Structural Health Monitoring Using Smart Sensors - ideals ...

On reception of a NACK, the sender resends the last packet. Simplicity is a main
advantage of this method. The IEEE 802.11 wireless LAN adapts the stop-and-wait
protocol. This protocol is, however, not efficient in the sense that the sender needs to wait
for acknowledgment of each packet.
In the go-back-N protocol, the sender transmits more than one packet without waiting
for acknowledgment. The receiver keeps receiving packets replying ACK until an error is
detected. If an error is found in the N-th packet, the N-th packet and all subsequent packets
are discarded. The receiver sends back a NACK with the number corresponding to the lost
packet. The sender resends packets from the N-th packet. Discarding the received packets
after the N-th packets lowers the efficiency of this protocol.
Selective-repeat protocol addresses the inefficiency in the go-back-N protocol. The
sender continues sending unsent packets. Once the receiver detects an error in a packet, a
NACK is sent back to the sender. Packets received after detection of an error are stored in
a reception buffer. Only the packet with an error is disregarded. On reception of a NACK,
the sender temporary stops sending unsent packets and resends the lost packet. Then the
sender continues to send the remaining packets.
These concepts of ARQ can be utilized to guarantee reliable communication among
smart sensors. The reliable communication protocol developed by Mechitov et al. (2004)
sends a set of packets and then waits for acknowledgment. If the receiver does not receive
ACK, the same set of packets is sent again. Upon reception of ACK, the sender moves on
to the next set of packets. Even when one of packets sent in a group is lost, all of the
packets are sent again, reducing the efficiency of the protocol.
In this section, concepts from the ARQ protocol are modified for reliable wireless
communication. The RF component on the Imote2 is in either a listening mode or a
transmission mode. During transmission, the Imote2 cannot receive packets. However, in
selective-repeat protocol, transmission and reception are deeply interwoven. The receiver
may send an acknowledgment while the sender is in transmission mode. Packet loss and
retransmission are expected to be more frequent if the ARQ protocol is implemented
directly. Scheduling interwoven transmission and reception may result in long waiting
times. In the proposed protocols, the sender transmits all of the packets before this node
expects an acknowledgment packet. The receiver stores all of the received data in a buffer.
Once the receiver gets a message indicating the end of data transmission, the receiver
replies to the sender, indicating which packets are missing. Only missing packets are sent
again. In this way, the number of acknowledgments and retransmissions can be greatly
reduced.
This protocol is designed to send either 64-bit double precision data, 32-bit integers,
or 16-bit integers. Many ADCs on traditional data acquisition systems have a resolution
less than 16 bits, supporting the need for transfer of 16-bit integer format data. Some
ADCs have a resolution better than 16 bits, necessitating data transfer in 32-bit integer
format. Once an acceleration record is processed, the outcome may need more bits.
Onboard data processing such as FFTs and SVDs is usually performed using double
precision calculations. Even when the effective number of bits is smaller than 32,
debugging of onboard data processing greatly benefits from transfer of double precision
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