Murata Electronics North America DNT500FP Manual Do Utilizador
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DNT900 - 11/05/08
Access Mode
Description
Max Number of Remotes
Slot Size
0
CSMA polling
1024
manual
1
CSMA contention
1024
manual
2
TDMA dynamic slots
up to 16
automatic
3
TDMA fixed slots
up to 16
automatic
4
TDMA with PTT
up to 16
automatic
Table 2.7.1
2.7.1 CSMA Modes
When using CSMA, each remote with data to send listens to see if the channel is clear and then trans-
mits. 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 254 remotes can be supported 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. Figure 2.7.1 .1 below compares
TDMA to CSMA operation.
mits. 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 254 remotes can be supported 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. Figure 2.7.1 .1 below compares
TDMA to CSMA operation.
TDMA and CSMA operation
Figure 2.7.1.1
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 attempting to send the packet
again (back-off interval). The CSMA_ Predelay parameter controls the maximum time that a remote will
randomly backoff when it finds the channel available before transmitting.
defines the maximum time that a remote will wait after a collision before attempting to send the packet
again (back-off interval). The CSMA_ Predelay parameter controls the maximum time that a remote will
randomly backoff when it finds the channel available before transmitting.
CSMA polling (Mode 0)
- is used for point-to-point systems and point-to-multipoint systems 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_Predelay parameter is ignored on the first transmission attempt and no
predelay is used for minimum latency. This mode provides maximum throughput since there is no conten-
tion 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 CSMA_MaxBackoff, BaseSlotSize and RemoteSlot-
Size
transmits at a time, the CSMA_Predelay parameter is ignored on the first transmission attempt and no
predelay is used for minimum latency. This mode provides maximum throughput since there is no conten-
tion 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 CSMA_MaxBackoff, BaseSlotSize and RemoteSlot-
Size
parameters when using this mode. Note that a CSMA_Delay parameter setting of 0x00 would lead to
collisions if more than one remote tried to transmit. Applications where more than one remote can receive