MAC Protocols
The dominant challenge in developing body area networks (BANs) designed to support implantable medical devices is the hard energy constraint. The MAC research of the WISAR Lab concentrates on designing a MAC which will satisfy the QoS demands of the BAN while delivering ultra low power consumption. The QoS demands include coexistence with other networks, latency, discovery/ network set up time, heterogeneous devices, sleep modes, network synchronisation, guaranteed bandwidth, emergency events, channel quality variation, packet size variation, and adaptability to changing QoS, security and privacy.
As part of this research the WiSAR Lab is evaluating the IEEE 802.15.6 MAC, which is under development. This evaluation and analysis includes the development of an 802.15.6 Simulation Model in Omnet, an Energy Analysis of IEEE 802.15.6, a QoS Characterisation of IEEE 802.15.6 and QoS and energy efficiency comparisons of IEEE 802.15.6 and IEEE 802.15.4. A further area concerns Interference and Co-existence and the aim of this research strand is to develop techniques to mitigate interference and co-existence between different radio technologies and WBAN networks between themselves.
The primary function of the MAC layer is to decide which radio device will have access to the hub at a given time and a given channel. Some classes of sensors report in a largely predictable and manner, except for when devices enter or leave the network and in these cases a Time Division Muliplex Access (TDMA) protocol may be most suitable to allow these reports to be made in an efficient manner. Less predictable sensor devices or consumer applications may require a Carrier Sense Multiple Access/Collision Avoidance scheme to resolve contention for the channel when messages are sent. Medical services on the BAN can be categorised as periodic and non-periodic. Periodic data is traditionally best suited to a TDMA type protocol i.e. monitoring temperature, glucose levels etc, whereas non-periodic data is best suited to a contention style MAC, i.e. medical emergency.
Unlike other wireless networks, it is generally difficult or impractical to charge or replace exhausted batteries, and therefore battery lifetime defines node lifetime. A battery powered implanted medical device ideally must have a lifetime of up to 10 years, while also supporting functionality including sensing, data processing, and network management. This places a severe demand on a small battery.
Since the communication operations between nodes will consume more energy than their computational operations, it is a primary objective to minimise transmit and receive operations while maintaining connectivity. This generally means that nodes must effectively sleep for most of the time, only waking up when required to carry out essential sensing and networking operations.
The main energy saving features a well designed MAC protocol must exhibit are: collision avoidance, overhearing, control packet overhead, receiver idle listening, and transmitter over emitting. In contrast with other wireless networks, important attributes such as latency, throughput, and bandwidth utilisation, may be secondary in priority in BANs. Therefore, the medium access control (MAC) protocol in a BAN, composed of nodes with very low duty cycles must be highly energy efficient.
