The Bluetooth Link Layers

The Bluetooth link layer outlines the way Bluetooth devices can use the raw transmission facility given by the radio layer to exchange information. The functions of the link layer is very close to MAC (medium access control) sublayer of the OSI model.

The following diagram shows the position of link layers in the Bluetooth protocol architecture −

Functions of the Bluetooth Link Layer

  • Defining procedures for discovering Bluetooth devices.

  • Establishing logical links between the Bluetooth devices that are communicating. One of the devices is assigned as master and the other is the slave.

  • Broadcasting data to be sent. Managing the links between the devices throughout data communications.

  • Sending data by converting the raw bit streams of the radio layer into frames and defining key formats.

  • Taking into consideration, the challenges of wireless transmission like interference, noise and deep fades.

Protocols in the Bluetooth Link Layer

There are two main protocols in the link layer, namely, Link Manager Protocol (LMP) and Logical Link Control and Adaptation Protocol (L2CAP).

  • Link Manager Protocol (LMP): LMP establishes logical links between Bluetooth devices and maintains the links for enabling communications. The other main functions of LMP are device authentication, message encryption and negotiation of packet sizes.

  • Logical Link Control and Adaptation Protocol (L2CAP) L2CAP provides adaption between upper layer frame and baseband layer frame format. L2CAP provides support for both connection oriented as well as connectionless services.

Working Principal

Bluetooth devices are categorised into a master device and slave devices. In the simplest method, time division multiplexing is used for master – slave communications. Time slots of 625 µsec are defined the master starts transmitting in odd slots while the slaves start transmitting in even slots. Length of frames can be 1, 3 or 5 slots. Each frame is associated with 126-bits overhead for access code and header, as well as a 250 µsec/hop setting time.

The LMP sets up links by pairing method. Secure simple paining method is used where the master generates a passkey and both devices display the same passkey. The users confirm the passkeys to pair the devices.

Once paired, the LMP sets up links. The two types of links used are −

  • Synchronous Connection Oriented (SCO): Here, a fixed slot is allotted in each direction. It is used for real-time data.

  • Asynchronous Connectionless (ACL): Here, communication is done on best-effort basis. It is used for packet switched data, which is available at irregular intervals.