
Transmission, addressing, and routing
Transmission timeouts
XBee/XBee-PRO® S2C ZigBee® RF Module
98
The ZigBee stack includes some provisions for a device to detect if the destination is an end device.
The ZigBee stack uses the unicast timeout unless it knows the destination is an end device.
The XBee API includes a transmit options bit that you can set to specify the extended timeout used for
a given transmission. If you set this bit, the extended timeout will be used when sending RF data to
the specified destination. To improve routing reliability, applications set the extended timeout bit
when sending data to end devices if:
n
The application sends data to 10 or more remote devices, some of which are end devices.
n
The end devices may sleep longer than the unicast timeout.
Equations for these timeouts are computed in the following sections.
Note
The timeouts in this section are worst-case timeouts and should be padded by a few hundred
milliseconds. These worst-case timeouts apply when an existing route breaks down (for example,
intermediate hop or destination device moved).
Unicast timeout
Set the unicast timeout with the
NH
command. The actual unicast timeout is computed as ((50 * NH)
+ 100). The default
NH
value is 30 which equates to a 1.6 second timeout.
The unicast timeout includes 3 transmission attempts (1 attempt and 2 retries).
The maximum total timeout is approximately:
3 * ((50 * NH) + 100)
For example, if NH=30 (0x1E), the unicast timeout is approximately 3 * ((50 * 30) + 100) or one of the
following:
n
3 * (1500 + 100)
n
3 * (1600)
n
4800 ms
n
4.8 seconds
Extended timeout
The worst-case transmission timeout when you are sending data to an end device is a larger issue
than when transmitting to a router or coordinator. As described in
, RF data packets
are sent to the parent of the end device, which buffers the packet until the end device wakes to
receive it. The parent buffers an RF data packet for up to (1.2 *
SP
) time.
To ensure the end device has adequate time to wake and receive the data, the extended transmission
timeout to an end device is:
(50 *
NH
) + (1.2 *
SP
)
This timeout includes the packet buffering timeout (1.2 *
SP
) and time to account for routing through
the mesh network (50 *
NH
).
If no acknowledgment is received within this time, the sender resends the transmission up to two
more times. With retries included, the longest transmission timeout when sending data to an end
device is:
3 * ((50 *
NH
) + (1.2 *
SP
))
The
SP
value in both equations must be entered in millisecond units. The
SP
command setting uses 10
ms units and must be converted to milliseconds to be used in this equation.
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