LOGICAL LINK CONTROL AND ADAPTATION PROTOCOL

               The logical link control and adaptation protocol or L2CAP is equivalent to the LLC sub layer in LAN's . It is used for data exchange on an ACL link . SCO channel do not use L2CAP .The below fig : shows the format of the data packet at this level .

1. The length field in the data packet represents the size of the data , coming from the upper layers . This length field is 2 - bytes .
2. The maximum data , it can support is 65,535 bytes .
3. The unique identifier for the virtual channel created at this level is defined by channel ID.

                The L2CAP ha specific duties : Multiplexing , Segmentation and Reassembly , Quality of service , and group management .

MULTIPLEXING:-    
                     This layer handles multiplexing and de-multiplexing of packets from different upper layer sources . The L2CAP can multiplexing . At the sender site , it accepts data from one of the upper - layer protocols , frames them , and delivers them to the base band layer . At the receiver site , it accepts a frame from the base band layer , extracts the data , and delivers them to the appropriate protocol layer .

SEGMENTATION AND REASSEMBLY:-  

                                             The maximum size of the payload field in the base band layer is 2774 bits , or 343 bytes . This includes 4 bytes to define the packet and packet length , so the size of the packet that can arrive from an upper layer can only be 339 bytes . The L2CAP divides these large packets into smaller segments and add extra i/f to define the location of the segments in the original packet . The L2CAP segments the packet at the source and reassembles them at the destination .

GROUP MANAGEMENT:-

                                       Another functionality of L2CAP is to allow devices to create a type of logical addressing between themselves . This is similar to multi casting .

QUALITY OF SERVICE:-

                                         This layer is responsible for quality of service when the link is in establishing state or when the link is in normal state of operation .