SEM910
the master automatically configures the radio in the master as a base radio. Similarly, configuring
a SEM as a slave automatically configures the slave radio as a remote radio.
When a master SEM is powered on, it becomes active immediately, even if no slave SEMs are
detected. It will attempt to send packets addressed to devices that it thinks are not on its local
network. When a slave SEM is powered on, it listens for a master SEM and attempts to register
with the radio in the master. This detection and registration process can take up to 2 seconds.
During this time, no packets will be sent or received over the RF link by the slave.
If the application is such that more than one point-to-point link needs to located in the same area,
each master/slave pair must be assigned different network numbers. This will allow the SEMs to
identify the appropriate other SEM to which they should communicate. Because different
network numbers have different hopping sequences, this also allows various pairs to operate in
the same area without interfering with each other. Refer to the section on radio commands for
details on setting network numbers.
Point-to-Multipoint Mode
In point-to-multipoint mode multiple slave SEMs link with a single master SEM. Similar to the
point-to-point mode, each slave must register with the master, a process that can take up to 2
seconds. All of the radio addressing and registration occurs automatically and is transparent to
the application. A maximum of 15 slave SEMs can be connected to a master SEM. The amount
of data a slave can transmit depends on the number of slaves connected and the hop duration of
the radio network. Because packets from one slave that are intended for another slave must pass
through the master SEM, the master SEM should always be allocated 50% of the data
throughput, even at the expense of some throughput for the slaves. This is the factory default
configuration.
In frequency hopping technology, the following applies... the shorter the hop duration, the lower
the data latency but also 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.
The factory configuration is set up that the master will have one-half of the bandwidth reserved
for it. 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
25ms
75 Kbps
75 Kbps
150 Kbps
2
25ms
37.5 Kbps
75 Kbps
150 Kbps
3
25ms
25 Kbps
75 Kbps
150 Kbps
4
25ms
18.75 Kbps
75 Kbps
150 Kbps
©
2000- 2004 Cirronet
™
Inc
19
M-910-0003 Rev -
