XBee‐PRO®900HP/XBee‐PRO®XSCRFModules
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Becoming a Sleep Coordinator
A node can become a sleep coordinator in one of four ways:
Preferred Sleep Coordinator Option
A node can be specified to always act as a sleep coordinator. This is done by setting the preferred sleep
coordinator bit (bit 0) in the sleep operations parameter (SO) to 1. A node with the sleep coordinator bit set
will always send a sync message at the beginning of a wake cycle. For this reason, it is imperative that no
more than one node in the network has this bit set. Although it is not necessary to specify a preferred sleep
coordinator, it is often useful to select a node for this purpose to improve network performance. A node which
is centrally located in the network can serve as a good sleep coordinator to minimize the number of hops a
sync message must take to get across the network. A sleep support node and/or a node which is mains
powered may be a good candidate.
The preferred sleep coordinator bit should be used with caution. The advantages of using the option become
weaknesses when used on a node that is not positioned or configured properly. The preferred sleep
coordinator option can also be used when setting up a network for the first time. When starting a network, a
node can be configured as a sleep coordinator so it will begin sending sleep messages. After the network is set
up, the preferred sleep coordinator bit can be disabled.
Nomination and Election
Nomination is an optional process that can occur on a node in the event that contact with the network sleep
coordinator is lost. By default, this behavior is disabled. This behavior can be enabled with the sleep options
command (SO). This process will automatically occur in the event that contact with the previous sleep
coordinator is lost. Any sleep compatible node which has this behavior enabled is eligible to become the sleep
coordinator for the network. If a sleep compatible node has missed three or more sync messages and is not
configured as a non-sleep coordinator (presumably because the sleep coordinator has been disabled) it may
become a sleep coordinator. Depending on the platform and other configured options, such a node will
eventually nominate itself after a number of cycles without a sync. A nominated node will begin acting as the
new network sleep coordinator. It is possible for multiple nodes to nominate themselves as the sleep
coordinator. If this occurs, an election will take place to establish seniority among the multiple sleep
coordinators. Seniority is determined by four factors (in order of priority):
1. Newer sleep parameters: a node using newer sleep parameters (SP/ST) is considered senior to a node
using older sleep parameters. (See the Changing Sleep Parameters section below.)
2. Preferred Sleep Coordinator: a node acting as a preferred sleep coordinator is senior to other nodes.
3. Sleep Support Node: sleep support nodes are senior to cyclic sleep nodes. (This behavior can be modified
using the SO parameter.)
4. Serial number: in the event that the above factors do not resolve seniority, the node with the higher serial
number is considered senior.
Commissioning Button
The commissioning button can be used to select a module to act as the sleep coordinator. If the
commissioning button functionality has been enabled, a node can be immediately nominated as a sleep
coordinator by pressing the commissioning button twice or by issuing the CB2 command. A node nominated in
this manner is still subject to the election process described above. A node configured as a non-sleep
coordinator will ignore commissioning button nomination requests.
Changing Sleep Parameters
Any sleep compatible node in the network which does not have the non-sleep coordinator sleep option set can
be used to make changes to the network’s sleep and wake times. If a node’s SP and/or ST are changed to
values different from those that the network is using, that node will become the sleep coordinator. That node
will begin sending sync messages with the new sleep parameters at the beginning of the next wake cycle.
Note #1: For normal operations, a module will use the sleep and wake parameters it gets from the sleep sync
message, not the ones specified in its SP and ST parameters. The SP and ST parameters are not updated with
the values of the sync message. The current network sleep and wake times used by the node can be queried
using the OS and OW commands.
Note #2: Changing network parameters can cause a node to become a sleep coordinator and change the
sleep settings of the network. The following commands can cause this to occur: NH, NN, NQ, and MR. In most
applications, these network parameters should only be configured during deployment.
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