Bluetooth

Controller stack 

Asynchronous Connection-Less (ACL) 

The normal type of radio link used for general data packets using a polling TDMA scheme to arbitrate access. ACL can carry packets of several types, which are distinguished by:

Length (1, 3, or 5 time slots depending on required payload size)
Forward error correction (optionally reducing the data rate in favour of reliability)
Modulation (Enhanced Data Rate packets allow up to triple data rate by using a different RF modulation for the payload)
A connection must be explicitly set up and accepted between two devices before packets can be transferred.

ACL packets are retransmitted automatically if unacknowledged, allowing for correction of a radio link that is subject to interference. For isochronous data, the number of retransmissions can be limited by a flush timeout; but without using L2PLAY retransmission and flow control mode or EL2CAP, a higher layer must handle the packet loss.

ACL links are disconnected if there is nothing received for the supervision timeout period; the default timeout is 20 seconds, but this may be modified by the master.

Synchronous Connection-Oriented (SCO) link 

The type of radio link used for voice data. A SCO link is a set of reserved timeslots on an existing ACL link. Each device transmits encoded voice data in the reserved timeslot. There are no retransmissions, but forward error correction can be optionally applied. SCO packets may be sent every 1, 2 or 3 timeslots.

Enhanced SCO (eSCO) links allow greater flexibility in setting up links: they may use retransmissions to achieve reliability, allow a wider variety of packet types, and greater intervals between packets than SCO, thus increasing radio availability for other link.

Link manager protocol (LMP) 

Used for control of the radio link between two devices, handling matters such as link establishment, querying device abilities and power control. Implemented on the controller.

Host controller interface (HCI) 

Standardised communication between the host stack (e.g., a PC or mobile phone OS) and the controller (the Bluetooth IC). This standard allows the host stack or controller IC to be swapped with minimal adaptation.

There are several HCI transport layer standards, each using a different hardware interface to transfer the same command, event and data packets. The most commonly used are USB (in PCs) and UART (in mobile phones and PDAs).

In Bluetooth devices with simple functionality (e.g., headsets), the host stack and controller can be implemented on the same microprocessor. In this case the HCI is obsolete, although often implemented as an internal software interface.

Low-energy link layer (LE LL) 

This is the LMP equivalent for Bluetooth Low Energy (LE), but is simpler. It is implemented on the controller and manages advertisement, scanning, connection and security from a low-level, close to the hardware point of view from Bluetooth perspective.