CSS means Chirp Spread Spectrum. For more information about the CSS radio layer and its power profiles, see the "Sub-GHz Protocol Stack" > "Amazon Sidewalk Endpoint connection modes" > "Asynchronous access mode profiles" section in the Amazon Sidewalk specification.
CSS is an asynchronous protocol, which means it does not keep a connection with the gateway. Though transmits are asynchronous, during an active downlink phase listening windows for reception occur every 5 seconds as shown below. These windows either repeat indefinitely or after a transmission for a limited time, depending on the active connection profile (also referred to as "power profiles" A and B).
Power Consumption and Energy Modes#
CSS Power Profiles#
Power profile A opens a limited number of listening windows after a transmission. Power profile B opens listening windows periodically while also assuring regular transmission to keep the connection alive. In power profile B, the gateway considers the endpoint inactive after 5 minutes without transmission and stops sending messages. To prevent this, the Sidewalk stack implements a keep-alive mechanism for power profile B that creates a power consumption overhead not present in power profile A. There is a trade-off between this additional overhead and lower latency, as in power profile A a device must re-perform time synchronization before transmission if too much time has elapsed since the last transmit. Power profile B is the default connection profile in the Sidewalk stack.
The CSS power profile can be modified by the
sid_option API call using option
SID_OPTION_900MHZ_SET_DEVICE_PROFILE. Then the structure
sid_device_profile_unicast_params can be updated with the following values:
The device profile ID possible values
The number of RX windows is either
SID_RX_WINDOW_CNT_INFINITEfor power profile B and any of the other values in enum
sid_rx_window_countfor power profile A.
The windows separation can be any of the values of enum
Values for the type of event for which the device wakes up are contained in enum