What is the difference between BLE advertising mode and connected mode?

Advertising mode is a broadcast-based, connectionless state where a BLE peripheral transmits short packets on three dedicated advertising channels (37, 38, 39) at a configurable interval (typically 20 ms to 10.24 s). Any scanner within range can receive these packets without establishing a connection. Advertising packets carry up to 31 bytes of payload (extended to 255 bytes with BLE 5.0 Extended Advertising), and a scan response can provide an additional 31 bytes if a scanner sends a scan request. This mode is ideal for beacons, broadcasting sensor readings to multiple observers, or making a device discoverable. The power consumption depends heavily on the advertising interval — a 1-second interval might draw 10-20 microamps average on an nRF52, while a 100 ms interval increases that by roughly 10x.

Connected mode establishes a dedicated, bidirectional, acknowledged link between exactly two devices (Central and Peripheral). The Central sends connection request during an advertising event, and both devices switch to data channels (0-36) using an adaptive frequency hopping scheme that avoids channels with interference. Data is exchanged in connection events at a regular connection interval (7.5 ms to 4 s). The Peripheral can use slave latency to skip a configurable number of connection events without responding, saving power when there is no data to send — for example, a slave latency of 4 with a 30 ms connection interval means the peripheral only needs to wake every 150 ms if it has nothing to transmit.

The tradeoffs are significant for system design. Advertising mode is one-to-many but unreliable (no acknowledgment, no retransmission), limited in payload, and cannot support bidirectional communication. Connected mode is one-to-one but provides reliable delivery, larger effective throughput (up to 1.4 Mbps with BLE 5.0 2M PHY and DLE), security (encryption via AES-128-CCM after pairing), and features like notifications and indications with acknowledgment. A common design pattern combines both: use advertising for initial discovery and broadcasting a device identifier, then establish a connection for configuration, firmware updates, or streaming sensor data. The choice between the two directly impacts battery life, latency, and maximum number of simultaneously communicating devices.