an energy-efficient and low-latency routing for wireless sensor networks
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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
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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;
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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)
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
• 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?
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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)
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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.
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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;
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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.
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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)
Node density = 275 nodes/100m^2
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
Node density = 125 nodes/100m^2
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)
Node density = 275 nodes/100m^2
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.
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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.
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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.
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
Thank you for your kind attention
UNIVERSITY COLLEGE DUBLIN DUBLIN CITY UNIVERSITYSMI || NCSR || CDVP
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
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