODV: Route Request (4/5)
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D does not forward RREQ, because it is the intended target of the RREQ
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AODV: Route Request (5/5)
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Represents links on path taken by RREP
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AODV: Route Reply (1/2)
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• An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S
• To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used
• Forward links are set up when RREP travels along the reverse path
AODV: Route Reply (1/2)
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AODV: Forward Path Setup
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F adds a forward path entry in its routing table:
Destination Next Hop
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forward path entry
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• Routing table entries used to forward data packet• Route is not included in packet header
DATA
Represents a link on the forward path
AODV: Data Delivery
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AODV: Route Maintenance - Route Error (RERR)
• When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message
• Node X increments the destination sequence number for D cached at node X
• The incremented sequence number N is included in the RERR
• When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N
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Information “Freshness” Assured
• Each originating node maintains a monotonically increasing sequence number.
• Used by other nodes to determine the freshness of the information.
• Every nodes routing table contains the latest information available about the sequence number for the IP address of the destination node for which the routing information is maintained.
• Updated whenever a node receives new information about the sequence number from RREQ, RREP, or RERR messages received related to that destination.
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Information “Freshness” Assured
• AODV depends on each node in the network to own and maintain its destination sequence number.
• A destination node increments its own sequence number immediately before it originates a route discovery
• A destination node increments its own sequence number immediately before it originates a RREP in response to a RREQ
• The node treats its sequence number as an unsigned number when incrementing accomplishing sequence number rollover.
• Destination information is assured by comparing the sequence number of the incoming AODV message with its sequence number for that destination.
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ADOV: Summary
• Nodes maintain routing tables containing entries only for routes that are in active use
• Routes are not included in packet headers
• At most one next-hop per destination is maintained at each node
• DSR may maintain several routes for a single destination
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Flooding of Control Packets
• How to reduce the scope of the route request flood ?
• LAR [Ko98Mobicom]
• Query localization [Castaneda99Mobicom]
• How to reduce redundant broadcasts ?
• The Broadcast Storm Problem [Ni99Mobicom]
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Location-Aided Routing (LAR) [Ko98Mobicom]
• Exploits location information to limit scope of route request flood
• Location information may be obtained using GPS
• Expected Zone is determined as a region that is expected to hold the current location of the destination
• Expected region determined based on potentially old location information, and knowledge of the destination’s speed
• Route requests limited to a Request Zone that contains the Expected Zone and location of the sender node
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Expected Zone in LAR
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X = last known location of node D, at time t0
Y = location of node D at current time t1, unknown to node S
r = (t1 - t0) * estimate of D’s speed
Expected Zone
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Request Zone in LAR
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Request Zone
Network Space
BA
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LAR
• Only nodes within the request zone forward route requests
• Node A does not forward RREQ, but node B does (see previous slide)
• Request zone explicitly specified in the route request
• Each node must know its physical location to determine whether it is within the request zone
• If route discovery using the smaller request zone fails to find a route, the sender initiates another route discovery (after a timeout) using a larger request zone
• the larger request zone may be the entire network
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LAR Variations: Adaptive Request Zone
• Each node may modify the request zone included in the forwarded request
• Modified request zone may be determined using more recent/accurate information, and may be smaller than the original request zone
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Request zone adapted by B
Request zone defined by sender S
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Location Aided Routing (LAR)
• Advantages
• reduces the scope of route request flood
• reduces overhead of route discovery
• Disadvantages
• Nodes need to know their physical locations
• Does not take into account possible existence of obstructions for radio transmissions
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Other Routing Protocols
• Plenty of other routing protocols
• Geographic Distance Routing (GEDIR)
• Energy-aware routing
• QoS routing
• Routing with Guaranteed Delivery
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MANET Routing: Issues
• How to compare these protocols fairly?
• Message overhead
• Routing delays
• Throughput
• Energy Consumption
• Comparisons are based mainly on empirical results
• Lack of analytical results
• Interaction with MAC protocols
• Scalability:
• >200 nodes is already a problem
• A possible solution for scalability:
• hierarchical organization of ad hoc networks
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• 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)
MANET Routing: Many Variations
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• 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
MANET Routing: Many Variations
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• 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
MANET Routing: Many Variations
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Research on Mobile Ad Hoc Networks
Variations in capabilities & responsibilities
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Variations in traffic characteristics, mobility models, etc.
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Performance criteria (e.g., optimize throughput, reduce energy consumption)
=
Significant research activity
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Questions
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More on IEEE 802.11 DCF
• Collision Avoidance Mechanism
• CSMA + RTS/CTS (physical/virtual carrier sensing)
• Collision Resolution Mechanism
• Binary Exponential Backoff (BEB)
• Does IEEE802.11 DCF work well in multi-hop networks?
• Answer: No! (has known fairness problems)
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What is Fairness?
• What does fairness mean in a multi-hop environment?
• Max-min fairness
• an allocation is max-min fair if there is no way to give more bandwidth to a flow without decreasing the allocation of a flow of lesser or equal bandwidth
• Practical approach
• ensure that each flow gets a minimum share of the bandwidth
• allow those flows that are not interfering with the current transmitting flows to send simultaneously
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Throughput of each flow:
f0: 1298
f1: 684
f2: 47
f3: 1173
f4: 1913
f0 f1 f2 f3 f4
A B C D E F
Unfairness in IEEE 802.11 DCF
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f0 f1 f2 f3 f4
A B C D E F
f0 f2 f4
f1 f3
The Flow Contention Graph
Nodes represent flows
Two flows connected by an edge means they conflict with each other
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f0 f2 f4
f1 f3
Flow Contention Graph
Scheduling:Slot 1: f0 + f3/f4Slot 2: f1 + f4Slot 3: f2Slot 4: f3 + f0Slot 5: f4 +f0/f1……Slot 6: f0 + f3/f4Slot 7: f1 + f4Slot 8: f2Slot 9: f3 + f0Slot 10: f4 +f0/f1……Each flow is guaranteed to be scheduled once in every 5 slots!
An example of scheduling
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f0 f2 f4
f1 f3
Improved schedule:Slot 1: f0 + f3Slot 2: f1 + f4Slot 3: f2
Slot 4: f0+f3Slot 5: f1+f4Slot 6: f2……Each flow is guaranteed to be scheduled once in every 3 slots!
Flow Contention Graph
One can do better…
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Challenges
• Which scheduling is better?
• Trade-off between fairness and overall throughput
• How to implement a fair scheduler in a fully distributed way?