An Energy-Efficient MAC Protocol for Wireless Sensor Networks

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21 Νοε 2013 (πριν από 3 χρόνια και 10 μήνες)

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Rahul

Mallampati

Department of Computer Science

Contents


Introduction to Wireless Sensor Networks


Introduction to Medium Access Control


Classification of MAC protocols for WSNs


S
-
MAC


Z
-
MAC


References


Introduction to Wireless Sensor
Networks
[1]


Wireless Sensor Network : Collection of sensor nodes


Sensor Nodes are


Used for sensing the environment around them



Components are controller, sensor, memory, power
supply and communication


Work for common goal that is sensing environment for
one event or set of events



Why do we need Medium Access
Control (MAC)?


MAC specifies

1.
Which nodes in the network need to talk?

2.
How the nodes need to talk?

3.
When and how long are the nodes going to talk?


If there is no MAC then all the nodes might try to
talk at the same time, resulting in collisions.

Medium Access Control in WSN


Since WSN is collection of many sensor nodes →
Contention for transmission channel for data transmission


Medium Access Control is required for systematic
transmission of data


Without MAC in WSN there will loss of energy in
collisions and resulting retransmissions


Three important requirements for good MAC are


1. Energy Efficiency


2. Scalability, Topology Changes


3. Application level fairness, Throughput, bandwidth and
latency

Problems of MAC in WSN


Resource constraints
: Energy, Bandwidth, Network
Topology changes


Signal loss in wireless channel


Collision at the receiver’s end
: Sender does not have any
idea about the collision


Hidden and exposed terminal problems


Categories of MAC for WSNs



Schedule based MAC

MAC protocols

Contention based MAC

Hybrid MAC

Sensor MAC (S
-
MAC)


Introduction to S
-
MAC protocol


States in S
-
MAC


Choosing and Maintaining Schedules


Collision Avoidance


Overhearing Avoidance


Message Passing


Introduction to S
-
MAC Protocol
[2]


Sources of energy wastage

1.
Collisions

2.
Overhearing

3.
Idle Listening

4.
Control Overhead


S
-
MAC introduces some latency for message transmission


Each node in network using S
-
MAC, has periodic sleep
and listen policy.

States in S
-
MAC Protocol


Two states in S
-
MAC are


1. Sleep State


Communication devices are turned off


Sensing devices are turned on


2. Wakeup State


Communication devices are turned on


Sensing devices are turned on

States in S
-
MAC
Protocol(Continued)


Every node spends time in wakeup state and after certain
amount of time, the nodes moves into Sleep State.


Nodes are free to follow their own schedules. Node with
the same schedule form virtual clusters.


Neighboring nodes in the network have same schedules.
But this might not be possible always.


A

B

C

D

Choosing and Maintaining
Schedules


Schedules

of

neighbors

stored

in

schedule

table
.


Three

types

of

nodes

depending

on

schedule


Synchronizer


Follower


Border nodes


Maintaining Schedule


Updating schedules done using SYNC packets.





Listen interval: 1. Receiving SYNC



2. Receiving RTS


Address of
the sender

Time of next
sleep

Collision Avoidance


Collision avoidance done using RTS/CTS mechanism


Each packet contains time remaining for data
transmission along with data.


NAV vector is used for collisions avoidance. Using NAV
we perform Virtual Carrier Sense.


Broadcast do not use RTS/CTS. Unicast use
RTS/CTS/DATA/ACK





Overhearing Avoidance


Nodes go to sleep, if RTS/CTS packet is received since it
takes long time for reception of Data/ACK packets.


Example


Covering collision and overhearing avoidance




A

B

E

F

C

D

Message Passing


S
-
MAC uses RTS/CTS mechanism for data
transmission


S
-
MAC combines the data packets into burst and the
data transmission is done.



After each data fragment is received, an ACK sent






Zebra MAC (Z
-
MAC)


Introduction to Z
-
MAC


Setup Phase of Z
-
MAC


Contention Levels in Z
-
MAC


Transmission Rule in Z
-
MAC


Reception Rule in Z
-
MAC


Introduction to Z
-
MAC
[3]


Z
-
MAC is a hybrid MAC protocol, which

combines
Scheduled based MAC and Contention based MAC


Overhead of the protocol is Setup Phase which is performed
at the start of the protocol.


Z
-
MAC is Contention based MAC(uses RTS/CTS for data
transmission, Explicit Contention notification packets ) and
Scheduled based MAC (uses DRAND for slot assignment
for transmission of data)


Contention Levels


Two levels for the nodes in Z
-
MAC are


High Contention Level(HCL)(Z
-
MAC acts as TDMA)


Low Contention Level(LCL) (Z
-
MAC acts as CSMA)


Once the nodes receive ECN messages are sent, they
move into HCL.


Two methods for measuring the Contention level

1.
To measure the noise level of the channel

2.
To measure the packet loss in the acknowledgements

Setup Phase


Step 1:

Neighborhood Discovery
: Two hop
neighborhood


Step 2:

Slot Assignment: DRAND Algorithm:
Distributed Version of RAND:GRANT, IDLE,
REQUEST, RELEASE


Step 3:

Local Framing : Maximum Slot Number


Step 4:

Global Time Synchronization: Time Protocol for
Sensor Network (Node arranged in hierarchy)


Transmission Rule


Node checks whether it is owner of the slot


If it is owner of the slot, then it checks whether the channel
is unoccupied and if channel is unoccupied, then the node
transmits the data.


If channel is occupied, then the node waits for random
amount of time. After the time, the node repeats the same
process.


If the node is in HCL, then the node waits for some amount
of time and applies back off within contention window.


Reception Rule


Low Power Listening (LPL) used in Z
-
MAC.


In LPL, each node maintains listening duty cycle.


Data transmission is preceded by preamble as large as the
check time.

Categories of MAC for WSNs



Schedule based MAC

MAC protocols

Contention based MAC

Hybrid MAC

TRAMA, LEACH, etc

S
-
MAC, T
-
MAC, IEEE 802.11

Z
-
MAC, B
-
MAC

References


[1] “Protocols and architectures for wireless sensor
networks”, by H. Karl and A.
Willig


[2] “ An Energy
-
Efficient MAC Protocol for Wireless
Sensor Networks”, by W. Ye, J.
Heidemann
, D.
Estrin


[3]

“ZMAC: a Hybrid MAC for Wireless Sensor
Networks”,

by
Injong

Rhee,
Ajit

Warrier
, Mahesh
Aia

and
Jeongki

Min