AG Electronica XBee 2 Series Manual page 24

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XBee Series 2 OEM RF Modules ‐ ZigBee ‐ v1.x2x [2007.07.019]
R1
R1
Legend
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Figure 3‐06. Sample Route Request (Broadcast) Transmission Where R3 is Trying to Discover a Route to R6t
R3
C
R2
When the destination node receives a route request, it compares the 'path cost' field against
previously received route request commands. If the path cost stored in the route request is better
than any previously received, the destination node will transmit a route reply packet to the node
that originated the route request. Intermediate nodes receive and forward the route reply packet
to the source node (the node that originated route request).
Figure 3‐07. Route Reply Sample Route Reply (Unicast) Where R6 Sends a Route Reply to R3.
R3
R2
First Route Reply
Second Route Reply
Note: R6 could send multiple replies if it identifies a better route.
Retries and Acknowledgments
ZigBee includes acknowledgment packets at both the Mac and Application Support (APS) layers.
When data is transmitted to remote device, it may traverse multiple hops to reach the destination.
As data is transmitted from one node to its neighbor, an acknowledgment packet (Ack) is
transmitted in the opposite direction to indicate that the transmission was successfully received. If
the Ack is not received, the transmitting device will retransmit the data, up to 4 times. This Ack is
called the Mac layer acknowledgment.
In addition, the device that originated the transmission expects to receive an acknowledgment
packet (Ack) from the destination device. This Ack will traverse the same path that the data
traversed, but in the opposite direction. If the originator fails to receive this Ack, it will retransmit
© 2007 Digi International, Inc.
R4
R6
R5
R4
C
R5
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Chapter 3 ‐ ZigBee Networks
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23
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