Murata Electronics North America DNT900 Manual Do Utilizador
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©2008 by RF Monolithics, Inc.
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DNT900 - 11/05/08
The minimum required hop duration for a DNT900 configuration is:
T
HD
= T
BRO
+
NRS*TRO
+
TRFB*(BBSS
+ N
RS
*
B
RSS
)
Where:
T
HD
is the minimum required hop duration in milliseconds
T
BRO
is the base and registration request overhead time for each hop (RF data rate dependent)
N
RS
is the number of remote slots
T
RO
is the remote overhead time for each hop (RF data rate dependent)
T
RFB
is the transmission time for one user byte (RF data rate dependent)
B
BSS
is the BaseSlotSize parameter in bytes
B
RSS
is the RemoteSlotSize parameter in bytes
The constants in the equation for each RF data rate are given in the following table:
RF Data Rate kb/s
T
BRO
ms
T
RO
ms
T
RFB
ms
38.40
11.620
4.817
0.2080
115.2
4.953
2.039
0.0694
200
3.540
1.450
0.0400
500
2.388
0.970
0.0160
Table 2.8.2
For example, consider a point-to-point CSMA Mode 0 system operating at 38.4 kb/s with the BaseSlot-
Size
Size
parameter set to 133 bytes and the RemoteSlotSize parameter set to 128 bytes. The minimum hop
duration needed to support one-hop transmissions of full slot size messages in both directions for this
configuration is:
configuration is:
= 11.620 + 1*4.817 + 0.2080*(133 + 1*128)
= 16.437 + 0.2080*261
= 70.725 ms
The closest programmable hop duration is 70.750 ms.
It should be noted that the base station operating system will commandeer 5 bytes from the BaseSlotSize
allocation in Mode 0 and up to 13 bytes in Mode 1 to send reception acknowledgements (ACKs) back to
the remotes. The BaseSlotSize should be sized accordingly. In the above example, the BaseSlotSize
parameter is set five bytes larger than the RemoteSlotSize parameter to accommodate the ACK bytes.
allocation in Mode 0 and up to 13 bytes in Mode 1 to send reception acknowledgements (ACKs) back to
the remotes. The BaseSlotSize should be sized accordingly. In the above example, the BaseSlotSize
parameter is set five bytes larger than the RemoteSlotSize parameter to accommodate the ACK bytes.
When running a point-to-multipoint network with uncoordinated remotes using CSMA Mode 1, it is useful
to set
to set
NRS
to a value of 3 or higher in the equation. Although CSMA does not create reserved time slots
for remotes, extending the hop duration this way allows several uncoordinated transmissions of user data
and/or periodic/event reports to arrive in the same slot with a relatively few collisions.
and/or periodic/event reports to arrive in the same slot with a relatively few collisions.
The performance of a CSMA Mode 1 system can often be helped by setting the MinPacketLength and
TxTimeout
TxTimeout
parameters on any remotes running transparent mode to non-zero values, especially if host
messages only contain a few bytes each and transmission latency is not critical. For starting point values,
set the MinPacketLength equal to one-half the RemoteSlotSize and TxTimeout to at least three times the
hop duration. This will help avoid excessive transmission collisions due to having many packets transmit-
ted, each carrying only a small amount of user data on top of the relatively large packet overhead
set the MinPacketLength equal to one-half the RemoteSlotSize and TxTimeout to at least three times the
hop duration. This will help avoid excessive transmission collisions due to having many packets transmit-
ted, each carrying only a small amount of user data on top of the relatively large packet overhead