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L2CAP Channel

* L2CAP is based around the concept of ’channels’. * Each one of the endpoints of an L2CAP channel is referred to by a **channel identifier (CID)**.
## CHANNEL IDENTIFIERS * The null identifier (0x0000) shall never be used as a destination endpoint. * Identifiers from 0x0001 to 0x003F are reserved for specific L2CAP functions. * Identifiers from 0x0001 to 0x003F are referred to as **Fixed Channels**.
## Signaling channel * Fixed Channel 0x0001 is the L2CAP Signaling channel. * Fixed Channel 0x0005 is the L2CAP LE Signaling channel. * Those two should be supported at a minimum.
## The characteristics of fixed channel * defined on a per channel basis. * include configuration parameters (e.g., reliability, MTU size, QoS), security, * the ability to change parameters using the L2CAP configuration mechanism.
## Fixed Channels * Fixed channels are available as soon as the ACL-U or LE-U logical link is set up. * Fixed channels should be initialized when the ACL-U or LEU logical link is set up. * Fixed channels shall only run over ACL-U, ASB-U, or LE-U logical links. * Fixed channels shall not be moved.
## Non-Fixed Channels * Implementations are free to manage the remaining CIDs * Two simultaneously active L2CAP channels shall not share the same CID. * A different CID name space exists for each ACL-U, ASB-U, and LE-U logical link. * The AMP-U logical link shares the CID name space with its associated ACL-U logical link.
![01](http://docs.ifjy.me/contents/wireless/025/images/004.png)
## Dynamically allocated CID * CID allocattion is local to a particular logical link. * When shared the same CID on the same channel with several remote devices, it still can be distinguished by associating each remote CID with a different device. * When shared the same CID with the same remote device, they will be bound to a different logical link.
## OPERATION BETWEEN DEVICES * The connection-oriented data channels represent a connection between two devices. * a CID, combined with the logical link, identifies each endpoint of the channel.
* for connectionless channel, when * used for broadcast transmissions, data flow in a single didrection. * used for unicast transmissions, data flow in either direction between a master and a slave.
## Fixed channel 0x0001(Signal channel) * used to create and establish connection-oriented data channels * negotiate changes in the characteristics of connection-oriented channels * discover characteristics of the connectionless channel operating over the ACL-U logical link.
## Fixed channel CID (0x0002) * reserved for all incoming and outgoing connectionless data traffic, whether broadcast or unicast.
![03](http://docs.ifjy.me/contents/wireless/025/images/002.png)
## OPERATION BETWEEN LAYERS * L2CAP implementations transfer data between upper layer protocols and the lower layer protocol. * L2CAP implementation should export a number of services. * L2CAP implementation shall also support a set of signaling commands.
* L2CAP implementations should also be prepared to * accept certain types of events from lower layers. * generate events to upper layers.
![02](http://docs.ifjy.me/contents/wireless/025/images/003.png)
## MODES OF OPERATION * Basic L2CAP Mode (equivalent to L2CAP specification in Bluetooth v1.1) * Flow Control Mode * Retransmission Mode * Enhanced Retransmission Mode * Streaming Mode * LE Credit Based Flow Control Mode
## MAPPING CHANNELS TO LOGICAL LINKS * L2CAP maps channels to Controller logical links, which in turn run over Controller physical links. * All logical links going between a local Controller and remote Controller run over a single physical link.
* There is one ACL-U logical link per BR/EDR physical link and one LE-U logical link per LE physical link. * There may be multiple AMP-U logical links per AMP physical link.

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