Specifications
XBee®/XBee‐PRO®SERFModules
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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 on
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 XBee Smart Energy 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.
Device Registration
The trust center (coordinator) is responsible for deciding which devices can join the smart energy
network. To prevent unwanted devices from joining the network, the coordinator sends the
network key encrypted by the trust center link key. For a device to join a smart energy network
and receive the network key, it must either:
• Have its 64-bit address and initial link key registered with the trust center, or
• Be pre-configured with the same trust center link key used by the trust center.
The Register Joining Device API frame (0x24) is used to provide the coordinator with the address
and key information for each device that will join the network. See Chapter 9 for details.
Key Establishment
The trust center supports the key establishment cluster on endpoint 0x5E. If a device attempts to
perform key establishment with the trust center, the trust center may spend up to 4 seconds
performing computations. During this time, its Associate LED will cease blinking, and the
coordinator will not be responsive to serial or RF traffic. CTS will de-assert during these periods,
indicating when the application should avoid sending serial data.
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.