© 2014 Digi International Inc. 44
XBee/XBee-PRO
®
DigiMesh 2.4 User Manual
• Transmitting a unicast with a broken route.
Note: The timeouts in this section are theoretical timeouts and not precisely accurate. The
application should pad the calculated maximum timeouts by a few hundred milliseconds.
When using API mode, use the Tx Status API packets as the primary method of determining if
a transmission has completed.
Unicast One Hop Time
A building block of many of the calculations presented below is the unicastOneHopTime. As its
name indicates, it represents the amount of time it takes to send a unicast transmission between
two adjacent nodes. It is largely dependent upon the mac retry setting (RR). DigiMesh networks
assume that the average number of mac level retries across a multi-hop wireless link will be 3 or
less. It is defined as follows:
Transmitting a Broadcast
A broadcast transmission must be relayed by all routers in the network. The maximum delay
would be when the sender and receiver are on the opposite ends of the network. The NH, NN,
and MT parameters define the maximum broadcast delay as follows:
BroadcastTxTime=NN*NH*(MT+1)* 18ms
Transmitting a Unicast with a Known Route
When a route to a destination node is known, the transmission time is largely a function of the
number of hops and retries. The timeout associated with a unicast assumes the maximum
number of hops is necessary (as specified by NH). The timeout can be estimated in the following
manner:
knownRouteUnicast=2*NH*MR*unicastOneHopTime
Transmitting a Unicast with an Unknown Route
If the route to the destination is not known the transmitting module will begin by sending a route
discovery. If the route discovery is successful and a route is found then the data is transmitted.
The timeout associated with the entire operation can be estimated as follows:
unknownRouteUnicast=BroadcastTxTime+NH*unicastOneHopTime +knownRouteUnicast
Transmitting a Unicast with a Broken Route
If the route to a destination node has changed since the last time a route discovery was
completed a node will begin by attempting to send the data along the previous route. After it
RR (mac retries) Unicast One Hop Time
0 unicastOneHopTime = 5ms
1 unicastOneHopTime = 24ms
2 unicastOneHopTime = 40ms
3 unicastOneHopTime = 63ms
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