Modified OSI Architecture for Low-Power Wireless Networks Jay Bruso Mike Matranga

Agenda Review Thesis Physical Layer Fundamentals Modulation / Demodulation Front-end circuitry PLL Data Link Layer Fundamentals Methods of data processing Proposed solution Project objectives

Thesis Proposal Applying standard OSI model to WSNs results in poor energy utilization Physical Layer lacks knowledge of data being sent across the channel Packet Format, Length, Data Type, etc. Data Link Layer forced to do work that could be done in Physical Layer Cost of performing work in Data Link Layer will be more expensive than equivalent function in Physical Layer For energy optimization Physical Layer needs to be smarter

Transceiver Block Diagram

Modulation to the CC2400 Frequency Shift Keying The CC2400 uses FSK/GFSK

Receiver Front End Review Part 1 MIXER RF AMP RF IN RF IN * A IF OUT LO IN RF amplifier: amplifies a weak RF signal coming out of the antenna. Mixer: together with the local oscillator downconverts the RF signal to the IF frequency band.

Receiver Front End Review Part 2 Power Divider Frequency Synthesizer Control Logic To Mixers N, R from TXCR Control Logic SPI from Microcontroller 90 CLK to Microcontroller Reference Clock Frequency Synthesizer is a PLL

Phase Locked Loop N and R are programmed by the microcontroller, and stored into registers in the transceiver.

Receiver Front End Review Part 3 Filter AGC ADC RSSI CLK Bandpass Filter: Removes out of band signals Automatic Gain Control (AGC): Adjusts the IF amplifier gain according to the signal level (to keep the average signal amplitude almost constant) Analog to Digital Converter : Digitizes the analog signal

Demodulation V1 V2 PLL Tracking Voltage FSK Signal In 1 This data is digitally filtered & buffered. Once the signal is recognized, the outputs CLK, SOM, RXDATA are sent to the Microcontroller

Typical WSN Block Diagram Controller configures XCVR for receive mode XCVR notifies controller with SOM signal Controller processes incoming data Controller disables XCVR when done

Methods of Data Processing Method 1, Polling for Bytes Controller continuously checks for new data received Checks if message is complete, and if so, powers off transceiver Most inefficient because CPU always running

Methods of Data Processing Method 2, Interrupt Driven Controller interrupted after every byte is received Checks if message is complete, and if so, powers off transceiver More inefficient than polling because CPU can go to sleep (enter low-power) mode in between receiving data

Methods of Data Processing Method 3, DMA Controller interrupted after a block of data is received More efficient than interrupt because CPU is woken up less often More difficult than interrupt because size of DMA transfer must be predetermined Difficult when receiving packets of unknown length Usually results in setting DMA transfer size small (5 bytes, etc)

Data Processing Comparison Method Response Time Power Consumption Polling High Interrupt Low DMA Med

Where is Power Wasted Finite amount of CPU time required to process and detect end of message Finite amount of time required for CPU to disable power to XCVR Wasted Power = (Tproc+Tpwroff)*Ireceive

What Can Be Done Provide physical layer with intelligence for detecting EOM condition Packet Format Message length encoding Offset into data stream where to find length Character frame encoding Provide register for escape character (0x80) Provide register for EOM character (0xFE) No Data Received Timer Automatically powers off if no message received within programmed time interval

Focus of Project Quantify how much power is really being wasted Design and implement a small WSN Measure power wasted Various protocols Various processing methods Various data rates MSP430 as controller ChipCon CC2400 as transceiver

Focus of Project (cont’d) Quantify how much power can be saved if physical layer were smarter Implement proposed solutions in small CPLD (XPLA3 … XCR3512XL) Repeat previous tests and measure power saved Various protocols Various processing methods Various data rates

Project Model Add DLL functionality into PL using CPLD Demonstrate improved performance Quantify energy savings Report results