Bluetooth data file transfer, links & codec

- overview or tutorial of the basics of Bluetooth data file transfer including synchronous and asynchronous Bluetooth links and details of the voice codec.

Bluetooth data transfer can be achieved using a variety of different data packet types and using different forms of links - asynchronous links and synchronous links

These different Bluetooth data file transfer formats provide flexibility, but they are invisible to the user who sees a connection being made and Bluetooth data being transferred.

Bluetooth links

There are two main types of Bluetooth link that are available and can be set up:

  • ACL   Asynchronous Connectionless communications Link
  • SCO   Synchronous Connection Orientated communications link

The choice of the form of Bluetooth link used is dependent upon the type of Bluetooth data transfer that is required.

Bluetooth ACL

The ACL or Asynchronous Connectionless Communications Link is possible the most widely used form of Bluetooth link. The ACL Bluetooth link is used for carrying framed data - i.e. data submitted from an application to logical link control and adaptation protocol channel. The channel may support either unidirectional or bidirectional Bluetooth data transfer.

There is a variety of different ACL formats that can be used - most of them incorporate forward error coding, FEC as well as header error correction to detect and correct errors that may occur in the radio link.

The Asynchronous Bluetooth link provides connections for most applications within Bluetooth. Data transfers like this are normally supported by profiles which allow the data to be incorporated into frames and transferred to the other end of the Bluetooth link where it is extracted from the frames and passed to the relevant application.

The ACL is enables data to be transferred via Bluetooth 1 at speeds up to the maximum rate of 732.2 kbps. This occurs when it is operating in an asymmetric mode. This is commonly used because for most applications there is far more data transferred in one direction than the other. When a symmetrical mode is needed with data transferred at the same rate in both directions, the data transfer rate falls to 433.9 kbps. The synchronous links support two bi-directional connections at a rate of 64 kbps. The data rates are adequate for audio and most file transfers.

When using Bluetooth 2 enhanced data rate, data rates of 2.1 Mbps may be achieved. Also asynchronous links can be granted a quality of Service, QoS by setting the appropriate channel parameters.

Bluetooth SCO

The SCO or Synchronous Connection Orientated communications link is used where data is to be streamed rather than transferred in a framed format.

The SCO can operate alongside the ACL channels, and in fact needs one ACL to configure the SCOs.

A Bluetooth master node can support up to three simultaneous SCL channels and these can be split between up to three slave nodes.

The idea of the SCO is to ensure that audio data can be streamed without suffering delays waiting for frames or packet slots to become available. The SCO communications links is assigned guaranteed time slots so that they will be transported at the required time with a known maximum latency.

A further form of link known as an eSCO or Extended SCO was introduced with version 1.2 of the Bluetooth standard. Originally no acknowledgement had been sent, whereas using the eSCO greater reliability is provided to the Bluetooth link by sending an acknowledgement and allowing a limited number of re-transmissions if data is corrupted. In view of the latency requirements, re-transmissions are only allowable until the next guaranteed time slot, otherwise new data would be delayed.

Bluetooth codec

Within the core specification, there are a number Bluetooth codec types that are included. These Bluetooth codecs are relatively basic and are not used for audio, including stereo music applications which would use the ACL.

Any Bluetooth codec is intended to provide telephone standard audio, limiting the audio bandwidth to around 4 kHz.

The codecs are often CVSD, Continuously Variable Slope Delta modulation, based and their advantage is that they provide a minimum latency solution so there are no issues with synchronisation. As a result they may often be used with applications such as video phones, etc..

By Ian Poole


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