3) Logical Link Control and Adaptation Protocol (L2CAP)
It is an adaptation protocol which shield baseband protocol for high-level transfer layer protocol and application layer protocol. It provides high-level application layer and transfer layer with more effective data packet format. It is used to multiplex multiple logical connections between two devices using different higher level protocols. It provides segmentation and reassembly of on-air packets.In Basic mode, L2CAP provides packets with a payload configurable up to 64kB, with 672 bytes as the default MTU, and 48 bytes as the minimum mandatory supported MTU.
In Retransmission & Flow Control modes, L2CAP can be configured for reliable or isochronous data per channel by performing retransmissions and CRC checks.
Bluetooth Core Specification Addendum 1 adds two additional L2CAP modes to the core specification: Enhanced Retransmission Mode (ERTM) [4] and Streaming Mode (SM) [5]. These modes
effectively deprecate original Retransmission and Flow Control modes. Reliability in any of these modes is optionally and/or additionally guaranteed by the lower layer Bluetooth BDR/EDR air interface by configuring the number of retransmissions and flush timeout (time after which the radio will flush packets). In-order sequencing is guaranteed by the lower layer.