I NTRODUCCIÓ

In order for a wireless technology to be suitable for BANs it must satisfy certain criteria. It must be efficient such that it is capable of long term use (i.e., days, weeks or months) operating on battery power. It must be secure through authentication and encryption. The data rates must be able to satisfy the applications and the wireless technology must be adaptable to accommodate different network architectures and quantity of network nodes. The two most commonly used wireless technologies in BANs are Bluetooth [40–43] and ZigBee [11,32,44,45]. Others include ultra-wideband (UWB), GPRS, WiFi, IEEE 802.15.6 and in-body radio frequency communications, as described by Yang [46].

UWB over IEEE 802.12.3a was a proposed technology for low energy, high data rate (100s of Mbps) Wireless Personal Area Network (WPAN) applications but the Task Group was dissolved in 2006. Following this WiMedia, Bluetooth Special Inter-est Group (SIG), Wireless Universal Serial Bus (USB) Promoter Group and the USB Implementers Forum all attempted to move the UWB standard forward. Blumrosen et al. [47] are researching UWB in BANs but similar works are limited. Common Off The Shelf (COTS) UWB modules are difficult to find or unavailable. Due to the lack of current research, difficulties and uncertainty with standards and limited availability of parts, UWB has not been given further consideration in this research.

Both GPRS are WiFi are suited for Tier 2 communications but not specifically for intra-BAN communications due to the relatively high power requirements. GPRS also requires a formalized network connection with a telecommunications provider.

For these reasons, GPRS and WiFi are not considered suitable wireless technologies for Tier 1 of the BAN.

IEEE 802.15.6 is the Task Group working on the standard for BAN

technolo-gies [48]. According to the Task Group this standard is still in draft and has recently been shared with sponsors to test for support. Since COTS components will not be available for some time this technology will not be further addressed in this research.

Bluetooth technology was originally adopted by the IEEE 802.15.1 Task Group in 2004. Bluetooth operates in the unlicensed 2.400 to 2.4835 GHz Industrial, Scientific and Medical (ISM) band. The physical layer can be set up to limit power consump-tion to 1mW, which provides a usable range to about 10m, or as much as 100mW, allowing a range of up to 100m. To avoid interference with other devices operating in the same frequency band and for an added security feature, Bluetooth applies a Frequency Hopping Spread Spectrum (FHSS) method for data transmission. While transmitting data, a Bluetooth device will hop between 79 channels, each of 1MHz bandwidth, 1600 times per second using a pseudo random hopping sequence that is known by the receiver and transmitter. The typical network configuration is an ad hoc network, called a piconet. In a piconet, a master can connect with up to seven slaves. In Bluetooth networks there are three distinct security modes according to Healy et al. [35]. Mode 1 is unsecured, Mode 2 initiates security features after a link is established and Mode 3 initiates security features prior to establishing a link. To add to the security features, network nodes can choose to be discoverable or non-discoverable. Non-discoverable nodes are not visible to other nodes in the network.

The E0 stream cipher is used to encrypt data. The most recent technology release is Bluetooth v4.0. Bluetooth v4.0 includes a low energy subset called Bluetooth Low Energy (BLE). Prior to BLE, Bluetooth technology had some drawbacks that made it less suitable for a BAN than competing technologies such as ZigBee. However, BLE has reduced power consumption by reducing the duty cycle of transmission to approximately 20% while maintaining the wireless data rate at 1Mbps. This of course impacts the throughput, but at 200kbps this technology still meets the needs of most BAN applications. Network setup time was also addressed in the v4.0 release, going from 6s to 3ms. As a result, BLE is gathering momentum with respect to BANs according to Yu et al. [49].

25

ZigBee is designed to be a very low power, low data rate technology and is used in many applications including industrial, building and home automation, home enter-tainment, WSNs, toys, smoke and intruder alarms, and consumer electronics. ZigBee is built on top of the IEEE 802.15.4 standard, which only defines the Physical (PHY) and Media Access Control (MAC) layers for Low Rate Personal Area Networks (LR-PANs). ZigBee defines the network layer and Application Support Sublayer (APS).

Two PHYs are specified by the IEEE 802.15.4 standard; the 868/915MHz band used in Europe, U.S. and Australia and the 2.450GHz ISM band used ubiquitously. The wireless data rate is 250kbps per channel and the useable range is 10-100 meters. Zig-Bee has a useable range of 10-100m. Support for peer to peer and star topologies is available. Three types of devices are defined by ZigBee, Coordinator, Router and End Device. The Coordinators and Routers are defined as Full Function Devices, mean-ing they include all aspects of IEEE 802.15.4. End Device are defined as Reduced Function Devices, meaning they do not use all aspects of IEEE 802.15.4. Security is based on an open trust model, which means that all layers (i.e., MAC, Network and APS) on a device trust each other. If a layer is responsible for generating a packet then that layer is also responsible for securing it. Security between devices in a network is based on the exchange of 128 bit Advanced Encryption Standard (AES) encrypted keys. ZigBee has an entity called a Trust Center (TC), which is a single device that is trusted by all other devices in the network. The TC is respon-sible for distributing keys, authenticating devices and managing end to end security in the network [35, 50]. ZigBee supports 64 bit addressing, allowing for over 65000 devices per network. Bandwidth is subdivided into 16 channels, with 5MHz spacing and each ZigBee network uses a distinct channel. In applications that have many ZigBee networks deployed in the same locale there is a possibility that two or more networks will attempt to use the same channel. Sahandi and Lui [51] have researched ZigBee networks in remote patient monitoring in a general hospital ward. The study investigated potential interference problems when ZigBee networks in the same locale used the same channels. The study concluded that multiple ZigBee networks could be applied in the same area provided that appropriate transmission time intervals are

used.

Table 3.1 summarizes several of the important aspects of both Bluetooth and ZigBee, as related to BANs. Two noteworthy points are the number of devices per network and the design complexity. In the near term, eight devices per network may be suitable for BANs but likely this number will be unacceptable as more BAN systems become commercialized. Similarly, the Bluetooth SIG will likely increase this number to respond to market demands. Design complexity is a significant challenge for Bluetooth. If these two protocols are comparable in all other ways, the extra

Table 3.1 Comparison of Low Power Wireless Technologies

Detail Bluetooth ZigBee

Data rate 200kbps-24Mbps 250kbps

Devices/ntwk 8 65536

Security E0 stream cipher 128b AES Engineering complexity high low

Power consumption low very low

Frequency ISM band ISM band

Related Standard 802.15.1 802.15.4

engineering effort required will have a significant impact on the technology selection for both researchers and commercial designers. However, Bluetooth has something very important that ZigBee doesn’t: it is nearly ubiquitous on cell phones. According to Yu et. al. [49] Bluetooth v4.0 will be on all smart phones by the end of 2012. In order for BANs to become mainstream in today’s society, integration with smart phones will have to be accommodated. In addition, Bluetooth offers higher data rate services, which may be useful for streaming of sensor data as applications and sensors evolve. However, these higher data rate services are not available with the low power consumption BLE technology. The selection of wireless technology is very difficult and readers are encouraged to allow the applications, and not current momentum, drive the selection.

27

The system designed in this work uses ZigBee because it satisfies the requirements of BANs in that it uses low power, its secure and it has the required throughput.

ZigBee was selected over Bluetooth due to the reduced engineering effort required to configure a network, ZigBee allows more devices per network and because ZigBee is the lower power solution (with the exception of BLE).

Based on the selections made to address each of the four major design challenges identified in Section 3.2, the following chapter includes a detailed description of the BAN system designed in this work.