The L2CAP layer loosely matches the data-link layer of the OSI model. Apart from framing and multiplexing of packet streams, it also supplies QoS for the ACL links. Two alternatives services are provided:

» Connectionless service: Datagram-like service without establishing a connection.

» Connection-mode service: A connection is required before a data exchange between the master and a slave.

» Three types of channels are provided by the L2CAP layer. For each type, a channel identification (CID) is assigned to identify the channel in use:

1. Connectionless: Unidirectional channel used primarily for broadcasting by the master. A slave can only have one connectionless channel to the master. Its CID is 2. Connectionless channels are used to implement connectionless service.

2. Connection-oriented: Full-duplex channel for connection-mode service. Between a slave and the master there can be multiple connection-oriented channels, each of which is identified by a unique CID larger than 63.

3. Signaling: This is not for data exchange but for signaling between L2CAP entities.

Protocol Data Units (PDUs) handled by the L2CAP layer are of a similar format across the three types of channels. In addition to the payload data (in the case of a signaling command PDU, the payload is the command representation), a field of PDU length and a field of CID are encapsulated. CIDs of connection-oriented channels are used to conduct multiplexing and demultiplexing of upper layer data sources. For connectionless channels, a PDU that is carried by the channel has a protocol/service multiplexing (PSM) field to indicate its upper layer source. On the transmitter side, the L2CAP-layer PDUs may be fragmented into small segments if the underlying logical channel cannot send packets of that length.

Source of Information : Elsevier Wireless Networking Complete 2010


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