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Bluetooth/BLE Notes

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BLE

Application Layers                                                             
┌────────────────────────────────────────────────┐                              
│   Profiles, Services                           │                              
└────────────────────────────────────────────────┘                              
                                                                                
Host Layers                                                                     
┌─┬──────────────────────────────────┬───────────────────────────────────┐      
│ │                                  │                                   │      
│ │ Generic Attribute Profile (GATT) │ Generic Access Profile (GAP)      │      
│ ├──────────────────────────────────┼────────────────────────────────┐  │      
│ │ Attribute Protocol (ATT)         │ Security Manager Protocol (SMP)│  │      
│ │   MTU Payload Bytes:             │                                │  │      
│ │   L2CAP_MTU - 4 (header) = 19    │                                │  │      
│ └──────────────────────────────────┴────────────────────────────────┴──┤      
│   Logical Link Control and Adaptation Protocol (L2CAP)                 │      
│     Default MTU: 23 bytes, up to 251; Header uses 4 bytes              │      
└────────────────────────────────────────────────────────────────────────┘      
                                                                                
Controller Layers                                                               
┌─────────────────────────────────────────────────────────────────────┐         
│   Host Controller Interface (HCI - Data/Control)                    │         
├─────────────────────────────────────────────────────────────────────┤         
│  Isochronous Adaptation Layer                                       │         
│  Link Layer - 27 bytes, up to 251 bytes (Bluetooth 5)               │         
│  Physical Layer - Depending on PHY (1Mbps/2MBps) or coding scheme   │         
└─────────────────────────────────────────────────────────────────────┘

Application Layers

  • Defines the use case and functionality. Specifies how to interpret data coming from specific GATT Services
  • Unit and format of the data is calculated and normalized here

Host Layers

  • L2CAP provides multiple “logical channels” defined by the Channel ID (CID)
  • This is NOT the same as the 40 radio channels at the physical layer
    • The CID namespace is defined in the Bluetooth spec, Volume 3, Part A, Table 2.3
    • Usually, CID 0x0004 Attribute Protocol (ATT) is used for the actual data because that goes up to GATT and then the Attributes themselves
    • Dynamic CIDs are used for custom applications
    • Generic Access Profile (GAP) defines how devices discover, connect, and secure communication with each other. GAP uses L2CAP’s signaling capabilities to manage connection parameters and updates. Similar to a Control Plane.
  • If ATT_MTU is larger than physical packet size (Link Layer), it will be fragmented automatically. L2CAP is responsible for dividing up and re-assembling the packets, such that from ATT’s perspective it just gets one payload blob.
  • GATT Services, Characteristics and Descriptors have their own UUIDs for identification
    • You can design your own GATT Services and Characteristics; or use existing well-known ones.

Physical Layers

  • Physical Layer has Protocol Data Unit (PDU) of 27 bytes (up to 251 bytes in Bluetooth 5.0)

More Details

Bluetooth Classic

TBD