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Unicast Routing
Ad Hoc networks
(under standardization in the IETF
MANET WG)
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What is an ad hoc networkA wireless multi-hop infrastructure-lessnetwork whose devices act as source/destination of messages & as relay forpackets generated by a node s and addressed
to a node z (iff they are on a s-z route)Pros: No need for infrastructure low cost,enables communication where it is usuallynot needed or is not viable
Must be: Self-organizing, self-configuring,self-maintaining
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Application scenariosCollaboration between users in officeenvironments
Disaster recovery applications
Military networksPersonal Area Networks
Home Networking
Wireless Sensor Networks (WSNs)Inter-vehicular communication
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Features of ad hoc networksHighly dynamic networks device mobility, energysaving sleep/awake modes
Need for low energy/resource-consuming, simpleprotocols
Bandwidth and resource constrained environmentTraffic: All-pairs in general ad hoc networks, from sensors to sink(s) in
sensor networks
In many case not high
Scale: Application dependent 10-100 nodes in traditional ad hoc networks
1000-10000 in sensor networks
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Features of highly resource constrained ad hocnetworks (WSNs)
Highly dynamic networks due to device mobility (only insome specific applications), to the fact the active node setchanges in time for sake of energy saving (always to beconsidered)
Need to design low energy/resource-consuming, simpleprotocols very critical, energy consumption a realbottleneck
Traffic from sensors to sink(s)
Scalability is a major issueCode must be simple (small storage capability, verysimple, inexpensive, resource constrained devices)
First solutions we will see for traditional ad hoc networksdo not scale to high numbers and are not energy-saving
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Routing-Traditional approach(from Reti 1)
Intra-AS routing in the Internet
Link State Approaches
(info on the topology graph gathered atnodes which run shortest path algorithms-Dijkstra- to decide the routes to thedifferent destinationse.g. OSPF routingprotocol)
Distance Vector approaches (e.g. RIP)
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Bellman-FordGiven a graph G=(N,A) and a node sfind the shortest path
froms
to every node in N.A shortest walk from sto i subject to the constraint that the walkcontains at most h arcs and goes through node sonly once, is denotedshortest(
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Bellman-Ford
Dh+1i=mink [wi,k + Dhk]
Can be computed locally.
What do I need?
For each neighbor k, I need to know-the cost of the link to it (known info)
-The cost of the best route from the neighbor k to the destination(this is an info that each of my neighbor has to send to me viamessages)
In the real world: I need to know the best routes among each
pair of nodes we apply distributed Bellman Ford to get the bestroute for each of the possible destinations
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Distance Vector Routing Algorithm-Distributed Bellman Ford
iterative:continues until nonodes exchange info.
self-terminating: nosignal to stop
asynchronous:nodes need notexchange info/iteratein lock step!
Distributed, based onlocal info:
each nodecommunicates onlywith directly-attachedneighbors
Distance Table data
structureeach node has its own
row for each possibledestination
column for each directly-attached neighbor to node
example: in node X, for dest.Y via neighbor Z:
D (Y,Z)X
distance from X toY, via Z as next hop
c(X,Z) + min {D (Y,w)}Z
w
=
=
Cost associated to the (X,Z) link
Info maintained at Z. Min mustbe communicated
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Distance Vector: link cost changesLink cost changes:
good news travels fastbad news travels slow-count to infinity problem!
X Z
14
50
Y60
algorithmcontinues
on!
Y detects link costIncrease but think can
Reach X through Z at atotal cost of 6 (wrong!!)
The path is Y-Z-Y-X
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Count-to-infinityan everyday lifeexample
Which is the problem here?the info exchanged by the protocol!! the best route to X I
have has the following cost (no additional info on the
route)
A Roman example-assumption: there is only one route going from Colosseo to
Altare della Patria: Via dei Fori Imperiali. Let us now consider
a network, whose nodes are Colosseo., Altare della Patria,
Piazza del Popolo
Colosseo Altare PatriaPiazza del
Popolo
1 Km 1 Km
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Colosseo Al.Patria P.Popolo1Km 1Km
The Colosseo. and Alt. Patria nodes exchange the following info
Colosseo says the shortest route from me to P. Popolo is 2 Km
Alt. Patria says the shortest path from me to P. Popolo is 1Km
Based on this exchange from Colosseo you go to Al. Patria, and from there toPiazza del Popolo OKNow due to the big dig they close Via del Corso(Al. PatriaP.Popolo)
Al. Patria thinks I have to find another route from me to P.Popolo.
Look there is a route from Colosseo to P.Popolo that
takes 2Km, I can be at Colosseo in 1Km I have founda 3Km route from me to P.Popolo!! Communicates the new cost to
Colosseo that updates OK I can go to P.Popolo via Al. Patria in 4Km
VERY WRONG!! Why is it so? I didnt know that the route from
Colosseo to P.Popolo was going through Via del Corso from Al.Patriato P.Popolo (which is closed)!!
Count-to-infinityeveryday life example (2/2)
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Routing in ad hoc networks-Goals
Minimal control overhead
Minimal processing overhead
Multi-hop path routing capability
Dynamic topology maintenance
No loops
Self-starting
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2 Primary Approaches
Proactive Based on traditional distance-vector and link-state
protocols Each node maintains route to each other network node
Periodic and/or event triggered routing update exchange
Higher overhead in most scenarios
Longer route convergence time Examples: DSDV, TBRPF, OLSR
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Highly Dynamic Destination-SequencedDistance-Vector (DSDV) RoutingProactive, distance vector approach (uses distributedasynchronous Bellman Ford). Updates on routes coststransmitted periodically or when significant new informationis available.
Difference wrt Bellman Ford: in ad hoc networks there arefrequent changes in the topology, solutions must try to avoidloops (approaches such as Poison reverse non effective inbroadcast channels, we seek solutions which are simple andfully distributed)
Metrics: fresh routes better than stale routes, number ofhops used to select among the fresh routes
How to identify fresh routes? By means of sequencenumbers identifying the freshness of the communicated
information. When changes occur, the sequence numberincrease.
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Highly Dynamic Destination-SequencedDistance-Vector (DSDV) RoutingPeriodically destination nodes transmit updates with a newsequence number (and such updates are propagated by theother nodes). Updates contain information on the costs toachieve the different destinations and the freshness of thedelivered information
Data broadcast include multiple entries each with: Destination address
Number of hops required to reach the destination
Sequence number of the information received regarding that
destination as originally stamped by the destinationIn the header the data broadcast also include: Address (HW address/Net address) of the sender of the message
Sequence number created by the transmitter
Two types of updates (full dump or incremental-only
changes- to decrease bandwidth consumption.
Hi hl D i D i i S d
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Highly Dynamic Destination-SequencedDistance-Vector (DSDV) RoutingHow can the costs be modified? Cost=number of hops, target:
using fresh routes as short as possible a link cost changesfrom 1 to inf and from inf to 1
How do we detect that a link is broken? At layer 2 (no hellomessages received for some time, or attempts to retransmit aframe exceeds the MAC protocol threshold) or at layer 3 (do
not receive periodic updates by a neighbor)Link cost increase (1 inf): The nodes incident to that link (A,B) discover it (see above)
Routes going through that link get assigned an inf cost in nodes A and Brouting tables
A new sequence number is generated by the mobile node. Mobile nodesdifferent from the destination use odd SN, the destination even SN.
Updates with routes with infinite cost are immediately transmitted bynodes
Link cost decrease (inf1): Immediately transmits updates
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Highly Dynamic Destination-SequencedDistance-Vector (DSDV) RoutingWhen a node receives updates it sees if costs to reach thedifferent destinations can be improved: routes with more recent sequence numbers to a given destination are
used
if more routes available with the same SN the shortest is used
Newly recorded routes are scheduled for immediateadvertisement (inf finite value)
Routes with improved metric are scheduled for advertisementat a time which depends on the estimated average settling
time for routes to that particular destination (based onprevious history)
As soon as a route cost changes the node may delayinforming its neighbors but immediately starts using the newinformation for its forwarding
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Highly Dynamic Destination-SequencedDistance-Vector (DSDV) Routing-Correctness
Assuming routing tables are stable and a change occurs let G(x) denotes the routes graph from the sources to x BEFORE the
change (assume no loop)
change occurs at iwhen 1) the link from ito its parent p(i) in G(x)
breaks isets to inf that route (no loop can occur) 2) node ireceivesfrom one of its neighbors k a route to x with sequence number SNxkand metric m which is selected to replace the current metric ihas toreach x (this occurs only if SNxkgreater than the previous SN I hadstored Snxi or if the two SN are equal but the new route has a lowerhop cost in the first case if selecting k leads to a loop then SNx
k
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Why traditional approacheshave limits?Proactive protocols are costly in terms of overhead(the bandwidth and energy are critical resources)
The cost of maintaining routes updated may not
make sense in an environment in which Medium-high mobility
Medium-high dynamicity (awake/asleep states)
Often make the opt. Route change (requiring updates)while
Traffic is generally low (so the cost of maintaining alwaysupdated routes is not balanced by their use)
If this is the scenario what can we do?
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2 Primary Approaches (cont.)
Reactive (on-demand) Source build routes on-demand by flooding
Maintain only active routes Route discovery cycle
Typically, less control overhead, better scalingproperties
Drawback: route acquisition latency Example: AODV, DSR
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Ad hoc On-Demand DistanceVector (AODV) RoutingReactive (nodes that do not lie on active paths neithermaintain any routing information nor participate in anyperiodic routing table exchange; a node does not have todiscover/maintain a route to a destination till it is on a path
to it or has to send messages to it)Route discovery cycleused for route finding
Maintenance ofactiveroutes
Sequence numbers used for loop prevention and as routefreshness criteria
Descendant of DSDV (standard distance vector approachmapped to ad hoc networks), in AODV no periodic updatesbut pure on-demand operation.
Provides unicast and multicast communication
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AODV: Route Discovery
1. Node Sneeds a route to D AND does nothave routing info for it in its table
A
B
C D
S
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AODV: Route Discovery
1. Node Sneeds a route to D
2. Creates a Route Request (RREQ)
Enters Ds IP addr, seq#,Ss IP addr, seq#
hopcount (=0), broadcast ID
A
B
C D
S
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AODV: Route Discovery
1. Node Sneeds a route to D2. Creates a Route Request (RREQ)
Enters Ds IP addr, seq#,Ss IP addr, seq#
hopcount (=0), broadcast ID
3. Node Sbroadcasts RREQ to neighbors
A
B
C D
SRREQ
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AODV: Route Discovery
4. NodeAreceives RREQ Makes reverse route entry for S
dest=S, nexthop=S, hopcnt=1,expiration time for reverse path
Source node, Source node SN,D,broadcastID also maintained It has no route to D, so it rebroadcasts RREQ (hopcount
increased)
If it has already received that request (same source andbroadcast ID) it discards the RREQ
if it knows a valid path to D it will send back a reply to thesource
A
B
C D
SRREQ
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AODV: Route Discovery
4. NodeAreceives RREQ Makes reverse route entry for S
dest=S, nexthop=S, hopcnt=1 It has no route to D, so it rebroadcasts RREQ
A
B
C D
S
RREQ
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AODV: Route Discovery
5. Node Creceives RREQ Makes reverse route entry for S
dest=S, nexthop=A, hopcnt=2 It has a route to D, and the seq# for route to D
is >= Ds seq# in RREQ
A
B
C D
S
RREQ
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AODV: Route Discovery
5. Node Creceives RREQ (cont.) Ccreates a Route Reply (RREP)
Enters Ds IP addr, seq#
Ss IP addr, hopcount to D(= 1), lifetime of theforward route
Unicasts RREP toA
A
B
C D
S
RREQ
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AODV: Route Discovery
5. Node Creceives RREQ (cont.) Ccreates a Route Reply (RREP)
Enters Ds IP addr, seq#Ss IP addr, hopcount to D(= 1).
Unicasts RREP toA
A
B
C D
S RREP
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AODV: Route Discovery
6. NodeAreceives RREP Makes forward route entry to D
dest = D, nexthop = C, hopcount = 2 Unicasts RREP to S
A
B
C D
SRREP
RREP
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AODV: Route Discovery
7. Node Sreceives RREP Makes forward route entry to D
dest = D, nexthop =A, hopcount = 3
Also the latest SN of the destination is updated when receivingthe RREP
Nodes not along the path determined by the RREP will timeout
after ACTIVE_ROUTE_TIMEOUT (3000ms) and will deletethe reverse pointer
A
B
C D
SRREP
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AODV: Route Discovery
7. Node Sreceives RREP Makes forward route entry to D
dest = D, nexthop =A, hopcount = 3 Sends data packets on route to D
A
B
C D
S
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What if.
A node receives further RREPs for the samerequest? (e.g. more neighbors of a node hadpaths to the destination in cache)? upon reception of another RREP the node
updates its routing information and propagatesthe RREP only if the RREP contains either agreater destination SN, or the same destination SN
with a smaller hopcount
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Other info maintained
Each node maintains the list of activeneighbors, neighbors sending to a givendestination through it useful for route maintenance
Routing table entries: dest,next hop,hopcount, dest SN, active neighbors for thisroute, expiration time for route table entry
(updates each time the route is used fortransmitting data routes entries aremaintained if the route is active)
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AODV: Route Maintenance
1. Link between Cand Dbreaks2. Node Cinvalidates route to Din route table3. Node Ccreates Route Error (RERR) message
Lists all destinations which are now unreachable
Sends to upstream neighbors
Increases of one the SN of the destination
A
B
C D
S RERR
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AODV: Route Maintenance
4. NodeAreceives RERR Checks whether Cis its next hop on route to D
Deletes route to D Forwards RERR to S
A
B
C D
SRERR
RERR
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AODV: Route Maintenance
5. Node Sreceives RERR Checks whetherAis its next hop on route to D
Deletes route to D Rediscovers route if still needed (in that case it
sends a RREQ with a SN which is equal to thelast known destination Sequence Number +1)
A
B
C D
SRERR
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AODV: Optimizations
Expanding Ring Search
Prevents flooding of network during route
discovery Control Time To Live (TTL) of RREQ to search
incrementally larger areas of network
Advantage: Less overhead when successful Disadvantage: Longer delay if route not found
immediately
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AODV: Optimizations (cont.)
Local Repair Repair breaks in active routes locally instead of
notifying source
Use small TTL because destination probably hasntmoved far
If first repair attempt is unsuccessful, send RERR tosource
Advantage: repair links with less overhead, delay andpacket loss
Disadvantage: longer delay and greater packet losswhen unsuccessful
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AODV: Summary
Reactive/on-demand
Sequence numbers used for route
freshness and loop preventionRoute discovery cycle
Maintain only active routes
Optimizations can be used to reduceoverhead and increase scalability
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Dynamic Source Routing (DSR)
Reactive
Route discovery cycleused for route
findingMaintenance ofactiveroutes
Utilizes source routing
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DSR: Route Discovery
1. Node Sneeds a route to D
2. Broadcasts RREQ packet
1. RREQ identifies the target of the route discovery,contains a route record in which the traversed route isaccumulated, contains a pair uniquely identifying the request
A
B
C D
S
RREQ: S
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DSR: Route Discovery
1. Node Sneeds a route to D2. Broadcasts RREQ packet
3. NodeAreceives packet, has no route to DAND is NOT D Rebroadcasts packet after adding its address to
source route
A
B
C D
S
RREQ: S
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DSR: Route Discovery
1. Node Sneeds a route to D2. Broadcasts RREQ packet
3. NodeAreceives packet, has no route to D Rebroadcasts packet after adding its address to
source route
A
B
C D
S
RREQ: S, A
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DSR: Route Discovery
4. Node Creceives RREQ, has no route to D Rebroadcasts packet after adding its address to
source route
A
B
C D
S
RREQ: S, A
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DSR: Route Discovery
4. Node Creceives RREQ, has no route to D Rebroadcasts packet after adding its address to
source route
A
B
C D
S RREQ: S, A, C
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DSR: Route Discovery
4. Node Creceives RREQ, has no route to D Rebroadcasts packet after adding its address to
source route5. Node Dreceives RREQ, unicasts RREP to C Puts source route accumulated in RREQ into
RREP
A
B
C D
S RREQ: S, A, C
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DSR: Route Discovery
4. Node Creceives RREQ, has no route to D Rebroadcasts packet after adding its address to
source route5. Node Dreceives RREQ, unicasts RREP to C Puts source route accumulated in RREQ into
RREP
A
B
C D
S RREP: S, A, C, D
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DSR: Route Discovery
6. Node Creceives RREP Unicasts toA
A
B
C D
S RREP: S, A, C, D
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DSR: Route Discovery
6. NodeAreceives RREP Unicasts to S
A
B
C D
SRREP: S, A, C, D
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DSR: Route Discovery
8. Node Sreceives RREP Uses route for data packet transmissions
A
B
C D
SRREP: S, A, C, D
General node operation upon
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General node operation uponreceiving RREQ
If the pair has recently been seen, DISCARD
If the node ID is already listed in thesource route DISCARD avoids loops
If Im the destination, send a RREP
Otrherwise, append my ID in the sourceroute and rebroadcast (orange casesalready seen in the previous slides)
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Route maintenance
The two endpoints of a failed link aretransmitted to the source in a route errorpacket
Upon a receiving a RERR packet a nodeinvalidates all the routes going through thatlink
If the route is invalidated and it is needed, anew route must be discovered
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Optimizations (1)Extensive use of caching (caching source routes
means that I already know all the route tointermediate destinations, discovery a betterroute to an intermediate destination also bringsme to improve the route to the final destination).
Transmitting packets or sending back repliesmake me learn routes.
A node that knows a route to a given destination(has a source route in cache) can immediately
answer a RREQ Broadcast storm? Each nodes waits for a time which is
C*(h-1+r), r random in (0,1), h length of the route Imadvertising. Only if I havent received other routes listen to other routes tx in the meanwhile-I transmitmine.
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Optimization (2)
Operation in promiscuous mode (I keepdiscovering new routes by transmission of routesby my neighbours)
RREQ overhead minimization: first set a TTL=1, if
I do not get answer I set it to infinityPath shortening: if Y receives a packet by node X
but in the source route we have X, B,,C,Y, Ysignals the path can be shortened (unsolicited
RREP)What if the network is disconnected? Exponentialback-off to decrease the quantity of RREQ sent
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AODV and DSR Differences
DSR uses source routing;AODV uses next hop entry
DSR uses route cache;
AODV uses route tableDSR route cache entries do not havelifetimes;
AODV route table entries do have lifetimes
DSR nodes respond to each RREQ duplicate;AODV nodes only respond to first RREQ,unlessone arrives along a better path
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Geographically-Enabled Routing
Outline Problems with proactive approaches
Problems with reactive approachesA new way of naming\locating the
destination node: geographic routing
Two seminal protocols
DREAM & LAR Geo-enable routing costs: I need to know
where I am, where the destination is.
Proactive Solutions:
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Proactive Solutions:Drawbacks
Updates overhead, especially inpresence of high mobility
Overhead for enforcing loop freedomLarge routing tables
Low scalability
Is it really necessary to maintain aconsistent view of the networktopology?
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Reactive Protocols: DrawbacksThe discovery phase introduces long delaysRoute discovery and maintenance is very sensitive tonode mobility
Route caching is memory greedy
The size of the header of a data packet can becomecumbersome in approaches such as DSR (noscalability)
Operating in promiscuous mode is energy-consuming.
Relying on flooding based route discovery is resourceconsuming.
Is the dependency on the network topologyavoidable?
Location-Enabled Ad Hoc
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Location Enabled Ad HocRouting
Nodes are equipped with positioningsystem devices (e.g., Global PositioningSystem receivers) that make themaware of their positionThis enables directional routing
Possible solutions differ on how the
location information of the destinationnodes is achieved
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Strengths
No need to update big routing tables, no
need to piggyback routes to the packet
Destination position must be known at the
source.
No need to know the nodes on the way to the
destination: they can be moving while the
packet travels
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Drawbacks
Needs extra hardware
Depends on the extra hardware limitation
(and resource requirements)
Scalability is an issue (indeed the problem
here translates to how to maintain correct
estimates of the nodes positions)
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DREAM
Distance routing effect algorithm formobility [Basagni+, 1998]
A proactive, effective way to spreadlocation information
Directional routing
Disseminating Location
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Disseminating LocationInformation: Problems
Need to periodically update thelocation of a moving node.
Efficient broadcast of location information Determining how far each location packet
should travel
Determining how often a location packetshould be sent
Disseminating Location
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Disseminating LocationInformation: Solutions
Distance effect
Rate of updates is bound to the mobilityof the node
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The Distance Effect
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The Distance Effect
Closer nodes look like they are movingfaster
Need to receive more location updatesfrom closer node
Each location update packet is
associated with an age that determineshow far that packet must travel
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DREAM: Rate of updates
Triggered by the mobility of the nodes
The faster the node the more updates itsends
A plus: slow moving nodes impose littleoverhead
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DREAM: Directional Routing
Source S determines the location ofdestination D at time t0 based on its locationtable
Based on the current time t1 and t0 Sdetermines the area in which D can be found(hence, Ds direction)
S transmits the data packet to all its
neighbors in Ds directionEach neighbor does the same till D is reached
DREAM: Routing a Data
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DREAM: Routing a DataPacket
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DREAM, Strengths
First of its kind: after that, the deluge!
Robustness: multiple routes to thedestination
directional flooding
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DREAM, Weaknesses
It is flooding, although only directional
It is not that scalable, geographic infoupdates have to be periodically
transmitted (even if mechanisms to
limit such overhead are enforced)
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Location-Aided Routing (LAR)
Exploits location information to limit scope ofRREQ flood
Expected Zone: region that is expected to holdthe current location of the destination Expected region determined based on potentially old
location information, and knowledge of thedestinations speed
RREQs limited to a Request Zonethat contains
the Expected Zone and location of the sendernode
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LAR: Expected Zone
D
D'
r
D = last known location of node
D, at time t0
D' = location of node D at current
time t1, unknown to node S
r = (t1 t0) * estimate of Ds speed
Expected Zone
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LAR
The request zone is the smallest rectangle thatincludes the current location of the source andthe expected zone
Only nodes within the request zone forward
route requests Node A does not forward RREQ, but node B does
Request zone explicitly specified in the RREQ
Each node must know its physical location todetermine whether it is within the request zone
LAR R t Z
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LAR: Request Zone
D
D'
r
S
Request Zone
Network Space
BA
(XS, YS)(XD+ r, YS)
(XS, YD + r) (XD+ r, YD+ r)
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LAR, Possible Failures
If route discovery using the smallerrequest zone fails to find a route, thesender initiates another route discovery
(after a timeout) using a larger requestzone The larger request zone may be the entire
network
Rest of route discovery protocol similar toDSR
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LAR, the Routing
The basic proposal assumes that, initially, locationinformation for node X becomes known to Y only duringa route discovery
This location information is used for a future routediscovery
Variations
Location information can also be piggybacked on anymessage from Y to X
Y may also proactively distribute its location information
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LAR, Pros and Cons
Advantages
Reduces the scope of RREQ flood
Reduces overhead of route discoveryDisadvantages
Nodes need to know their physical locations
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In a sensor network we seek..
For solutions which scaleWhich are energy savingWhich are well integrated with awake/asleep schedulesWhich do not require to maintain routing tablesWhich are simple
Solutions such as AODV and DSR have been proven to work well iffthey exploit intensively caching and promiscuous mode operation(energy inefficient work by L. Feeney et al, 2001) and have beenshown not to scale to the volumes of nodes expected in sensor networks
(work by E. Belding Royer and S.J. Lee)What can we use? communication sensors sink Info such as localization and some level of synchronization often needed
by the application (if I sense an event I have to say WHERE and WHEN itoccurred, otherwise the information is not very interesting)
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An example: GeRaF
Integrates
geographic routing
awake/asleep schedule MAC
How do nodes alternate between
awake and asleep states? According toa duty cycle (time ON/time ON+ OFF)
ON ON
OFF OFF
GeRaF basic idea
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GeRaF basic idea
Geographic routing:each node needs toknow its location, thedestination (sink)
location, and thelocation of whom istransmitting(communicated in thepacket)Greedy approach:
tries to select relaysso to advance asmuch as possibletoward the destinationat each hop
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GeRaF: operations
Main problem to be solved: how to make acontention-based scheme/routing work in thepresence of sleep modes Flat solution
Integrated MAC/routing/awake-asleep but awake-asleep schedule and routing decoupled each nodedoes not know its neighbor and their schedules low overhead
Tightly integrated with the routing layer (no
clear separation really) Without requiring routing tables/routing table updates
Based on the knowledge of the nodes location and onthe knowledge of the sink location
Example of GeRaF operation
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Example of GeRaF operationRTS invites all awake neighbors to become
a relayNodes in best position should win
Nodes within tx range are divided in areas
depending on how close they are to the finaldestination (the closest the better as relay)
Need of location awareness
GeraF operation
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GeraF operationNode i sends RTS with the identity of the area it
is polling now (starting from the closer to the sink,among the slices in which its tx range has beendivided)
Each node, upon receiving the RTS, decides
whether it belongs to the polled area or not(based on location info)
Only nodes in the polled area answer with a CTS No node answers node i polls next area (no node
available for forwarding in the area-there are no nodesor they are sleeping)
One answer, CTS correctly received, send DATA
Multiple answer COLLISION, sender sends a collision
packet, MAC needed to solve collision (next slide)
GeraF operation (how to
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handle collision)
1)A node receiving a collision packet tosses acoin and with probability p transmits a CTS iffit was participating to the previous phase (it
had previously sent a CTS resulting incollision) if only one node answers node i sends data
If no node answers node i asks these nodes to
toss a coin again.. if more nodes answer COLLISION. Collision
packet is sent. GO TO 1) (only the nodes whichhave lead to collision survive to the next phase)
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What If
All areas are polled unsuccessfully?
Try again after some time (exponentialbackoff)
Can I always reach the destination inthis way?
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What if (answer)
No. Here is
An example
Solutions? (mechanisms have to be added torecognize the problem, do backtracking andtry another route)
sink
Tx radius
How to solve dead-endsA problem only at low density
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A problem only at low density
We can set a maximum number of attempts
to find a relay. When a node fails to find arelay it starts decreasing its duty cycle/or theprobability to propose itself as relay overtime nodes along paths to dead-ends are lessand less selected as next hop relays andother paths able to bring to the destinationare instead found
Still..we may have problems
sink
Tx radius
GeRaf performance
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GeRaf performance
square area 320m x 320m
Transmission range=40m
100-1000 randomly deployed nodes (avgdegree 5-50)
Duty cycle =0.01,0.1,0.5
Comparable costs for tx/rx/idle
Poisson packet arrivalChannel data rate 38Kbps
Casari, Marcucci,Nati, Petrioli, Zorzi IEEE MILCOM 2005
GeRaf performance d=0 1
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GeRaf performance, d=0.1Casari, Marcucci,Nati, Petrioli, Zorzi IEEE MILCOM 2005
GeRaf performance d=0 1
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GeRaf performance, d=0.1Casari, Marcucci,Nati, Petrioli, Zorzi IEEE MILCOM 2005
GeRaf performance d=0 1
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GeRaf performance, d=0.1Casari, Marcucci,Nati, Petrioli, Zorzi IEEE MILCOM 2005
Con i meccanismi per evitare dead end si sale a % di successfullyDelivered packets nel caso di 200 nodi pari a 93-97% (evitandoNel tempo cammini che portino a dead ends). Non si risolve ilProblema nel caso n=100. Soluzione completa in uno schema che
abbiamo proposto: ALBA.
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Localization in sensornetworks
Thanks to Prof. Mani Srivastava
These slides have been derived
From his tutorial on sensor networks
Given at Rome Un. On July 2003
LocalizationUseful info
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Useful info Helps with some protocols (e.g. GeraF)
Needed for being able to identify where events occurWhy not just GPS (Global Positioning System) atevery node? Large size
High power consumption Works only when LOS to satellites (not in indoor, heavy
foliage)
Over kill often only relative position is needed (e.g.enough to know that relative to a coordinate systemcentered in the sink the event occurred in a position(x,y). Starting from relative info if some nodes haveglobal coordinates global coordinates of events can beinferred.
Works only on earth
Localization
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Localization
Basic step is to evaluate distance betweentwo nodes (ranging). Different techniquesdepending on the available HW: AoA (e.g. directional antennas)
RSS ToA
Range free approaches (number of hops
between nodes used to estimate thedistance between them without using anyextra HW)
Techniques for LocationSensing (AoA)
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Sensing (AoA)Measure direction of landmarks Simple geometric relationships can be used to
determine the location by finding the intersectionsof the lines-of-position
e.g. Radiolocation based on angle of arrival (AoA)
measurements of beacon nodes (e.g. basestations) can be done using directive antennas + a compass
need at least two measurementsBS
BS
BS
MS
12
3
Measure distance to landmarks, or RangingTechniques for Location Sensing (RSS or ToA)
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, g g
e.g. Radiolocation using signal-strength or time-of-flight also done with optical and acoustic signals
Distance via received signal strength use a mathematical model that describes the path loss attenuation with
distance each measurement gives a circle on which the MS must lie
use pre-measured signal strength contours around fixed basestation(beacon) nodes
can combat shadowing location obtained by overlaying contours for each BS
Distance via Time-of-arrival (ToA) distance measured by the propagation time
distance = time * c
each measurement gives a circle on which the MS must lie active vs. passive
active: receiver sends a signal that is bounced back so that the receiver knowsthe round-trip time
passive: receiver and transmitter are separate
time of signal transmission needs to be known
N+1 BSs give N+1 distance measurements to locate in N dimensions
d l d SS
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Radiolocation via ToA and RSSI
x1
x2
x3
d1
d3
d2
MS
BS
BS
BS
But what if the circles
do not intersect due tomeasurement errors (e.g.
due to fading etc.)?will have to identify the best guess given errors
L i i 3D
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Location in 3D
L ti i 3D
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Location in 3D
L ti i 3D
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Location in 3D
A possible solution: AbsoluteL li ti
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Localization
Beacon
Unkown Location
Randomly Deployed Sensor Network
Beacon nodes
A small fraction of the nodesis aware of their locations
Rest need to collaborate to
estimate their locations
Atomic Multilateration
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22
,, )()( uiuiiuiu yyxxrf
2
,min),( iuuu fyxF
),( uu yx - initial position estimate for node u
u
1
2
3
1,ur
3,ur
2,ur
Beacon nodewith known
location
Our objective function is:
Metrica di interesseErrore quadratico medio
Nodi che hanno
almeno 3 vicini
(in 2D, se si usaAd esempio RSS)
beacon
possono stimare
la loro posizione(triangolarization)
In presenza di errori
Iterative Multilateration
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Iterative MultilaterationNodes that estimate their locations can become
beacons and help other nodes discover theirlocations.
Some observations: Can work for small networks, if ranging is accurate
Energy efficient
Still requires quite a lot of initial beacons
Suffers from error accumulation
Bad geometry yields bad results => unpredictableperformance
Still a useful primitive for Distributed Collaborative Multilateration
Primo approccio semplice, vasta letteratura
Ref: based on slides by Andreas Savvides
R f l li ti
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Range free localization
Non usa ranging, ma solo informazioniche si possono ottenere tramite algo dirouting tradizionali
Idee su come possa funzionare?
Qualche idea sullapproccio
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Qualche idea sull approccio Servono degli anchor, nodi che
conoscono la propria posizione in unsistema di coordinate comune
X1,Y1
X2,Y2
X3,Y3
Qualche idea sullapproccio
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X1,Y1
X2,Y2
X3,Y3
Tutti I nodi calcolano il numero min. di hop tra
loro e gli anchor Anche gli anchor lo fanno tra loro
Qualche idea sullapproccio
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Anchor A: conosco la posizione esatta mia edegli altri anchor, il num. di hop, posso stimare la
lunghezza media di un hop
Questa informazione e usata per stimare ledistanze da tutti I nodi agli anchor. Sulla base ditali distanze, le corrette coordinate degli anchor,per triangolarizzazione ciascun nodo stima leproprie coordinate
Pro: Non serve extra HW
Cons: si perde in precisione
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Scalability Problems andCl t i
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Clustering
What happens to protocols when the numberof network nodes grows? Especially crucial in WSNs
A traditional networking solution: Hierarchicalorganization of the nodes
Network nodes are grouped into clusters
Some nodes, locally the best, are selected
to coordinate the clustering process:Clusterheads
How to Select the Best Nodes
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How to Select the Best Nodes
Independence of the clusterheads
Dominance of the clusterheads
Possibility to express preferences
Distributed operations
Fast and simple implementation
Previous Approaches
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Previous Approaches
Heuristics based on Independent Sets
Minimum ID approach (Gerla & al.)
Maximum degree (Ephremides & al.)Heuristics based on Dominating Sets
The concept of spine
Minimum connected dominating set
Maximum Independent Set(MIS)
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(MIS)
A subset Vof the vertices V of a graphG=(V,E) is independent when for eachu,v V the edge {u,v} E
MIS is an Optimization Problem
Input: A Graph G=(V,E) with n vertices
Output: A subset V of V that isindependent and has maximum size
MIS: Hardness
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MIS: Hardness
No known algorithm computer a MIS inpolynomial time
Need for approximate solutions
And approximation algorithm is an algorithmthat produces a solution that is not optimal,but that approximates it
We sacrifice optimality in favor of a goodsolution that can be computed efficiently
MIS is HARD to Approximate
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MIS is HARD to Approximate
Bad news
Not only MIS is computationally hard
It is also hard to approximate:Approximate solutions are not so good
They are unboundedly far from the optimum
We consider the simple greedy heuristicfor the MIS
Greedy Heuristic for MIS 1
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Greedy Heuristic for MIS, 1
Select the vertex with minimum degreeand put it in the MIS
The degree of a vertex is the number of itsneighbors Cardinality of its adjacency list
Keep going till all the vertices are either in
the MIS or COVERED by a vertices in theMIS
Greedy Heuristic for MIS 2
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Greedy Heuristic for MIS, 2
MIS(V,E,d)// d is the vector of degrees
mis =
while V dov = vertex with min degree
mis = mis U {v}
V = V {{v} U N(v)}return mis
Greedy MIS: Maximal Solution
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Greedy MIS: Maximal Solution
The greedy solution provides a maximalindependent set An independent set is maximal when, if you add a
vertex, the set is no longer independentYou cannot make an maximal independent set bigger
This solution is also a minimal dominating set A dominating set D subset of V is a set such that a
vertex v V is either in D or it has a neighbor in D
Solutions we will see are variant of thisapproach
On MDSwhat if we look at UDGgraphs?
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graphs?
Bad news: Still computationally hard
Better news: Minimum DS It is
approximable up to a constant It means that the ratio between the size of
a DS computed by MIS greedy on UDGsand the size of a MDS is < c, c a constant
This constant is 5
Greedy MIS for MDS on UDGis 5 approximable 1
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is 5-approximable, 1
Key fact: In a UDG disk (radius 1) thereare at most 5 independent nodes
Consider an Optimal solution and aGreedy solutionSince Opt is dominant, it dominatesGreedy
Assign every vertex of Greedy to onedominator in Opt (choose one if more)
Greedy MIS for MDS on UDGis 5 approximable 2
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is 5-approximable, 2
For each u in Opt consider its assignedvertices v1(u), v2(u), , vk(u) of Greedy
How big is k?Well, all vi(u) must be distant 1 from uand they also have to be independent
Greedy: at most 5 times bigger thanOpt
MIS and Dominating Sets andWireless Networks
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Wireless Networks
UDGs model ad hoc networks
IS and DS are useful for clustering ad hoc
networks Gives the network a hierarchical organization
Decreases the amount of information at each node
Enhances scalability
Helps in resource assignment
Advantages of hierarchicalorganization
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organization
routing always through the clusterhead
data aggregation at the clusterhead
easy to locally synchronize nodeswithin the cluster, using TDMA MACprotocol for intra-cluster communicationand different MAC protocols (e.g.CDMA) for inter-cluster communications
MWIS-Based Clustering
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MWIS-Based Clustering
MWIS = Maximal Weight IndependentSetClustering selection based on generic
weights (real numbers > 0)
Mobility/node related parameters
Generalizes previous IndependentSet solutions
Two Protocols
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Two Protocols
Distributed Clustering Algorithm (DCA) Quasi-mobile networks, periodical
reclustering. Allow complexity analysis, fast
and simpleDistributed and Mobility-AdaptiveClustering (DMAC) Algorithm Same rules/procedures for clustering set
up and maintenance, adaptive to nodesmobility and node/link failures
DCA: Distributed ClusteringAlgorithm 1
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Algorithm, 1
Assumptions
Knowledge of IDs and weights of one-hop
neighbors
Broadcast transmission of a packet in finitetime (a step)
Nodes do not move during clustering
DCA 2
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DCA, 2
(Only) Two messages:
CH(v): Sent by a clusterhead v
JOIN(u,t): Sent by ordinary node u when itjoins the cluster of clusterhead t
Three (simple) procedures:
Init (start up)
OnReceivingCH(v), OnReceivingJOIN(u,v)(message triggered)
DCA
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DCA
Ogni nodo conosce i suoi vicini ed il loro pesoUn nodo init se ha il peso pi grande deipesi dei suoi nodi viciniGli init node diventano clusterhead e invitanoi loro vicini a far parte del loro clusterUn nodo x aspetta di ricevere messaggi daivicini di peso maggiore prima di prendere unadecisione Se un vicino di peso maggiore lo invita a far parte
del suo cluster allora x entra a far parte del clusterdel vicino di peso maggiore che lo contatta(inviando un messaggio di Join) nodo ordinario
Altrimenti diventa clusterhead lui stesso e inviaun messaggio di CH
DCA
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DCA
Due tipi di messaggi
CH(v) usato da un nodo v per rendereconsapevoli i suoi vicini del fatto che ha
assunto il ruolo di clusterhead
JOIN(v,u) usato dal nodo v percomunicare ai suoi vicini che sar parte di
un cluster il cui clusterhead il nodo u
DCA
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DCA
Variabili Cluster(v) indica linsieme dei nodi che fanno parte
del cluster di cui clusterhead v
Clusterhead una variabile che identifica ilclusterhead del mio cluster
Ch(u) vero quando o ha mandato un messaggioCH (u==v) oppure quando ha ricevuto un
messaggio di CH dal nodo u La variabile booleana Join (u,t) vera se il nodo v
ha ricevuto un JOIN(u,v) dal nodo u
DCA-Procedure (eseguite dalnodo v)
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nodo v)
Init
Se tutti i nodi vicini hanno un peso
minore di vinvia CH(v);
Cluster(v)=Cluster(v)U{v};
Ch(v)=true;Clusterhead=v;
DCA-Procedure (eseguite dalnodo v)
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nodo v)
On receiving CH(u)
Ch(u)=true;
Se u ha un peso maggiore di me e ivicini di peso maggiore di v con pesomaggiore di u hanno tutti mandato unJoin, allora
Clusterhead=u;invia JOIN(v,Clusterhead);
DCA-Procedure (eseguite dalnodo v)
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nodo v)On receiving JOIN(u,t)
Join(u,t)=true;Se v un clusterhead allora se t==v
Cluster(v)=Cluster(v)U{u};Se ho ricevuto Join da tutti i vicini pi piccoli EXIT
Altrimenti si verifica se tutti i vicini di peso maggiore hanno presouna decisione sul ruolo.
Se questo il caso e tutti i vicini di peso maggiore hannomandato JOIN
mandiamo un CH(v);Cluster(v)=Cluster(v)U{v};Clusterhead =v;Se si ricevuto JOIN da tutti i vicini minori EXIT.
Altrimenti se uno o pi vicini di peso maggiore hanno mandato unCH
Clusterhead=il vicino di peso maggiroe con peso pigrande tra quelli che sono diventati clusterhead e mi hannoinvitato.
manda JOIN(v,Clusterhead);
EXIT;
Example
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Example
4(9)
5(8)
7(5)
6(1)
2(3)
1(6)
8(1)
3(2)
Cluster 1Cluster 2
Cluster 3
clusterhead clusterhead
clusterhead
I Step II Step III Step IV Step V Step
DCA: Provable Properties
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DCA: Provable Properties
Consider
t: V {1,2,3, , 2k}
V = set of network nodes, k = number of
clustersProposition: Each node v in V sends exactlyone message by t(v) steps
Corollary 1: DCA message complexity is n
=|V|Corollary 2: DCA terminates correctly in atmost 2k steps (
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Node/Link Failures: DMAC
DMAC is for clustering set up ANDmaintenance
Nodes can move during the clustering
Each node reacts to Reception of a message
Presence of a new link
Link failure
Same assumptions of DCA, plus knowledge ofneighbors roles (no role = ordinary role)
Quando muore un nodo ordinario se ne tiene solo tracciaQuando muore un gw bisogna riselezionare un nuovo gwQuando muore un CH un nodo che stava in quel clusterhead
vede se c un altro vicino CH a cui riaffiliarsi ed in casosi affilia a quello di peso maggioreAltrimenti se ha il peso maggiore tra i vicini restanti diventa CHLarrivo di un nuovo nodo o la comparsa di un link puoCostringere a manutenzioni del backbone (per mantenere la
proprieta che il nodo di peso maggiore CH)
DMAC: The Procedures
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DMAC: The Procedures
INIT
Link-dependent procedures:
Link_Failure New_Link
Message-triggered procedures:
OnReceivingCH(v) OnReceivingJOIN(u,t)
Joining Clusterheads: DynamicBackbone
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Backbone
A theorem from Chlamtac and Farago:
If a network is connected, and DCA is
used, then if and only if eachclusterhead is linked to all theclusterheads at most three hops away,the resulting backbone network is
connected
Example
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Example
4(9)
5(8)
7(5)
6(1)
2(3)
1(6)
8(1)
3(2)
Cluster 1Cluster 2
Cluster 3
clusterhead clusterhead
clusterhead
I Step II Step III Step IV Step V Step
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4 Backbone Formation Protocols
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4 Backbone Formation Protocols
3 representatives of major approaches Selection of independent set of nodes and
backbone construction (DCA)
Rich dominating set formation and pruning(WuLi)
Two-phase algorithm with theoreticalguarantees (WAF)
1 proposal after the performancecomparison (DCA-S)
Distributed Clustering Algorithm(DCA)
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(DCA)
Distributed and localized implementationof the greedy for independent set
Takes node status into account for node
selectionIndependent nodes are joined into aconnected backbone (connectivity is
guaranteed) via gatewaysLow degree of parallelism (dependencychains)
A DCA Backbone
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A DCA Backbone
WuLi: Wu and Li protocol
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WuLi: Wu and Li protocol
Distributed and localized protocols forforming a connected dominating set
Build a rich connected dominating setApplies localized rules for pruningunnecessary nodes/links
High degree of parallelism (alllocalized)
WuLi: Wu and Li protocol
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WuLi: Wu and Li protocol
Distributed and localized protocols forforming a connected dominating set
Build a rich connected dominating set
Applies localized rules for pruningunnecessary nodes/links
High degree of parallelism (alllocalized)
If a vertex v has two neighbors which are notin visibility range it enters the set C
WuLi: Wu and Li protocol
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u u a d p o o o
Distributed and localized protocols forforming a connected dominating set
Build a rich connected dominating set
Applies localized rules for pruningunnecessary nodes/links
High degree of parallelism (alllocalized)What ia needed is, from the neighbors, whetherthey are in C and their list of neighbors.
WuLi-dominance propertyda On Calculating Connected Dominating Set for Efficient Routing in Ad Hoc Wireless Networks, Wu and Li
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Assume V is the set of vertices that are marked T in V, i.e., the set of vertices whichinitially enter the CDS since they have at least two neighbors which are notneighbors of each other. The reduced graph G is the subgraph of G induced by V.The following two theorems show that G is a dominating set of G and it isconnected.
THEOREM 1: Given a G = (V, E) that is connected, but not completely connected,the vertex subset V, derived from the marking process, forms a dominating set of G.PROOF: Randomly select a vertex v in G. We show that v is either in V (a set ofvertices in V that are marked T) or adjacent to a vertex in V . Assume v is marked F,if there is at least one neighbor marked T, the theorem is proved. When all itsneighbors are marked F, we consider the following two cases: (1) All the othervertices in G are neighbors of v. Based on the marking process and the fact thatm(v)=F, all these neighbors must be pairwise connected, i.e., G is completelyconnected. This contradicts to the assumption that G is not completely connected.(2) There is at least one vertex u in G that is not adjacent to vertex v. Construct ashortest path, {v,vi,v2, . . . . u}, between vertices v and u. Such a path always existssince G is a connected graph. Note that v2 is u when v and u are 2-distance apart inG. Also, v and v2 are not directly connected; otherwise, {v, v2,. .. u} is a shorterpath between v and u. Based on the marking process, vertex vi, with both v and v2as its neighbors, must be marked T. Again this contradicts the assumption that vsneighbors are all marked F. CVD
WuLi (first step produces a CDS)
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( p p )
THEOREM 2: The reduced graph G = G -V is aconnected graph.PROOF: We prove this theorem by contradiction.Assume G is disconnected and v and u are twodisconnected vertices in G. Assume dis
G(v,u) =k+1 >
1 and{v,v1,v2,...,vk,u} is a shortest path betweenvertices v and u in G. Clearly, all v1,v2, . . ..vk aredistinct and among them there is at least one vi suchthat m(vi) = F (otherwise, v and u are connected inG). On the other hand, the two adjacent vertices of
vi, vi-1 and vi+i, are not connected in G (otherwise,{v, ..vi, vi+1, . . . . vk,u} would be a shorter path).Therefore, m(vi) =T based on the marking process.
WuLi: Wu and Li protocol
Rule 1: for each pair of nodes u and v in Cthe one with the smallest ID, say v, can be removed
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p
Distributed and localized protocols forforming a connected dominating set
Build a rich connected dominating set
Applies localized rules for pruningunnecessary nodes/links
High degree of parallelism (alllocalized)
from C if v and all its neighbors are covered by u
Rule 2: Assume nodes u,v, and w are in C andassume that vs ID is the smallest. If u and w areneighors of v and are in each other transmissionrange and if each neighbor of v is covered by uand w, then v can be removed from C.
Mantengono la propriet di connessione e di dominanza
WuLi: Wu and Li protocol
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p
Distributed and localized protocols forforming a connected dominating set
Build a rich connected dominating set
Applies localized rules for pruningunnecessary nodes/links
High degree of parallelism (alllocalized)What ia needed is, from the neighbors, whetherthey are in C and their list of neighbors.
A WuLi Backbone
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WAF: Wan, Alzoubi andFrieder
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Two phases Leader election: One node is chosen
among all network nodes to be the root ofa tree
Nodes at different levels of the trees canbe chosen to form a connected dominatingset
The leader election tree is quite
expensiveVery low degree of parallelism
A WAF Backbone
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DCA-S: DCA Sparsified
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Build a connected dominating set (say, withDCA) and consider its spanned sub-graph H(include gateways)Erds: If a graph does not have small cycles
then it is sparseFind and break small cycles (small=log n) In practice we search and break cycles with 3 and
4 links
Breaking cycles does not compromiseconnectivity
Simulation Results
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Metrics (all averages)
1. Protocol duration
2. Operation overhead (in bytes)
3. Energy consumption (per node)
4. Backbone size
5. Route length
6. Backbone robustness (node deaths fordisconnections)
Simulation Results, 2
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Parameters of ns2-based simulations
Nodes: 300, IST EYES prototype Tx range: 30m
Initial (residual) energy: 1J
Tx, Rx, idle power: 24, 14.4, 0.015 (mW)
Area: 200 x 200m
Six scenarios with increasing densities(avg. degrees: 3.5 to 20)
Protocol Duration
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WuLi is fastest Simple operation; parallelism
DCA: Reasonably fast
Possible dependencies and gateway selectionDCA-S: As DCA The sparsification phase is executed by fewer
nodes and requires little info exchange
WAF: Slower Non-trivial leader election
Protocol Duration, 2
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Protocol Overhead
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Protocol Overhead, 2
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Average number of protocol bytes pernode
WuLi: Best performing Simple list exchangeDCA(-S): Almost twice as much Bit more info needed (weight, IDs, )
WAF Leader election complexity
Energy Consumption
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Important metric per backbone set upand maintenance
Similar to overhead results
WuLi and DCA perform quite well
DCA-S performs similarly: No difference
in breaking cycles with 3 or 4 linksWAF: High consumption due to firstphase
Energy Consumption, 2
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Backbone Size
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Important metric: Aggregation andawake/asleep cycles
Small backbone + role rotation: key for
WSNs
Decrease with n increasing (biggerclusters)
WAF: Slimmer backbone (tree like)
DCA-S, 4 < DCA-S, 3 < DCA < WuLi
Backbone Size, 2
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Backbone Robustness
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Number of nodes needed to disconnect thebackbone
Useful for planning backbone re-orgs
Increases with network densityWuLi and DCA: More robust Resilient to up to 25 death when n = 300
WAF: Quite a disaster (tree-like topologies)DCA-S: In the middle
Backbone Robustness, 2
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Route Length
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Flat topology (visibility graph) as abase
Expected increase: Hierarchy routes arelonger
DCA & WuLi: 7 to 34.7% longer routes
DCA-S: Up to 9% more than DCAWAF: Up to 33.4% longer than DCA
Route Length, 2
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To Go
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Hierarchical organization is effective forprolonging network lifetime
Four protocols for backbone formation: DCA, WuLi, WAF and DCA-S
Nice theoretical features hard to implement
Simple solutions (WuLi, DCA): Good startingpoint for efficient implementations
DCA-S: Slimmer backbone at a reasonable cost