Digi XBee Product Manual

Modes of operation for the XBee/XBee-PRO RF Modules
XBee / XBee-PRO RF Modules 802.15.4 Product Manual 30
The RF data packet structure follows the 802.15.4 specification. Refer to XBee / XBee-PRO addressing
on page 28 for more information.
Direct and indirect transmission
There are two methods to transmit data:
• Direct transmission - data is transmitted immediately to the Destination Address
• Indirect transmission - A packet is retained for a period of time and is only transmitted after the
destination module (source address = destination address) requests the data.
Indirect transmissions can only occur on a Coordinator. Thus, if all nodes in a network are End
Devices, only direct transmissions will occur. Indirect transmissions are useful to ensure packet
delivery to a sleeping node. The Coordinator currently is able to retain up to two indirect messages.
Direct transmission
A Coordinator can be configured to use only direct transmission by setting the SP (Cyclic Sleep
Period) parameter to "0". Also, a Coordinator using indirect transmissions will revert to direct
transmission if it knows the destination module is awake.
To enable this behavior, the ST (Time before Sleep) value of the Coordinator must be set to match the
ST value of the End Device. Once the End Device either transmits data to the Coordinator or polls the
Coordinator for data, the Coordinator will use direct transmission for all subsequent data
transmissions to that module address until ST time occurs with no activity (at which point it will revert
to using indirect transmissions for that module address). "No activity" means no transmission or
reception of messages with a specific address. Global messages will not reset the ST timer.
Indirect transmission
To configure Indirect Transmissions in a Personal Area Network (PAN), the SP (Cyclic Sleep Period)
parameter value on the Coordinator must be set to match the longest sleep value of any End Device.
The sleep period value on the Coordinator determines how long (time or number of beacons) the
Coordinator will retain an indirect message before discarding it.
An End Device must poll the Coordinator once it wakes from Sleep to determine if the Coordinator
has an indirect message for it. For Cyclic Sleep Modes, this is done automatically every time the
module wakes (after SP time). For Pin Sleep Modes, the A1 (End Device Association) parameter value
must be set to enable Coordinator polling on pin wake-up. Alternatively, an End Device can use the FP
(Force Poll) command to poll the Coordinator as needed.
Clear Channel Assessment (CCA)
Prior to transmitting a packet, a CCA (Clear Channel Assessment) is performed on the channel to
determine if the channel is available for transmission. The detected energy on the channel is
compared with the CA (Clear Channel Assessment) parameter value. If the detected energy exceeds
the CA parameter value, the packet is not transmitted.
Also, a delay is inserted before a transmission takes place. This delay is able to be set using the RN
(Backoff Exponent) parameter. If RN is set to “0”, then there is no delay before the first CCA is
performed. The RN parameter value is the equivalent of the “minBE” parameter in the 802.15.4
specification. The transmit sequence follows the 802.15.4 specification.
By default, the MM (MAC Mode) parameter = 0. On a CCA failure, the module will attempt to re-send
the packet up to two additional times.
When in Unicast packets with RR (Retries) = 0, the module will execute two CCA retries. Broadcast
packets always get two CCA retries.

Digi XBee Specifications

General

Operating Frequency	2.4 GHz
RF Data Rate	250 kbps
Model	XBee S2C
Category	Control Unit
Operating Voltage	2.1 to 3.6V DC
Operating Temperature	-40 to +85 °C
Interface	UART
Dimensions	24.38 x 27.61 x 9.14 mm