Campbell Scientific LoggerNet Datalogger Support Software - Manual (Page 214)

Section 6. Network Status and Resolving Communication Problems
LoggerNet responds by sending an RF Test command packet, containing a
definition of the RF path to be tested, to the RFBase-TD. The RFBase-TD
executes the RF Link Quality test by sending an RF Test Packet across the
network to the RFRemote-TD/PB at the end of the RF path. As the end of path
modem receives the RF Test Packet, it internally logs a TD-RF Quality Report
detailing information about the size of the packet and its ability to decode the
data contained in the packet. (For details, see Section 6.5.3.2 TD-RF Quality
Report.) The end of path modem will then send the RF Test Packet back across
the network toward the RFBase-TD. As the RF Test Packet makes the return
trip across the network, each RFRemote-TD/PB modem in the path as well as
the RFBase-TD will log a TD-RF Quality Report based on the received packet.
The RFBase-TD then sends a command across the network to collect the TD-
RF Quality Report from each of the modems involved in the test. The RFBase-
TD sends the collected reports to LoggerNet in the RF Test response packet.
The TD-RF Quality Reports are displayed in the right-hand pane of the TD-RF
Quality Test window in the order (top-down) they were collected; starting with
the end of path modem and ending with the RFBase-TD.
6.5.3.2 TD-RF Quality Report
An RF modem primarily functions as an interface bridging two distinctly
different modes of data transport; the wired medium of dataloggers and PCs,
and the wireless medium of RF transceivers. The RF modem accomplishes this
by converting the serial data stream from the wired medium into a waveform of
proper amplitude and frequency for driving the FM modulator circuits of the
RF transceiver, and vice versa.
In a process known as line coding, the RF modem encodes the binary data from
the wired data stream onto a 3 KHz waveform. The line coding utilized in
CSI’s TD-RF modems is called Miller Encoding. This encoding scheme
employs a method of differentiating the binary “1s” and “0s” of the data stream
based on the timing of transitions in the waveform from one level to another
within the bit period (approximately 333 microseconds for a 3 KHz bit rate). A
binary “1” is represented by a level transition occurring in the middle of the bit
period. A binary “0” is represented by either there being no transition occurring
within the bit period or, in the case of consecutive “0”s, the transition occurs at
the end of the bit period.
In order to properly decode the encoded data from a received signal, one must
precisely detect when the level transitions are occurring in relation to the
middle and end of the bit period; the T1 and T2 transition points respectively.
The OS in the RF modem does this by establishing a detection window
centered about the time a transition is expected to occur. The detection window
is 204 units wide, so the optimal transition timing would occur in the center of
the window; unit 102. In the real world, the optimal transition timing is not
likely to occur that often so there will always be some deviation to where the
transitions occurs within the detection window. The more noisy the received
signal, the greater the deviations. If the transition occurs outside the detection
window, the proper bit will not be detected and a data error will occur. The
demodulated data stream and the associated detection windows for the T1 and
T2 transition points are illustrated in the graphic below.
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