Spread Spectrum Wireless Data Transceiver User Manual
Choosing Point-to-MultiPoint or Point-to-Point Operation
A Point-to-Point system is limited to one Master and one Slave transceiver. Repeaters may be added in
to extend the reach of the network, but no other Master or Slave may be added.
In a MultiPoint system the transceiver, designated as a Master, is able to simultaneously be in
communication with numerous Slaves. In its simplest form, a MultiPoint network functions with the Master
broadcasting its messages to all Slaves and the Slaves responding to the Master when given data by the
device connected to the data port.
It is important to note the differences between Point-to-Point and MultiPoint systems. In a Point-to-Point
system all packets are acknowledged, whether sent from the Master to the Slave or from the Slave to the
Master. In a MultiPoint system, outbound packets from the Master or Repeater to Slaves or other
Repeaters are sent a user defined number of times. The receiving transceiver, Slave or Repeater, will
accept the first packet received that passes the 32 bit CRC. However, the packet is not acknowledged.
On the return trip to the Master, all packets sent are acknowledged or retransmitted until they are
acknowledged. Therefore, the return link in a MultiPoint system is generally very robust.
Traditionally, a MultiPoint network is used in applications where data is collected from many instruments
and reported back to one central site. As such, the architecture of such a system is different from Point-
to-Point applications. The number of radios in a MultiPoint system is influenced by the following
parameters:
1.
Size of the blocks of data. The longer the data blocks, the smaller the system capacity.
2.
Baud rate.
3.
The amount of contention between Slaves. Polled slaves vs. timed slaves.
4.
Use of Repeaters. Using the Repeater setting in a MultiPoint network will decrease overall
system capacity by 50%.
For example, if the network will be polling Slaves once a day to retrieve sparse data, several hundred
Slaves could be configured to a single Master. However, if each Slave will be transmitting data at greater
levels, then fewer Slaves should be linked to the Master. The overall system will be closer to capacity
with fewer Slaves.
For examples and additional information on data communication links, see the section Examples
of Data Communication Links later in this document.
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