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Bluetooth® Low Energy (BLE) Link Layer Device Address Roles and States

Last modified by Microchip on 2023/11/10 11:16
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From the Bluetooth® Low Energy (BLE) link layer perspective, the following role pairs are defined:

  • Advertiser/Scanner (Initiator)
  • Slave/Master
  • Broadcaster/Observer

The roles exist and evolve during various phases of discovery/connection during Unicast (peer-peer) or Broadcast connections.

Unicast (Peer-Peer) Connection

The following diagram depicts two BLE hosts, initially in a Standby (unconnected) state. They enter a Discovery state whereby the device wishing to be discovered becomes the Advertiser and the host wishing to connect becomes a Scanner. The Advertiser sends advertising packets containing basic information about the host. All Scanners receive these packets.

Unicast (Peer-Peer) Connection Example

At some point, the Scanner (after filtering/analyzing information contained in the advertising packets) becomes an Initiator and decides to initiate a connection with a specific advertiser. This is known as the Connecting phase and is highlighted by the Initiator sending a CONNECT_REQ advertising packet to the Advertiser:

Connecting phase

Finally, the Advertiser accepts the connection request, thus becoming the Slave while the Initiator becomes the Master. This is known as the Connected phase:

Connected phase

Note that the Link Layer Master is also the GAP Central and GATT Client, while the link layer Slave is the GAP Peripheral and GATT Server.

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Broadcast Connection

For broadcast connections, the link layer roles do not change. The defined roles are Broadcaster (the host sending the packets) and Observer.

Note:

  • Messages are one-way
  • Messages are one-to-many
  • ADV_IND is one of three advertising packet types that can be used by Broadcasters to broadcast data to Observers

Broadcast Connection

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Link Layer States

As shown in the accompanying image, these roles are defined in the five states of the link layer state machine:

Link Layer State Machine

It's important to note that vendor-specific state labels are often substituted for the standard ones mentioned above. For example, the BM70 radio defines the following labels for its link layer states:

BM70 radio link layer states

Note the additional state (Shutdown) which is not defined in the BLE specification.

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