Performance Analysis of IEEE 802.15.4 For Intra-Vehicle Wireless Networks

Utayba Mohammad

Department of Electrical and Computer Engineering, University of Detroit Mercy, Detroit, MI, USA

Abstract - In the past decade, cars have evolved to become very complex systems, offering a verity of safety and infotainment options. This evolution was made possible by the development of advanced intra-vehicle communication networks that allowed for implementing highly distributed information processing systems and achieving significant cost reduction in manufacturing. Many efforts have been directed to improve the efficiency of intra-vehicle networks, enhance its scalability, and reduce its cost. Wireless technology presents a potentially viable approach to meet all of these goals. Recently, industries in general, and automotive in particular, have been exploring the deployment of wireless technologies in harsh environments and attempting to identify the underlying challenges to these networks.

This paper evaluates IEEE 802.15.4 as an intra-vehicle wireless network protocol. It explores implementing in-vehicles space division multiplexing with IEEE 802.15.4, develops an intra-vehicle wireless connectivity diagram with IEEE 802.15.4, and evaluates the performance of IEEE 802.15.4 under extreme interference conditions with coexisting IEEE 802.11 networks.

Keywords: Intra-vehicle Network; IEEE 802.15.4; wireless sensor networks; real-time wireless communication

1 Introduction Multiplexed vehicle communications emerged to

support the increased complexity of automotive system, and to enable scalable, upgradable, efficient, and low-cost designs [1][2]. In multiplexed vehicle networks, a single bus is shared among multiple devices and is usually accessed, either by using a Time Division Multiplexing Access (TDMA) technique, or by using bus contention resolution techniques, such as CSMA/CR. Many multiplexing protocols have been proposed in this area. SAE classified those protocols, according to speed, into three classes A, B, and C [3]. Class A is used for interfacing with simple switches or sensors. It represents low speed protocols with a bit rate up to 10 Kbps and a message rate about 100 ms. Class B protocols provide a bit rate between 10 and 125 Kbps and a message rate around 20 ms, allowing for the communication protocol to handle some parts of the engine and transmission controllers. Finally, class C protocols provide bit rates starting from 125 Kbps and up to 1 Mbps, and a message rate of less than 5 ms, which makes these protocols most suitable for real time and critical control

system communications. However, several protocols have emerged recently, offering bit rates over 1 Mbps and marking a new class of protocols, “Class D.”

The main two objectives of multiplexed vehicle networks are: reducing the wiring costs and complexity of the network, and distributing th in-vehicle processing load over multiple low-end processors. In this context, a wireless communication system is better than the single bus multiplexing system, since it supports the concept of distributed systems, and yet does not require any wires. Moreover, intra-vehicle wireless networks become even more attractive when plug-and-play sensors are implemented. Currently, the potential wireless protocols for intra-vehicle networks are IEEE 802.15.4, IEEE 802.15.1/Bluetooth, and the IEEE 802.11 family [4][5]. This paper presents the potential use of each of these protocols and evaluates the IEEE 802.15.4 performance for intra-vehicle network applications.

The rest of the paper is organized as follows: IEEE 802.11 family is discussed in section II, and Bluetooth is reviewed in section III. IEEE 802.15.4 is then presented in section IV and evaluated experimentally in section V. Finally, the paper is concluded in section VI.

2 IEEE 802.11 IEEE 802.11 standards were developed to serve wireless

LANs. These protocols operate at the ISM 5.9 GHz or 2.4 GHz bands, and offer high bit-rates that range from 11 Mbps in 802.11b to 300 Mbps in the draft 802.11n. Since designed for LANs, these protocols were optimized to deal with big chunks of data, such as images and video files, and address some of the common challenges in wireless LANs, such as hidden and exposed terminal problems. As a result 802.11 protocols are not best suited for intra-vehicle networks, especially when it comes to transmission power and protocol overhead. Therefore, researchers have not considered 802.11 protocols for limited area wireless sensor networks, and rather, they focused more on the Personal Area Networks (PANs) that resemble in many ways the intra-vehicle networks.

3 IEEE 802.15.1/Bluetooth Bluetooth is one of the most common WPAN protocols.

It was first developed by Ericson in 1994 and released in 1998 as a standard by the Bluetooth Special Interest Group (SIG), including Ericson, IBM, Intel, Nokia, and Toshiba [6], [7], [8]. The initial idea was to replace short cable connectivities with wireless devices that are cheap, battery

operated, and have power saving modes. However, these features were extended later on to meet the requirements of an ad-hoc network’s nature and emphasize short-range communication, as well as, satisfy the low cost constraints.

Since Bluetooth started as a cable replacement, it was reasonable for it to adopt a master-slave, peer-to-peer communication approach. However, the need for networking support extended the master-slave concept to a one-master, multiple-slaves network concept, which is called piconet. The piconet must have one, and only one, master, and can contain up to seven slaves and 255 parking nodes. Bluetooth uses the Frequency Hopping Spread Spectrum (FHSS) technique to provide immunity against noise and interference. Therefore, a frequency-hopping sequence is generated based on the master node address and is used by all piconet members. Each piconet’s hopping sequence is unique because their master’s address is unique, which explains why one piconet’s slaves cannot communicate with another piconet’s master directly. This raises the question of how we can propagate data between different piconets. The answer came with the scatternet concept. A scatternet is a network of piconets. A master of one piconet can be a slave in another piconet, and a slave in one piconet can be slave in three other piconets.

Bluetooth radio operates in the 2.4 GHz unlicensed Industrial Scientific Medical (ISM) band and uses a fast Frequency Hopping Spread Spectrum (FHSS) and Gaussian Frequency Shift Keying (GFSK) modulation (generally) to transmit its data. Throughout the transmission, Bluetooth radio uses 79 channels (carrier frequencies), spaced 1 MHz from each other. The time between two frequency hops is called a slot and is equal to 625 µsec. Power-wise; Bluetooth is categorized into three classes. Class 1 has a maximum transmission power of 100 mW and a minimum transmission power of 1 mW, and can reach typically to 100 m. Class 2 ranges from 1mW to 2.5 mW and can reach up to 20m. Finally, Class 3 has a maximum transmission power of 1 mW and can reach up to 10 m [9].

Although supports many of the in-vehicle network requirements, Bluetooth faces some serious problems when it comes to its architecture, especially, the piconet limited scalability, the involved delays in scatternet and operating mode switching, and the master-slave topology that increases the bandwidth demand [10].

4 IEEE 802.15.4 IEEE 802.15.4 standard was developed to meet the

different needs of Wireless Personal Area Networks (WPANs), which includes low power consumption, self-organizing, self-healing, and the ability for expansion [9], [10], [11], [12]. Since it was designed for WPANs that satisfy home, industry, and environment applications’ requirements, it was necessary to choose its operational frequencies from the free unlicensed frequency range. Therefore, IEEE 802.15.4 has been chosen to operate in three different free unlicensed frequency ranges, with each one having a different number of channels, bandwidth, and data rate. These frequency ranges are:

• 868 – 868.6 MHz, used in Europe, has one channel only and provides an ideal bit rate of 20 Kbps.

• 902 – 928 MHz, used in USA, has ten channels and provides an ideal bit rate of 40 Kbps.

• 2400 – 2483.5 MHz, used in most of the world and has sixteen channels with a maximum bit rate of 256 Kbps.

4.1 Network topology The network devices in IEEE 802.15.4 are classified

according to their actual physical specifications or their logical task in the network. Physically, the network devices are classified as Full-Function Devices (FFDs) or Reduced-Function Devices (RFDs); while logically they are classified as PAN coordinator, routers, and end devices. These two classifications intersect with each other, i.e. a FFD can operate as PAN coordinator, router, or regular end device; on the other hand, a RFD can only operate as an end device. FFDs have higher physical requirements than RFDs, such as more memory, higher computational capabilities and, consequently, larger power consumption requirements (usually powered by a main supply). This level of complexity makes FFDs capable of communicating with other FFDs or RFDs. On the other hand, RFDs have limited resources of memory, computational capabilities, and power (usually powered by batteries), and, hence, can only communicate with FFDs only.

Based on the three WPAN logical components (coordinator, router, and end device), IEEE 802.15.4 supports three Network topologies. The first one is the Star topology, in which all data transfers occur between the PAN coordinator and the other network devices as shown in Fig.1-a. The second topology is the Peer-to-Peer (Mesh Network) topology, which still has one PAN coordinator, but its devices can communicate among each other’s without accessing the coordinator as shown in Fig.1-b. Since only FFDs can communicate with other FFDs and RFDs, they are the most commonly used devices in the mesh networks, while RFDs only reside on the leaves of these networks to provide sensing and actuating. The third topology is a special formation of the peer-to-peer topology, and is called the Cluster-Tree topology. The cluster-Tree topology is a trade off option, offering less power consumption and connectivity than the peer-to-peer topology and more, of both, than the star topology. Fig.1-c depicts the cluster tree topology.

4.2 IEEE 802.15.4 operation schemes: The communication management in IEEE 802.15.4 can

operate in one of two modes, the beacon-enabled mode or the nonbeacon-enabled mode. In the beacon-enabled mode, the coordinator sends periodic beacons to synchronize all devices with the PAN superframe. The superframe is bound by two beacons, and divided into two main periods, active and inactive as shown in Fig. 2. The active period comprises two main periods, the Contention Access Period (CAP) and the Contention Free Period (CFP). The CAP is divided into time slots, so that devices willing to transmit shall use a slotted CSMA/CA algorithm to compete for the medium.

On the other hand, the CFP is divided into Guaranteed Time Slots (GTS), where each slot is allocated to a predefined device to insure critical data transmission.

Fig. 1. ZigBee Topologies, a- Star, b- Mesh Network, and c-Cluster Tree

The Nonbeacon-enabled mode is much simpler than the beacon-enabled one. The devices in this mode compete for the medium with CSMA/CA algorithm. Hence, delays in the physical layer are less than those in the beacon-enabled mode, but there is no guaranteed delivery mechanism.

Fig. 2. IEEE 802.25.4 Superframe

4.3 IEEE 802.15.4 MAC layer: The IEEE 802.15.4 MAC layer plays a major role in

driving and determining the efficiency of the communication system. It setups the superframe period division, allocates GTSs, and orchestrates the association process to form up the whole network. The superframe structure in the beacon-enabled IEEE 802.15.4 has a significant importance in optimizing the communication among the different devices. The MAC layer is responsible of structuring the superframe using several user-defined constants, such as the Beacon Order (BO) and the Superframe Order (SO). As described previously, the superframe is bounded between two beacons, therefore, the period of complete superframe is called the Beacon Interval (BI), while the active part of the superframe is called the Superframe Duration (SD). The Beacon Interval and Superframe Duration are defined by Superframe Order and Beacon Order respectively according to Eq. (1) and Eq.(2), respectively.

BI = B * 2BO, where, 0 ≤ BO ≤ 15 (1)

SD= B * 2SO, where, 0 ≤ SO ≤ BO (2)

Where B is a MAC layer constant called aBaseSuper-frameDuration.

In case BO=BI=15, the system functions in a nonbeacon-enabled fashion; while BO=BI=0, will lead to a minimum beacon interval and fully active superframe, resulting in maximum utilization of the superframe period.

With its flexible and configurable topology, IEEE 802.15.4 has a good potential to serve as an intra-vehicle communication protocol. The rest of the paper evaluates the IEEE 802.15.4 protocol for intra-vehicle applications, and tries to find what type of communication class it can support in comparison to the vehicle wired multiplexed communication protocols.

5 IEEE 802.15.4 evaluation To assess IEEE 802.15.4’s potential for automotive

applications a set of experiments were conducted. In [14], ZigBee’s performance was evaluated thoroughly in a General Motors 2005 Cadillac STS. Although that study covered a wide range of measurements, it was very particular to the case study. In this section the physical layer performance of IEEE 802.15.4 in vehicles is evaluated with a more general approach. Three experiments are conducted: the first, determines the major factors that affect the wireless signal strength at different parts of a vehicle; the second, develops an approximate wireless connectivity diagram inside vehicles; and the third evaluates IEEE 802.15.4 coexistence with WiFi. Throughout the experiments MICAZ nodes running the TinyOs embedded operating system were used to determine the Received Signal Strength and Packet Error Rate (PER).

5.1 Main factors in intra-vehicle wireless signal propagation

This experiment was designed with three factors and four replicates. the factors are:

• Vehicle Type (factor A): Two vehicles are used as test plants, a Chevrolet Prizm 1999 and a Mazda LX 1996.

• Node level (factor B): Both nodes are installed at the same level (height), and then one node is installed at the ground level and the other at the engine level.

• Engine Status (factor C): Engine is OFF, and Engine is ON running idle.

A 23 factorial analysis with 32 measurements was performed to study the effect of the aforementioned factors, and their interactions, on the Received Signal Strength (RSS). However, due the dependencies between the mean and the variance of the RSS a linear 1/Y variance stabilization transform was applied to the data sample and the insignificant 2-way and 3-way factor interactions were discarded. Fig. 3 shows the residual plots of the transformed data and the associated 23 factorial analysis. Fig. 3 further shows that the residuals do approximate a normal distribution and have a constant variance, which satisfies the assumptions of a factorial design. As a result the reported P-values can be used from the analysis to conclude that we are

CFP

GTS

BI SD

Beacon

CAP

Active Period

almost certain and 94.9% confident, that the node level and the “vehicle type * engine status” interaction, respectively, affect wireless signal propagation in the engine compartment. This conclusion indicates that a connectivity diagram needs to be developed before planning to deploy wireless nodes in a vehicle, and that this diagram is unique to each vehicle based on the vehicle’s internal component distribution and its electronic and electrical wiring.

5.2 Wireless connectivity diagram in vehicles

This section attempts to draw a rough diagram of wireless connectivity inside vehicles. MICAZ nodes were used in this experiment, with one transmitter, one receiver, and a transmission power of -25 dBm. The transmission power was chosen as the minimum possible value in order to guarantee the least interference between adjacent vehicles.

Fig. 3. The 23 factorial experiment analysis results after after applying a linear transformation Y* = 1/Y and aliasing the insignificant terms

The vehicle under test was virtually divided into three sections: the engine compartment, the cabin, and the trunk. The wireless connectivity in each of these sections and among themselves was evaluated and a general connectivity diagram was deduced. The following sub-sections show the node placement and the corresponding received power levels, while the conclusion of these measurements is presented at the end of this experiment.

5.2.1 Engine compartment test The transmitter and receiver are placed in three different

configurations: at the same level under the hood, at the same

level on the ground, and at the two ends of the engine compartment diagonal. Fig. 4 shows the placement configurations for the engine compartment test, and Table I elaborates on the received signal power at different points in the engine compartment.

Fig. 4. Nodes’ placements for the engine compartment test

TABLE I. RECEIVED SIGNAL POWER IN THE ENGINE COMPARTMENT

Transmitter Receiver Received Power [dBm] A B -73 C D -72 A   D   -­‐91  

5.2.2 The Vehicle Cabin Test

The main communication in the vehicle cabin is usually between the seat modules, door modules, and the dashboard cluster. To emulate such communications, the transmitter was placed in the Glove Box and the receiver was moved around the front seat pocket, back seat pocket, and under the front seat and for both sides of the vehicle. The received power is recorded as shown in Table II.

TABLE II. RECEIVED SIGNAL POWER IN THE VEHICLE CABIN

Transmitter Receiver Received Power [dBm]

Glove Box Front Left door pocket -75 Glove Box Front Right door pocket -61 Glove Box Back Left door pocket -75 Glove Box Back Right Door Pocket -78 Glove Box Under the Front Left Seat -75 Glove Box Under the Front Right Seat -62

5.2.3 Trunk test:

The communication in the trunk section is mainly concerned with the tail and the center light signaling. Therefore, our measurements were taken in three locations in the trunk as shown in Fig. 5. Table III shows the received signal power at different locations in the trunk.

Fig. 5. Communication testing in the vehicle trunk

B

A

CD

A

B

C

TABLE III. RECEIVED SIGNAL POWER IN THE VEHICLE TRUNK

Transmitter Receiver Received Power [dBm] B A -65 C A -54

5.2.4 Inter-section Connectivity test:

After the connectivity within the individual section was evaluated, the inter-section connectivity was studied. Table IV shows the received power level between the different sections when the vehicle is empty. “No connectivity” indicates that the received power falls below the receiver sensitivity, which is -95 dBm for MICAZ (CC2420).

5.2.5 Conclusions on intra-vehicle wireless connectivity

In an empty vehicle and with a -25 dBm transmission power, the wireless connectivity provided by IEEE 802.15.4 compliant transceivers could be divided into seven main sub-networks: the headlights, engine compartment, dashboard, left door module, right door module, seat controls, and the trunk. The connectivity within each of these sub-networks is preserved. However the connectivity among the different areas is not guaranteed. More specifically, the engine compartment nodes are connected with the dashboard and headlight modules; however, the last two are not connected. Also, the dashboard is connected to both door modules and, sometimes, with the seat control; but it fails to connect with the trunk, as well as the seat modules when the vehicle is loaded with passengers. Finally, the seat controls are connected with the trunk module all the time. Fig. 6 depicts a rough wireless connectivity diagram in a vehicle when using IEEE 802.15.4 transceivers.

TABLE IV. RECEIVED SIGNAL POWER DURING THE INTER-SECTION COMMUNICATION TEST IN THE VEHICLE

Transmitter Location

Receiver Location Received Power [dBm]

Left Tail Light in the Trunk

Front Head Lights (Both Left & Right)

No Connectivity

Rear Left and Right Side – ground level

Front Left & Right Side – ground level

-84

Glove Box Center of the Engine Compartment

-83

Glove Box Front Headlight No Connectivity

Left Door Pocket Center of the Engine Compartment

-88

Glove Box Left Tail Light in the trunk -87 Back Seat Trunk -72

Fig. 6. Wireless connectivity Diagram in a vehicle when using IEEE 802.15.4 compliant transceivers

5.3 Coexistence with Wifi Nowadays, It is very common to come across Wifi-

enabled devices, E.g. cell phones, wifi hotspots, wifi-based remote controls, security cameras ...etc. This poses the questions: how well will ZigBee/IEEE 802.15.4 tolerate the co-existence of Wifi-enabled device? And would it still be able to serve under extreme conditions of interference and spectrum contention?

To answer these questions two MICAz nodes were used as a transmitter and a receiver. The transmitter was set up to send bulk messages of 200 packets each every 5 seconds with transmission power of -25 dBm, in compliance with the previous connectivity test. The receiver was set up to report the number of received packets from each message to a monitoring computer. The MICAz nodes were alternated to operate at IEEE 802.15.4’s channel 26 and 22, while two computers were used to provide an active Wifi file transfer over channel 11. The experimenter occupyied the passenger seat and used a notebook to transmit a data file to nearby WiFi base station. At the same time the IEEE 802.15.4 transmitter was positioned in the glove box and the receiver was placed in the close door pocket. This setup was meant to imitate a real life scenario of a user using WiFi inside a vehicle while sensory data is exchanged wirelessly via IEEE 802.15.4. The experimental setup is illustrated in Fig. 7.

Fig. 7. The setup for testing IEEE 802.15.4 coexistence with WiFi

Door Cluster C1

Door Cluster C2

Engine Cluster

C3

back light + trunk

Cluster C4

dashboard Cluster C5

head lights + Sonar cluster

C6

Seat Control Cluster C7

Receiving WiFi Macbook Pro

Transmitting WiFiMacbook Pro

TransmittingIEEE 802.15.4 Node

ReceivingIEEE 802.15.4 Node

Spectrum Analyzer

Fig. 8. Box plots for the IEEE 802.15.4’s interference with channel 11 of WiFi

In IEEE 802.15.4, channel 26 and 22 are centered at 2.48 GHz and 2.46 GHz respectively, with a channel bandwidth of 2 MHz. On the other hand, channel 11 of WiFi is centered at 2.462 GHz and has a bandwidth of 22 MHz. In this setup, overlapped channels (22 of IEEE 802.15.4 and 11 of WiFi) were used to measure the maximum effect of interference. On the other hand, channel 26 of IEEE 802.15.4 and 11 of WiFi were used as two adjacent non-overlapping channels in order to measure the minimum interference effect on IEEE 802.15.4 performance. Fig. 8 shows the box plots, over 15 measurements, for the number of received packets during active and inactive channel 11 WiFi and for both IEEE 802.15.4’s channels 22 and 26. We indicate here that we did not distinguish between channels 22 and 26 when WiFi is inactive because 100% of the messages were received in both cases and for all measurements.

Although channel 26 of IEEE 802.15.4 is, theoretically, spaced 7 MHz from the upper band of WiFi’s channel 11 (2.473 GHz), Fig. 8 reveals a significant drop in number of received packets when WiFi is active vs. when WiFi is not active. The box plots show more resilience to WiFi when using channel 26 vs channel 11, but suggest that they are both not practically usable for control application purposes.

These results are further explained by using a spectrum analyzer. It is found that channel 11 of WiFi still overlaps with Channel 26 of IEEE 802.15.4, but at a much lower level than is the case with channel 22. Fig. 9 shows snapshots of the spectrum during the experiment for channel 11 of WiFi, during active and inactive transfers, and for channel 26 and 22 of IEEE 802.15.4 during inactive WiFi transfer.

6 Conclusion The experimental results show that IEEE 802.15.4 is

capable of operating in an intra-vehicle environment, and that it exhibits good immunity against vehicular electromagnetic noise when the engine is on and when it is off. The propagation characteristics throughout the vehicle are shown to be affected by the obstructing metal parts in the vehicle. At the lowest transmission power of -25 dBm

the wireless signal is decently contained within the vehicle vicinity, and therefore a vehicle wireless connectivity diagram of 7 connected sections is derived at this power. Further experimental work indicated that full connectivity could be achieved by increasing the transmission power but with risking the result of inter-vehicle interference.

Fig. 9. (a) channel 11 spectrum, (b) channel 22 spectrum, (c) channel 26 spectrum.

2.462 GHz-70 dBm

22 MHz

2.48 GHz-93 dBm

2.46 GHz-65 dBm

2.48 GHz-103 dBm

2.46 GHz-100 dBm

(a)

(b)

(c)

In contrast to the general assumption that IEEE 802.15.4 and WiFi can coexist without degradation in the communication quality, our expiremnt shows that using the -25 dBm transmission power of IEEE 802.15.4 in the close vicinty of active WiFi transmitter can degrade the communication quality significantly. The worst case takes place IEEE802.15.4 and WiFi are operating at in overlapping channles with less effect being exerted whe operating in adjacent channles. Hence, using IEEE 802.15.4 for intra-vehicle communication requires coordinating its active channels with the active WiFi channels all the time.

7 References [1] Mark Thompson, “Replacing wirings with multiplexed in-vehicle

communication system Papers: Thick and thin of car cabling”, Motorola, IEEE Spectrum, v33, n 2, February 1996, pp.42-45

[2] Pretson, N. C. G. N. (University of Liverpool, Engl), J. Lucas, “Multiprocessor Implementation Of The Logic Function Of A Multiplexed Wiring System For Automotives”, IEEE Proceeding, Part E: Computers and Digital Techniques, v 129, n 6, November 1982, pp. 223-228.

[3] Christopher A. Lupini, “Vehicle Multiplex Communication - Serial Data Networking Applied to Vehicular Engineering”, SAE, April 2004.

[4] A. Willig, K. Matheus, A. Wolisz, “Wireless Technology in Industrial Networks”, Proceedings of the IEEE, June 2005, Volume (93), Issue (6), pp. 1130- 1151.

[5] T. Nolte, H. Hanssonlo, L. Bello, “Automotive communication past current and future”, Emerging Technologies and Factory Automation, 2005. ETFA 2005. 10th IEEE Conference on, Volume (1), pp. 985- 992.

[6] C. Bisdikian, “An overview of the Bluetooth wireless technology”, IEEE Communications Magazine, v 39, n 12, December, 2001, p 86-94.

[7] Specification of the Bluetooth System, Covered Core Package version: 2.0 + EDR Current Master TOC issued: 4 November 2004 volume (0), Available: http://www.bluetooth.com/Bluetooth/Learn/Technology/Specifications/.

[8] M. Hayoz, “ The Bluetooth Wireless Technology An Overview”, Master Seminar in Telecommunications, University of Fribourg, Switzerland, 2003.

[9] M. Petrova, J. Riihijarvi, P. Mahonen, S. Labella, “Performance study of IEEE 802.15.4 using measurements and simulations”, Wireless Communications and Networking Conference, 2006. WCNC 2006. IEEE, 3-6 April 2006, volume(1), pp. 487- 492.

[10] Mohammad, U., Al-Holou, N., “Development of Wireless Protocols for Automotive Applications”, Worldcomp ICWN’07, Las Vegas, pp.

[11] IEEE Std 802.15.4™-2003, Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs), 2003.

[12] Jin-Shyan Lee, “An Experiment on Performance Study of IEEE 802.15.4 Wireless Networks”, Emerging Technologies and Factory Automation, 2005. ETFA 2005. 10th IEEE Conference on Volume 2, Issue , 19-22 Sept. 2005 Page(s): 451 – 458.

[13] Electronics Design, Strategy News. Shreharsha Rao, “Estimation ZigBee transmission range in the ISM band”, Texas Instrument, EDN Europe, 01 July 2007. [online] available: http://www.edn-europe.com/estimatingzigbeetransmissionrangeintheismband+article+1608+Europe.html

[14] Hsin-Mu Tsai; Tonguz, O.K.; Saraydar, C.; Talty, T.; Ames, M.; Macdonald, A.; “Zigbee-based intra-car wireless sensor networks: a case study”.Wireless Communications, IEEE, Issue 6, December 2007, pp. 67-77.

[15] Ns2 Network Simulator website: http://www.isi.edu/nsnam/ns/.

Mohammad, U., Al-Holou, N., Balas, C.,“Performance Evaluation of IEEE 802.15.4/ZigBee Protocol for Automotive Applications”, SAE Congress 2008, In-Vehicle Networks and Software, SP-2197, pp. 69-74