A cyclic MAC layer synchronisation approach for time-critical low-power body sensor networks

Emerging areas of body sensor networks (BSNs) include closed-loop applications with the need for a frequent exchange of sensor data between implanted and/or body-worn devices. An example is the Artificial Accommodation System, a set of two micro-mechatronic systems implanted into the capsular bags of the eyes, which require continuous exchange of sensor data at 10 Hz. This paper will present a light-weight medium access control (MAC) protocol capable of synchronising two sensor nodes through the cyclic dissemination of regular data messages and time stamping on MAC layer on the receiver side. The inter-arrival time of two messages is used to successively predict the time of arrival of a future message. Thus, idle listening, which is a major source of energy wastage in time division multiple access (TDMA) protocols, can be reduced to a minimum just dependent on the underlying clock accuracy of the system without requiring additional protocol overhead for synchronisation purposes. The protocol was implemented in an experimental setup of the Artificial Accommodation System using off-the-shelf components. Due to space restrictions, the internal RC oscillators of standard microcontrollers were employed as clock sources. Despite their low frequency accuracy, remarkably low duty cycles below 0.6% were obtained for a bidirectional data exchange of 8-byte messages at 10 Hz and 250 kbit/s.