Real-Time Wireless Sensor Network Platform


  FireFly is a low-cost wireless sensor network platform capable of data acquisition, processing & multi-hop mesh communication. Each battery-operated node functions with scalable & economical global time-synchronization and delivers a lifetime of 1.5-2 years. Fixed and mobile nodes can dynamically form a network and facilitate applications such as utility monitoring, surveillance, location tracking and voice communication. It is easy to add-on custom modules for user-specific applications.


  Each FireFly node features an IEEE 802.15.4 transceiver capable of short-range (50-100m) data communication with a maximum raw data rate of 250Kbps. An 8-bit microcontroller processes data from (optional) on-board light, motion, audio, temperature and acceleration sensors. Each node has a SD Flash card slot for add-on data acquisition and storage modules. In addition, FireFly nodes feature a low-power AM/FM radio receiver to periodically acquire a time synchronization pulse. The time sync pulse is radiated through the building using an AM carrier current transmitter (FCC part 15 compliant). The AM radio transmitter is plugged into a power outlet in the building and uses the building’s power grid as an extended AM antenna to radiate a periodic pulse. For example, in our test-bed at Carnegie Mellon University, we are able to radiate a 50us pulse once every 5 seconds across an 8-story campus building. Our 2nd generation design features a digital Radio Data Broadcast System (RDBS) receiver so any public radio station can broadcast the time sync pulse across cities. FireFly runs the Nano-RK real-time sensor operating system for easy deployment.

How it Works

  Global time synchronization enables extremely energy efficient operation and extends the lifetime of each node by a factor of 4-5X more than current sensor networking platforms. Global time synchronization maximizes the common sleep time between nodes and enables high throughput operation with bounded end-to-end delay. Each node is scheduled to transmit and receive data on dedicated 5ms time slots and are shutdown at all other times. As shown in the figure on the right, nodes operate with Low Power Listening (small pulses) where they wake up for 25us, check if there is any transmission and go back to sleep if there is none. If there is data to be received, they wake up on pre-assigned slots (long pulses) and promptly go back to sleep after activity. Nodes also operate in contention mode (series of short pulses) where a mobile or unscheduled node may send data in one of many available contention slots. This way we cater to both delay sensitive periodic sensing and communication tasks and also to asynchronous environment-triggered events in an energy efficient manner.


  Using this tightly time synchronized regime of operation, nodes with pre-programmed or dynamically assigned schedules are able to achieve the maximum possible throughput in a collision-free manner. The table on the right lists the low-power operation values for each FireFly node. Real-time applications such as delay sensitive voice communication (24ms packet intervals), image capture and forwarding for surveillance (150kbps goodput) and asset tracking (in-network processing) are examples where FireFly nodes have been used.

  Through experiments and analysis we have demonstrated that the Real-Time Link (RT-Link) protocol running over a network of FireFly nodes is capable of operating with an effective lifetime of 1-2 years on 2AA Li-ion batteries. RT-Link outperforms Low-Power Listen CSMA protocols such as the Berkeley B-MAC sensor protocol by the use of hardware-based global time synchronization.

Deployment Experiences

  We have successfully deployed a network of 42 FireFly nodes in the NIOSH coal mine for people tracking (bottom left figure). The network was configured to deliver both low duty-cycle sensor data and high rate voicemail communication for stranded miner. At all times, the latest position of the miners was tracked. The sensor network functions with a low duty-cycle during normal operation and is able to switch over to a high-rate mode for voice communication during emergencies. RT-Link supports on-demand rate control and can switch the networks operation based on the current application’s throughput and end-to-end delay requirements.

  A multi-hop mesh network of FireFly nodes was deployed in a campus building to report surveillance sensor data (bottom right figure) and detect people presence across the floor.

FireFly supports both hardware-based global time synchronization and in-band software-based sync. (a) Shows the TDMA schedule generated to support both high-speed voice streaming and low-speed sensor probing. (b) Shows the hybrid time sync with hardware sync along the mine backbone and software sync everywhere else.

Click on Images to Enlarge

FireFly Nodes with Hardware-based Time Sync. Each node features light, audio, temperature, humidity, acceleration

Carrier-Current AM Radio Transmitter plugs into an ordinary power outlet and periodically sends out a 50us synchronization pulse throughout the building

Tight Global Time Synchronization with sub-100us accuracy and 5ms transmit time slots. The RT-Link TDMA protocol supports fixed nodes with explicitly scheduled slots and contention slots.

The RT-Link MAC protocol provides near optimal node lifetimes and out performs Low-Power Listening CSMA. RT-Link provides practical lifetimes of 1.5 to 2 years on 2AA batteries.

Inside the NIOSH Coal Mine: FireFly nodes are hung from the ceiling and multi-hop along the mine corridors.

Real-Time & Multimedia Systems Lab

Carnegie Mellon University
Dept. of Electrical and Computer Engineering
5000 Forbes Ave.
Pittsburgh, PA 15213
Phone: 412.268.6064
FAX: 412.268.3690

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