Westermo RM-80 - Manual (Page 27)

27 6193-4201
Fragmentation Threshold
The maximum transmission
unit (MTU) of data over the
radio.
This selects the maximum number of bytes that will be
transmitted in one message. If more than this number
of bytes is input into the RM-80, the module will
transmit more than one message.The default value
is 500 bytes. If fixed radio rates are configured, this
value can be increased and will reduce radio transmis-
sion overhead. For 76.8 Kbps , the fragmentation
threshold can be increased to 2000, and at 38.4Kbps,
to 1000. However if the radio path is poor, or there
is high radio interference, increasing this value will
decrease system performance as the number of re-try
messages will increase. If Ethernet traffic is only small
packets sizes (ie <300 bytes), an improvement in overall
throughput rate can be achieved by reducing MTU size.
This improvement is achieved through a reduction in
the retry holdoff time required for units in the system.
As packets in the system are smaller, units will holdoff
for shorter periods of time and be confident that their
retry transmission will not cause interference with
other units.
RSSI Threshold
The received signal strength
level at which beacons from
Access Points are to be
ignored.
This should be used to prevent Clients and Access
Points establishing links beyond a sustainable range.
The default value is set below the noise floor at -150
dBm. This allows all messages received to be processed.
If a value of –90 is entered, any beacons weaker than
–90dBm will be ignored, resulting in the link eventually
resetting if the radio path continues at less than –90.
Contention Window Size
The number of time slots
allocated by the Access Point.
The Access Point allocates a “time slot” to each Client,
to minimise transmission contention between Clients.
Each Client can only transmit during its allocated time
slot. The width of each time slot varies from 7msec to
12 msec depending on the Beacon time – the time slot
is 5msec at the shortest Beacon and 12 msec at the
longest beacon.The number of time slots can be manu-
ally set (1 to 8) or a value of zero (default) allows the
Access Point to automatically determine the number
of time slots, which it does based on the number of
Clients connected. If there are more than 8 Clients,
then more than one Client is allocated to some time
slots.In a large system where there is inherently lit-
tle transmission overlap (such as in a polling system),
then manually setting the Contention Window size to
a smaller number than 8 will increase the overall data
rate.