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Routing Protocols

References…2

H. Karl and A. Willing. Protocols and Architectures for Wireless Sensor Networks. John Wiley & Sons, 2005. (Chapter 11)K. Sohraby, D. Minoli, and T. Znati. Wireless Sensor Networks: Technology, Protocols, and Applications. John Wiley & Sons, 2007. (Chapter 6)E M Royer and C -K Toh A Review of Current Routing Protocols for E. M. Royer and C. K. Toh. A Review of Current Routing Protocols for Ad-Hoc Mobile Wireless Networks. IEEE Personal Communications. Vol. 6, No. 2, pp 46–55, April 1999X. Hong, K. Xu, and M. Gerla. Scalable Routing Protocols for Mobile Ad Hoc Networks. IEEE Network, Vol. 16, No. 4, pp. 11–21, July/August 2006

Introduction...3

WSNs can be deployed to support a wide range of applications, whether they have stationary or mobile nodesOnce deployed, the nodes self-organize into a autonomous wireless ad hoc network, with little or no

imaintenanceThe nodes collaborate to perform the task(s) of the application for which they are deployedTheir main task is to collect data, process data, forward the information back to the application; achieving this task efficiently requires an efficient routing protocol to set up paths between the sensors and the data sink

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Forwarding…4

Whenever a node cannot send its data directly to the sink, it has to rely assistance of intermediate nodes to forward the data, thus form a multihop network

SC

Node S wants to send a packet to node D; no direct link, has to pass (forward) the packet to its neighbor first

A

B

C

D

Forwarding…25

The simplest forwarding technique is “flooding”, where every packet is sent to every neighbor until it reaches the sinkAlternatively, a packet is forwarded only to a randomly selected neighbor, in a technique called “gossiping”Using a technique (in the middle grounds), called “controlled flooding” a packet is sent to a “select” set of controlled flooding , a packet is sent to a select set of neighborsThe performance of forwarding is measured in terms of number of packets sent or delay, and usually the above techniques demonstrate a poor performance“suitability” of a neighbor is needed (captured as a set of metrics)

Is the forwarding node capable? Is it closer to the destination? …

Routing tables…6

Nodes gather and store metrics about their neighbors in “routing tables”

Destination Nexthop

Cost

A A 1

Destination Nexthop

Cost

S S 1

Node S Node A

Determining these tables is performed by the “routing protocols”

B B 1C C 1D C 2D B 2

B S 2C S 2D C 3

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Routing protocols…7

In “wired” networks, the routing protocols are usually based on the link state or distance vector algorithms.In wireless (mobile, multihop) networks, different strategies are required; should best ateg es a e equ ed; s ou d be

distributedlow overheadself configuring

Ad hoc routing can be classified asTable driven (proactive)Source initiated (demand driven)

Ad hoc protocols…8

Ad hoc routing protocols

Table driven Source-initiatedOn-demand

DSDV

CGSR

WRP

TORA SSR

ABRLMRDSRAODV

DSDV—Destination Sequenced Distance VectorCGSR—Cluster Gateway Switch RoutingWRP—Wireless Routing Protocol

AODV—Ad hoc On-demand Distance VectorDSR—Dynamic Source RoutingLMR—Lightweight Mobile RoutingTORA—Temporally Ordered Routing AlgorithmABR—Associativity Based RoutingSSR—Signal Stability Routing

Routing in WSNs…9

The crucial aspects in WSNs areEnergy efficiency

Selecting the most energy efficient routesLow overhead of table construction

Stability and dependability of routesStability and dependability of routesSize of the routing tableResilience

Unicasting, broadcasting, and multicastingGeographic routing

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Gossiping and agent-based unicast forwarding…10

The idea is to eliminate the need of a routing tableThe overhead is high orThe tables are to be created in the first place!

A parallel is drawn between the distribution of data in a replicated data base system and epidemics occurring in h l tihuman populations

Rumor mongering…Once an update is received, the site propagates this periodically and randomly to another siteThe goal is to spread the updates as fast as possible while minimizing the message overhead

The idea of randomly choosing forwarding nodes can be applied to WSNs

Randomized forwarding… 11

In gossip based randomized forwarding, the key is the probability with which a node retransmits an incoming packet

It is shown that the critical threshold is 65-75%The boundary nodes are critical as they have fewer neighborsThe boundary nodes are critical, as they have fewer neighbors

Alternatively, a data packet (acting as an agent) can wander through the network in search for its destination (this is known as random walking)

All packets wander floodingSome packets wander gossiping

Rumor routing…12

Rumor routingDetect an event and install a few paths in the network

Agents propagate from node to node installing routing information

?

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Energy-efficient unicast…13

Take the network graphAssign a cost value (reflecting the energy consumption) to each linkUse an algorithm that would compute the least-cost path(s)

3D

2E

2B

2G

4H

4F

4A

1C

3

3

2

21

1

2

21

2

Send data from A to H

Energy-efficient unicast…214

Minimize energy/bitExample: A-B-E-H

Maximize network lifetimeTime until the first node fails

Routing considering available battery capacity Maximum available battery levelsy

Example: A-C-F-HMinimum battery cost routing (MBCR)

Example: A-D-HMin-max battery capacity routing (MMBCR)

Only take battery level into account when below a given level Conditional min-max capacity routing (CMMBCR)Minimize variance in power levels

Minimum total transmission power routing (MTPR)

Some unicast protocols…15

Attracting routes by redirectingDistance vector routing over topology controlMaximizing time to first node failure as a flow problemMaximizing time to first node failure by max-min optimizationMaximizing number of messagesBounding the difference between routing protocols

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Multipath unicast routing…16

Instead of constructing a single energy-efficient path, focus can be shifted on choosing the best possible path from a set of paths

Provides redundancy as a “standby”Goal is to find k paths that do not have either links or nodes in Goal is to find k paths that do not have either links or nodes in commonIncreased robustness Well studied

Multipath unicast routing…217

Sequential assignment routing (SAR)…Based on the observation that the nodes close to the sink are likely to fail soonerConstruct trees outward from each sink neighbor

Energy-efficient secondary pathsgy y pConstruct “braided” paths where various paths may share nodes

Simultaneous transmission over multiple pathsSend several copies of a packet over multiple node-disjoint paths

Random selection of the next upstream hop

Multipath unicast routing…318

Unicast routing is the broadest research topic in the context of ad hoc and wireless sensor networksA variety of other topics considered (but not covered) include:

Routing and topology controlRouting and topology controlMaximizing data flow for multiple source/destination pairsAll costs consideredIntegration of scheduling and power controlRouting and link qualityRouting and lifetime guaranteesRouting for one-shot queries

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Broadcast and multicast…19

While transmitting data from one node to another, nodes may also need to perform a broadcast operationEfficient broadcasting to restrict the set of f di d h ibl hil ll th forwarding nodes as much as possible, while all the nodes receive dataMulticasting has a similar goal

Options…20

Broadcast Multicast

Source-based tree

Shared core-based tree Mesh

Minimize total cost

Minimize maximum cost per destination

Single core

Multiple core

Source-based tree approach…21

The idea is to construct, for each source, a tree, rooted at the given source, that contains all the destinations for this source, as well as other nodes that are needed

Try to find a tree for which the sum of all link costs is Try to find a tree for which the sum of all link costs is minimum (Steiner tree)Try to find the minimum cost to each destination (shortest-path tree)

Steiner treesrc

dst 1

dst 2

2

2

1

Shortest path treesrc

dst 1

dst 2

2

2

1

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Source-based protocols…22

A greedy heuristic—Shortest Path TreeCalculate the shortest (cheapest) path to each destination and overlay these paths onto a tree

Broadcast using minimum cost spanning tree—Prim’s algorithmg

Start with a tree consisting of the source node and, in |V|-1 steps, add one nodeFor every next node, construct a shortest path to some other node already on the tree

Other Steiner tree approximationsBroadcasting or multicasting with a finite set of powers

Wireless advantage…23

Broadcasting or Multicasting in wireless medium is different from the same in wired medium

Wired: locally distributing a packet to n neighbors: n times the cost of a unicast packet

Wireless: sending to n neighbors can incur costsAs high as sending to a single neighbor—if receive costs are neglected completelycompletelyAs high as sending once, receiving n times—if receives are tuned to the right momentAs high as sending n unicast packets—if the MAC protocol does not support local multicast

If local multicast is cheaper than repeated unicasts (a realistic assumption), then we claim the wireless multicast advantage…

Broadcast incremental power (BIP)…24

In the previous algorithms, the cost of transmitting to multiple neighbors (multiple children) increases exponentiallyBIP differs by exploiting the wireless multicastingA node already transmitting to some other node can A node already transmitting to some other node can increase its transmitting power to further the transmission to other nodes, without the cost of a new transmissionThe additional cost is only the difference between the current cost and the needed (higher) transmission power

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Broadcast incremental power (BIP)…225

// Initialize…Vt = {source node}P(source node) = 0 // transmission power assigned to a nodeforeach (v in V\Vt) {Set candidate edge to (source node, v)Set candidate edge weight to transmission power to reach

reach v from source node}// Compute tree…while (V ≠ V) {while (Vt ≠ V) {

Select v ∈ V\Vt with smallest candidate edge weightAdd v to Vt using this candidate edge (u,v)Increase P(u) to smallest power that reaches v// Re-compute candidate edges and their weightsforeach (v in V\Vt) {Select u which minimized P’(u)-P(u)// where P’(u) ≥ P(u) is smallest power to reach from v to uSet candidate edge to (u,v)Set candidate edge weight to P’(u)-P(u)

}}

BIP in action…26

Other multicast algorithms…27

Exploiting wireless multicast advantageMulticast Incremental Power (MIP)

Embedded wireless multicast advantageTransforming existing trees

A distributed, position based approach

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Shared, core-based protocols…28

The challenge is to find the “core”The problem can then be solved with a source-based tree algorithm with the core as the sourceOne approach is “merge point formation”, where a merge point for a tree with a few sinks is to be found

Mesh-based protocols…29

Tree-based protocols suffer from scalability and robustnessA structure with high level of connectivity is needed to connect multiple sources to their destinationsCore-Assisted Mesh Protocol (CAMP)

The mesh (a sub-graph of the original graph) has to contain all sources and destinations and provide at least one path from each source to each destinationRedundancy in a mesh can enable shorter paths than in a core-based tree, based on the forwarding procedure

Other broadcast and multicast approaches…30

Gossiping for multicastE.g., to improve the reliability

Directed antennas for multicastRobustness and traffic carrying capacity improvements

Relationship to topology controlDifference is the source-based protocols

Optimal solutions by linear programmingOptimal solutions by linear programmingLP is NP-hard, but provides better approximations

Optimal solutions for tree networksCollecting and distributing data

Time to complete a multicastTime is important, too…

Data replacementCaching

Cooperative multihop broadcast

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Geographic routing…31

In geographic routing protocols:Known positions of the source and destination, as well as the intermediate nodes can be used to assist the routing position-based routingIt is necessary (for many applications) to address physical y ( y pp ) p ylocations (e.g., any node in a given region) geocasting

For wireless sensor networks the latter is more important

Nodes are interchangeable and distinguishable only by their external aspects, the location service is usually not necessary

Position-based routing basics…32

Simple forwarding strategies:Most forward within rNearest with forward progressDirectional routingRestricted floodingHmmm. Dead ends...

Basic idea to get out of a dead end: Put right hand to the wall, follow the wall

Does not work if on some inner wall—will walk in circlesNeed some additional rules to detect such circles

Send the packets around the face using the right-hand rule

Greedy perimeter stateless routing (GPSR)33

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Greedy perimeter stateless routing (GPSR)34

When stuck in a dead-end, one way of escaping is to keep the right hand to the wall and keep walking...

Practically, backtrack the packet out of the dead end, counter-clockwise around the obstacle

Similar approach will allow the discovery of a node Similar approach will allow the discovery of a node closer to the destination in WSNsIn GPSR, a packet is greedily forwarded, until an obstacle is reached at, when the algorithm switches to “perimeter routing” (essentially, sending a packet around a plane using the right-hand rule)

A GPSR example…35

Route packet from node A to node Z

E I

Leave face routing

AZ

D

C

B F

G

H

J

K

LEnter face routing

Other position-based routing…36

ID-based routing, hierarchiesRandomized forwarding and adaptive node activity (GeRaF)Geographic routing without positions (GEM)

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Geocasting…37

Sending data to a subset of nodes that are located in an indicated region (an example of multicasting!)

Geographically restricted floodingLocation based multicast

Static zone, adaptive zone, adaptive distanceFinding the right directionFinding the right direction

Voronoi diagrams and convex hullsTessellating the plane

Simpler forms (tessellations) of the planeMesh-based geocasting

Unicast routingGeocasting using unicast (GeoTORA)

Trajectory-based forwarding (TBF)

Further issues in geocasting…38

Impact of localization errorsImpractical assumption in real systems, that all the nodes know their location

Location servicesMapping node IDs to node locations; more important in ad hoc networks

Location-aided Routing (LAR)Location aided Routing (LAR)Use location information to assist in the flooding phase(s)

Energy aware geocast (GEAR)Load splitting among neighbors when forwarding

Geographic routing without geographic coordinatesDoes not really need exact location

Link asymmetryGeographic routing fails badly

Mobile nodes…39

Sources of mobility in WSNs:The sensor nodesData sinksObserved event(s)

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Multiple sinks… 40

Source

Source

Sink movesdownward

SourceSource

SourceSource

Sink movesdownward

Source

Source

Sinkmovesupward

SourceSource

SourceSource

Sinkmovesupward