Quality of Service in WSN-A Review

Abstract

The capability of sensor nodes to observe and react to physical phenomenon has made wireless sensor networks a new technological vision. They are small devices having distributed sensing capabilities with wireless interfaces and communicate using Radio Communication (RF) techniques. As these networks are used for applications that emphasize on timeliness or reliability or both QoS becomes an important parameter. But due to resource constrained and power hungry nature of these devices very few work is done in this area and there is currently no standardization, on a framework and/or general guidelines in the networking community on how QoS can be achieved in WSNs. This paper presents the Quality of service parameters, issues and about the work done in this area.

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International Journal of Computer Applications (0975 – 8887)
Volume 113 – No. 18, March 2015
42
Quality of Service in WSN-A Review
Sukhkirandeep Kaur
Student, CSE Deptt.
National Institute of Technology
Srinagar
Roohie Naaz Mir
Professor, CSE Deptt.
National Institute of Technology
Srinagar
ABSTRACT
The capability of sensor nodes to observe and react to
physical phenomenon has made wireless sensor networks a
new technological vision. They are small devices having
distributed sensing capabilities with wireless interfaces and
communicate using Radio Communication (RF) techniques.
As these networks are used for applications that emphasize on
timeliness or reliability or both QoS becomes an important
parameter. But due to resource constrained and power hungry
nature of these devices very few work is done in this area and
there is currently no standardization, on a framework and/or
general guidelines in the networking community on how QoS
can be achieved in WSNs. This paper presents the Quality of
service parameters, issues and about the work done in this
area.
General Terms
Wireless Sensor Networks, Quality of Service, Routing
Keywords
Wireless Sensor Networks, Quality of Service, Timeliness,
Reliability, End to end delay
1. INTRODUCTION
Wireless Sensor Networks has been identified as most
important technology for low power wireless communication.
The ability of small, low cost devices called sensor nodes to
cooperatively monitor physical or environmental conditions
such as pressure, humidity, temperature has increased their
importance. Visual Sensor Networks that involves cameras
are also emerging form of WSN for real time applications.
WSN have wide range of applications in military,
environmental monitoring, healthcare, habitat monitoring and
surveillance. But limited memory, low power and limited
processing nature of WSN impose several issues also. Energy
conservation is an important issue to be considered. WSN are
also used in real time applications, the transmission of
imaging and video data requires careful handling in order to
ensure that end-to-end delay is within the acceptable range
and that the variation in such delays is acceptable [1] so
reliability and timeliness becomes important QoS parameters
in real time applications. QoS has become very important
topic in WSN research.
Different QoS parameters were used in traditional networks
such as packet loss, delay, jitter, bandwidth etc. However, the
QoS requirements in WSNs such as data accuracy,
aggregation delay, coverage, fault tolerance and network
lifetime etc. are application specific and they are different
from the traditional end-to-end QoS requirements due to the
difference in application domains and network properties [2].
The QoS models used in internet and adhoc networks like
Differentiated Services (DiffServ) and Integrated Services
(IntServ) cannot be used in WSN due to their different nature.
In case of WSN QoS is provided by layered and cross layered
approach. Layered approach deals with specific layer and
cross layer requires simultaneous interaction of different
layers. Both layers provide required QoS to users and
application but at the cost of reduced energy. QoS is an
umbrella term for a collection of technologies that allow
network-aware applications to request and receive predictable
service levels in term of QoS requirements [3].
The rest of the paper is organized as follows. In Section 2 we
have discussed the characteristics of WSN which pose
challenges for QoS support. Section 3 discusses the
classification of QoS in WSN. In Section 4 detailed survey of
the existing approaches for QoS support in WSN is done and
some open research issues for QoS support in WSNs are listed
in Section 5.
2. CHALLENGES FOR QoS SUPPORT
IN WSN
Nature of WSN is very different from adhoc networks and
internet. So the requirements with respect to these networks
are also different. Moreover resource constraint and power
hungry nature of WSN impose several challenges also which
are discussed below
 Resource Constraints: As we know sensor nodes are
deployed in hostile environment and the battery can
be drained quickly and there is no source to replace
or recharge the battery so energy becomes an
important constraint. Other constraints are memory,
limited processing and transmission power.
 Scalability: Depending upon the application
requirements sensor nodes can increase therefore
QoS should support scalability. Performance of
network should not be degraded as number of nodes
increases.
 Fault tolerance: Due to depletion of energy node can
fail. In case of node failures network should cope up
as soon as possible but network dynamics such as
node failures, link failures imposes challenges for
QoS.
 Packet Criticality: Depending upon the application
criticality of packet is identified and priority is
assigned. For real time packets higher priority is
assigned as they are delay bounded. QoS
mechanisms should differentiate packet importance
and set up priority structure.
 Multiple sinks and traffic types: Depending upon
the application requirements multiple sinks and
sensors may be involved for different purposes. This
makes QoS more challenging.
 Real time requirements: Multimedia applications
and Performance critical applications have different
delay requirements. Hard real time delay guarantees
become difficult to achieve because of the limited
resource and energy issues of WSN. Multimedia
data requires high bandwidth and low delay. So

International Journal of Computer Applications (0975 – 8887)
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43
predefined QoS is required in these applications to
achieve required service.
3. QoS IN WSN
QoS in WSN can be application specific or network
specific.WSN is application specific, for some application like
forest monitoring delay is a very important factor. Information
regarding any harmful event should reach within fraction of
seconds. For applications involving object tracking due to
improper coverage of any sensor or lack of active sensors
target can be missed. So in this type of applications we can
define coverage or active sensors as parameters to measure
QoS in WSN. And for multimedia applications involving
camera and video sensors real time delivery of data is
important. Depending upon the application different QoS
requirements can be imposed on WSNs. Different application
specific parameters are coverage, optimum number of active
sensors, fault tolerance, data accuracy and aggregation delay.
At application layer as whole different parameters which are
considered are System lifetime, Response time, Data
reliability etc.
Network QoS specifies how underlying communication
network can deliver QoS constrained data while efficiently
utilizing network resources. Basically network QoS deals with
the delivery of data and there are different data delivery
models i.e. Continuous, Query Driven and Event Driven.
1) Continuous Data delivery model sends the data
periodically to real and non real time applications.
Real time applications are delay intolerant but in
case of non real time delay can be tolerated to a
certain extent.
2) In Query Driven applications query is generated by
sink and sensor nodes respond to that query. These
are used in mission critical applications and Delay
tolerance is query specific. Reliability is of main
concern in these applications. Query driven is also
used to manage the sensor nodes. If there is any
change that is to be made to sensor node, sink can
send out command to execute these changes.
3) In case of Event Driven Applications whenever
sensor nodes observe any event, that event should
be reported to sink in timely fashion and appropriate
action should be taken depending upon the event.
So these applications are mission critical and delay
intolerant. This differs from Query driven as in case
of event driven data is pushed to the sink and in
query driven data is pulled by the sink.
At the network layer Path latency, Routing maintenance,
Congestion probability and Energy efficiency are the main
QoS parameters which are considered. Various techniques
used to achieve QoS in network layer are:
 Multipath Routing: Multiple copies of each packet
are sending over multiple paths to improve
reliability, load balancing, security and to achieve
QoS.
 Data aggregation: In network data aggregation
reduces amount of data to be sent over the network.
As multimedia applications require huge amount of
data to be sent and due to bandwidth limitations
aggregation is done to reduce redundancy and to
utilize bandwidth.
 Energy aware routing: Energy efficient path is
found that meets end to end delay requirements.
 Minimum cost forwarding: Minimum cost path in
terms of bandwidth, energy is found. So depending
upon the Data delivery model different QoS
requirements can be imposed on the network
4. QoS AWARE ROUTING
PROTOCOLS
QoS routing protocols can be differentiated based on
timeliness or reliability. Timeliness is needed in applications
which have severe delay requirements and reliability is
needed in cases where data is of foremost importance. For
measuring QoS important parameters which are considered
are Throughput, jitter, Delay and Packet loss. A lot of work
has been done and still going on in improving QoS in WSN.
Energy Conservation is also considered as an important
parameter. Many protocols have been developed and
evaluated on the basis of above parameters. Some of them are
listed below
4.1 Sequential Assignment Routing (SAR)
SAR is the first routing protocol that included notion of QoS.
Both energy efficiency and fault tolerance are included in this
protocol. It is a multi path, table driven routing protocol which
creates a tree of sensor nodes having root at the one hop
neighbour of the sink node. Paths are established by taking
into account the QoS metrics, energy resource in each path
and priority of each packet. Based on QoS and energy
resource of each path multiple paths are selected. In case of
large number of sensor nodes overhead increases as large
number of nodes are employed.
4.2 SPEED
SPEED is a localized protocol which is used when node
density is high and resources are scarce. Information about
nodes is exchanged through neighbour beacon exchange
message and geographic forwarding is used to forward
messages. Delay is calculated via Receive Delay Estimator
and next hop is chosen based on Stateless Non Deterministic
Geographic forwarding, forwarding candidate set of nodes is
chosen and relay speed is calculated by dividing the distance
by hop delay. Packets are forwarded to nodes belongs to
forwarding set, if no such node is found packet are dropped
Nodes whose relay speed is higher than Ssetpoint, they are
chosen as next hop. If no such node is found then neighbor
feedback loop is used. Limitation of this protocol is that it
provides in network wide speed which is not suitable for
differentiating various traffic with different deadlines. Same
preference is given to both real and non real time traffic. It
does not consider any energy and reliability metric.
Figure1. Basic architecture of SPEED [6]

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4.3 MMSPEED
Multi-Path and Multi-SPEED Routing is a multipath
multispeed protocol that provides real time guarantee. It is an
extension to SPEED Protocol QoS is provided in two
domains, reliability and timeliness. By guarantying multiple
packet delivery speed options timeliness is provided and
reliability is provided by multipath forwarding. When an
event is detected packet is created to be transmitted. Based on
content of packet appropriate end to end deadline and required
reaching probability is found. Then packet forwarding is done
by MMSPEED protocol. Packets are divided into different
speed layers according to their priorities and their scheduling
is done accordingly.
Reliability is provided by sending high reliable packet over
multiple paths. MMSPEED is used only for short lived
applications and it does not consider energy efficiency at all
[7].
4.4 QEMPAR
QEMPAR is QoS based protocol which also considers energy
consumption as an important parameter. It is a location based
protocol and it is assumed that nodes know their location,
their remaining energy and remaining energy of other nodes
in their transmission range. Energy model use both open space
and multipath channels by taking the distance between
transmitter and receiver. Then link suitability is found by
using PPS (Probability of packet sending) and APPR
(Average probability of packet receiving) and IB(Interference
of link A and B). Then node disjoint paths are found. After
path discovery Real time packets are broken into smaller
packets and are sent over consecutive paths. Results are
compared with MCMP protocol and it shows better
performance in terms of End to End delay and average energy
consumption.
4.5 EAQoS
Energy Aware QoS routing protocol[5] is QoS aware
protocol for real time traffic. It also maximize throughput for
non real time traffic. Best delay constrained path based on
cost function is found from multiple paths. Optimal path is
found considering energy for real time consumption and error
rate while considering end to end delay required for real time
data. Queuing model is employed which divides the traffic
into Real Time and Non-Real Time and allocates bandwidth
according to value of r (Bandwidth ratio). Paths are found and
Dijkastra’s algorithm paths are arranged according to their
least cost. R.
4.6 EQSR
Energy efficient and QoS based routing protocol (EQSR) is
multipath routing protocol which considers reliability, energy
efficiency and delay as important QoS parameters and also
includes XoR based FEC technique. Next hop is selected
based on residual energy, available buffer size and Signal to
noise ratio. Message is splitted into segments; correction code
is added and is transmitted over multiple paths. Service
differentiation is provided by using Queuing model which
handles both real and non real time traffic.
4.7 RTLLR
Real-time link-Reliability routing is QoS based protocol
which considers both reliability and timeliness by considering
link reliability, Delay and energy efficiency. It considers two
hop neighborhood information for making routing decisions.
It mainly focuses on two parameters Link Reliability and Link
delay and based on that information next hop is selected. In
RTLRR if a link with higher PRR is selected probability of
successful packet delivery to the forwarding node is
increased. It is compared with SPEED and THVR and shows
better performance in terms of delay and energy consumption.
4.8 RRQRP
Reliable and Robust QoS routing protocol for WSNs based on
a Combined Weight (CW) Value. The CW is based on the
QoS parameters link quality, residual energy and available
bandwidth. To estimate Link Quality node mobility and RSSI
are considered. Available bandwidth is estimated from
AW
BW = MaxBW  Idle t
Int t (1)
Residual energy is calculated by subtracting energy consumed
by the node from initial energy and link quality is estimated
from SNR. If CW value is higher than MCW (minimum
combined weight value) then all copies of route request are
forwarded otherwise negative acknowledgement is sent. It
shows better performance as compared to QBRP in terms of
energy, delay.
4.9 QEMH
QoS Aware and Multi-path Hierarchical protocol is energy
aware clustering based protocol that gives preference to real
time traffic over non real time so that real time data gets
delivered with a very little delay. Cluster head is chosen based
on residual energy and node Distance to sink. Every node
broadcast these two parameter values and node having
maximum value is chosen as cluster head. After selecting
cluster head, next hop is chosen by cluster head based on
energy factor, available buffer and link performance factor.
RREQ message is sent to next preferred node until sink is
reached. Alternative paths are found in this way and path
having lower delay is chosen for real time traffic. To improve
reliability correction codes are added using XoR based
correction methods. Comparison is done with MCMP and
EAP protocols and it achieves more energy saving, low
average end to end delay, more lifetime and high packet
delivery ratio.

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Table 1: Comparison of different Protocols
Routing Protocol Multi-path Energy efficiency Reliability QoS
SAR YES HIGH - YES
SPEED YES LOW LOW YES
MMSPEED YES LOW HIGH YES
QEMPAR YES HIGH HIGH YES
EAQoS YES HIGH HIGH YES
EQSR YES HIGH HIGH YES
RTLLR NO HIGH HIGH YES
RRQRP YES HIGH LOW YES
QEMH YES HIGH HIGH YES
5. RESEARCH ISSUES
As already discussed WSN are very different from adhoc
networks so Diffserv and Interserv models cannot be used in
WSN. To achieve QoS simple models should be used using
cross layer approaches. Mobility is the main issue to be
considered as most of the time it is assumed that sensor node
and sink are stationary but there exist certain scenarios, for
example, in military environment, the sensor nodes and the
sink will be made mobile. So efficient techniques for QoS
considering mobility should be developed. Also, the topology
of the network may also keep on dynamically changing.
Therefore, efficient routing protocols are required to address
mobility and dynamicity of the wireless sensor network [2].
In case of real time applications Reliability and timeliness is
an important concern in providing QoS. Data redundancy may
exploit reliability; if we use fusion techniques it will effect
timeliness and introduces delay. So optimum techniques
should be developed to overcome this issue. Multipath
techniques should be developed to ensure delivery of data in
timely and reliable fashion. Moreover different cross layer
techniques should be developed and different QoS control
mechanisms should be developed to achieve Quality of
Service in WSN. Most importantly the major issue of WSN
i.e. Energy consumption should be considered while
developing new protocols and techniques.
6. CONCLUSION
So far very few work and effort has been done in field of QoS
in WSN. Due to the development of both real time as well as
non real time application providing QoS is a very important
and major issue to be considered. Energy efficiency is the
main issue to be considered while developing protocols and
algorithms. Moreover to meet timeliness and reliability
demand of WSN application different QoS techniques like
multipath routing to increase reliability, Queuing model to
differentiate real and non real time traffic, clustering
techniques to overcome problems of scalability should be
used.
In this paper we have discussed challenges imposed by
several characteristics and discussed useful parameters of
providing QoS in WSN. We have discussed several routing
protocols that include the notion of QoS and compared them
on basis of different parameters. Different issues related to
QoS have been identified and discussed.
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