www.RFM.com
Technical s1.678.684.2000
Page 16 of 82
©2009 by RF Monolithics, Inc.
E-mail:
DNT2400 - 08/18/09
affect the throughput. Nevertheless, it is important to note that the throughput for the remote in the exam-
ple above is substantially less than the remote slot size in bytes divided by the hop duration.
It is not a DNT2400 requirement that the complete application message be sent in a single hop, nor that
the remote response is returned in a single hop, when in transparent mode. If either transmission occurs
over more than one hop, then depending on the length of the data, the RF data rate and the serial port
data rate at the receiving end, there may be a gap in the serial data. Some protocols, such as Modbus
RTU, use gaps in data to determine packet boundaries.
2.10.3 CSMA Throughput
In CSMA mode, remote radios do not have a fixed throughput, which is why applications requiring guar-
anteed throughput should use polling or a TDMA mode. The reason that the throughput of a CSMA
remote is not fixed is because its ability to transmit at any given time depends on whether another radio is
already transmitting. The throughput of a remote is further affected by how many other remotes are
waiting for the channel to become clear so they can transmit. This is not a problem when remotes, even a
large numbers of remotes, only send data infrequently. The DNT2400 includes several configuration
parameters that can be used to optimize the performance of a CSMA network.
It is often desirable to limit the amount of data a CSMA remote can send in one transmission. This pre-
vents one remote from hogging network throughput. To accommodate this, the DNT2400 provides a
CSMA_RemtSlotSize parameter that is user configurable. When a remote has transmitted CSMA_
RemtSlotSize bytes on a given hop, it will stop transmitting until the next hop. Note that this remote will
have to contend for the channel on the next hop, so it is not guaranteed that it will be the first remote to
transmit on the next hop or that it will be able to transmit on the next hop at all. To allow multiple remotes
a chance to transmit on the same hop, the HopDuration parameter must be set long enough to support
the BaseSlotSize, plus the number of remotes to transmit per hop multiplied by the CSMA_ RemtSlotSize,
plus the number of remotes to transmit per hop multiplied by the CSMA_Backoff. Because of the way
CSMA channel access works, this does not guarantee that the desired number of remotes to transmit on
a hop will always be able to transmit on a single hop. This is due to the fact that when a remote with data
to send finds the channel busy a second time, it waits for a longer period to time before testing the chan-
nel again. This time will continue to increase until the remote finds the channel clear. In practice this is
unlikely to present a problem, as CSMA networks are used with devices that infrequently have data to
send.
The DNT Throughput Calculator can be used to determine the HopDuration, but it will be necessary to
increase the number of remotes in DNTCalc to a value greater than the number of remotes to transmit on
a single hop to account for the backoffs. It is indeterminate how many backoffs may occur during a single
hop, which is why the number of remotes that transmit on a given hop cannot be guaranteed. Note that
the CSMA_ Backoff parameter sets the length of time a remote will wait to recheck the channel when it
has detected that the channel is busy. The second time a remote detects that the channel is busy, it will
increase the amount of time it waits until it checks again. Every subsequent time it detects a busy channel
it will increase the amount of time it will wait in a geometric fashion. This continues until it detects an idle
channel. So while a short CSMA_Backoff can decrease the time between when one remote transmits and
the next remote transmits, it can actually lead to a longer time between remote transmissions than a
longer backoff. This can occur when the remote checks the channel multiple times during the transmitting
remote’s transmission causing the back-off time to be increased.
