Once a Bluetooth connection is established, Smart Remote performs several actions in order to ensure secure and reliable operation.
In order to provide satisfactory level of security, the HID service is allowed to send data only over an encrypted link. As a result, link encryption must be performed as soon as possible in order to enable data flow.
Because Smart Remote is a Bluetooth peripheral device, it cannot enforce link security. After a connection from a bonded host, the remote waits for CONFIG_ENC_REQ_DELAY miliseconds for an encryption request sent from the host. If during that time a link encryption procedure is not initiated, the remote sends its own request. If a secure link cannot be established, the remote disconnects and either starts advertising (if there is no bond stored or power management is disabled) or triggers a shutdown procedure.
Smart Remote implements Bluetooth connection parameters negotiation in order to obtain the most efficient link configuration. If the host chooses parameters different than the ones set in the Smart Remote configuration file (CONFIG_MIN_CONN_INTERVAL_MS, CONFIG_MAX_CONN_INTERVAL_MS, CONFIG_SLAVE_LATENCY, and CONFIG_CONN_SUP_TIMEOUT_MS), the remote tries to renegotiate them. In such case, the first connection parameter update request is sent CONFIG_FIRST_CONN_PARAMS_UPDATE_DELAY miliseconds after establishing the connection. If the negotiation is not successful, or the negotiated parameters are outside of the configured range, the remote waits CONFIG_NEXT_CONN_PARAMS_UPDATE_DELAY seconds and sends another connection parameter update request. The process repeats up to CONFIG_MAX_CONN_PARAMS_UPDATE_COUNT times unless satisfactory parameters are negotiated.
Smart Remote supports both 1 Mbit/s and 2 Mbit/s PHYs (accordingly with Bluetooth 5.0 specification). By default, the remote answers on PHY update request and advertises support for both rates. The fastest PHY supported by both the host and the remote is then used. The remote can also initiate PHY negotiation if CONFIG_PHY_SEND_UPDATE_REQUEST option is set to 1. List of the available PHYs can be altered by respective options (CONFIG_PHY_TX_1MBPS, CONFIG_PHY_TX_2MBPS, CONFIG_PHY_RX_1MBPS, CONFIG_PHY_RX_2MBPS).
In order to make negotiations described above as fast as possible, the remote temporarily disables slave latency. It is enabled back when link security is established (in case of connection with bonded host), or when all HID characteristics are enabled (in case of a first connection with the given host). Such approach minimizes the time needed to set up the Bluetooth link, allowing for fast bonding and reconnection.