an energy-efficient and low-latency routing for wireless sensor networks

UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP

An Energy-Efficient and Low-Latency Routing for Wireless

Sensor NetworksAntonio G. Ruzzelli, Richard Tynan,

G.M.P. O’Hare.

Adaptive Information Cluster project (AIC) Smart Media Institute (SMI)

Department of Computer ScienceUniversity College Dublin

Ireland.http://www.adaptiveinformation.ie/home.asp


Overview of Wireless Sensor Networks

• Large number of tiny sensors (nodes) distributed in an area network;

• Sensor nodes:– have sensing devices attached;– are self-organizing;– are usually battery operated and of low cost

hence power limited • multi-hop communication to save energy;


Objectives

Experimental evaluation of two essential scheduling regimes within the MERLIN* protocol to be injected dynamically in the

network.

↓An increase of network adaptivity to save energy

by choosing the appropriate scheduling with respect to the application requirements.

* Mac Energy-efficient, Routing and Localization Integrated, (see reference in the paper)


• Separated MAC and Routing layers for WSNs cause an extremely high latency (e.g. SMAC and DSR tens of seconds delay for packets of nodes in hop 10 or more)

• MERLIN integrates MAC+Routing features into the same simple architecture;

• No usage of handshake mechanisms;• Latency is considerably reduced while

ensuring a very low energy consumption

Motivation for MERLIN?


What is the main IDEA behind the MERLIN protocol?

Gateway

Node

Why Time Zones?

Nodes with the same color are in the same time zone

Nodes within the same subset belong to the same gateway

---------------------------------

Nodes within the same zone wake up and go inot sleep simultaneously

(European EYES project, NL)


Data traffic• Subnet flooding by gateway: Gateway msgs are

forwarded to all nodes in the subnet• 2. Local broadcast by node : Nodes send msgs to

all of the direct neighbours. No forwarding is performed.

• 3. To gateway Transmission by node : Nodes closer to the gateway forward msgs until it reaches the gateway.


Transmission Mechanism (I)

1. Nodes in the same zone share the medium;

2. Nodes in the same zone use a contention period (CP) for CSMA;

3. Nodes in the neighbouring zones are listening;

4. Nodes 2 hops away are sleeping;

5. A collision report period (CR) is provided at the end of the slot.

6. The CR message is a short burst tone.

Zone 1 Zone 2Zone 3

Zone 4Zone 5

A

B

DATACP

A B

S L O T

CR

Note: MERLIN, not addressing the receiving neighbouring node, can cause multiple copy of the

same msg sent increase overhead


Transmission Mechanism (II)• Packets are organised in multiple msgs of the same data traffic

type;• Packets start with an index containing the ID of included msgs;• Nodes, which lose the contention, keep on listening to the

beginning of the transmitted packet then go into sleep;• Nodes discard from their queue the msgs already fowarded.

Pro : Reduce overhead in transmission!

Con : Small increase of node activity;

Increase complexity.

Channel contention

messagesMsg-index

Discard msgs already forwarded from their queue

P a c k e t

Listen to the packet index


Transmission Mechanism (III)Mitigating the Hidden terminal problem (HTP) [WiseMac]:•The CSMA can be Adjusted by the SNR;

•Interference range of a signal can be set greater than the range of correct signal reception;

•MERLIN sets a certain received power threshold for a correct reception of a packet;

• E.g. A threshold of 15 dB SNR for a correct reception results in a maximum of 50m transmitting range and over 100m interference range.

Zone N

Zone N+1

A B

A B

CP

C

Con: throughput reduction

Transmitting range

Interfering range


Scheduling tables: V-schedule vs. X- Schedule

gate

Zone 1

Zone 6

Zone 2

Zone 3

Zone 4

Zone 5

Zone 7

Zone 8

Zone 9

Time

Sp

ace

gate

Zone 1

Zone 6

Zone 2

Zone 3

Zone 4

Zone 5

Zone 7

Zone 8

Zone 9

Slot

F R A M E

CSMA / Transmit / Listen

Receive

Idle

•Frame is divided in 8 slots;

•Nodes in the same zone transmit simultaneously

•The X scheduling is obtained by super positioning 2 V-sched one of which upside-down

•Nodes go into sleep immediately after the transmission


Simulation and resultNodes with the same colors are in the same zone (same hop Count Number).

Number slot /frame = 4

Contention period = 30ms

DataRate = 115200 bits/sec

DataSize = 16+8 Bytes (data + 3 bytes preamble + starting code)

Parameters Values

Energy Transmitting 21 mW

Energy Receiving 14.4 mW

Energy stand-by 15 µW

Switch time Tx/Rx 518 µs

Switch time Rx/Tx 12 µs

Switch stanby/Rx 518 µs

Switch stand-by/Tx 15 µs

Eyes node


X-scheduling vs V-scheduling

0

50

100

150

200

250

300

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Frametime (sec)

Network Lifetime (days)

X Scheduling

V-Scheduling

1 Gateway 100 Nodes rand. Distributed.800*500 area network Min signal strength(12 m)50 msg/min sent by 5 rand. nodes Static network

V and X scheduling Network lifetime.

•The network is considered to fail when 30% of nodes are depleted.•Lifetime calculated for a linear depletion of 2 AA batteries.

The network lifetime depends linearly on the frame length;

•The usage of V-sched results in a longer network lifetime than X-sched;


X and V scheduling setup time

X-Scheduling Network Setup

0

1

2

3

4

5

0 50 100 150 200 250 300

Node Density(nodes/100m^2)

Tim

e(s

ec

)

V-Scheduling Network Setup

0

2

4

6

8

10

0 50 100 150 200 250

Node Density (nodes/100 m^2)

Tim

e (

se

c)

•V-sched shows double network setup time with respect to X-sched;

•X and V scheduling can be setup in less than 10 seconds for 250 nodes network density.


Node density = 125 nodes/100m^2

0.761.02 1.01 1.07

2.76 2.772.52 2.54

3.76 3.71 3.77

0

1

2

3

4

1 2 3 4 5 6 7 8 9 10 11

Node hop count

Late

ncy

(se

c)


0.771.01

0.77

1.27

2.162.44 2.27 2.27

3.17 3.16 3.17

3.77

4.263.88

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Node hop count

Late

nc

y (

se

c)

End-to-end packet delayX-scheduling


2.312.87

2.52

5.125.52 5.45

4.12

6.315.52

7.51 7.52

0

1

2

3

4

5

6

7

8

1 2 3 4 5 6 7 8 9 10 11Node hop count

Lat

en

cy (

sec)


1.61 1.52 1.51

4.013.52

4.52 4.18

6.18 6.386.85 6.52

7.518.02

8.52

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Node hop count

La

ten

cy

(s

ec

)

V-scheduling• The controlled multiple path

mechanism may cause a lower delay for nodes farther from the gateway than other;

• A periodic and discontinuous increase of latency at the intersection of data traffic flows due to:

• X-sched: Packet Collisions hence retransmission;

• V-sched: Periodical stop of nodes activity that go into sleep.


Average end-to-end packet delay

0

1

2

3

4

5

6

7

1 2 3 4 5 6 7 8 9

Hop count number

Ave

rag

e la

ten

cy

X-scheduling V-scheduling

•X-sched presents a more accentuated linear behaviour than the V-sched;

•V-sched shows a considerable increase of packet delay than X-sched;

• X-sched has a greater throughput than V-sched.

Which scheduling to adopt should be based on both the application requirements and

network conditions.


Conclusion and future work•The absence of handshake mechanisms like RTS/CTS by means of MERLIN can considerably reduce the end-to-end packet delay;

•The X scheduling should be used for applications in which some energy can be traded off for a decrease of latency of messages and for applications in which latency is a tighter constraint;•V-scheduling is more suitable for low data traffic applications where the need of saving energy is of paramount importance.

•Future work: As a result of the same nature of X and V scheduling together with a fast setup time, MERLIN is suitable for a dynamic scheduling switching in accordance with change of network conditions, e.g. by means of migrating agents to be injected.


Thank you for your kind attention


Implicit Multiple path Performing

1

23

3

4

4 5

56

6

7

6

• Forwarding “Transmission to Gateway” results in multiple copies of the same msg;

•Nodes can detect copies of arriving msgs by combination of Source ID and message ID contained in the msg;

•Messages arriving at gateway follow multiple paths.

•Pro : Greater reliabilty!

•Con : Increase overhead!

7

9

8

an energy-efficient and low-latency routing for wireless sensor networks

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