XBee®/XBee‐PRO® ZB SMT RF Modules
If a valid PAN is not found, the end device performs the PAN scan on the next channel in its scan channels list
and continues this process until a valid network is found, or until all channels have been scanned. If all channels
have been scanned and a valid PAN was not discovered, the end device may enter a low power sleep state and
scan again later.
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 9 scans per minute for the first 5 minutes, and 3 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.
The initial release of software on this platform supports up to 20 end devices when configured as a coordinator
or a router.
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
chapter 5 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), it will leave the network and try to discover and join a new network. When the end device leaves a
network, the previous PAN ID and operating channel settings are lost.
© 2010 Digi International, Inc.
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DCD [ DIGITAL CORE DESIGN ]
