Transmission Timeouts; Unicast One Hop Time; Transmitting A Broadcast; Transmitting With A Known Route - Digi XBee Manual

XBee/XBee‐PRO DigiMesh 2.4 RF Modules

Transmission Timeouts

When a node receives an API TX Request (API configured modules) or an RO timeout occurs (modules configured for Transparent
Mode) the time required to route the data to its destination depends on a number of configured parameters, whether the trans-
mission is a unicast or a broadcast, and if the route to the destination address is known. Timeouts or timing information is pro-
vided for the following transmission types:
•

Transmitting a broadcast

• Transmitting a unicast with a known route
• Transmitting a unicast with an unknown route
• 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, Tx Status API packets should be 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 wire-
less link will be 3 or less. It is defined as follows:
RR
(mac retries)
0
1
2
3
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 time-
out 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 fails a route discovery will be initiated and, upon completion
of the route discovery, the data will be transmitted along the new route. The timeout associated with the entire operation
can be estimated as follows:
brokenRouteUnicast=BroadcastTxTime+NH*unicastOneHopTime +2*knownRouteUnicast
unicastOneHopTime = 5ms
unicastOneHopTime = 24ms
unicastOneHopTime = 40ms
unicastOneHopTime = 63ms
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