Wireless Sensor Networks

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Università di Bologna. Networks. IEEE 802.15.4 / Zigbee Protocol Stack. Application Objects. End Manufacturer. Application Layer. Network Layer. Zigbee.
Networks

Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks Chiara Buratti [email protected] +39 051 20 93549 Office Hours: Tuesday 3 – 5 pm @ “CSITE” Building within main campus, first floor

Credits: 6

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Wireless Sensor Networks

Syllabus 1. NET Protocols 2. Localization 3 Tyme Synchronization 3. 4. Data Aggregation 5. Case Studies

WiLab @ DEIS, Università di Bologna

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Protocol Stack

Time Synchronization

Application Layer

Energy Management

Network Layer

Localization

MAC Layer

PHY Layer

Data aggregation ti

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Wireless Sensor Networks

Outline 1. Zigbee Tree-based Topology 2. Zigbee Mesh Topology 3 Other protocols 3.

WiLab @ DEIS, Università di Bologna

Networks

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

IEEE 802.15.4 / Zigbee Protocol Stack

Application Objects

End Manufacturer

Application Layer Zigbee Alliance

Network Layer

MAC L Layer

Physical Layer

IEEE 802.15.4

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Wireless Sensor Networks

IEEE 802.15.4 / Zigbee Protocol Stack

WiLab @ DEIS, Università di Bologna

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Why Zigbee?

• Open Standard • Radio + Protocol • Mesh Networking



Up to 65536 network nodes



Full Mesh Networking Support



Multiple channels in the global 2.4 GHz band

Network coordinator ZigBee Router ZigBee End Device Communications flow



250 Kbps data rate

Virtual links

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Network Topologies Star

Zigbee Coordinator ZigBee Router ZigBee End Device Communications flow Virtual links

Tree

Mesh

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Comparing Topologies



Star Topologies ƒ ƒ ƒ

Suitable for small networks Adv: Simplicity Disadv: No scalability

Zigbee Coordinator ZigBee g Router ZigBee End Device Communications flow Virtual links

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Comparing Topologies



Tree-based Topologies ƒ ƒ ƒ ƒ

Suitable for large networks with a small set of destination nodes Only one path between couples of nodes Adv: Minimal overhead (simple routing) Disadv: No rubustness to link failures Level 0 Zigbee Coordinator

Level 1

ZigBee g Router ZigBee End Device Communications flow Virtual links

Level 2

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Comparing Topologies



Mesh Topologies ƒ ƒ ƒ ƒ

Suitable for large networks with a large set of destination nodes Multiple paths between couples of nodes Adv: Robustness to link failures Disadv: Need of routing tables, complex forwarding strategies

Zigbee Coordinator ZigBee g Router ZigBee End Device Communications flow Virtual links

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Wireless Sensor Networks

Outline 1. Zigbee Tree-based Topology tree-based based topology • The Zigbee tree • Designing a three-level tree • Comparing star and tree-based topologies 2. Zigbee Mesh Topology 3. Other protocols

WiLab @ DEIS, Università di Bologna

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Wireless Sensor Networks

Outline 1. Zigbee Tree-based Topology tree-based based topology • The Zigbee tree • Designing a three-level tree • Comparing star and tree-based topologies 2. Zigbee Mesh Topology 3. Other protocols

WiLab @ DEIS, Università di Bologna

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Tree-based Topologies • Nodes work in Beacon-Enabled mode • The Router providing the strongest signal power is selected as parent

Level 0

Level 1

Zigbee Coordinator ZigBee g Router ZigBee End Device

Level 2

Communications flow Virtual links

Wireless Sensor Networks

Networks

Tree Formation Superframe Duration Baecon Interval

Parent

Beacon tracking

Child

Beacon Tx offset

WiLab @ DEIS, Università di Bologna

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

The Access to the Channel / 1

Router 1 transm

B

PAN Coordinator Router 1

B

Level 1 nodes

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

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The Access to the Channel / 2 Inactive part of PAN Coordinator superframe Inactive part of Router 1 superframe

SD=960*T SD 960 Ts*2 2SO BI=960*Ts*2BO

Inactive part of all the superframes Beacon PAN C Coordinator

Beacon Route B er 1 Level 1

Be eacon PAN C Coordinator

Tx level 2 nodes toward Router 2 Beacon Route er 2 Level 1

Tx level 2 nodes toward Router 1

Tx level 1 nodes

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

The Access to the Channel / 3

PAN Coordinator

Level 1 nodes

Number or Routers having at least one child

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Wireless Sensor Networks

Outline 1. Zigbee Tree-based Topology tree-based based topology • The Zigbee tree • Designing a three-level tree • Comparing star and tree-based topologies 2. Zigbee Mesh Topology 3. Other protocols

WiLab @ DEIS, Università di Bologna

Networks

Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Three-Level Tree

Once SO and BO are set PAN Coordinator

A Router will have a part of the superfarme allocated, with probability: Level 1 nodes (N1) Level 2 nodes (N2)

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Performance Evaluation

• Nodes generate one packet per Beacon Interval • No connectivity problems are present Æ pCON=1 in each link (ps=pMAC) • Query-Based Q B d application li ti Æ nodes d iin a given i llevell start t t th the CSMA/CA protocol t l att the same time, that is when they receive the beacon from their parent • No retransmissions are considered

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Success Probability

Success from level 1 to the Coordinator

PAN C di t Coordinator

& Success from level 2 to level 1 nodes

Level 1 nodes (N1)

Level 2 nodes (N2)

N = N1 + N2 Æ Number of nodes in the network L = 3 Æ number of levels in the tree

Wireless Sensor Networks

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WiLab @ DEIS, Università di Bologna

Success Probability

p s tree

1 = N

p s tree =

N

∑p i =1

si

Success probability for the i-th node

L −1

∑p k =1

k

p sk

Success probability for a node being at level k

Probability of being at level k

p s tree = p1 ⋅ p s ( N 1 ) + p 2 ⋅ p frame 1 ⋅ p s ( N 1 ) ⋅ p s 2 N1 N1 + N 2

N2 N1 + N 2

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Optimising N1 BO=5

N=20 N=50

SO=0 Æ 31 routers has a part of superframe allocated SO=1 Æ 15 routers has a part of superfarme allocated

SO=1

N 12opt + N 1opt ≅ N SO=0 SO 0

N 1opt ≅

−1+ 1+ 4N 2

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Wireless Sensor Networks

Comparing Tree and Star: ps

Tree

Star

SO, BO

WiLab @ DEIS, Università di Bologna

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Average Delay

1 superframe is needed for packets coming from Level 1 nodes

PAN C di t Coordinator

2 superframes are needed for packets coming from Level 2 nodes

N = N1 + N2 Æ Number of nodes in the network L = 3 Æ number of levels in the tree

Level 1 nodes (N1)

Level 2 nodes (N2)

Wireless Sensor Networks

Networks

WiLab @ DEIS, Università di Bologna

Average Delay

D mean tree

1 = N

D mean tree =

N

∑D i =1

Average delay for the i-th node

mean i

L −1

∑p k =1

k

D mean k

Average delay for a node being at level k

Probability of being at level k

D mean tree = p1 ⋅ D mean 1 ( N 1 ) + p 2 ⋅ D mean 2 D mean 1 = T ⋅

SD / T −1



j =0

P{Z j ( N 1 )} ( j − 1) ps ( N1 )

D mean 2 = D mean 1 ( N 1 ) + BI

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Wireless Sensor Networks

Comparing Tree and Star: Dmean

SO, BO Tree

Star

WiLab @ DEIS, Università di Bologna

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Summing-Up

• In terms of success probability tree-based topologies performs better than star topologies. topologies • In terms of delays star topologies performs better that tree topologies. • Trees allows to create networks distributed over larger area and/or having a larger number of nodes

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

To fix the ideas... Consider the thee shown in the Figure. Assume that: 1. No connectivity problems are present (pCON=1 in each link); 2. SO=0; 2 Th 2. The average S Success P Probability b bilit related l t d tto MAC MAC, ps(N), (N) b being i N th the number b off nodes competing for the channel , when SO=0, is equal to: ps(N=1)=1; ps(N=2)=0.9; ps(N=3)=0.8; ps(N=4)=0.7; Evaluate: 1. The value of BO s.t. all routers have a portion of superframe allocated? 2 The average (for whatever a node) Success Probability 2. Probability, pstree (use the value of BO obtained in point 1). 1. Decrease BO by 1 and evaluate pstree

ZED ZR

ZC

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

To fix the ideas... Consider the tree shown in the Figure. Assume that: 1. SO=0; 2. The Average Delay, Dmean(N), in a single hop, when N nodes are competing for th channel the h l and db being i SO SO=0, 0 iis equall tto: Dmean (N=1)=5 ms; Dmean(N=2)=10 ms; Dmean(N=3)=15 ms; Dmean(N=4)=20 ms; Question: Q estion 1. Compute the Average Delay in the tree, Dmean_tree, when BO=2. ZC

ZED ZR

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Wireless Sensor Networks

Outline 1. Zigbee Tree-based Topology 2. Zigbee Mesh Topology 3 Other protocols 3.

WiLab @ DEIS, Università di Bologna

Networks

Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Mesh Topologies • Nodes work in non Beacon-Enabled mode • Need of defining a Routing Protocol

Zigbee Coordinator ZigBee g Router ZigBee End Device Communications flow Virtual links

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Zigbee mesh topology • Based on AODV • The path, P, minimising the total path cost, C(P), is selected C(P1)=3

⎧⎪ ⎢ 1 ⎥ ⎫⎪ C (l ) = min ⎨7 , ⎢ 4 ⎥ ⎬ ⎪⎩ ⎣ p s ⎦ ⎪⎭ G Generally, ps=pCON L −1

C ( P ) = ∑ C (li ) i =1

2

1

1

1 2 C(P2)=4

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Wireless Sensor Networks

Zigbee mesh topology formation

WiLab @ DEIS, Università di Bologna

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Zigbee mesh

Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Performance Evaluation

• Nodes generate a packet every Tq • Query-Based (QB) and non QB applications are considered: • QB Æ nodes start the CSMA/CA protocol at the same time, that is when they receive i th the query • Non QB Æ nodes generate the packet in an instant that is uniformed and randomly distributed in Tq • 3 retransmissions are allowed

Node 1 Node 2

Node 1

Node 2

t Tq Tq

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Zigbee mesh topology: Scenario 1

Coord

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Wireless Sensor Networks

Simulation Results: Packet Error Rate No QB application

WiLab @ DEIS, Università di Bologna

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Zigbee mesh topology: Scenario 2

ZC

ZED0

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WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Data Transfer: From ZC to ZED0 • Sleeping ZED0 Æ ZED0 polls ZR every 200 ms • Data from ZC to ZED0 Æ 18 bytes; • Data from ZED0 to ZC Æ 45 bytes; • Ack=11 bytes. (3) Data Request (1) Data

ZR0

ZC (2) Ack

(4) Ack (5) Data (6) Ack

ZED0

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Experimental measurement Results: Average Delay

From ZED0 to ZC

From ZC to ZED0

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Wireless Sensor Networks

Experimental measurement Results

Data from ZC to the ZED0

Data from ZED0 to ZC

No QB application

QB application

WiLab @ DEIS, Università di Bologna

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Summing-Up

• Mesh topologies are more flexible and robust to link failures • The average delay is approx. 5 ms per hop • Non QB applications (generating asynchronous traffic) perform better than QB applications in terms of packet error rate and delays.

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Networks

To fix the ideas.. Consider the set of nodes and virtual links (with the relative costs) shown in the figure. • Evaluate the set of paths used by all nodes to reach the ZC, in the case of Zigbee mesh routing protocol and in case of Zigbee tree. B

C

1

1

2 2

A

3

ZC 4

1

ZED ZR

D

Wireless Sensor Networks

Networks

WiLab @ DEIS, Università di Bologna

To fix the ideas.. In the case of tree, set SO=0 and BO to the minimum value s.t. pframe=1. The Success Probability related to MAC MAC, pMAC(N), (N) when N the number of nodes competing for the channel, in the case of non beacon-enabled (BE) mode and in the case of BE mode (when SO=0), is equal to: pMAC(N=1)=1;

pMAC(N=2)=0.9;

pMAC(N=3)=0.8; pMAC(N=4)=0.7;

The Average g Delay, y, Dmean((N), ), in a single g hop, p, when N the number of nodes competing for the channel, in the case of non BE mode and in the case of BE mode (when SO=0), is equal to: Dmean (N=1)=5 ms; Dmean(N=2)=10 ms; Dmean(N=3)=15 ms; Dmean(N=4)=20 ms; 1) Which is the topology maximising the average Success Probability? 2) Which is the topology mininising the average Delay?

Networks

Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

To solve the problem.. Recall that: 1 1.

⎧⎪ ⎢ 1 ⎥ ⎫⎪ C ((ll ) = min ⎨ 7 , ⎢ 4 ⎥⎬ p ⎪⎩ ⎣ CON ⎦ ⎪⎭

Therefore if the cost is lower than 7 Æ

p CON =

1 4 C

2. The total success p probability y in a g given link is the p product of the p probability y of having connectivity on the link (pCON) and the probability to have success in the access to the channel (pMAC).

p s = p CON (C ) ⋅ p MAC ( N )

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Networks

Mesh – Success Probability B

C

1

p CON (1) = 1

1

2 2

A

3

ZC 4

1

ZED

D Step 1: 4 nodes accessing the channel. AÆB, BÆC, CÆZC, DÆZC Step 2: 2 nodes accessing the channel. BÆC (packet of node A) C ÆZC (p (packet of node B)) Step 3: 1 node accessing the channel. CÆ ZC (packet of node A)

ZR

p CON ( 2 ) = 0 .84 p CON ( 4 ) = 0 .7

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Networks

Mesh – Success Probability B

C

1

p CON (1) = 1

1

2 2

A

3

ZC 4

1

ZED

D

ZR

p s A = p CON A ⋅ p MAC A = 0 .52 p CON A = p CON ( 2 ) ⋅ p CON (1) ⋅ p CON (1) = 0 .84 p MAC A = p MAC ( 4 ) ⋅ p MAC ( 2 ) ⋅ p MAC (1) = 0 .63

p CON ( 2 ) = 0 .84 p CON ( 4 ) = 0 .7

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Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Mesh – Success Probability

p s B = p CON B ⋅ p MAC B = 0 .63 p CON B = p CON (1) ⋅ p CON (1) = 1 p MAC B = p MAC ( 4 ) ⋅ p MAC ( 2 ) = 0 .63 p sC = p CON C ⋅ p MAC C = 0 .7

p s D = p CON D ⋅ p MAC D = 0 .49

p CON C = p CON (1) = 1

p CON D = p CON ( 4 ) = 0 .7

p MAC C = p MAC ( 4 ) = 0 .7

p MAC D = p MAC ( 4 ) = 0 .7

1 p s = ( p s A + p s B + p sC + p s D ) = 0 .58 4

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Networks

Tree – Success Probability B

1

C 1 ZED

ZC

A 4

1 D

p s A = p CON A ⋅ p MAC A = 0 .63 p CON A = p CON (1) ⋅ p CON ( 4 ) = 0 .7 p MAC A = p MAC (1) ⋅ p MAC ( 2 ) = 0 .9

ZR

Networks

Wireless Sensor Networks

WiLab @ DEIS, Università di Bologna

Tree – Success Probability

p s B = p CON B ⋅ p MAC B = 0 .9 p CON B = p CON (1) ⋅ p CON (1) = 1 p MAC B = p MAC (1) ⋅ p MAC ( 2 ) = 0 .9 p sC = p CON C ⋅ p MAC C = 0 .9 p CON C = p CON (1) = 1 p MAC C = p MAC ( 2 ) = 0 .9

p s D = p CON D ⋅ p MAC D = 0 .63 p CON D = p CON ( 4 ) = 0 .7 p MAC D = p MAC ( 2 ) = 0 .9

1 p s = ( p s A + p s B + p sC + p s D ) = 0 .76 4

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Networks

Mesh – Average Delay B

C

1

ZED

1

2 2

A

3

ZR

ZC 4

1 D

D mean A = D mean ( 4 ) + D mean ( 2 ) + D mean (1) = 35 ms D mean B = D mean ( 2 ) + D mean (1) = 30 ms D mean C = D mean D = D mean ( 4 ) = 20 ms D mean

1 = ( D mean A + D mean B + D mean C + D mean C ) = 26 .25 ms 4

WiLab @ DEIS, Università di Bologna

Wireless Sensor Networks

Networks

Tree – Average Delay B

1

C ZED ZR

1 ZC

A

SO=0 BO=2 (s.t. (s t pframe=1) BI=61.44 ms

4

1 D

D mean A = D mean B = D mean ( 2 ) + BI = 71 .44 ms D mean C = D mean D = D mean ( 2 ) = 10 ms

D mean

1 = ( D mean A + D mean B + D mean C + D mean C ) = 40 .72 ms 4

Networks

Wireless Sensor Networks

Outline 1. Zigbee Tree-based Topology 2. Zigbee Mesh Topology 3 Other protocols 3.

WiLab @ DEIS, Università di Bologna

Networks

Wireless Sensor Networks

Routing Protocols 1. Flat routing 2. Hierarchical routing

WiLab @ DEIS, Università di Bologna

Networks

Wireless Sensor Networks

Routing Protocols 1. Flat routing 2. Hierarchical routing

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Wireless Sensor Networks

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Flat Routing Protocols In these protocols all sensors play the same role. No effort is made to organize the network or its traffic, only to discover the best route hop by hop to a destination by a any y pa path.

Main Advantage: g no need of organizing g g the network Main Disadvantage: redundant transmission of data

Examples: - Flooding - Gossiping

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Flooding Protocol

Each sensor receiving a data broadcasts it to all its neighbours

Wireless Sensor Networks

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Wireless Sensor Networks

Gossiping Protocol

Each sensor receiving g a data transmit it to one neighbour randomly selected

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Wireless Sensor Networks

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Hierarchical Routing Protocols In these protocols higher energy equipped nodes can be used to process and send the information while low-energy nodes can concentrate in monitoring the environment. e o e C Creation ea o o of cclusters us e s o of nodes. odes Not o a all nodes odes will have a e the e sa same e role o e in the network. Main Advantage: g no redundant transmission of data Main Disadvantage: organization of the network required (e.g., cluster formation)

Example: - LEACH

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Wireless Sensor Networks

Routing Protocols 1. Flat routing 2. Hierarchical routing

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Wireless Sensor Networks

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Leach (Low-Energy adaptive Clustering Hierarchy)

Setup phase: creation of clusters (CH election)) Steady state phase: cluster nodes transmit data to their CHs. CHs collect data, aggregate them and and send data to the sink

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Wireless Sensor Networks

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Leach (Low-Energy adaptive Clustering Hierarchy) Randomized rotation of CH role among nodes.

At each round each node chooses a random number between 0 and 1 and elects ityself as CH if the extracted value is lower than:

P = optimal percentage of CHs r = current round G = sett off nodes d that th t have h been b CHs CH iin th the llastt 1/P rounds d

Each node will become CH within 1/P rounds