BLE Notes

Page Contents

The Basics

  • Low power, low bandwidth application focus
  • 2.4GHz ISM Band (Industrial Scientific Medical Band)
  • Range 10-30 meters depending on the environment
  • Optimised for low power
  • Peak current under 15mA typically.
  • v4 in 2010
  • v4.2 in 2014
  • v5 in 2016
  • Operates over 40 RF channels
  • Discovery occurs over 3 channels (quicker connection)

Device Types

  • Bluetooth Smart - BLE only - single mode
  • Bluetooth Smart Ready - BLE and bluetooth classic - dual mode
  • Bluetooth - Classic only (BR - basic rate/EBR - enhanced basic rate)

BLE architecture blocks

  • Application - uses the host softeare to build use cases
  • Host - manages how two or more devicesd commuinicater
    • Generic Access Profile
    • Generic Attributre Profile
    • Attribute Protocol | Security Manager
    • Logical Link Control and Adaption Protocol
    • -- HCI --
  • Controller - a physical device that can transmit and receive radio signals.
    • -- HCI 00
    • Link Layer
    • PHysical Layer | Direct Test Mode
  • Can have as a single chip solution or 2-bip solution with host on an MCU and Controller on separate RF chip.
┌─────────────────┐
│ Application     │
└─────────────────┘
┌─────────────────┐
│ GATT            │
└─────────────────┘
┌────┐ ┌──┐ ┌─────┐
│ GAP│ │SM│ │ATT  │
└────┘ └──┘ └─────┘
┌─────────────────┐
│ L2CAP           │
└─────────────────┘
┌─────────────────┐
│ HCI             │
└─────────────────┘
┌─────────────────┐
│ Link Layer      │
└─────────────────┘
┌─────────────────┐
│ RF and PHY      │
└─────────────────┘

Layers

Physical Layer (PHY)

  • The radio hardware
  • 2.4GHz ISM band
  • Spectrum is segmented into 40 RF channels, each spearated by 2MHz
  • 3/40 channels are primary advertising channels, the rest secondary advertising and data transfer.
  • Frequency hopping spread spectrum (FHSS)
    • Minimises effect of ratio interferenced from other technologies in the same spectrum.
  • v4, v4.1, v4.2, data rate fixed at 1MBps - called 1M PHY
  • v5, 2 new physical layers introduced.
    • BLE 5 optimised for IoT. x2 speed, x4 range, x8 advertising capacity of BLE 4.
    • 2 Mbps PHY. x2 speedup! Reduces power consuption because same data transfered in less time == less radio on time.
    • Coded PHY. Longer range compared to v4 BLE. Increases power consuption because multiple symbols needed transmitted for 1 bit of data. Reduces speed since more bits required for same data.
  • Bit stream encoded using Guassian Frequency Shift Keying (GFSK).
  • Responsible for advertising, scanning and creating/maintaining a connection.
    • Advertises - sends advertising packets.
    • Scanner - scans for adverttising packets.
    • Master - initialites and manages a connection.
    • Slave - accepts a connection request.
  • Ensures packets are structured properly
                      ┌─────────────┐
                      │             │
                      │ Scanning    │ (Scans for devices that are advertising)
                      │             │
                      └──────▲──────┘
                             │
                             │
                             │
                             │
(Sends out ad packets)       │ (Radio does not tx or rx)
┌─────────────┐       ┌──────▼──────┐        ┌─────────────┐
│             │       │             │        │             │ (Scanning device decides to
│ Advertising ◄───────► Standby     ◄────────► Initiating  │  establish a connection with
│             │       │             │        │             │  advertising device)
└──────┬──────┘       └──────▲──────┘        └──────┬──────┘
       │                     │                      │
       │                     │                      │
       │                     │                      │
       │                     │                      │
       │                     │                      │
       │              ┌──────┼──────┐               │
       │              │             │               │
       └──────────────► Connected   ◄───────────────┘
                      │             │
                      └─────────────┘
                      (Device has establish a link with another device.)

Packets

  • Advertising - For finding and connecting to other devices.
  • Data - For sending/receving data once a connection is made.
PACKET STRUCTURE FOR LR UNCODED PHYs
┌─────────┐┌──────────────────┐┌──────────┐┌────────────────────┐┌─────┐┌─────┐┌───────┐
│         ││                  ││ PDU Hdr  ││ PDU Payload        ││ MIC ││ CRC ││ TERM2 │
└─────────┘└──────────────────┘└──────────┘└────────────────────┘└─────┘└─────┘└───────┘


PACKET STRUCTURE FOR LE CODED PHYs
┌─────────┐┌──────────────────┐┌──────┐┌─────────┐┌──────────┐┌────────────────────┐┌─────┐┌─────┐┌───────┐
│Preamble ││ Access Address   ││ CI   ││ TERM1   ││ PDU Hdr  ││ PDU Payload        ││ MIC ││ CRC ││ TERM2 │
└─────────┘└──────────────────┘└──────┘└─────────┘└──────────┘└────────────────────┘└─────┘└─────┘└───────┘

  • Access address

    • 32-bites in size.
    • Advertising - for broadcasting data or when advertising, scanning, or intiatin connections
    • Data access address - used in connection adter a connection has been establish between two devices

  • Header

    • Different for ad vs data packet.

  • The shortes allowable packet is with no data is 80 microseconds long.

  • Longest allowable packet is a fully loaded advertising packet, 376 microseconds long.
  • Typical advertising packet is 128 microseconds long.
  • Typical data packet is 144 microseconds long.

HCI - Host Controller Inteface

  • Sends commands to controller and receives events back.
  • Sends and receives data from a peer device.
  • Interface between the physical and logical interface between devices.
    • Loigical interface defines packet formats for commands, events and data.
    • Physical interface defines how logical packets are send between host and controller.

The Bluetooth Logical Link Control and Adaptation Protocol (L2CAP) supports higher level protocol multiplexing, packet segmentation and reassembly, and the conveying of quality of service information.

  • A protocol multiplexing layer - multiple protocols from upper layers into standard BLE.
  • Handles the Attribute Protocol (ATT) and Security Manager Protocol.
  • Enables segmentation and reassembly of packets that are larger than what the radio can deliver.
  • Error control and retransmissions.

Attribute Protocol (ATT)

To allow devices to read and write small data values held on a server, an Attribute Protocol (ATT) is defined. Each stored value, typically only a few octets, is known as an attribute. This protocol allows attribute to be self identifying using UUIDs to identify the type of data.

Attribute protocol messages are sent over L2CAP channels, known as the ATT bearers.

Attribute protocol defines two roles: Client and Server

GAT

Generic Attribute Profile (GATT) is built on top of the Attribute protocol (ATT) and establishes common operations and a framework for the data transported and stored by the Attribute protocol...

... The Bluetooth system defines a base profile which all Bluetooth devices implement. This profile is the Generic Access Profile (GAP), which defines the basic requirements of a Bluetooth device. For instance, or BR/EDR, it defines a Bluetooth device to include the Radio, Baseband, Link Manager, L2CAP, and the Service Discovery protocol functionality; for LE, it defines the Physical Layer, Link Layer, L2CAP, Security Manager, Attribute Protocol and Generic Attribute Profile. This ties all the various layers together to form the basic requirements for a Bluetooth device. It also describes the behaviors and methods for device discovery, connection establishment, security, authentication, association models and service discovery.

In LE, GAP defines four specific roles: Broadcaster, Observer, Peripheral, and Central ... In LE, GAP defines four specific roles: Broadcaster, Observer, Peripheral, and Central.

Generic Access Profile (GAP)

Application interoperability in the Bluetooth system is accomplished by Bluetooth profiles. Bluetooth profiles define the required functions and features of each layer in the Bluetooth system from the PHY to L2CAP and any other protocols outside of this specification ...

... The Bluetooth system defines a base profile which all Bluetooth devices implement. This profile is the Generic Access Profile (GAP), which defines the basic requirements of a Bluetooth device.

  • Descibes:
    • Device roles.
      • Broadcaster - (Beacon) A device that sends advertising packets. Does not use connectable ad packets - no one can connect to a broadcaster.
      • Observer - Device that scans for broadcasters and reports this information to an app.
      • Peripheral - Device that advertises by using connectable advert packets.
      • Control - Device that initialates connections to peripherals.
    • Device roles
    • Advertisement
    • Connection establishment
Broadcaster Observer Peripheral Control
Radio Radio receiver not required Radio transmitter not required Both required Both required
Transfer One way data One way data Two way data Two way data
Stack Reduced h/w and stack Reduced h/w and stack Full h/w and stack Full h/w and stack

In the advertising stateL

  • Device sends out packets at fixes intervals.
  • Same packet transmitted on each of the 3 primary advertisement channels.
    • Primary channel limited to 31 bytes
    • Secondary limited to 254 bytes
  • Scan request and response to send additional data that exceeds above limits. This is active scanning
    • Receiving scans for advertisment data and sends a scan request
    • Advertiser provides scan response if supported.
  • Passive scanning is where the device just listends for advertisment data without sending scan requests subsequently.

Pairing

Paring means that devices establish a connection with these properties:

  • Connection it capable of data encryption to keep the communication confidential.
  • Connection it capable of data authentication, ensuring the data comes from a trusted source.
  • Connection offers device authentication, confirming the identity of the devices.
  • Connection provides device tracking protection, preventing unwanted tracking.

Steps

  1. Feature exchange: paring request and response packets whcih detail a device's features and capabilities.
  2. Key generation.
  3. Transport-specific key distribution.

Bonding

  • Devices remember the security information exchanged during a successful pairing.
  • Enables automatic re-connect without having to re-pair.