DNT500
2008 by RF Monolithics,
Inc.
8
M-0500-0000 Rev D
Access Mode
Description
Max # of Remotes
Remote Slot Size
0
CSMA
polling
1024 manual
1
CSMA
contention
1024 manual
2 (default)
TDMA dynamic slots
up to 15
automatic
3
TDMA fixed slots
up to 15
automatic
4
TDMA
with
PTT
1024 automatic
2.7.1 CSMA Modes
When using CSMA, each remote with data to send listens to see if the channel is clear
and then transmits. If the channel is not clear, a remote will wait a random period of time
and listen again. CSMA works best when a large or variable number of remotes transmit
infrequent bursts of data. There is no absolute to the number of remote radios that can be
supported in this mode. For a DNT500 network, a maximum of 255 remotes can be sup-
ported if base station join-leave tracking is required, or a maximum of 1024 remotes is
suggested if base station join-leave tracking is not required. The illustration below com-
pares TDMA to CSMA operation.
There are two important parameters related to CSMA operation. The CSMA_MaxBackoff
parameter defines the maximum time that a remote will wait after a collision before at-
tempting to send the packet again (back-off interval). The CSMA_Persistence parameter
sets the probability that a remote will transmit immediately rather than first waiting for a
pre-transmit delay interval. Persistence is a one-byte parameter with a range of 0x00 to
0xFF:
0xFF = 100% probability
0x00 = 0% probability
CSMA polling (Mode 0) is used for point-to-point systems and point-to-multipoint sys-
tems where only one remote at a time can receive data to transmit (ModBus, etc.). Since
only one remote will attempt to transmits at a time, the CSMA_Persistence parameter is
fixed at 0xFF for minimum latency. This mode provides maximum throughput since
there is no contention between remotes and the entire portion of the hop frame following
the base station transmission is available for a remote to transmit. The user can set
