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XBee/XBee‐PRO DigiMesh 2.4 OEM RF Modules
Routing
A module within a mesh network is able to determine reliable routes using a routing algorithm and
table. The routing algorithm uses a reactive method derived from AODV (Ad-hoc On-demand
Distance Vector). An associative routing table is used to map a destination node address with its
next hop. By sending a message to the next hop address, either the message will reach its
destination or be forwarded to an intermediate node which will route the message on to its
destination. A message with a Broadcast address is broadcast to all neighbors. All receiving
neighbors will rebroadcast the message and eventually the message will reach all corners of the
network. Packet tracking prevents a node from resending a broadcast message twice.
Route Discovery
If the source node doesn't have a route to the requested destination, the packet is queued to
await a route discovery (RD) process. This process is also used when a route fails. A route fails
when the source node uses up its network retries without ever receiving an ACK. This results in
the source node initiating RD.
RD begins by the source node broadcasting a route request (RREQ). Any node that receives the
RREQ that is not the ultimate destination is called an intermediate node.
Intermediate nodes may either drop or forward a RREQ, depending on whether the new RREQ has
a better route back to the source node. If so, information from the RREQ is saved and the RREQ is
updated and broadcast. When the ultimate destination receives the RREQ, it unicasts a route reply
(RREP) back to the source node along the path of the RREQ. This is done regardless of route
quality and regardless of how many times an RREQ has been seen before.
This allows the source node to receive multiple route replies. The source node selects the route
with the best round trip route quality, which it will use for the queued packet and for subsequent
packets with the same destination address.
Sleeping Routers
Sleeping routers under DigiMesh allows for all nodes in the network to synchronize their sleep and
wake times. All synchronized nodes enter and exit a low power state at the same time. This
forms a cyclic sleeping network. Nodes synchronize by receiving a special RF packet called a synch
message which is sent by a sleep coordinator. Any node in the network can become a sleep
coordinator through a process called nomination. The sleep coordinator will send one synch
message at the beginning of each wake period. The synch message being a broadcast packet is
repeated by every node in the network.
There are two modes of operation:
A network should consist of nodes operating in the same mode. Mesh route discovery, and thereby
routing, is incompatible between nodes operating in different modes. However, during network
setup and maintenance, a mix of nodes is useful, and can be tolerated provided route discovery is
not attempted.
Sleep Mode
A node in cyclic sleep mode sleeps for a programmed time, wakes in unison with other nodes,
exchanges data and synch messages, and then returns to sleep. While asleep, it cannot receive RF
messages, neither will it read commands from the UART port. Sleep and wake times are set by SP
and ST respectively. These parameters must be set the same for all nodes in the network. If D7 =
1 (CTS Flow control) CTS is de-asserted while asleep, and asserted while awake.
An unsynched sleeping node, newly powered, will wake and poll for a synch message and then
return to sleep, repeating the cycle until it becomes synched by receiving a synch message. Once
synched, the node will wake to exchange messages for the programmed time interval and then
return to sleep.
SM0 - Normal (default)
SM4 - Cyclic Sleep (low power)
© 2008 Digi International, Inc.
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