[Bluetooth] Bluetooth Link Layer

1. Link Layer 像是網路七層裡的Data Link層，主要是負責接收Phyiscal Layer送上來了封包進行分析處理，在一需求回應至上層(如圖一)。



圖一: Bluetooth System Architecture

2. Link Layer 的運作可以被表示成一個狀態機，並且在任何時間下，只允許運作於某一狀態。狀態共分成五種 (如圖二):



圖二 : State Diagram of the Link layer state machine
圖片來源 : Bluetooth 4.0 Spec



• Standby State
• does not transmit or receive any packets
• The Standby State can be entered from any other state.
• Advertising State
• the state will be transmitting advertising channel packets
• possibly listening to and responding to responses triggered by these advertising channel packets.
• the Advertising State can be entered from the Standby State
  
• Scanning State
• will be listening for advertising channel packets from devices that are advertising
• The Scanning State can be entered from the Standby State.
• Initiating State
•  will be listening for advertising channel packets from a specific device(s) and responding to these packets to initiate a connection with another device.
•  The Initiating State can be entered from the Standby State.
• Connection State
• can be entered either from the Initiating State or the Advertising State.
• two roles are defined
• Master Role
• Slave Role
• When entered from the Initiating State, the Connection State shall be in the
  Master Role. 
• When entered from the Advertising State, the Connection State
  shall be in the Slave Role.
 
3.  在Link layer只有兩種Packets
• Advertising Packet
• 共三組Advertising Channel(圖三)
• Data Packet 
• 共三十七組Data Channel(圖三)



圖三 : Channel Index
圖片來源 : Bluetooth 4.0 Spec

Reference:
[1]  Core Version 4.0 - Bluetooth
[2] How Bluetooth LE works? - Link layer