STM32WB-WBA Connection Oriented Channel – COC

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1. STM32WB-WBA - Connection Oriented Channel

A Connection Oriented Channel - COC allows the exchange of Bluetooth^® Low Energy data at L2CAP layer without GATT layer. There is no notion of client/server, only central and peripheral.
To set a COC logical link, it is necessary to establish first a Bluetooth^® Low Energy link.Over this Bluetooth^® Low Energy link, one or several COC can be established.
The central or the peripheral may be COC initiator.

Bluetooth^® LE architecture

1.1. General L2CAP Terminology

Bluetooth® LE General L2CAP Terminology

Term	Description
L2CAP channel	The logical connection between two endpoints in peer devices, characterized by their Channel Identifiers (CIDs)
SDU or L2CAP SDU	Service Data Unit: a packet of data that L2CAP exchanges with the upper layer and transports transparently over an L2CAP channel
PDU or L2CAP PDU	Protocol Data Unit: a packet of data containing L2CAP protocol information fields, control information, and/or upper layer information data
Maximum Transmission Unit (MTU)	The maximum size of payload data, in octets, that the upper layer entity can accept (that is, the MTU corresponds to the maximum SDU size).
Maximum PDU Payload Size (MPS)	The maximum size of payload data in octets that the L2CAP layer entity can accept (that is, the MPS corresponds to the maximum PDU payload size).

1.2. L2CAP Connection Oriented Channel Example

L2CAP COC example
File:Connectivity L2CAP COC example.png

1.3. L2CAP COC establishment

ACI_L2CAP_COC_CONNECT

L2CAP COC connect command
File:Connectivity L2CAP COC Connect.png

ACI_L2CAP_COC_CONNECT_CONFIRM

L2CAP COC connect confirm command
File:Connectivity L2CAP COC Connect Confirm.png

MTU frames are fragmented into K-frames of MPS size at application level on transmitter side.
MTU frames are reassembled at application level on receiver side.
Initial Credits are handled at application level once they have been exchanged through both previous commands.

1.4. L2CAP COC data exchange

ACI_L2CAP_COC_TX_DATA

L2CAP COC Tx Data command
File:Connectivity COC Tx Data.png

This command sends a K-frame packet on the specified connection-oriented channel.
Note: for the first K-frame of the SDU, the Information data shall contain the MTU Length coded on two octets followed by the K-frame information payload. For the next K-frames of the SDU, the Information data shall only contain the K-frame information payload.
Application must handle MTU length, fragmentation in K-frame, length of current transmitted frame and credits number.

Reception of ACI_L2CAP_COC_RX_DATA_EVENT

L2CAP COC Rx Data event
File:Connectivity COC Tx Data.png

Application must handle MTU length received in first K-frame, K-frame number, length of current received frame and credits number.

1.5. L2CAP COC flow control

ACI_L2CAP_COC_FLOW_CONTROL

L2CAP COC Flow Control command
File:Connectivity COC Flow Control.png

ACI_L2CAP_COC_FLOW_CONTROL_EVENT

L2CAP COC Flow Control Event
File:Connectivity COC Flow Control.png

Application must handle credits number on transmitter and receiver side.

1.6. Bluetooth^® Low Energy connection and COC establishment initiated by the central

1.6.1. On central side

Initiate a Bluetooth^® Low Energy link: send aci_gap_create_connection
Wait for HCI_LE_CONNECTION_COMPLETE_EVENT
Initiate a CoC connection: ACI_L2CAP_COC_CONNECT
Wait for ACI_L2CAP_COC_CONNECT_CONFIRM_EVENT (get channel_index_list)

Range of channel_index_list for the COC initiator: from 0 to 0x1F

1.6.2. On peripheral side

Start advertising: send aci_gap_set_discoverable
Wait for HCI_LE_CONNECTION_COMPLETE_EVENT
Wait for ACI_L2CAP_COC_CONNECT_EVENT
Send ACI_L2CAP_COC_CONNECT_CONFIRM
In hci_command_complete_event, get Channel_number and Channel_Index_List values

Range of channel_index_list for the COC peripheral: from 0x20 to 0x3F

1.7. COC data exchange

1.7.1. On sender side

Send ACI_L2CAP_COC_TX_DATA (Channel_Index, length, data)

1.7.2. On receiver side

Reception of ACI_L2CAP_COC_RX_DATA_EVENT

1.8. Example of Bluetooth^® Low Energy connection, COC establishment and data exchange

Bluetooth^® Low Energy and COC link establishment

L2CAP COC data exchange
File:Connectivity COC data.png

1.9. Code Example

A STM32WB COC code example has been shared on STM32-Hotspot ^[1]
A STM32WBA COC code example is available with BLE_SerialCom application

1.10. References

↑ COC code example

[1] COC code example

[1]