CHAPTER 4

CHAPTER 4

Performance Analysis of IEEE 802.15.4 for Wireless Sensor

Networks

4.1 INTRODUCTION

IEEE 802.15.4 was designed uniquely inorder to suit personal

wireless network requirement consuming low power, provides low data

rate and low cost. Wireless Local Area Networks standard (Wi-Fi) and the

WPAN standard (Bluetooth and Zigbee) products utilize the same

unlicensed 2.4 GHz ISM band. It becomes crucial to ensure that each

wireless technology maintains and provides its desired performance

requirements when co-existence between such wireless technologies within

the same frequency spectrum. Wireless Personal Area Networks (WPAN)

is formed using tiny sensor devices that have several resource constraints.

IEEE 802.15.4 is an established set of specifications for WPAN i.e. digital

radio connections between computers and related devices. By using these

networks, use of physical data buses as USB and Ethernet cables, could be

eliminated. The devices could include telephones, hand-held digital

assistants, sensors and controls located within a few meters of each other.

IEEE 802.15.4 [34,60] is one of the global standards of communication

protocol. WPAN Low Rate/ZigBee [55] is a standard that provides

64

specifications for devices tliat have low data rates, consume very low

power and are thus characterized by long battery life. The design of

protocols optimized for the constraints of sensor nodes and requirements of

data dissemination in the network are few of the central research topics in

wireless sensor networking. Disseminations requirements are very specific

data from source nodes, potentially highly correlated, may be generated

and periodically, on a query, or on a particular event routed, either directly,

towards the observer nodes (sinks), or towards aggregation nodes for

further processing. Sensing areas may be queried by many observer nodes

connected at arbitrary nodes. Each query may specify required fidelity,

timeliness and reliability. In the past, many specific solutions optimized for

particular sensing tasks have been proposed [26] and analyzed, which

brought a better understanding of the necessary functionality of the general

energy efficient communication stack for WSNs.

IEEE 802.15.4 standard was designed for Low-Rate Wireless

Personal Area Networks (LR-WPANs). WPAN is an all-wireless

deployment of sensor nodes. It includes a sink, known as PAN coordinator

that is used for short-range communication . The network architecture is

such that a virtual backbone is formed, with the PAN coordinator serving

as the core node while other devices function as child nodes that rely on

their parent, in this case the PAN coordinator, during network

65

establishment and communication. This basic topology can be extended to

a multi-tiered hierarchical network by electing one or more child nodes as

a coordinator or cluster-head to manage their own WPAN. Immobile

operation of wireless sensors is assumed in present discussions on WSNs,

and LR-WPANs in particular.

Three network growing scenarios, in which the network is enhanced

by incrementally adding new nodes, and by incremental introduction of

new sensor network applications is defined here. Starting from a simple

scenario and moving towards more challenging ones followed by

examination of how IEEE 802.15.4 networks (WPAN) can self organize to

support sensor application coexistence and interworking is considered m

this work. IEEE 802.15.4 has been designed as a flexible protocol in which

a set of parameters can be configured to meet different requirements. The

topology of this network is irregular. Topologies considered are Scattemet,

Piconet and Peer-to-Peer networks.

The sensor nodes are placed randomly on the phenomenon, and this

is known as ScatterNets that are used to extend the coverage and numbers

of devices are varied. Second topology considered is known as one-hop

star or Piconet, which consists of one coordinator and up to seven devices.

In a piconet [17], a device only communicates with its coordmator. ZigBee

employs two modes to enable the to-and-fro data traffic i.e. beacon or non-

66

beacon mode. Beacon mode is used when the coordinator runs on batteries

and thus offers maximum power savings, whereas the non-beacon mode

finds favour when the coordinator is mains-powered. In the beacon mode,

a device watches out for the coordinator's beacon that gets transmitted

periodically, locks on and looks for messages addressed to it. If message

transmission is complete, the coordinator dictates a schedule for the next

beacon so that the device 'goes to sleep'; in fact, the coordinator itself

switches to sleep mode. While using the beacon mode, all the devices in a

mesh network know when to communicate with each other. In this mode,

necessarily, the timing circuits have to be accurate, or wake up sooner to

be sure not to miss the beacon. This in turn means an increase in power

consumption by the coordinator's receiver, entailing an optimal increase in

costs. The non-beacon mode will be included in a system where devices

are 'asleep' nearly always, as in smoke detectors and burglar alarms. The

devices wake up and confirm their continued presence in the network at

random intervals. On the detection of activity, the sensor spring to

attention, as it were, and transmit to the ever-waiting coordinator's receiver

as it is mains-powered. However, there is the remotest of chances that a

sensor finds the channel busy, in which case the receiver unfortunately

would 'miss a call'. Thus, third topology considered is peer-to-peer

topology with beacon enabled mode.

67

4.2 OVERVIEW OF IEEE 802.15.4

802.15.4 [44] standard defines physical (PHY) and medium access

control(MAC) layer protocols for supporting relatively simple sensor

devices that consume minimal power and operate in the area of 10m or

less. The point of service (POS) may be extended beyond 10m, but this

requires additional energy to operate. Two types of topologies such as a

simple one hop star or a self configuring peer-to-peer network to be

established are allowed by this standard.

802.15.4 operates in three license free Industrial Scientific Medical

(ISM) frequency bands are: data rates of 250 kbps in the 2.4 GHz band, 40

kbps in the 915 MHz band and 20 kbps in the 868 MHz band in terms of

wireless links.

Number of channels allotted to bands is: the first band has 16 chaimels

while the second has 10. The latter was allocated one channel. Additional

channels allow the flexibility of switching to another in case the existing

becomes not conducive even though only one channel is used at a time.

Two categories of devices exist in 802.15.4. One of them is called full-

function device (FFD) while the other is reduced-function device (RFD).

RFD is a crude device supporting simple application such as a switch or

sensor. It is usually controlled by FFD device. RPDs can be used to

communicate among themselves and with FFDs. The former is desired in

68

this research work as it can take on the role of a router that enables peer-to-

peer communication. The protocol assumes the use of either 16bit short or

extended 64-bit IEEE addresses in terms of addressing. The latter is

available in all devices by default and is commonly known as physical

(MAC) address while the previous is allocated by the PAN coordinator

which the device is associated with. IEEE 802.15.4 [94] standard

particularly the MAC and PHY layer are as described below:

4.2.1 PHY Layer:

The PHY layer provides an interface between the MAC sublayer and

the physical radio channel. It provides two services: PHY data service and

the PHY management service accessed through two service access points

(SAPs). The PHY protocol performs Energy Detection (ED) scan and

Clear Channel Assessment (CCA) on the channel to detect any ongoing

activities and relay the results to the MAC layer. A channel is considered

busy if the activity levels detected exceed certain threshold value. Another

important assessment is link quality. Upper layers protocols (MAC and

network) depend on this information before deciding on using a particular

channel as network performance could affect because external

interferences such as noise and electromagnetic signal. If a particular

channel is not feasible, there are 26 other channels available under

802.15.4 to be selected. As part of 802.15.4 efforts in preserving energy,

69

the radio transceiver can be turned off if inactive (not receiving or

transmitting).

4.2.2 MAC Layer:

This layer provides an interface between upper layers and the PHY

layer. It handles channel access, link management, frame validation,

security, and node synchronization. Beaconless mode that implies

unslotted CSMA/CA mechanism is adopted in this work. For this mode,

the PAN coordinator is responsible for handling only device association/

disassociation and (short) address allocation in case the 64-bit IEEE

addressing is not used. The CSMA/CA protocol is an important

mechanism for channel access but does not include the RTS/CTS

handshake, considering low data rate adopted in 802.15.4. This mechanism

evaluates the channel and allows data packets to be transmitted if the

condition is suitable (free of activities). Otherwise, the algorithm shall

back off for certain periods before assessing the channel again. Without the

RTS/CTS handshake, it would appear to encourage packet collisions due to

hidden nodes [26, 32]. If nodes are out of signal range of each other then

nodes are said to be hidden.

4.3 RELATED WORK

Present work is motivated by the tremendous potential of IEEE

802.15.4 in supporting simple, low-rate, and low-power applications for

70

LR-WPANs [33]. Several efforts on performance evaluations were

conducted since the inception of IEEE 802.15.4.

Zheng [16] implemented the IEEE 802.15.4 network only. IEEE

802.15.4 standard was implemented by Zheng et al. subsequently they

produced the first performance evaluation on 802.15.4. The research work

describes the first experiment on multi-hop ad hoc networks unlike the one

conducted in [16]. 802.15.4protocol as well as simulations on various

aspects of the standard is defined comprehensively in literature. Lee [27]

[29] focused on simple one-hop star network. G. Lu et al. [28]

implemented their own NS2 version of 802.15.4 and studied its

performance in beacon-enabled mode while Lee [28] performed a realistic

experiment using hardware devices. Timmons and Scanlon [31] presented

an analytical analysis of the protocol in body area networking (BAN).

Howitt et al. [25] presented a brief technical introduction of the IEEE

802.15.4 standard and analyzed the coexistence impact of an IEEE

802.15.4 network on the IEEE 802.11b devices. Misic et al. [30] have

considered this technology in their cross-layer activity management

scheme.

This research work covers simulation and different topological

experiments focusing on small-scale networks with seven sensor nodes,

71

thus providing simulated as well as actual performance measurements and

hence significantly different from other existing works.

4.4 EXPERIMENTAL SETUP and SIMULATION RESULTS

Performance of 802.15.4 is evaluated using following metrics:

Packet Delivery Ratio (PDR), Hop Delay and Routing Overhead.

Packet Delivery Ratio (PDR): The ratio of packet successfully sent to

packets received in MAC Sub-layer.

PDR: Number of packets received / Number of packets sent X 100.

Hop Delay: The transaction time of passing a packet to a one-hop

neighbour including time of all necessary processing, back off as well as

transmission, and averaged over all successful end-to-end transmissions

within a simulation run.

Hop Delay: (End Time - Start Time) / Total number of received packets.

Routing Overhead: The total number of control packets transmitted during

communication.

General simulation parameters are as depicted in table 4.1.

Simulation Parameter Link BER Packet Error Rate

Simulation Duration Application Traffic Traffic Type Radio Propagation Model

Value 10-̂ to 10'' 0.2%

1000 seconds 20 to 900 seconds CBR/FTP/Poisson Two-ray groimd

72

Application Packet Size reflection 90 bytes

Table 4.1 General simulation parameters

Performance behaviour of IEEE 802.15.4 for measuring PDR, delay

and control packet overhead is explained under three scenarios. Scenario

one explains by considering scattemet topology, scenario two deals with

piconet topology and scenario three deals with peer-to-peer topology. Both

pure CSMA-CA and slotted CSMA-CA are used by LR-WPAN. Set of

parameters can be configured to meet different requirements as IEEE

802.15.4 has been designed as a flexible protocol.

The topology of ScatterNets is irregular. The sensor nodes are

placed randomly on the phenomenon. This is also known as ScatterNets

that are used to extend the coverage and number of devices is varied. The

simulation starts from 10 nodes and adding 10 nodes up to 50 nodes with 9

meter transmission range which covers the neighbours along diagonal

directions. 802.15.4 operates at an over air data rate of 250 kbps in non-

beacon enabled mode. Each of the iteration will consist of three set traffic

flows like: CBR, FTP and Poisson. Table 4.2 indicates simulation setup

considered.

Simulation Parameter Node Density Simulation Area

Value 10,20,30,40,50 50 X 50 m̂

73

Traffic flow Transmission Range Traffic Type Routing Protocol Duration

9->6, 4->2 and 3 -> 2 9 meters CBR/FTP/Poisson AODV 900 seconds

Table 4.2 Simulation parameters for scenario-1

Next topology considered is the star topology with beacon enabled

mode. This topology considered is known as one-hop star or piconet,

which consists of one coordinator and up to seven devices. A device only

communicates with its coordinator in case of piconet. Piconet topology is

as shown in Figure 4.1. Table 4.3 indicates simulation setup considered for

this scenario-2.

<1> ®

<2> P A N C o o i

<s> <I>

<I>

Figure 4.1 Piconet

Simulation Parameter

Node Density

Simulation Area Traffic flow Transmission Range

Value

07 (including PAN coordinator) 50 X 50 m^ 0 to all other devices 10 meters

74

Traffic Type Routing Protocol Duration Beacon order Data rate

CBR/FTP/Poisson AODV 900 seconds Oto8 250 kbps

Table 4.3 Simulation parameters for scenario-2

All the other nodes serve as both coordinator (to its children) and

device (to its parent) except PAN coordinator (node 0) and the leaf nodes

depicted in grey in Figure 4.1, which are pure devices. A device can only

reach the coordinator and two devices adjacent to it whereas all other

devices are hidden. Simulation data captured for Piconet is as shown in

table 4.4.

Traffic

Type

CBR

FTP

Poisson

No. of

Packets

sent

24195

17463

24627

No. Of

Packets

Received

25503

24553

25623

No. of

Packets

Dropped

16529

19213

16782

PDR

94.87

78.12

96.11

Hop

Delay

0.0039

0.0040

0.0039

Routing

Overhead

66231

61231

67036

Table 4.4 Simulation data for Piconet PAN

It is clear from table 4.4 that the number of packets dropped is high

in all three cases. PDR of CBR and Poisson is high. But the application

traffic FTP generates less routing overhead compared with CBR and

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Poisson. Also, traffic FTP has low PDR (78%). Hop delay of CBR and

Poisson is same whereas FTP has recorded high delay. The reason for this

is that the flow of traffic in Poisson uses probability distribution fiinction

which makes packet generation and transmission time consuming.

Finally, the topology considered for scenario-3 is peer-to-peer with

beacon enabled mode. Peer-to-Peer topology is as shown in Figure 4.2.

Table 4.5 indicates simulation setup considered for this scenario-3. Figure

4.2 is known as Peer-to-peer beacon enabled network, which consists of

one coordinator and up to twenty one devices. A device only

communicates with its coordinator in case of Peer-to-peer network.

<E>

<E>

<Z>

<1>

<E>

<1>

C2>

<Q>

<£2>

Figure 4.2 Peer-to-Peer beacon enabled network

Simulation Parameter

Node Density

Simulation Area Traffic flow Transmission Range

Value

21 (including PAN coordinator) 80 X 80 m2 0 to all other devices 15 meters

76

Traffic Type Routing Protocol Duration Beacon order Data rate

CBR/FTP/Poisson AODV 900 seconds 0 to 20 250 kbps

Table 4.5 Simulation parameters for scenario-3

Traffic

Type

CBR

FTP

Poisson

No. of

Packets

sent

24195

17463

24627

No. of

Packets

Received

25503

24553

25623

No. of

Packets

Dropped

16529

19213

16782

PDR

94.87

71.12

96.11

Hop

Delay

0.0039

0.0040

0.0039

Routing

Overhead

66231

61231

67036

Table 4.6 Simulation data for peer-to-peer network

Table 4.6 shows the captured simulation data from the peer-to-peer

network with beacon enabled mode in WPAN. The number of packets

dropped is very high in all the three cases. This is due to beacons where

device watches out for the coordinator's beacon that gets transmitted at

periodically, locks on and looks for messages addressed to it. Due to this

huge amount of control packets are generated and gets collided resulting in

dropping of packets. But the CBR, application traffic FTP and Poisson

generates huge routing overhead with also high PDR (96%-94%). But

there is a slight drop in the PDR for FTP (71%). The Hop Delay of CBR

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and Poisson are same, and FTP has recorded slight high delay because of

the flow of traffic in Poisson uses probability distribution function for

generating packets and transmission.

4.5 RESULTS and DISCUSSIONS

Initial setup consists of placing the nodes in random places. Initially,

simulation starts with 10 nodes and node density is incremented by 10

nodes up to 50 nodes. Each of the iteration will consist of three set traffic

flows as shown in Figure 4.3. These combinations of the nodes are chosen

randomly, and user can make the selection of any source and destination.

The performance of WPAN is analysed by changing the node density. In

the beginning, CBR performs well as the number of nodes is minimum. As

there is an increase in the nodes at 50, the PDR drops drastically to 35 %.

In all other cases, an average of 80% PDR is resulted. This is because of

peer to peer application traffic will have an impact of delivery ratio drop

sharply from 86% to 35%). 802.15.4 maintains a high packet delivery ratio

for application traffic up to 40 (86%), but the value decreases quickly as

node density increases. The main advantage of 802.15.4 is not using

RTS/CTS as in the case of 802.11 standards. Performance of FTP is

average as the number of nodes is minimum. As the nodes increase, the

PDR also increases up to 96 % and start dropping slowly towards 35%>

when node density is 50. In all other cases, an average of 70% PDR is

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resulted. Poisson performs less when compared to CBR and FTP as the

number of nodes is minimum initially. As the nodes are increased then

gradually the PDR also increases up to 95 % and start dropping slowly

towards 34% when node density is 50.

= 120

o 18 Q£

I 1 s.

ion 80

60

40

20

0

PDRvs.Ntimher of Nodes

•CBR FTP Poission

10 20 30 40

Niiitii>ei of Nodes

SO

Figure 4.3 Packet Delivery vs. Number of Nodes

Average hop delay for different traffic is measured in, and the

results are depicted in Figure 4.4. Initially, FTP results show that 802.15.4

has higher delay than CBR and Poisson. Nevertheless, this comparison is

unfair to 802.15.4 as it operates at a data rate of 250 kbps whereas 802.11

operates at 2 Mbps in most of the simulation experiments. The hop delay

according to the media data rate is normalised, which gives us a different

view that the hop delay of 802.11 is around 3.3 times of that of 802.15.4.

The hop delay for sink-type application traffic is 22% (for Poisson) to 66%

(for CBR) higher than that for peer-to-peer application traffic FTP is 80%.

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The increment of delay is expected, since all the traffic flows now need to

converge on the sink node.

0.8

"5 0.6

I 0.4 Q

8 Q2 X

H 01) D elay vs. N ii ml) er of N o des

10 20 30 40

Number of Hodtt

50

•CBR

•FTP

Poission

Figure 4.4 Hop Delay vs. Number of Nodes

The routing overhead is depicted in Figure 4.5 with changing

node density. All the traffic type will almost maintain the steady stage as

there is no much exchange of control or routing packets as in the case of

802.11. There is a huge rise (almost 80%) in routing overhead packets at

node density 40 due to burst in application traffic. The routing overhead

drops to normal and continues at node density 50 due to adjustment in the

traffic load.

80

Routing Overhe<)cl vs Number of nodes

100000

I 80000

« 60000

,1 40000

I 20000

A T H

\ V

-CBR

-FTP

Poission

10 20 30 40

Numbf r of Nodu

50

Figure 4.5 Routing Overhead vs Number of nodes

4.6 CONCLUSIONS

Inorder to evaluate the general performance of this standard NS2

simulator is used, which covers all the 802.15.4 MAC and network layer

primitives. Three sets of experiments are carried out that involve

comparing the performance of 802.15.4 between CBR, FTP and Poisson

traffic with varying node density considering ScatterNet topology, PicoNet

and Peer-to-peer beacon enabled network. Detailed experimental results

are presented along with analysis and discussions. The obtained simulation

data from the peer to peer with beacon enabled mode in WPAN indicates

that the number of packets dropped is very high in all the three cases. This

is due to beacons where device watches out for the coordinator's beacon

that gets transmitted periodically, locks on and looks for messages

addressed to it. Due to this huge amount of control packets are generated

81

and gets collided resulting in dropping of packets. Association and tree

formation in 802.15.4 proceed smoothly in both beacon enabled mode and

non-beacon enabled mode, which implies 802.15.4 possesses a good self-

configuration feature and can shape up efficiently without human

intervention. The orphaning and coordinator relocation (recovery from

orphaning) mechanism provides for a device a chance of self-hewing 6:0m

disruptions. No significant difference has been observed in the packet

delivery ratio among the three data transmission methods. Nevertheless,

the hop delay varies, which will affect the packet delivery ratio in upper

layers. The hop delay indirect data transmission is much shorter than those

in indirect and GTS data transmissions. For the lack of RTS/CTS, 802.15.4

is expected to suffer from hidden terminal problems. The experiment

results match this expectation. But for low data rates up to one packet per

second, the performance degradation is minor. The default CSMA-CA

back-off period in 802.15.4 is too short, which leads to frequent repeated

collisions. Superframes with low beacon orders can also lower the slotted

CSMA-CA back-off efficiency and lead to high collision probability at the

begiimings of superframes. Thus, it is indicated in this simulation study

that 802.15.4 is an energy-efficient standard which favours low data rate

and low power consumption applications.

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