Download 20:32, 11 October 2010

BIOGRAFISCHE ANGABEN
TECHNISCHE UNIVERSITÄT
DRESDEN
FAKULTÄT ELEKTROTECHNIK
UND INFORMATIONSTECHNIK
INSTITUT FÜR
NACHRICHTENTECHNIK
DIPLOMARBEIT
Thema:
The Simulative Investigation of Zigbee/IEEE 802.15.4
Autor:
Betreuer:
Prakash Rao,
Vaddina
1996 - 1998
1998 - 2002
Vaddina, Prakash Rao
Eingereicht am:
10. November 2005
Dipl. –Ing. Dimitri Marandin
Hochschullehrer:
Prof. Dr. -Ing. Ralf Lehnert
2002 - 2005
04.2004 - 04.2005
Dipl. –Ing. Falk Hofmann, ZMD AG
05.2005 - 11.2005
Secondary Education, India
Bachelor of Engineering,
CBIT, Osmania University,
India
Master(MSc.), ET, TU Dresden
Internship, Auvidea GmbH,
Denklingen.
Thesis, TUD & ZMD.
1. Abstract
The IEEE 802.15.4 is a new personal wireless area network standard
designed for applications like wireless monitoring and control of lights,
security alarms, motion sensors, thermostats and smoke detectors.
IEEE 802.15.4 specifies physical and media access control layers that
have been optimized to ensure low-power consumption. The MAC layer
defines different network topologies, including a star topology (with one
node working as a network coordinator, like an access point in IEEE
802.11), tree topology (where some nodes communicate through other
nodes to send data to the network coordinator), and mesh topology
(where routing responsibilities are distributed between nodes and
master coordinator is not needed).
In this thesis a star network topology according to the 802.15.4
standard has to be simulated with the simulator ns-2. The goals of the
thesis are to build a simulation model and to investigate different
functional modes of IEEE 802.15.4 and their impact on energy
consumption and network performance. Different application scenarios
have to be evaluated. The simulation results must be generated with
input from ZMD for their transceiver ZMD44101.
With reference to the above graphs and the energy analysis, we can
deduce the following statements:
•
Maximum achievable data bandwidth = 5.3Kbps
•
Achievable Data Bandwidth @ 99% delivery ratio = 5.25kbps
•
The battery of a device transmitting @ 10.07 Kbytes/day will
last for 4 months
5. Simulation Parameters
The simulations are conducted using the following simulation
parameters
Parameter
Value
2. Introduction
Topology
STAR
ZIGBEE is a new wireless technology guided by the IEEE 802.15.4
Personal Area Networks standard. It is primarily designed for the wide
ranging automation applications and to replace the existing nonstandard technologies. It currently operates in the 868MHz band at a
data rate of 20Kbps in Europe, 914MHz band at 40Kbps in the USA,
and the 2.4GHz ISM bands Worldwide at a maximum data-rate of
250Kbps. Some of its primary features are:
Number of nodes
15
Number of Flows
8
Traffic Type
CBR
Traffic Direction
Node  Coordinator
Packet Size
70 Bytes
Radio Propagation Model
Two-Ray Ground
Antenna Type
OmniAntenna
Queue Type
DropTail
Queue Length
150
Transmit Power
0dBm (1mW)
Receiver Sensitivity
-97dBm
Carrier sensing threshold
-97dBm
Capture Threshold
10
Antenna Height
1.0m
Transmitting Power
0.0744W
Receiving Power
0.0648W
• Standards-based wireless technology
• Interoperability and worldwide usability
• Low data-rates
• Ultra low power consumption
• Very small protocol stack
• Support for small to excessively large networks
• Simple design
• Security, and
• Reliability
3. Outline
The current study firstly tries to build a reliable, definitive and
deterministic simulation environment, identify the simulation
parameters and provide the performance metrics: throughput, delay
analysis, delivery ratio and energy consumption for the 868Mhz. The
results obtained for the 868Mhz band, can be in a way, applied to the
frequency band of 915Mhz, due to their close proximity in the spectral
space.
This report also focuses on the network congestion giving out reasons,
and highlighting the inefficiency in terms of backoff exponent
management on part of the channel access mechanism CSMA-CA. The
degradation in performance because of this inefficiency is explained
and a proposal has been made to have an efficient backoff exponent
mechanism for the devices involved in transmission, called the Adaptive
Backoff Exponent Algorithm, and results are presented to support the
performance improvement achieved by it.
4. Functional Overview
4.1 Network Topologies
The IEEE 802.15.4 can support two types of network formations.
Firstly is the Star topology where communication is only possible with
the coordinator and the peer-to-peer topology where communication
among nodes is also possible if the devices are capable.
7. Adaptive Backoff Exponent
A study of the system performance at collision scenarios reveals an
exponential increase in the number of packet drops, for higher datarate
operation. Poor link quality is a direct consequence of the hidden node
problem. Even if the nodes can detect each others presence, collisions are
caused when two or more of them choose an identical backoff time
duration. Hence they transmit without being aware that an other node
has also detected an idle channel. This result in frequent confrontations
among nodes often resulting in collisions. The reason can be traced back
to the inefficient backoff mechanism. An a efficient backoff maintenance
algorithm has been proposed called the Adaptive backoff Exponent.
The new mechanism will effectively allow the devices to choose any value
within 1 and 7 based on their contribution to the network congestion
(called the decision algorithm). Thus minimizing the odds of using the
same backoff wait duration. The more a node congests the network the
more will be its BE. It has been observed that at lower datarates this
mechanism allows the network to perform identical to the original
scenario while considerably improving its performance at higher rates
before succumbing to network congestion.
Table-1: Simulation Parameters
6. Performance Analysis at 868Mhz
The graphs inidcate the performance measure of a standard Zigbee
network with a star network topology at 868Mhz. The results are
produced with a 95% confidence level. The impact due to receiver
sensitivity is studied. The graphs predict the performance of the system
with different receiver sensitivities and taking their sensing threshold
similar to that of the receiver sensitivities.
Observing figure-2, for the throughput analysis, in the case of -97dBm of
receiver sensitivity and a -97dBm of carrier sensing range, the
throughput increases linearly through the datarates from 0.1 to 1.0, and
following the trend until, 2.0, where it achieves a stagnation due to
network congestion. These experiments reveal a maximum of
5100bits/second can be achieved at this sensitivity. The peak
performance is considered at datarates of 1.0-2.0 pkts/sec. Taking the
delivery ratio into consideration, ideally a peak datarate of 1.0pkts/sec is
achievable for the said frequency band, receiver and simulation
characteristics. And in the case of lower receiver sensitivity and
corresponding sensing ranges, the performance at higher datarates has
been considerably lower, and during congestion the performance reaches
their lowest point. Their peak performance can be viewed at rates of 0.70.8 packets per second where the nodes were able to receive 2700bits/sec
for -92dBm sensitive receiver and nearly 2200bits/sec for the other
sensitive ranges.
The following improvement is brought about by the application of
Adaptive Backoff Exponent Algorithm.
•
Maximum achievable data bandwidth = 6.73kbps
•
Improvement of 27% in maximum throughput
•
Achievable Data Bandwidth @ 99% delivery ratio = 6.554kbps
•
25% Improvement in the maximum throughput achievable @ 1%
PER
8. Conclusions
4.2 Superframe Structure
The superframe structure is an optional part of a WPAN. It is the time
duration between two consecutive beacons. The structure of the
superframe is determined by the coordinator. The coordinator can also
switch off the use of a superframe by not transmitting the beacons. The
superframe duration is divided into 16 concurrent slots. The beacon is
transmitted in the first slot. The remaining part of the superframe
duration can be described by the terms, CAP, CFP and Inactive.
The superframe is used to provide vital statistics like synchronization,
identifying the PAN and the superframe structure, to the devices
connected in a Wireless PAN. This information is critical for the
operation of the PAN in a Beacon enabled network.
With reference to figure-3 a system when using a receiver capable of
detecting packets with power as low as -97dBm, the delivery ratio is close
to 100% in the range of 0.1-1.0 packets/second. As higher traffic is
applied to the network, more and more packets are dropped due to
collisions and bad link quality of the transmitted packets. This would
increase the number of retransmissions exponentially, effectively
congesting the network even further.
A detailed study of the Zigbee technology with its internal architecture,
and the layered structure has been conducted. It is followed by a detailed
study of the simulation environment (ns-2) and the Zigbee modules built
under ns-2. Later a well behaved and near realistic simulation scenario is
built in TCL. The Zigbee modules have been exclusively tested for the
simulator environment and several loopholes have been identified. Some
of the inconsistencies that have been modified are the Routing
mechanism, the Address resolution mechanism, the active and passive
scan, the energy threshold and the carrier sense threshold, etc.
A detailed performance study has been done and the working areas have
been identified. This has highlighted the inconsistency in the CSMA-CA
algorithm which has been fixed by the Adaptive Backoff Exponent
mechanism. A comparative analysis focusing on the gain in performance
when using the new algorithm is presented to validate the claim of better
performance.