Bluetooth logical link control and adaptation (L2CAP) structure

ission for each channel through flow control and retransmission mode.

The L2CAP layer has a logical channel, that is, the L2CAP channel, which is mapped to the L2CAP logical link supported by ACL logical transmission.

Basic operation

L2CAP is based on the "channel" concept. Each endpoint of the L2CAP channel is called a channel identifier (CID)

Channel identifier

The channel identifier (CID) is the local name, which represents the logical channel endpoint of the device. CID allocation is related to a specific device, and a device can be assigned a CID independently of other devices (unless it needs to use any of the several reserved CIDs)
Operations between devices

The figure above illustrates the use of CID when communicating between corresponding L2CAP entities of the same type on separate devices. The connection-oriented data channel refers to the connection between two devices, and the CID identifies each end of the channel. The connectionless channel restricts data flow in a single direction. These channels are used to support channel "groups", where the CID of the source device represents one or more remote devices. There are many CIDs reserved for special purposes. One example of reserved channels is the signaling channel. This channel is used to create and establish connection-oriented data channels and to agree on changes in the characteristics of connection-oriented and connectionless channels.

Operating mode

L2CAP can run in one of three different modes selected for each L2CAP channel.