Digi XBee User Manual

© 2014 Digi International Inc. 42
XBee/XBee-PRO
®
DigiMesh 2.4 User Manual
support all 16 channels and XBee-PRO modules support 12 of the 16 channels. The operating
channel on a module is selected using the CH command.
For modules to communicate, the channel (CH) and network identifier (ID) must be equal on all
modules in the network.
DigiMesh Data Transmission and Routing
Unicast Addressing
The use of retries and acknowledgements ensures reliable data delivery when transmitting using
Unicast communications. The number of retries is determined by the NR (Network Retries)
parameter. RF data packets are sent up to NR + 1 times and ACKs (acknowledgements) are
transmitted by the receiving node upon receipt. If the node does not receive a network ACK
within the time it would take for a packet to transverse the network twice, the data is
retransmitted. If a network ACK is not received within the time it would take for a packet to
traverse the network twice, a retransmission occurs.
To send Unicast messages, set the DH and DL on the transmitting module to match the
corresponding SH and SL parameter values on the receiving module.
Broadcast Addressing
Broadcast transmissions is received and repeated by all routers in the network. Broadcast
transmissions do not use ACKs. Therefore, the originating node will send the broadcast multiple
times. By default a broadcast transmission is sent four times. Essentially the extra transmissions
become automatic retries without acknowledgments. This results in all nodes repeating the
transmission four times as well. In order to avoid RF packet collisions, a random delay is inserted
before each router relays the broadcast message. (See NN parameter for details on changing this
random delay time.) Sending frequent broadcast transmissions can quickly reduce the available
network bandwidth and as such should be used sparingly.
The broadcast address is a 64 bit address with the lowest 16 bits set to 1. The upper bits are set
to 0. To send a broadcast transmission set DH to 0 and DL to 0xFFFF. In API mode the
destination address would be set to 0x000000000000FFFF.
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 router which will route the message on to its
destination. A message with a broadcast address is broadcast to all neighbors. All routers
receiving the message will rebroadcast the message MT+1 times and eventually the message
will reach all corners of the network. Packet tracking prevents a node from resending a broadcast
message more than MT+1 times.

Digi XBee Specifications

General

Operating Frequency	2.4 GHz
RF Data Rate	250 kbps
Model	XBee S2C
Category	Control Unit
Operating Voltage	2.1 to 3.6V DC
Operating Temperature	-40 to +85 °C
Interface	UART
Dimensions	24.38 x 27.61 x 9.14 mm