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Explain me Bluetooth Protocol stack and Bluetooth hardware

By Applied Electronics - Wednesday, April 19, 2017 No Comments
Today bluetooth is everywhere from Personal Computer(PC) to cell phones. They are also used in industrial electronics and healthcare systems. From Bluetooth developer point of view it is a complicated technology. A Bluetooth IC is a complete radio system on its own operating at 2.5GHz frequency. The IC is small but contains everything like RF front end, ADC(Analog to Digital Converter), DAC(Digital to Analog Converter), DSP(Digital Signal Processors), Microcontroller, RAM and various connection interfaces. Shown below is a typical Bluetooth Architecture of BC417143B bluetooth IC.


As if this hardware aspect is not complicated, the software architecture is equally complicated. The Bluetooth software aspect is dependent on its protocol stack. A pile or stack of predefined functions and protocol that must be followed and dictates how the Bluetooth system works. Shown below is the protocol stack of the Bluetooth.

Lets briefly see what these components of the stack are and what they do. Overall these all protocol stacks are there to help user application connect reliable and quality wirelessly.

Beginning at the bottom is the radio interface related protocols. As we go up higher the stacks, the protocol becomes more software oriented, that is application oriented. At the top we have the user application interacting with the rest of the protocol stack. Now user application varies widely. So in order to support such variety of application, the protocol stack is complicated.

Thus at the bottom of the stack there is link manager, link controller and radio controller. These all have to do with physical wireless air connection. They format the over-air transmissions, handle error detection and re-transmission, and manages the links between devices.

The L2CAP stands for Logical Link Control and Adaptation Protocol. It is responsible to package or multiplex the data from the upper level(applications) into Asynchronous ConnectionLess (ACL) connection between two bluetooth devices. First remember that L2CAP only deals with data traffic and not voice. In case, the device is a master, the L2CAP directs the data to the slave device. It packages data coming from the upper level and/or unpackages the data coming from the lower level. In reality, the I2CAP is performing simulateous operation on coming data either up or down. And the application to which the data belong are identified by have each I2CAP logical channel with a unique Channel Identifier (CID).

RFCOMM emulates full 9-pin RS232 serial communication over an L2CAP channel. It is based on the TS 07.10 standard for a software emulation of the RS232 hardware interface.TS 07.10 includes the ability to multiplex several emulated serial ports onto a single data connection using a different Data Link Connection Identifier (DLCI) for each port. However, each TS 07.10 session can only connect over a single L2CAP channel and thus only communicate with one device.A master device must have separate RFCOMM sessions running for each slave requiring a serial port connection.

OBEX stands for Object Exchange which is a standard developed by the Infrared Data Association (IrDA) to facilitate operations common to IR-enabled devices like personal digital assistants (PDAs) and laptops. Rather than develop a new standard, the Bluetooth SIG took OBEX largely as is, detailed a few specifics regarding Bluetooth implementation (e.g., making some optional features mandatory),
and used it in the File Transfer, Synchronisation, and Object Push profiles. OBEX allows users to put and get data objects, create and delete folders and objects, and specify the working directory at the remote end of the link. IrDA has also provided formats for data objects, while the Bluetooth specification has adopted the vCard format for business card exchange and the vCal format for exchanging calendars.

Telephony Control Protocol Specification Binary (TCS Binary, also called TCSBIN), is based on the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) Q.931 standard for telephony call control. It includes a range of signaling commands from group management to incoming call notification, as well as audio connection establishment and termination. It is used in both the Cordless Telephony and Intercom profiles.

The Service Discovery Protocol(SDP) is like a service database. It is different from other layers in the protocol stack because SDP is specifically Bluetooth oriented. The user application is registered as available services on the database. Then remote devices once connected can query the database to find out what services are available and how to connect to them.

In the protocol stack picture above you can see device manager, security manager and connection manager. These are rather function blocks rather than protocol. The device manager handles the lower level operation of the bluetooth device. The security manager mainly checks that the device has sufficient service privileges. The connection manager handles application connection to bluetooth devices.


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