User's Manual

FreeWave Technologies Spread Spectrum Transceiver User Manual
900 MHz and 2.4 GHz
V5.0S
11
Multipoint Operation
Theory of Operation
In a Multipoint system a 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 slaves responding to the master when given data by the device
connected to the RS232 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 (those sent from the master or repeater out to slaves or
other repeaters) are sent a fixed number of times (see Master Packet Repeat).
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 (data going back 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 completely different from point-to-point
applications. The theoretical maximum number of slaves that can be configured
into a Multipoint network is a function of the data throughput needed from each of
the slaves. 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. If, on
the other hand, each slave will be transmitting data at greater levels then fewer
slaves may be connected to the master (the overall system will be closer to
capacity with fewer slaves). The theoretical limit of 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. RS232 baud rate.
3. The amount of contention between slaves.
4. Use of repeaters. A single repeater in a Multipoint network will decrease
overall system capacity by 50%; more than one repeater does not further
decrease network capacity.
Installing Multipoint Systems
When installing multipoint systems it is important that some planning is done up
front. Unlike point to point systems (where the master will set certain system
parameters), a multipoint system requires that many parameters are set
consistently on all transceivers in the system. This includes RF data rate, min and
max packet size, number of repeaters, and frequency key.
Furthermore, if several independent multipoint systems are to be located in close
proximity the planning becomes much more critical. In this scenario it becomes
very important to include as much frequency and time diversity as possible through
use of different frequency keys, min and max packet sizes, and frequency banks.
Please contact FreeWave Technologies if you have any questions about the
installation of multipoint systems.