XBee‐PRO®900HP/XBee‐PRO®XSCRFModules
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The MAC layer is the building block that is used to build repeater capability. Repeater mode is implemented with a
network layer header that comes after the MAC layer header in each packet. In this network layer there is additional
packet tracking to eliminate duplicate broadcasts. In this delivery method, unicasts and broadcast packets are both
sent out as broadcasts that are always repeated. All repeated packets are sent to every radio. Broadcast data will be
sent out the serial port of all radios that receive it.
When a unicast is sent, it specifies a destination address in the network header. Only the radio that has the matching
destination address then will send it out the serial port. This is called a directed broadcast. Any node that has a CE
parameter set to route will rebroadcast the packet if its broadcast hops (BH) or broadcast radius values have not
been depleted. If a repeated broadcast has already been seen, the node will ignore it. The NH parameter sets the
maximum number of hops that a broadcast will be repeated. This value is always used, unless a BH value is specified
that is smaller.
By default the CE parameter is set to route all broadcasts. As such, all nodes that receive a repeated packet will
repeat it. By changing the CE parameter, you can limit which nodes repeat packets, which can help dense networks
from becoming overly congested while packets are being repeated.
Transmission timeout calculations for directed broadcast/repeater mode are the same as for DigiMesh, and can be
found below in the DigiMesh section.
DigiMesh Networking
Related Command: MR
In the same manner as the repeater delivery method, DigiMesh builds on P2MP and repeater modes. In DigiMesh,
broadcasts always use repeater delivery method, but unicasts use meshing technologies. In the DigiMesh network
layer, there are additional network layer ACKs and NACKs. Mesh networking allows messages to be routed through
several different nodes to a final destination. DigiMesh firmware allows manufacturers and system integrators to bol-
ster their networks with the self-healing attributes of mesh networking. In the event that one RF connection
between nodes is lost (due to power-loss, environmental obstructions, etc.) critical data can still reach its destina-
tion due to the mesh networking capabilities embedded inside the modules. Note that if you disable network ACKs,
the network will never heal.
DigiMesh Feature Set
DigiMesh contains the following features
• Self-healing
Any node may enter or leave the network at any time without causing the network as a whole to fail.
• Peer-to-peer architecture
No hierarchy and no parent-child relationships are needed.
• Quiet Protocol
Routing overhead will be reduced by using a reactive protocol similar to AODV.
• Route Discovery
Rather than maintaining a network map, routes will be discovered and created only when needed.
• Selective acknowledgements
Only the destination node will reply to route requests.
• Reliable delivery
Reliable delivery of data is accomplished by means of acknowledgements.
• Sleep Modes
Low power sleep modes with synchronized wake are supported with variable sleep and wake times.
Data Transmission and Routing
Unicast Addressing
When transmitting while using DigiMesh Unicast communications, reliable delivery of data is accomplished
using retries and acknowledgements. The number of mesh network retries is determined by the MR (Mesh
Network Retries) parameter. RF data packets are sent up to MR + 1 times across the network route, and
ACKs (acknowledgements) are transmitted by the receiving node upon receipt. If a network ACK is not
received within the time it would take for a packet to traverse the network twice, a retransmission occurs.
Note that when sending a DigiMesh Unicast that both MAC and NWK retries/acknowledgments are used.
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