11/26/13

1. P14251 Underwater Acoustic Communication • Framing Parameters • Sentinels and Header Information • Control frame data • Frame sizes • Frame transmission times • Error Detection/Correction 11/26/13 Rochester Institute of Technology 1

2. P14251 Underwater Acoustic Communication • Frame Sentinels • Sentinels are a unique pattern of bits that signify the start and end of the framei.e. 01110 –data– 01110 • When preparing the frame for transmission, if the pattern appears anywhere in the data, bit stuffing is used to eliminate it (i.e. 01110 -> 011010) • Commonly used sentinel which we’ll use is 01111110 • Frame Header • 1 bit to signify the type of frame: control or message • 1 bit that gets flipped each time a new frame is sent 11/26/13 Rochester Institute of Technology 2

3. P14251 Underwater Acoustic Communication • Control frame formats • Since it’s especially important to interpret control frames correctly, 4 bits are used to display the unique patterns • 0000 – Request to Send (RTS) • 0110 – Clear to Send (CTS) • 1001 – Acknowledgement (ACK) • 1111 – Done (Signifies that all frames have been sent) 11/26/13 Rochester Institute of Technology 3

4. P14251 Underwater Acoustic Communication • Frame Sizes • - 15kbits per second, after compression and encoding = 9kbits • - 9kbits = Number of Frames * Message Bits/Frame • Minimum frame sizes:- Control Frame: 18 (overhead) + 4 (type) = 22 bits- Message Frame: 18(overhead) + X (message) = 18+X bits 11/26/13 Rochester Institute of Technology 4

5. P14251 Underwater Acoustic Communication • Frame Transmission Times • (per 2 bits) • Frames that get transmitted for a message: • Send RTS – 22 bits • Receive CTS – 22 bits • Send all message frames – 18+x bits/frame • Receive all acknowledgements – 22 bits/frame • Send Done – 22 bits • Receive Acknowledgement – 22 bits • - Tx time for x bits = 20ms + (40μs/2)*(18+x) 11/26/13 Rochester Institute of Technology 5

6. P14251 Underwater Acoustic Communication • Frame Transmission Times • - Tx time for x bits = 20ms + (40μs/2)*(18+x) • Frames that get transmitted for a message: • Send RTS = 20.4 ms • Receive CTS = 20.4 ms • Send all message frames (20ms + (40μs/2)*(18+x))*Y • Receive all acknowledgements 20.4 ms * Y • Send Done = 20.4 ms • Receive Acknowledgement = 20.4 ms • Where x = number of bits, Y = number of frames, and x*Y = 9000 11/26/13 Rochester Institute of Technology 6

7. P14251 Underwater Acoustic Communication • Frame Transmission Times • -From (40ms + (40μs/2)*(40+x))*Y and x*Y = 9000, with the maximum time left for transfer being under 0.9s: • Maximum number of frames Y = 15, • where x = 600 bits/frame, • and the total transmission time = 0.792s • - This is much better than a 9kbit frame, but is still a risk area for error correction/detection- The propagation delay is by and large the biggest limiting factor (ignoring frame sizes, it takes 40ms per message/ACK pair, * 15 frames = 0.6s) 11/26/13 Rochester Institute of Technology 7

8. P14251 Underwater Acoustic Communication • Brief note Error Detection/Correction • If a 600 bit codeword takes too long to decipher: • - Partition the frame into five 120 bit codewords (or possibly smaller) • - If necessary, use multi-threading to try and decode all five 120 bit codewords at the same time • - Alternatively, it may be possible to send fewer frames, if the propagation time reduction grants enough code execution time • - Either BCH or Reed-Solomon will be used 11/26/13 Rochester Institute of Technology 8