How Bluetooth LE works? - Link layer

Table of Contents

• 1 Bluetooth LE system architecture
• 2 Bluetooth LE Physical layer
• 3 Bluetooth LE Link Layer states
• 4 Bluetooth LE Link Layer packets

1 Bluetooth LE system architecture

This is the first post and hopefully a series of upcoming postings about Bluetooth Low Energy. Above figure is the protocol stack of a whole Bluetooth LE system.

The Bluetooth LE is one part of Bluetooth v4.0 specification, it was designed not to pursue high transmission speed but for lower energy consumption. So the Bluetooth LE products can be powered by a button-cell battery for years. How did it do that? This article is a review of Robin Heydon's excellent book – Bluetooth Low Energy, the developer's handbook.

Bluetooth LE is completely redesigned of classic Bluetooth protocol, It seems that both the Bluetooth LE and classic Bluetooth has the same architecture from above figure, but actually they are so different inside, an event in the physical layer.

2 Bluetooth LE Physical layer

BLE still works in 2.4GHz ISM band of course, but it have 40 channels with 3 of them as advertising channels and the rest of them as data channels, and each channel's bandwidth is 2 MHz. There is a website named 37channels, which is made to advertising Robin Heydon's book Bluetooth Low Energy The Developer's Handbook. Did you get the meaning of that name? BLE does has 37 data channels. This name was pretty cool, huh.

So, just to remember, BLE have 3 advertising channels and 37 data channels, and the packets transmitted between that channels are positioned in two kinds of events: Advertising and Connection events.

3 Bluetooth LE Link Layer states

The Link Layer of BLE is a state machine, which has five states: Standby, Advertising, Scanning(Active, Passive), Initiating, Connection(Master, Slave). While the scanning states include two sub-states: active and passive, and the connection state also includes two sub-states: Central & Peripheral.

Here is some terminology explanation, A Bluetooth devices can be considered in one of the following roles according to the states of its link layer:

• Advertisers: Devices that transmit the advertising packets on the advertising channels are referred to as advertisers.
• Scanners: Devices that receive advertising on the advertising channels without intention to connect to that are referred to as scanners.
• Initiators: Devices that need to form a connection to another device listens for connectable advertising packets.
• Central/Peripheral: Once a connection is established, the initiator become the Central device, the advertising device becomes the Peripheral device.

4 Bluetooth LE Link Layer packets

There are only two kinds of packets in Link Layer: Advertising packet and data packet, advertising packet was transmitted in the 3 advertising channels and data packet was transmitted in the 37 data channels.

The Link Layer's packet structure is much annoying, so I didn't decide to involve in this mess issues so mush. Maybe the upcoming articles should be focused on host protocol stacks and application levels.