SAS Medium Access Protocol
Up until now, network protocols have been optimized for either high performance or low power consumption precluding the deployment of large networks of autonomous, battery powered nodes. Fantastic Data's Synchronous/Asynchronous (sas) medium access protocol achieves both goals. The sas protocol rapidly switches from asynchronous, contention service to synchronous, collision free service depending upon data load and criticality. The hybrid approach optimizes for performance when that is critical and for low power consumption when performance is not as important. It is a completely distributed protocol with no special nodes, control points, boundary conditions, or limit on the size of the network. The sas protocol provides the following benefits:
Distributed and local control
Slots may be scheduled for communication to or from a specific node, subset of nodes, or all nodes. Before scheduling a slot, the node checks its knowledge of slot schedules for conflicts. After scheduling a slot, the node informs its neighbors, which then pass the information on to their own neighbors. Slots may be scheduled for transmit or receive and may be scheduled for point-to-point, fan-in (1 receiver from multiple potential transmitters) or fan-out (1 transmitter to multiple receivers) operation. Collision detection and a special comb backoff algorithm manage multiple access on fan-in slots.
Schedule conflicts are rare since the number of slots is far greater than the number required at a particular time. Conflicts are resolved by having one of the conflicted nodes change its schedule. Conflicts occur when 2 nodes in a neighborhood simultaneously schedule the use of the same slot, or more often during network formation when 2 nodes have scheduled the use of the same slot before they become connected. Slots are scheduled with an effective first and last use time and are only used within that time period. Slots scheduled by the transmitter are not used until acknowledged by the intended receiver. Time slots and frequency channels are scrambled on every frame based on a shared key and time of day.
The sas protocol may be configured to alter the slot schedule upon a number of types of events. Some events (such as excess load or a high collision rate) are measured within the communication stack, while other events may be generated by application specific code.
By far the most interesting event type for a sensor network is a cue from an external process. The sas protocol reacts to a cue from the surveillance system that sensor data may be forthcoming to produce a broadcast slot. With approximately 12 seconds of leadtime, the sas protocol can reconfigure the network neighborhood from a laid-back, barely active state to a state supporting the rapid movement of sensor data.
Once two-way communication is established, the nodes exchange other information about themselves and their neighbors, so that both nodes may build knowledge of activities in a neighborhood about themselves. Upon learning of a neighbor of a neighbor, the node attempts to contact that neighbor of a neighbor. This procedure establishes additional links more quickly and more efficiently than the unscheduled search that discovered the first link.
Hammel, T., Rich, M. A higher capability sensor node platform suitable for demanding applications. In Information Processing in Sensor Networks Conference, 2007 (IPSN 2007) (Cambridge, Massachusetts, April 25, 2007). ACM Press, 2007. paper presentation
Sensor Node User's Guide. May 25, 2006. manual