This paper analyzes the impact of network scalability on various physical attributes of Zigbee networks. Simulations were conducted using Qualnet to evaluate the performance of the Zigbee physical layer based on energy consumption and throughput. Energy consumption was analyzed for different modulation schemes (ASK, BPSK, OQPSK), network sizes (2-50 nodes), and clear channel assessment modes. The results showed that OQPSK and ASK had lower energy consumption than BPSK. Throughput was highest for OQPSK. While carrier sense had slightly higher throughput than other CCA modes, the energy consumption differences between CCA modes were minor.

1. Impact of Network Scalability on Zigbee Physical
Attributes
A. Yadav1
, A. Kalra2
, A. Swami3
and P. Kaur4
Department of EECE, ITM University, Gurgaon, India
Email:1
ashish26yadav@gmail.com,
2
abhi.kara10@yahoo.com,3
swami.avinash16@gmail.com,4
prabhjotkaur@itmindia.edu
Abstract— ZigBee has been developed to support lower data rates and low power consuming
applications. This paper targets to analyze various parameters of ZigBee physical (PHY).
Performance of ZigBee PHY is evaluated on the basis of energy consumption in
transmitting and receiving mode and throughput. Effect of variation in network size is
studied on these performance attributes. Some modulation schemes are also compared and
the best modulation scheme is suggested with tradeoffs between different performance
metrics.
Index Terms— ZigBee, Qualnet, throughput
I. INTRODUCTION
IEEE 802.15.4 (ZigBee) is recent standard that brought about lower data rate operations at low cost. It
consumes less power, has lower duty cycle with high battery life & with low hardware complexity. It ranges
from short to medium distance of 10 to 100 meters for personal area network (PAN). The main aim of
designing IEEE 802.15.4 open standard is to support the wireless connectivity for various applications like
for industry, home automation and automotive monitoring and control. This standard describes lower
hardware complexity as modulation methods used are binary phase shift keying (BPSK), quadrature phase
shift keying (QPSK) and orthogonal quadrature phase shift keying (O-QPSK). It uses a simple direct
sequence spread spectrum (DSSS) which is robust enough to work in low power approximately at (1mW)
transmitter output in unlicensed band. Lower power consumption requires efficient design that uses minimal
hardware and clocking in efficient technology such as complementary metal oxide semiconductor (CMOS).
In this paper we are analysing ZigBee physical layer. Evaluation of ZigBee physical layer is done on the
basis of energy consumption in transmitting and receiving mode. We have used ZigBee 802.15.4 radio type
and clear channel assessment (CCA) modes using constant bit rate (CBR) application, for ZigBee network
designed in Qualnet simulator.
This paper structured as follows. Section II describes basic structure of ZigBee which briefs the different
layers of ZigBee wireless technology. Section III focusses on the ZigBee physical layer and describes the
services provided by physical layer and its primitives. Section IV describes the simulation parameters used in
this paper and presents an analysis of the results obtained.
II. ZIGBEE OVERVIEW
The reference model for ZigBee is shown in Table 1 depicting various layers of ZigBee wireless technology
DOI: 02.ITC.2014.5.80
© Association of Computer Electronics and Electrical Engineers, 2014
Proc. of Int. Conf. on Recent Trends in Information, Telecommunication and Computing, ITC

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architecture [1]. These layers facilitate the features like reliable data transfer and easy implementation of
Zigbee which makes it very attractive. It also provides features like interoperability and testing.
TABLE I. ZIGBEE REFERENCE MODEL TABLE II. SIMULATION PARAMETERS
The physical layer of this reference model specifies the network interface component, their parameters and
operation. It offers the transmission and reception on the physical radio channel. The medium access control
(MAC) layer manages network association and disassociation and regulates access to the medium. Its
operation is achieved through two modes; beacon and non-beacon. The network layer provides the function
to support network configuration and device discovery, topology management, MAC layer management,
security and routing. The topologies like star, cluster tree and mesh are supported. The application layer has
application support sublayer (APS), ZigBee device object (ZDO) and manufactured defined objects. The
APS sublayer maintains table for binding devices together based on their services and need.
III. ZIGBEE PHYSICAL LAYER
ZigBee operates in unlicensed band. The standard offers two PHY options based on the frequency band.
They are based on direct sequence spread spectrum (DSSS). The data rate is 250kbps at 2.4GHz, 40kbps at
915MHz and 20kbps at 868MHz [2]. The higher data rate of 2.4GHz is attributed to a higher-order
modulation scheme. The lower data rate can be translated into better sensitivity and large coverage area. It
has been studied that the BER vs SNR values are affected by various communication parameters such as data
rates and number of bits/symbol [3]. Two types of services are provided by the physical layer. Firstly it
controls the radio link i.e. the transmission & reception of physical payload data unit (PPDU). Secondly, it
produces energy detection within the channel, clear channel assessment (CCA) before transmitting the
message and produces link quality indication (LQI) for received packets.
A. Primitives
IEEE 802.15.4 standard describes 14 physical & 35 MAC primitives. It supports two types of devices i.e. full
functional device (FFD) & reduced functional device (RFD). FFD supports all defined primitives and RFD
supports 38 primitives.
1) Physical Primitives
The physical primitives provide activation and deactivation of radio transceiver, energy detection, LQI
measurement, clear channel assessment, data transmission & reception.
a) Data transmission
Whenever data is to be transmitted, MAC layer management entity calls upon physical layer with these
primitives to transmit a data frame.
PD_ DATA request: The data request is generated by MAC sub layer entity and issued to physical entity to
request the transmission of a MAC-layer protocol data unit (MPDU). When the PD_DATA request is
received the physical layer first constructs a physical protocol data unit (PPDU) with supported physical
service data unit (PSDU).
PD _DATA confirm: It is generated by physical entity & issued to its MAC sub layer. It returns a status of
either success or an error code indicating that the transceiver is working in the receiver mode or the
transceiver is switched off.
PD_DATA indication: This primitive is generated by the PHY entity and issued to its MAC sub layer entity
to transfer a received PSDU. It will not be generated if the received PSDU length field is zero or greater than
a maximum PHY packet size.
Application layer
Network and security
MAC layer
Physical layer (915MHz, 2.4GHz)
Parameters Specifications
Channel frequency 2.4 GHz – OQPSK
915 MHz – ASK, BPSK
Radio type 802.15.4 ZigBee
Modulation schemes ASK, BPSK, OQPSK
CCA modes a. Carrier sense
b. Carrier sense with energy above
threshold c. Energy above threshold

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B. Services Provided by Physical Layer
The physical layer (PHY) provides the data transmission service, as well as the interfacing to the physical
layer management entity, which offers access to every layer management function and maintains a database
of information on related personal area network. Thus, the PHY manages the physical RF transceiver and
performs channel selection, energy and signal management functions. The data service enables the
transmission and reception of PHY protocol data units (PPDU) across the physical radio channel. The main
features are activation and deactivation of the radio transceiver, energy detection (ED), link quality indication
(LQI), channel selection, clear channel assessment (CCA) and transmitting as well as receiving packets
across the physical medium
The receiver energy detection is used by network layer as a part of channel selection algorithm. It calculates
the received signal power within the specified range of frequencies. Then channel assessment is processed
through clear channel assessment algorithm which operates in three modes in accordance to some threshold
level. When the energy is above threshold CCA reports that the medium is busy. In the second mode; it only
senses the carrier irrespective of the energy level. In the third mode; the carrier is sensed with energy above
threshold and reports that the channel is busy. Furthermore, the link quality is indicated using receiver energy
detection and signal to noise ratio estimation. LQI is the characterization of quality and strength of the
received packet.
IV. SIMULATION SETUP
We created different scenarios with varying network size of 2 nodes to 50 nodes using Qualnet [4]. The
sensor network in our scenario follows ZigBee (IEEE 802.15.4) standard with PAN coordinator, RFDs and
FFDs. The figure 1 shows the qualnet scenarios for 10 and 40 nodes and similiarly scenarios for 20, 30 and
50 nodes were formed using constant bit rate (CBR) application and results were analysed. Different
parameters like throughput and energy consumption have been analysed for these different scenarios. Various
simulation parameters used in our analysis are given in Table 2.
A. Result Analysis
Evaluation for different parameters like energy consumed in receiving mode and transmitting mode,
throughput has been done with varying number of nodes in wireless sensor networks. Simulations have been
done for different CCA modes i.e. carrier sense, carrier sense with energy above threshold and energy above
threshold. Various plots have been obtained using different modes for varying network size and are discussed
below.
(a) (b)
Fig. 1. Scenario and simulation for (a) 10 nodes and (b) 40 nodes
Figures 2 and 3 shows the comparison between different modulation schemes for energy consumption in
transmitting mode and receiving mode respectively. Result shows that the energy consumption is least in
ASK and OQPSK modulation scheme whereas BPSK consumes the maximum energy. This is the reason
why OQPSK is considered efficient for higher data rates. In this plot, we have used the scenarios

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Fig. 2. Energy consumption in transmitting mode with varying number of nodes using ASK, BPSK and OQPSK modulation schemes
Fig. 3. Energy consumption in receiving mode with varying number of nodes using ASK, BPSK and OQPSK modulation schemes
Fig. 4. Throughput of PAN cordinator with varying number of nodes using ASK, BPSK and OQPSK modulation schemes
with10,20,30,40 and 50 nodes and it shows that as the number of nodes are increasing, energy consumption
is also increasing.
Figure 4 is a plot between number of nodes and throughput for various modulation schemes. This plot shows
that the OQPSK modulation scheme has comparatively higher throughput than other modulation schemes. It
also shows that ASK modulation scheme has higher throughput than BPSK modulation scheme although

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BPSK is employed mostly in ZigBee standard. So this study also gives us an option of employing another
modulation scheme in form of ASK which can be used in ZigBee sensor network.
Fig. 5. Energy consumption in transmitting mode (PAN cordinator) with different CCA modes and varying number of nodes
Figure 5 is a plot between number of nodes and energy consumption in transmitting mode for various CCA
schemes. The result shows that there is a minor difference between these schmes in energy consumption.
This is the reason why we mainly employ carrier sense scheme as it has higher throughput than other CCA
schemes.
Fig. 6. Energy consumption in receiving mode (PAN cordinator) with different CCA schemes and varying number of nodes
Fig. 7. Energy consumption both for transmitting and receiving mode for all the three CCA schemes. CS: carrier sense, CSEAT: carrier
sense with energy above threshold and EAT: energy above threshold

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Figure 6 shows the plot between number of nodes and energy consumption in receiving mode for various
CCA schemes. As it was indicated previously in the transmitting mode there is a minor difference between
energy consumption in these CCA schemes. In the receiving mode also, there is a minute change in the
energy consumption of CCA schemes.
Figure 7 shows the plot between the energy consumption in all the three CCA schemes. It is shown that the
energy consumption in carrier sense with energy above threshold is maximum and least in case of energy
above threshold but carrier sense is preferred because it has higher throughput among all the CCA schemes.
V. CONCLUSION
In this paper various modes of operations of IEEE 802.15.4 ZigBee have been studied for different
modulations and CCA schemes with varying number of nodes. Energy consumption and throughput have
been considered for comparing various modulation schemes and modes of operation. We have observed the
OQPSK is the best modulation scheme in terms of energy consumption and throughput. Further it has been
observed that carrier sense scheme outperforms all other CCA schemes. In future we can analyse impact of
different routing protocols on ZigBee physical layer and can also devise a technique to evaluate LQI for
different CCA mechanisms and modulation schemes.
REFERENCES
[1] ZigBee Alliance ZigBee Specification[EB/OL]. http://www.zigbee.org/en/spec-download,2007
[2] IEEE Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless
Personal Area Networks[S] IEEE Std 802.15.4-2003, October 2003.
[3] M. Alnuaimi, K. Shuaib and I. Jawhar, “Performance evaluation of IEEE 802.15.4 physical layer using
matlab/simulink,” in innovations in information technology, Nov 2006.
[4] Qualnet simulator 5.02 purchased from scalable networks. web.scalable-networks.com/content/qualnet