Central vs peripheral roles?

The central (formerly master) initiates and manages the connection: sets timing parameters, channel map, and PHY. The peripheral (formerly slave) advertises and accepts connections. A phone is typically central; a sensor is peripheral. BLE 4.1+ allows a device to be both simultaneously.

Generic Attribute Profile structures data exchange using Services (groups of related data) and Characteristics (individual data values with read/write/notify properties). For example, a heart rate monitor exposes a Heart Rate Service with a Heart Rate Measurement Characteristic that sends notifications.

Wireless Protocols

BLE Connection

Q: Connection vs advertising?

Advertising is one-way broadcast with no acknowledgment. Connections provide bidirectional, acknowledged communication with error detection and retransmission. Use advertising for beacons and simple sensors; use connections for interactive communication, firmware updates, and reliable data transfer.

A BLE Connection establishes a bidirectional, acknowledged link between a central device and a peripheral. Connection events occur at regular intervals (7.5 ms to 4 s) on frequency-hopped data channels. Each event exchanges data packets using the GATT (Generic Attribute Profile) framework of Services and Characteristics. The connection provides error detection, retransmission, encryption (AES-128 CCM), and negotiable PHY selection.

Category: Wireless Protocols

Security: AES-128 CCM

Understanding BLE Connections

Connection establishment: the central scans for advertisers, sends CONNECT_IND, and both devices synchronize timing. From then on, the central sends a packet at every connection event; the peripheral responds within 150 μs. If no data is pending, empty packets maintain synchronization. Each event hops to a new channel per the channel map.

Key parameters: connection interval (how often events occur), slave latency (how many events the peripheral can skip), and supervision timeout (how long before a lost connection is declared). These trade off responsiveness, throughput, and power consumption.

Connection Parameters

Connection interval: 7.5 ms to 4 s
Slave latency: 0 to 499 events
Supervision timeout: 100 ms to 32 s

Effective interval with latency:
T_eff = CI × (1 + slave_latency)

Connection Lifecycle

Phase	Duration	Action
Discovery	10 ms - 10 s	Central scans, finds advertiser
Connect	~1.25 ms	CONNECT_IND sent
Service discovery	50-500 ms	GATT attribute enumeration
Data exchange	Ongoing	Read/Write/Notify via GATT
Disconnect	~1 ms	LL_TERMINATE_IND

Common Questions

Frequently Asked Questions

Central vs peripheral?

Central initiates/manages connection (phone). Peripheral advertises and accepts (sensor). BLE 4.1+ allows dual role.

What is GATT?

Structures data as Services and Characteristics with read/write/notify. Heart rate monitor: HR Service with HR Measurement Characteristic.

Connection vs advertising?

Advertising: one-way broadcast. Connection: bidirectional, acknowledged, encrypted. Use connections for interactive and reliable data.

Related Terms

← BLE Coded PHY Connection Interval →

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