Section 2:
Configuration
Revision 1.0a
26
© 2006 Intuicom, Inc.
the user may modify option (1) Master Packet Repeat, assigning a value between 0 (the
packet is transmitted once) to 9 (the packet is transmitted 10 times). For networks with
solid RF links, this parameter would be set at the lower end of the scale (0-1). If the
network has some weak or marginal links it would be set toward the higher values. If a
slave receives a good packet from a master more than once it will discard the repeated
packets received. In addition, once a multipoint repeater receives a good packet from the
master it will discard any of the repeated packets. In turn, the repeater will send the
packet out (to the next repeater or to the slaves) the number of times corresponding to
its
Master Packet Repeat setting. For more information on this, see the next section,
Master
Packet Repeat in Multipoint Systems with Repeaters
for additional detail.
It is important to keep in mind that increasing the master packet repeat will not only
increase the probability of a packet getting through, but will also increase latency in the
system because each packet from the master or repeater is being sent more often.
Therefore it is important to find the optimal mix between system robustness, throughput,
and latency. In general a setting of 2 to 3 will work well for most systems.
Master Packet Repeat in Multipoint Systems with Repeaters
The Master Packet Repeat parameter must also be set in multipoint repeaters when they
are used in a multipoint system. In a multipoint system a repeater looks like a master to a
slave. Therefore, the repeater will send the packet out the number of times corresponding
to its Master Packet Repeat parameter. If this parameter is set improperly the reliability
of the overall system may be compromised. For example, assume the master's Master
Packet Repeat parameter is set to 3, and the link between the master and repeater is
robust. Now assume that the repeater's Master Packet Repeat is set to 0, resulting in
marginal communications between the repeater and the slaves with which it is
communicating. The data communications between the master and those slaves
communicating through the repeater will be marginal, because it is only as strong as the
weakest link, which in this case is the link between the repeater and slaves.
Max Slave Retry and Retry Odds (Menu Option 2,3)
While packets transmitted from the master to the slaves in a Multipoint network are not
acknowledged, packets transmitted from slaves to the master are. However, it is possible
that more than one slave will attempt to transmit to the master at the same time, and it is
therefore important that a protocol exists to resolve contention for the master between
slaves. This is addressed through parameters (2) Max Slave Retry and (3) Retry Odds.
The Max Slave Retry setting defines how many times (0 to 9) the slave will attempt to
retransmit a packet to the master before beginning to use a back-off algorithm. Once the
slave has unsuccessfully attempted to transmit the packet the number of times specified
in Max Slave Retry it will attempt to transmit to the master on a random basis. The Retry
Odds parameter determines the probability that the slave will attempt to retransmit the
packet to the master; a low setting will assign low odds to the slave attempting to transmit