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Bluetooth uses a variety of protocols. Core protocols are defined by the trade organisation Bluetooth SIG. Additional protocols have been adopted from other standards bodies.
The bluetooth protocol stack is split in two parts: a “controller stack” containing the timing critical radio interface, and a “host stack” dealing with high level data. The controller stack is generally implemented in a low cost silicon device containing the bluetooth radio and a microprocessor. The host stack is generally implemented as part of an operating system, or as an installable package on top of an operating system. For integrated devices such as bluetooth headsets, the host stack and controller stack can be run on the same microprocessor to reduce mass production costs, this is known as a “hostless” system.
Contents
1 Controller stack
1.1 ACL (Asynchronous Connection Less) link
1.2 SCO (Synchronous Connection Oriented) link
1.3 LMP (Link Management Protocol)
1.4 HCI (Host/Controller Interface)
2 Host Stack
2.1 L2CAP (Logical Link Control and Adaptation Protocol)
2.2 BNEP (Bluetooth Network Encapsulation Protocol)
2.3 RFCOMM (Radio Frequency Communication)
2.4 SDP (Service Discovery Protocol)
2.5 TCP (Telephony Control Protocol)
2.6 AVCTP (Audio/Visual Control Transport Protocol)
2.7 AVDTP (Audio/Visual Data Transport Protocol)
2.8 OBEX (OBject EXchange)
3 See also
4 External links
//
Controller stack
ACL (Asynchronous Connection Less) link
The normal type of radio link used for general data packets using a polling TDMA scheme to arbritrate access. It can carry several different packet 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 (EDR - enhanced data rate - packets allow up to triple data rate by using a different RF modulation for the payload)
Confusingly, ACL links are actually connection oriented. 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 this requires L2CAP retransmission & flow control mode or EL2CAP.
ACL links are disconnected if there is nothing received for 20 seconds (supervision timeout)
SCO (Synchronous Connection Oriented) link
The type of radio link used for voice data. An 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.
eSCO (enhanced SCO) links use retransmissions to achieve reliability, and behave more like general data.
LMP (Link Management Protocol)
Used for control of the radio link between two devices. Implemented on the controller.
HCI (Host/Controller Interface)
Standardised communication between the Host stack (e.g. a PC or mobile phone OS) and the Controller (the bluetooth I.C.) This standard allows the host stack or controller I.C. 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 optional, although often implemented as an internal software interface.
Host Stack
L2CAP (Logical Link Control and Adaptation Protocol)
L2CAP is used within the Bluetooth protocol stack. It passes packets to either the Host Controller Interface (HCI) or on a hostless system, directly to the Link Manager.
L2CAP’s functions include:
Multiplexing data between different higher layer protocols.
Segmentation and reassembly of packets.
Providing one-way transmission management of multicast data to a group of other bluetooth devices.
Quality of service (QoS) management for higher layer protocols.
L2CAP is used to communicate over the host ACL (Asynchronous Connectionless) link. Its connection is established after the ACL link has been set up.
In basic mode, L2CAP provides reliable sequenced packets with a payload configurable up to 64kB, with 672 bytes as the minimum mandatory supported size. In retransmission & flow control modes, L2CAP can be configured for reliable or isochronous data per channel by configuring the number of…(and so on)
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