MERLIN: A synergetic Integration of MAC and
Routing for Distributed Sensor Networks
A.G.Ruzzelli, M.J.O’Grady, R.Tynan,
G.M.P.O’Hare.
Adaptive Information Cluster project (AIC)
Smart Media Institute (SMI)
Department of Computer Science
University College Dublin
Ireland.
http://www.adaptiveinformation.ie/home.asp

• Overview of WNSs and protocols
• Phase1: MERLIN design
– Motivation and objectives
– Fundamental concept
– MAC details
– Routing details
• Phase2: Simulation and results
– Scheduling performance
– Comparison against SMAC+ESR
Conclusion
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

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;
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

•Nodes must be cheap  Limited memory capabilities
 Limited processing capabilities
 Limited power capabilities
 Maybe not very reliable
•Limited energy  Power consumption
•High node number  Scalability issues
•High dynamic condition  Reactivity and Self-organization
•Always on radio  node depletion in few days (e.g. Mobile phone)
MAC issues
•Simultaneous msg to same device  Packet Collision
•Channel access delay
•Control packet overhead
•Multihop routing issues
•Route maintenance Overhead
•E-to-E latency
•Global addressing issue
•Only useful messages to deliver  In-network processing
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Wireless sensor network architecture
An example:
Sensing devices
•The most suitable network architecture for WSNs is still an open issue
Application
Data aggregation
•Each layer has its own task
Sensing coverage
•Any layer try to achieve the task using the smallest amount of energy possible
Localization
Routing
•Researchers are evaluating how to use the cross layer interaction at best
MAC
Physical
Antenna
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

The CSMA/CA approach (Carrier sense multiple access with collision avoidance)
Tx
Rx
•A potential transmitter listen to the channel for a random time in a CW to sense any ongoing transmission in progress
•Channel assumed free  Transmit the packet with procedeure RTS/CTS/Data/ACK
•Channel busy  Transmission postponed then node switches off the radio
•Adv Flexible
•Dis High latency and idle listening
Random
Time
Listen
RTS
Preamble+Data
Listen
CTS ACK
Sleep
Sleep
TX node2
Contention node3
The TDMA approach (Time division multiple access)
•Time is divided into slots that are (in some way) assigned to neighbouring nodes
•ADV: collision free and energy efficient
•DIS: Low flexibility
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Motivation for MERLIN
• Due to low duty cycle of WSNs, separate MAC and Routing layers cause an extremely high latency
– (e.g. SMAC and DSR  tens of seconds delay for packets of nodes in hop 10 or more)
• Layer modularisation requires higher memory capability
 Layer integration is beneficial
• Nodes are cheap and not reliable
– failure, interference, depletion, mobility  Addressing a single node can result in high error probability
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Objectives of MERLIN
•MAC+Routing integration features into a simple architecture;
•No usage of handshake mechanisms;
•No specific node addressing;
•Reduce latency while ensuring a very low energy consumption
•Controlled packet duplication to address sensor failure and bad channel condition;
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

What is the main IDEA behind the MERLIN protocol?
(European EYES project, NL)
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 into sleep simultaneously
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

SYNC packets from the gateway are forwarded to further nodes.
Every node sets its zone and forward the packet to more distant nodes.
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Nodes in the same timezone contend the slot for local broadcast only once each 4 frametimes
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

•
•
•
Downstream multicast : Packets are transmitted to higher zones
Local broadcast : Packets reach all neighbours. No forwarding is performed
Upstream multicast : Packets are forwarded to smaller zones
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

The minimum wakeup concept through CCA
• Alternation of long period of inactivity to tiny period of channel assessment;
• The Clear Channel Assessment CCA is the shortest time period needed for nodes to sense any activity on the channel (~2.5msec in BMAC)
• Much shorter CCA period than time required for a control packet
(e.g. 35msec for 5byte transmission with Tr1001)
• duty cycle reduced to less than 1%
Ts Sleep period
CCA
Time
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

• How can a Tx know when the Rx is awake?
• If not addressing a specific node (in multicast and broadcast), how can correct/incorrect receptions be notified?
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

• Properties
– Are signal impulse  Do not contain any coded information
– Are robusts  Several simultaneous burst can still be as one
– They are shorter that a normal ACK
• Utilization
Multicast: Bursts identify correct receptions Broadcast: Bursts identify reception errors
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Tx1
Tx2
Tx1
Tx2
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

1)
MERLIN does not address a specific receiving node  multiple copy of the same msg sent can be generated 
 increase overhead!
2)
Some collision due to the
Hidden Terminal Problem (HTP)
Zone 1
Zone 2
Zone 3
Zone 4
Zone 5
A
B
B
?
Zone 3
A
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Controlled multipath
• 3 small buffers of upstram, downstream and local broadcast are provided
• Packets organised in multiple msgs of the same data traffic type;
• Packets contain a msg-ID index 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 buffer the msgs already fowarded.
Channel contention
Msg-index
P a c k e t messages
Pro : Reduce overhead in transmission!
Con : Small increase of node activity;
Discard msgs already forwarded from their queue
Increase complexity.
Listen to the packet index
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Timezone maintenance
• Timezone update are sent periodically;
• Failed reception of timezone update from zone N-1 node to zone N node triggers a upstream multicast of Timezone Update request (TUR)
– N-1 node/s reply  Connection reestablished
• N-1 failed  local broadcast TUR
– At least one reply  change of zoen to N+1
• N failed  downstream broadcast TUR
1
2
2
3
4
4
3
1
2
3
TUR
4
1 2
3
4
TUR
1
6
2
3
5
4
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Phase 2: Simulation and Comparison with two existing protocol architecture:
SMAC (mac)+ ESR (routing)
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Nodes 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)
Eyes node
Parameters Values
Energy Transmitting
Energy Receiving
Energy stand-by
Switch time Tx/Rx
Switch time Rx/Tx
Switch stanby/Rx
Switch stand-by/Tx
21 mW
14.4 mW
15 µW
518 µs
12 µs
518 µs
15 µs
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

V-scheduling
300
250
200
150
100
50
0
V-Scheduling
0.4
0.6
0.8
1 1.2
Frametime (sec)
1.4
1.6
1.8
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
2
The network lifetime depends linearly on the frame length;
•The network is considered to fail when 30% of nodes are depleted.
•Lifetime calculated for a linear depletion of 2 AA batteries.
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

10
8
6
4
2
0
0
V-Scheduling Network Setup
50 100 150
Node Density (nodes/100 m^2)
200 250
V scheduling can be setup in less than 10 seconds up to 250 nodes/100m^2 of network density.
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

V-scheduling
Node density = 125 nodes/100m^2
6
5
4
3
2
1
0
8
7
2.31
2.87
2.52
5.12
5.52
5.45
4.12
6.31
5.52
1 2 3 4 5 6 7
Node hop count
8 9
7.51
7.52
10 11
Node density = 275 nodes/100m^2
9
8
7
6
5
4
3
2
1
0
1.61 1.52 1.51
4.01
3.52
4.52
4.18
6.18 6.38
6.85
6.52
7.51
8.02
8.52
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Node hop count
• The controlled multiple path mechanism may cause a lower delay for nodes farther from the gateway;
• An increase of latency at the intersection of data traffic flows due to periodical stop of nodes activity that go into sleep.
•V-scheduling delay obtained for 2sec frametime length
Frametime length should be based upon application requirements.
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

• SMAC divides time in two periods: active time and sleeping time;
• Active period = SYNC period for node sync update,
Request To Send (RTS), Clear to Send (CTS).
Transmitter
RTS
Receiver
CTS
• Communication establishing:
– neighboring nodes synchronize to the start of the active period then local broadcast of SYNC packets.
Data time
• Data message exchanges follow the
RTS/CTS/DATA/ACK;
–  nodes switch between different states periodically.
ACK
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Timing relationship of packet Tx/Rx listen t1 listen sleep t2 listen

•Scenario
•5 nodes two-hops
Sources
Forwarder
•70 nodes Random multihop
Destinations
•Metrics:
•Energy consumption per RX packet
•Network lifetime
•E-to-E latency
•Total packet overhead
•% sleeping time
•Parameters:
•Duty cycle (acting on CW and frametime size
•Low traffic conditions (12 packet/min)
•High traffic conditions (60 packet/min)
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

Note: These graphs have little relevance if not related to the EtoE latency
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

The MAC routing integrated nature MERLIN results in a smaller packet overhead than SMAC+ ESR.
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

• Description and simulated results of MERLIN have been presented;
• MERLIN is suitable for large scale sensor networks with energy consumption as main goal;
• MERLIN is suitable for communication to a from the gateway
• The multicast mechanism with burst ACK showed large improvement on the communication reliability
• The integrated nature, the absence of handshake mechanisms help reducing the EtoE packet delay
• EtoE delay can be traded-off for an longer network lifetime Results showed lifetime being extended by a factor of 2.5 of MERLIN with respect to SMAC
MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project

MERLIN: MAC and Routing Integration for WSNs by A.G.Ruzzelli
@ SAND project