Operation Modes of operation
XBee/XBee-PRO S1 802.15.4 (Legacy) User Guide
37
Indirect transmissions can only occur on a Coordinator. Thus, if all nodes in a network are End Devices,
only direct transmissions occurs. 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 reverts to direct transmission if it
knows the destination device 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 uses direct transmission for all subsequent data transmissions
to that device address until ST time occurs with no activity (at which point it reverts to using indirect
transmissions for that device address). "No activity" means no transmission or reception of messages
with a specific address. Broadcast messages do 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 retains 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 device
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, the device performs a CCA (Clear Channel Assessment) 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 device does not transmit the packet.
Also, the device inserts a delay before a transmission takes place. You can set this delay using the RN
(Backoff Exponent) parameter. If you set RN to 0, then there is no delay before the first CCA is
performed. The RN parameter value is the equivalent of the āminBEā 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 device attempts to re-send the
packet up to two additional times.
When in Unicast packets with RR (Retries) = 0, the device executes two CCA retries. Broadcast
packets always get two CCA retries.
Note Customers in Europe who have the XBee 802.15.4 module must manage their CCA settings. See
CA (CCA Threshold) for CA values.
Acknowledgment
If the transmission is not a broadcast message, the device expects to receive an acknowledgment
from the destination node. If an acknowledgment is not received, the packet is resent up to three
more times. If the acknowledgment is not received after all transmissions, an ACK failure is recorded.