RITS
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The Routing Integrated Time Synchronization (RITS) is a reactive timesync protocol, which can be used to obtain times of event detections at multiple observers in the local time of the sink node(s). From the application’s point of view, RITS is an extension of the routing service Upon detecting an event, the application on the sensor node generates a data packet containing the event information, and timestamps it with the value of the local time of detection. It forwards the packet with the timestamp to the routing service, which delivers it to the sink. RITS places no assumptions on the network topology or routing algorithm beyond those that are required by the application. Rather than performing explicit timesync after the event of interest is detected, RITS performs inter-node time translation along the routing path from an observer node to the sink: as the data packet travels from node to node in the network, RITS converts the corresponding timestamp from the local time of the sender to that of the recipient. When the packet arrives at the sink, the routing service signals an event to the application layer that a packet has been received. The application can then query the routing layer for the timestamp of the received packet, which is returned in the local time of the sink.
Integrating a reactive timesync service with message routing has several benefits over a standalone timesync service:
- Coupling of event data and event timestamps. There is a tight logical coupling between event information and the corresponding timestamps. RITS retains this coupling: in a data packet, event data and timestamps are physically collocated. RITS thus implements implicit timesync, that is, the flow of time information is embedded in the flow of data. There are no pure timesync messages, hence RITS has virtually no communication overhead.
- Network-transparent event timestamps. As data packets propagate in the network, RITS converts the corresponding time stamp hop by hop to the local time of the recipient node. As a result, all data packets received by a given node contain event timestamps in the recipient node’s local time, independently from where in the network the events originated.
- Packet aggregation. Packet aggregation helps decrease the number of message transmissions. In fact, not only does the number of radio messages decrease, but also the overall payload size. This is because in an aggregated radio message, n data packets (containing event information and event timestamp) share only one transmit timestamp.
- Packet filtering. Through packet filtering support, it is possible to discard outdated messages at intermediate nodes enroute to the destination, thus decreasing the message load.
