4RF Aprisa SR+ User Manual - page 32
30 | About the Radio
Aprisa SR+ User Manual 1.5.3
Listen Before Send
The Listen Before Send channel access scheme is realized using Carrier Sense Multiple Access (CSMA). In
this mode, a pending transmission requires the channel to be clear. This is determined by monitoring the
channel for other signals for a set time prior to transmission. This results in reduced collisions and
improved channel capacity.
There are still possibilities for collisions with this technique e.g. if two radios simultaneously determine
the channel is clear and transmit at the same time. In this case an acknowledged transaction may be used.
The transmitter requests an ACK to ensure that the transmission has been successful. If the transmitter
does not receive an ACK, then random backoffs are used to reschedule the next transmission.
Hop by Hop Transmission
Hop by Hop Transmission is realized in the Aprisa SR+ by adding a MAC address header to the packet. For
802.15.4, there are 2 addresses, the source and destination addresses.
Adaptive Coding Modulation
The Aprisa SR+ provides Adaptive Coding Modulation (ACM) which maximizes the use of the RF path to
provide the highest radio capacity available.
ACM automatically adjusts the modulation coding and FEC code rate in the remote to base direction of
transmission over the defined modulation range based on the signal quality for each individual remote
radio.
When the RF path is healthy (no fading), modulation coding is increased and the FEC code rate is
decreased to maximize the data capacity.
If the RF path quality degrades, modulation coding is decreased and the FEC code rate is increased for
maximum robustness to maintain path connectivity.