Central

Central

The central is the BLE device that initiates and manages connections with one or more peripheral devices. In earlier Bluetooth specifications this role was called "Master," but the Bluetooth SIG updated the terminology in Core Specification 5.3 to use the inclusive terms Central and Peripheral.

Responsibilities

The central device performs several critical functions in a BLE connection. It scans for advertising packets from peripherals, selects which device to connect to, and sends the connection request (CONNECT_IND). Once connected, the central controls the timing of all connection events, sets the connection interval, and manages the channel map for frequency hopping.

In the GATT layer, the central typically operates as the ATT">GATT client -- it discovers services and characteristics on the peripheral (GATT server) and initiates read/write operations. It also subscribes to notifications and indications by writing to the peripheral's CCCD.

Multi-Connection Capability

Modern BLE stacks allow a central to maintain multiple simultaneous connections. The nRF52840, for example, supports up to 20 concurrent BLE connections. The central's link layer scheduler interleaves connection events for different peripherals across the timeline, ensuring each peripheral gets its allocated airtime without collision.

Smartphones are the most common central devices, but BLE gateways, hubs, and embedded controllers frequently operate as centrals as well. An industrial gateway might connect to dozens of BLE sensors simultaneously, aggregating data and forwarding it over Wi-Fi or Ethernet to a cloud platform.

Central vs. Peripheral Power Profiles

In general, the central consumes more power than the peripheral because it must maintain the connection schedule and cannot skip events. However, the central is often a mains-powered or rechargeable device (phone, laptop, gateway), so this asymmetry aligns with typical deployment scenarios. Battery-constrained sensors and wearables operate as peripherals and benefit from slave latency to reduce their radio duty cycle significantly.