2000, 2001 Cirronet
Incorporated
7
M-2410-0007 Rev -
always be configured for maximum data throughput, even at the expense of some
throughput for the slaves. This is the factory default configuration.
In frequency hopping technology, the shorter the hop duration, the lower the data latency
but the lower the throughput. This is because the overhead required is the same regardless
of the hop duration. Thus at shorter hop durations, the overhead is a larger percentage of
the hop time. Longer hop durations provide more throughput but have a higher data
latency. If data from a slave appears just after the slave’s designated transmit time, the
slave will have to wait one hop duration before it can transmit the data. As the hop
durations of the SEM products are very short, ranging from 3.3ms to 17.7ms, the better
approach is to pick the hop duration to maximize data throughput without unnecessarily
penalizing latency.
The maximum number of bytes that can be transmitted on a single hop by either a master
or a slave is 208 bytes. The factory configuration is set up that the master will always be
able to transmit 208 bytes per hop. The slave devices split the remaining time equally and
transmit as much as the can each hop. The amount of time remaining will depend on the
hop duration. The table below gives data throughputs for multiple slaves based on the
default hop duration for a SEM.
Slaves
Hop Duration
Slave Throughput
Master
Throughput
Aggregate
Throughput
1
10ms
208Kbps
208Kbps
416Kbps
2
10ms
118.6Kbps
208Kbps
445.2Kbps
3
10ms
69.5Kbps
208Kbps
416.5Kbps
4
10ms
44.8Kbps
208Kbps
387.2Kbps
If the application has more slave devices, or if it is desirable to increase the throughputs
of the slaves at the expense of the master SEM, contact Cirronet technical support for
more details.
