Logical Link And Adaptation Layer

L2CAP provides connection-oriented and connectionless data services to upper layer protocols with protocol multiplexing capability, segmentation and reassembly operation, and group abstractions. L2CAP permits higher level protocols and applications to transmit and receive L2CAP packets up to 64 Kilobytes in length. L2CAP only supports ACL links. L2CAP uses the concept of channels to establish different pathways between different applications on Bluetooth devices. These channels are identified by Channel IDentifiers (CIDs) which represent a logical end point of a connection for each application on a device. CIDs are 16 bit numbers of which 0x0001 to 0x003F are reserved for specific L2CAP functions (0x0001 is a signalling channel, 0x0002 is a connectionless reception channel and rest are reserved or prohibited).

Connection Identifiers (CIDs)

The idea behind L2CAP is to provide an interface similar to TCP/IP function calls. In NSBLUE, the L2CAP connections and data are sent in the following manner:

cid = l2cap->openL2CAPConnection(ui->getContext());

l2cap->send(cid,data,len);

l2cap->recv(cid,data,len);

where `cid' is the channel identifier, `data' is the pointer to ``char'' and `len' is the length of the data. `getContext()' returns the local context like `Walmart', `JFK Airport' etc. CID is assigned from a pool of free CIDs (pool can be assigned in blocks to save memory). See `CONNECTION ESTABLISHMENT IN BLUETOOTH' for more information.

Protocol Multiplexing

L2CAP also does protocol multiplexing by using PSM field in the L2CAP Connection Request command. L2CAP can multiplex connection requests to upper layer protocols like Service Discovery Protocol (PSM = 0x0001), RFCOMM (PSM = 0x0003) and Telephony Control (PSM = 0x0005).

Segmentation and Reassembly

Segmentation and Reassembly (SAR) operations are used to improve efficiency by supporting a maximum transmission unit (MTU) size larger than the largest Baseband packet. This reduces the overhead by spreading the network and transport packets used by higher layer protocols over several baseband packets. L2CAP segments higher layer packets into `chunks' that can be passed to the Link Manager for transmission and reassembles those chunks into L2CAP packets using information provided through HCI and from packet header. SAR is implemented using very little overhead in Baseband packets. The two L_CH bits defined in the first byte of Baseband payload (also called the frame header) are used to signal the start and continuation of L2CAP packets (L_CH shall be `10' for the first segment and `01' for a continuation segment. To avoid any reassembly problems due to out of order packets, all L2CAP segments associated with an L2CAP packet must be passed through to the Baseband before any other L2CAP packet destined to the same unit may be sent. Also `Stop and Wait' protocol used by Baseband makes sure that a packet is received correctly by the other unit before the next packet is sent. This avoids most of the out of the order packets, as received in wireline TCP/IP connections because of `window' based transmissions.

L2CAP Events and Actions

L2CAP operates using events and commands which it receives or transmits from/to upper or lower layers. These events can be, for instance, a connection request from the upper layer, a data write request or may be a disconnection request. The lower layers can tell L2CAP about incoming connection, disconnection or other requests. If L2CAP of this unit needs to talk to the L2CAP on the other unit, then it uses some special commands which are called signalling commands. These commands are generally used to establish connection-oriented channels after a link level connection is created or present. L2CAP has seven operational states: CLOSED, W4_L2CA_CONNECT_RSP, W4_L2CAP_CONNECT_RSP, CONFIG, OPEN, W4_L2CAP_DISCONNECT_RSP, and W4_L2CA_DISCONNECT_RSP. These states further make L2CAP connections look similar to TCP connections.

Signalling command codes