What’s new in Bluetooth 5.2

Bluetooth 5.2 was introduced in CES 2020 which focuses majorly on LE Audio and its enhancements.

Three Major improvements are

  • Enhanced Attribute Protocol
  • LE Power Control
  • LE Isochronous Channels

Enhanced Attribute Protocol:

An improved version of the Attribute protocol (ATT), called the Enhanced Attribute protocol (EATT), has been introduced along with some associated improvements to the Generic Attribute  Profile (GATT).

Key Improvements:

  • EATT supports concurrent transactions. This can reduce the end- to- end latency of one or more of the applications and improve the user’s experience of responsiveness.
  • In support of EATT a new L2CAP Enhanced Credit Based Flow Control Mode introduced which provides flow control and so allows applications to regard the protocol as reliable.
  • EATT has security advantages over unenhanced ATT as it may only be used over an  encrypted connection.

Enhanced Attribute Protocol: Capabilities and Benefits

EATT modifies the sequential transaction model, making concurrent ATT transactions possible when conducted over distinct enhanced ATT bearers. A new mode has been added to the L2CAP layer and is used by EATT. The new L2CAP mode provides flow control so that EATT can be regarded as reliable. MTU values at the ATT layer and L2CAP layer have been made independently configurable, and this may reduce latency experienced by some applications that share the stack with other applications.

Technical Highlights:

L2CAP Enhanced Credit Based Flow Control Mode – Receiving device gives no of credits to sending device like no of LE frames can be sent. (e.g) 1 credit = 1 LE Frame. So that receiving knows how many frames are expected.

Parallel ATT Transactions – The sequential transaction rule still exists when EATT is used, but its scope is now defined as being per instance of the Enhanced ATT Bearer. In other words, EATT transactions may be executing in parallel if they are supported by distinct L2CAP channels which use the Enhanced Credit Based Flow Control Mode, i.e. distinct Enhanced ATT Bearers

Discovering Support for EATT – The Generic Attribute Profile service has been updated to allow a client to determine supports EATT and, conversely, to allow the client to inform the server that it supports EATT. A new characteristic called Server Supported Features has been defined and must be included in the Generic Attribute Profile service if EATT is supported by the server. Bit 0 of the first octet of the value of this characteristic set to 1 means that EATT is supported. In Version 5.2 of the Bluetooth Core Specification has assigned meaning to two more bits. Bit 1 indicates whether or not the Enhanced ATT Bearer is supported by the client. Bit 2 indicates whether or not a new ATT PDU called Multiple Handle Value Notifications is supported. The client must write an appropriate value to this characteristic to inform the server of the features it supports.

Attribute Protocol PDUs

A number of new PDUs which may only be used over an enhanced ATT bearer have been defined.

1.The following new ATT PDUs, for use solely over an enhanced ATT bearer have been defined.

  • ATT_READ_MULTIPLE_ VARIABLE_REQ and ATT_READ_MULTIPLE_ VARIABLE_RSP
  • ATT_MULTIPLE_HANDLE_ VALUE_NTF

2.   The following ATT PDUs have had their specified behaviors refined to reflect the improved definition of an ATT bearer

  • ATT_PREPARE_ WRITE_REQ and ATT_PREPARE_WRITE_RSP
  • ATT_EXECUTE_ WRITE_REQ and ATT_EXECUTE_WRITE_RSP

3.   ATT PDUs which may not be used over an enhanced ATT bearer

  • ATT_EXCHANGE_MTU_ REQ and ATT_EXCHANGE_ MTU_RSP
  • ATT_SIGNED_WRITE_ COMMAND

LE Power Control(LEPC)

This feature provides Bluetooth LE devices optimize transmit power levels dynamically based on RSSI levels.

Link Layer PDUs introcuced for LEPC

  • LL_POWER_CONTROL_REQ – Sends delta value by which the sending device is requesting that the recipient device adjust its TxPower.
  • LL_POWER_CONTROL_RSP – Responds actual change in the transmit power and Acceptable power reduction value.
  • LL_POWER_CHANGE_IND – used to notify a remote device whenever the sender’s transmit power level has been changed autonomously in one or more PHY’s.

Host Controller Interface Commands and Events:

HCI defines several new commands and events that are related to LE Power Control. Both power management and path loss monitoring use cases are provided for

New HCI Commands

  • LE Enhanced Read Transmit Power Level
  • LE Read Remote Transmit Power Level
  • LE Set Path Loss Reporting Parameters
  • LE Set Path Loss Reporting Enable
  • LE Set Transmit Power Reporting Enable

New HCI Events

  • LE Transmit Power Reporting
  • LE Path Loss Threshold

LE Isochronous Channels

Primarily designed to support LE Audio, the next generation of Bluetooth audio, allows the communication of time-bound data to one or more devices for time-synchronized processing. it Can be used over connections or be broadcast to an unlimited number of devices in a connectionless fashion. An audio source can transmit audio for synchronized playback by small, private groups of devices (personal audio sharing) or to large collections of devices of unlimited sizes in public spaces, such as cinemas. Music sharing is only one application, however. LE Audio, built on top of the new LE Isochronous Channels, will offer a new standard for hearing aids and support assisted hearing systems in diverse locations, such as theaters, conferences, lecture halls, and airports.

 LE Isochronous Channels: Capabilities and Benefits

Time-Bound Data and Synchronized Processing:

The Bluetooth LE Isochronous Channels feature is a new way of using Bluetooth LE to transfer timebounded data between devices. It provides a mechanism that makes sure that multiple sink devices, receiving data from the same source, will render it at the same time. Data has a time-limited validity period, at the end of which it is said to expire. Expired data which has not yet been transmitted, will be discarded. This means that receiver devices only ever receive data which is valid with respect to rules regarding its age and acceptable latency.

New Audio Use Cases:

LE Isochronous communication provides the means by which audio, delivered from a source to multiple sinks, can be rendered at the same time, for properly synchronized playback. Audio, which for some reason is delayed after being generated at the source, expires and is discarded so that it does not affect the listening experience at the sink(s).

Summary

We discussed Bluetooth 5.2 features which focused on Bluetooth Low Energy related improvements.

We will discuss more about LE ISOC and LE audio more in separate blog.

Hope this is helpful. Only your feedbacks will help us improve

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