TDMA Scheduling in Wireless Sensor Networks

flangeeasyMobile - Wireless

Nov 21, 2013 (3 years and 11 months ago)

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TDMA Scheduling in
Wireless Sensor Networks

Presented by:

Ang Tashi Lama Sherpa

Department of Communications and
Networking

Aalto University School of Science and
Technolgoy

Presentation Outline


Introduction


Wireless sensor network (WSN)


Issues in WSNs


Medium Access Control (MAC)


TDMA Scheduling


Wireless Sensor Network Model


Contributions


Centralized link scheduling and channel assignment with balance tree
formation algorthim


Distributed Broadcast TDMA (DB
-
TDMA) scheduling algorithm


Extension to DB
-
TDMA


Performance Metrics


Performance Evaluation


Conclusion and Future Work





Wireless Sensor Network (WSN)


Wireless sensor network is a collection of
sensor nodes, ranging from tens to thousands,
that are spatially distributed in a geographical
area to cooperatively monitor and gather data
about the environment.


Each sensor node in a WSN is a
battery
powered device

consisting of sensors such as
temperature and light, a small microcontroller,
and a transreceiver for wireless
communication.

Issues in WSNs


Jennifer Yick et.al [1] has
grouped the issues in WSNs
into three groups.


System:


In WSNs, each sensor node is an
individual system and to support
various application software,
there is a need to develop new
platforms, operating systems,
and storage schemes.


Communication protocols:


Communication protocols enable
communication between the
application and sensors, and also
between the sensor nodes.


Services:


Services are developed to
enhance the application, and
improve system performance and
network efficiency.

Medium Access Control (MAC)


MAC protocol plays an important role in
determining channel capacity utilization,
network delays, and power consumption.


Time division multiple access (TDMA) MAC is
a potential candidate for WSNs.


TDMA conserves energy by eliminating collisions, avoiding
idle listening, and entering into inactive states until their
allocated time slots.


TDMA can bound the delay of packets and guarantees
reliable communication.

One of the major challenge in adopting TDMA MAC
is finding efficient time
-
schedule.



TDMA Scheduling


TDMA scheduling can be defined as the process
of allocating time slots to the nodes or links
between each pair of neighboring nodes, to ensure
collision free channel access.


S. Ramanathan et.al. [2] has classified TDMA
scheduling into two types:


Broadcast Scheduling:
The stations themselves are scheduled. The
transmission of a station must be received collision
-
free by all its one
-
hop
neighbors


Link Scheduling:
The links between stations are scheduled. The transmission
of a station must be received collision
-
free by one particular neighbor.

A proper TDMA scheduling should avoid collision and also minimize
the number of time slots in a frame to reduce latency in the
network.


TDMA Scheduling


The objective of TDMA Scheduling is to get rid of
primary

and
secondary conflict
.


Primary conflict: Occurs when one node transmits and receives at the same
time slot or receivs more than one transmission destined to it at same time slot.


Secondary conflict: Occurs when an intended receiver of particular transmission
is also within the transmission range of another transmission intended for other
nodes


Wireless Sensor Network Model


Static sensor network:

All nodes remain in the same position
or has limited mobility throughout their lifetime in the
network.


Connectivity:

The network is assumed to be connected.


Single radio sensor node:

It is assumed that the sensor
nodes have only one radio transceiver.


Each node is assumed to have a
unique node ID
.


Global synchronization:

All the nodes in the network are
assumed to be synchronized with each other.


Bidirectional radio link:

The radio link between two nodes is
assumed to be bidirectional, i.e, if node v can hear node u,
then node u can hear node v.

Centralized link scheduling and
Channel assignment with balanced
tree formation algorithm


A
centralized link scheduling and channel assignment
with balanced tree formation algorithm
is designed to
provide joint routing and multi
-
channel TDMA link
scheduling for wireless sensor networks.


Balanced tree formation:

The algorithm generates a
balanced tree structured network topology that evenly
distributes data traffic generated by sensor nodes across
the different branches of the routing tree.


Channel allocation (CA):

The channel allocation is receiver
based, i.e., the node selects the receiving channel for itself.


Time Allocation:

CA is then followed by time slot allocation,
which consists of allocating time slots to the links between
the nodes for uplink and downlink.

Distributed Broadcast TDMA
Scheduling Algorithm (DB
-
TDMA)


A
distributed broadcast TDMA scheduling algorithm
(DB
-
TDMA)
that allows each node to assign itself a time
slot based on the two
-
hop neighborhood information.


The main objective of the proposed algorithm is to reduce the
number of message transaction taking place between the
nodes without compromising the convergence time of the
algorithm.


The algorithm provides an option of having global time frame
or local time frame in the network.


In global time frame, all the nodes have the same time frame.


In local time frame, the time frame of a node depends on its
number of two hop neightbours(F
i
), which is calculated as,





)]
(
[log
2
i
i
F
ceiling
rame
Localtimef

Extension to DB
-
TDMA


An
extension to distributed broadcast
scheduling algorithm

is also proposed.


This extension proposed for DB
-
TDMA when
local time framing is utilized.


The extension allows nodes with the largest
one
-
hop neighorhood size among its one
-
hop
neighbors to select multiple time slots for
itself. We term these nodes as priority nodes.

Example of DB
-
TDMA and its
extension with Local Time Framing

Performance Metrics


Schedule length
: The schedule length is defined as the
maximum number of allocated time slots after scheduling.


Message complexity
: The message complexity is the
average number of transmitted messages by each node,
to decide on the time slots for all executions of the
algorithm.


Running time
: The running time denotes the number of
rounds required for all the nodes in the network to decide
on their time slots. Each round is a period of time during
which, a node can: (a) send a request message, (b)
receive messages from all its one hop neighbors, (c)
select minimum time slot available, and (d) send one hop
broadcast of the selected time slot.



Performance Evaluation


In order to evaluate the algorithms we use a
simulation based approach using
Matlab
.


The network topology is generated by randomly
deploying
N

nodes in the area of
√N
×

√N

unit
2
,
so that when
N

varies, the node density within
the transmission range is kept constant. The
transmission range of the nodes are then
adjusted to control the density of deployment
keeping the number of nodes in the network
constant.

Centralized link scheduling and
channel assignment with balanced
tree formation algorithm


Here, 100 nodes are deployed
randomly in area of 10
×

10 unit
2

and
transmission range is varied from 1.5
to 2 units.


It is evident that there is a decrease
in schedule length with increase in
available channels in the network.


This decrease is observed to be less
significant when number of available
channels is more than two.


This behaviour is observed because
the interference limitation is
eliminated with two channels and
beyond this point, the connectivity
constraint, i.e., the number of
children connected to a node, limits
the performance

Scalability of DB
-
TDMA and its
extension


Here, number of nodes is
varied from 100 to 500 with
constant transmission range
of 1.5 units.


It is evident from the tables
that the proposed distributed
algorithms are scalable
making them suitable for
high density sensor
networks.

Schedule Length of DB
-
TDMA
and its extension


Here, 100 nodes are
deployed randomly in
area of 10
×

10 unit
2

and transmission
range is varied from 1
to 2 units.


The algorithms utilize
local time framing.


The schedule length of
both the algorithms is
similar.

Message Complexity and Running
Time of DB
-
TDMA and its extension


The message complexity and running time of extended DB
-
TDMA is
observed to be higher than that of DB
-
TDMA.


The main reason behind this is that additional message transaction are
required to ensure multiple slots to priority nodes.


Comparision of DB
-
TDMA with
DRAND and DD
-
TDMA


Here, 100 nodes are
deployed randomly in
area of 10
×

10 unit
2

and transmission range
is varied from 1 to 2
units.


Global time framing is
utilized for all the three
algorithms.


DB
-
TDMA, DRAND,
and DD
-
TDMA
achieves similar
schedule length.

Comparision of DB
-
TDMA with
DRAND and DD
-
TDMA


The message complexity of DB
-
TDMA is observed to
be lower than both DRAND and DD
-
TDMA.

Comparision of DB
-
TDMA with
DRAND and DD
-
TDMA


The running time of DB
-
TDMA is observed to be similar
to that of DD
-
TDMA and lower than that of DRAND.

Conclusion and Future Work


We have proposed two solution to the TDMA
scheduling problem: centralized link
scheduling and broadcast scheduling
algorithm.


Implement the algorithms on real
-
life sensor
network test beds.


Make the algorithms suitable for mobile
wireless sensor networks.


References

[1]J. Yick, B. Mukherjee, and D. Ghosal, Wireless sensor network
survey, The International Journal of Computer and
Telecommunications Networking, 2008

[2] S.Ramanathan, and E.L.Lloyd, Scheduling algorithms for
multihop radio networks, IEEE/ACM Transactions on
Networking (TON), vol.1, no.2, pp. 166
-
177, 1993


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