© Microhard Systems Inc. Confidential 53
3.0 Mesh Configuration
Routing Request TTL
When a modem gets a packet to deliver and it does not have a route listed for that packet in the routing
table it initiates a routing request. During the routing request, and route discovery, all transmissions from
this device are stopped until the request is complete. Since several routes may be available, the modem
will wait until the entire Routing Request TTL (S219) has completed, and then pick the best path based
on a Microhard path cost calculation discussed in the next section.
S219 = 10 - Routing Request Timeout in 10ms ticks (1-10000 ticks)
The default value for S219 is 10 ticks, which means that it will wait 100ms for all possible routes to be
returned, or for the request to simply timeout if no routes are found. If the routing request times out, the
request is repeated up to 3 times automatically before the packet is discarded.
Finding the right value for S219 can be difficult. Setting a value too low may not result in the best path
being reported, or maybe not even time enough at all to receive routing information. A value too long will
affect system performance because the devices will wait unnecessary amounts of time for routing dis-
covery to complete when they may have already received the best route.
The best option for determining an ideal value for S219, without knowing what the worst case scenario
is for the time it takes for packets to traverse a fully loaded network, would be to set the timeout to a
small value to where no data is being exchanged. Then slowly increase the value to where data is being
exchanged successfully, and then add a few more ticks.
Cost of Routes
During the routing request / route discovery process, more than one route may be returned as a possi-
ble candidate for the best route. Determining which route is the best route can be determined by calcu-
lating a cost metric using the number of hops and the signal strength between hops. The following for-
mula allows a user to influence the relationship between the number of hops and the overall health of
the path based on RSSI.
Cost = (Number of Hops x Cost of a Hop) + (Cost of RSSI)
The number of hops and the RSSI are actual measured values in the system, automatic to the calcula-
tion. The cost of RSSI is set by the modem based on the measured value between hops. The stronger a
signal, the lower its cost as seen in the relationship below.
Cost
RSSI
0 -20 -40 -60 -80 -100 =110…..
Very High
None
Drawing 3-8: Cost of RSSI vs RSSI
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