About the Radio | 35
Aprisa SR+ User Manual 1.11.1
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 and Modulation
The Aprisa SR+ provides bidirectional Adaptive Coding and Modulation (ACM) which maximizes the use of
the RF path on both downlink (base station/base-repeater to remote/repeater) and uplink (remote to base
station/base-repeater or to repeater) to provide the highest radio capacity available.
When the Modulation Type is set to Adaptive, in unicast packets, the transmitted modulation and coding
will be determined by the signal quality of the link to the destination radio. Link quality for each radio is
determined both concurrently and independently.
When a broadcast packet (such as broadcast IP, multicast IP, and serial) is sent from the base to a
remote/repeater, or from a repeater to a remote which is received by all remote radios, the signal quality
of the worst remote link is used.
When the RF path is healthy (no fading), modulation is increased and / or the FEC code rate is decreased to
maximize the data capacity.
If the RF path quality degrades, modulation is decreased and / or the FEC code rate is increased for
maximum robustness to maintain path connectivity.
Summary of Contents for Aprisa SR+
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