XBee ZigBee networks End Device operation
XBee/XBee-PRO ZigBee RF Modules User Guide 48
If scanning all SC channels fails to discover a valid PAN, XBee ZB modules will attempt to enter a low power state
and will retry scanning all SC channels after the module wakes from sleeping. If the module cannot enter a low
power state, it will retry scanning all channels, similar to the router. To meet ZigBee Alliance requirements, the
end device will attempt up to nine scans per minute for the first five minutes, and three scans per minute
thereafter.
Note The XBee ZB end device will not enter sleep until it has completed scanning all SC channels for a valid
network.
Joining a network
Once the end device discovers a valid network, it joins the network, similar to a router, by sending an association
request (to the device that sent a valid beacon) to request a join on the ZigBee network. The device allowing the
join then sends an association response frame that either allows or denies the join.
When an end device joins a network, it receives a 16-bit address from the device that allowed the join. The 16-bit
address is randomly selected by the device that allowed the join.
Parent child relationship
Since an end device may enter low power sleep modes and not be immediately responsive, the end device relies
on the device that allowed the join to receive and buffer incoming messages in its behalf until it is able to wake
and receive those messages. The device that allowed an end device to join becomes the parent of the end device,
and the end device becomes a child of the device that allowed the join.
End Device capacity
Routers and coordinators maintain a table of all child devices that have joined called the child table. This table is
a finite size and determines how many end devices can join. If a router or coordinator has at least one unused
entry in its child table, the device is said to have end device capacity. In other words, it can allow one or more
additional end devices to join. ZigBee networks should have sufficient routers to ensure adequate end device
capacity.
In the XBee ZB 2x6x firmware, a coordinator can support ten end devices, and a router can support 12 end
devices.
In ZB firmware, the NC command (number of remaining end device children) can be used to determine how many
additional end devices can join a router or coordinator. If NC returns 0, then the router or coordinator device has
no more end device capacity (its’ child table is full).
Also of note, since routers cannot sleep, there is no equivalent need for routers or coordinators to track joined
routers. Therefore, there is no limit to the number of routers that can join a given router or coordinator device;
there is no “router capacity” metric.
Authentication
In a network where security is enabled, the end device must then go through an authentication process. See ZB
security on page 74 for a discussion on security and authentication.
Persistent data
The end device can retain its PAN ID, operating channel, and security policy information through a power cycle.
However, since end devices rely heavily on a parent, the end device does an orphan scan to try and contact its
parent. If the end device does not receive an orphan scan response (called a coordinator realignment command),
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