Overview of BR/EDR Procedures and ModesV5.2 (2019-12-31)

The typical operational mode of a Bluetooth device is to be connected to other Bluetooth devices (in a piconet) and exchanging data with those Bluetooth devices. As Bluetooth is an ad-hoc wireless communications technology, there are a number of operational procedures that enable piconets to be formed so that the subsequent communications can take place. Procedures and modes are applied at different layers in the architecture and therefore a device may be engaged in a number of these procedures and modes concurrently.


Inquiry (discovering) procedure

Bluetooth devices use the inquiry procedure to discover nearby devices, or to be discovered by devices in their locality. The inquiry procedure is asymmetrical. A Bluetooth device that tries to find other nearby devices is known as an inquiring device and actively sends inquiry requests. Bluetooth devices that are available to be found are known as discoverable devices and listen for these inquiry requests and send responses. The inquiry procedure uses a special physical channel for the inquiry requests and responses.

Both inquiring and discoverable devices may already be connected to other Bluetooth devices in a piconet. Any time spent inquiring or occupying the inquiry scan physical channel needs to be balanced with the demands of the QoS commitments on existing logical transports.

The inquiry procedure does not make use of any of the architectural layers above the physical channel, although a transient physical link may be considered to be present during the exchange of inquiry and inquiry response information.

Extended Inquiry response

An Extended Inquiry Response can be used to provide miscellaneous information during the inquiry response procedure. Data types are defined for such things as local name and supported services, information that otherwise would have to be obtained by establishing a connection.

The extended inquiry response procedure is backwards compatible with the standard inquiry response procedure.


Paging (connecting) procedure

The procedure for forming connections is asymmetrical and requires that one Bluetooth device carries out the page (connection) procedure while the other Bluetooth device is connectable (page scanning). The procedure is targeted, so that the page procedure is only responded to by one specified Bluetooth device.

The connectable device uses a special physical channel to listen for connection request packets from the paging (connecting) device. This physical channel has attributes that are specific to the connectable device, hence only a paging device with knowledge of the connectable device is able to communicate on this channel.

Both paging and connectable devices may already be connected to other Bluetooth devices. Any time spent paging or occupying the page scan physical channel needs to be balanced with the demands of the QoS commitments on existing logical transports.


Connected mode

After a successful connection procedure over the BR/EDR Controller, there is a piconet physical channel to which both devices are connected, there is a physical link between the devices, and there are default ACL-C, ACL-U, ASBC, and ASB-U logical links. Two of these links (ACL-C and ASB-C) transport the LMP control protocol and are invisible to the layers above the Link Manager. The ACL-U link transports the L2CAP signaling protocol and any multiplexed L2CAP best-effort channels. The ASB-U link transports L2CAP channels that are broadcast to all slaves on the piconet. It is common to refer to a default ACL logical transport, which can be resolved by context, but typically refers to the default ACL-U logical link.

When in the connected mode

  • it is possible to create and release additional logical links and to change the modes of the physical and logical links while remaining connected to the piconet physical channel.
  • It is also possible for the device to carry out inquiry, paging or scanning procedures or to be connected to other piconets without needing to disconnect from the original piconet physical channel.
  • These actions are done using the Link Manager, which exchanges Link Manager protocol messages with the remote Bluetooth device

AMP physical links may be established in connected mode. Once an AMP physical link is created, one or more AMP-U logical links may be established to transport L2CAP user data.

During the time that a slave device is actively connected to a piconet there is always a default ACL logical transport between the slave and the master device. The only method of deleting the default ACL logical transport is to detach the device from the piconet physical channel, at which time the entire hierarchy of L2CAP channels, logical links, and logical transports between the devices is deleted.


Hold mode

Hold mode is not a general device mode, but applies to unreserved slots on the physical link. When in this mode, the physical link is only active during slots that are reserved for the operation of the synchronous link types SCO and eSCO. All asynchronous links are inactive. Hold modes operate once for each invocation and are then exited when complete, returning to the previous mode.


Sniff mode

Sniff mode is not a general device mode, but applies to the default ACL logical transports. When in this mode the availability of these logical transports is modified by defining a duty cycle consisting of periods of presence and absence. Devices that have their default ACL logical transports in Sniff mode may use the absent periods to engage in activity on another physical channel, or to enter reduced power mode. Sniff mode only affects the default ACL logical transports (i.e. their shared ACL logical transport), and does not apply to any additional SCO or eSCO logical transports that may be active. The periods of presence and absence of the physical link on the piconet physical channel is derived as a union of all logical transports that are built on the physical link.

Sniff subrating provides a mechanism for further reducing the active duty cycle, thereby enhancing the power-saving capability of Sniff mode. Sniff subrating allows a Host to create a guaranteed access-like connection by specifying maximum transmit and receive latencies. This allows the basebands to optimize the low power performance without having to exit and re-enter Sniff mode using Link Manager commands.

Broadcast logical transports have no defined expectations for presence or absence. A master device should aim to schedule broadcasts to coincide with periods of physical link presence within the piconet physical channel, but this is not always possible or practical. Repetition of broadcasts is defined to improve the possibilities for reaching multiple slaves without overlapping presence periods. However, broadcast logical transports cannot be considered to be reliable.


Role switch procedure

The role switch procedure is a method for swapping the roles of two devices connected in a piconet. The procedure involves moving from the physical channel that is defined by the original master device to the physical channel that is defined by the new master device. In the process of swapping from one physical channel to the next, the hierarchy of physical links and logical transports over the BR/EDR Controller are removed and rebuilt, with the exception of the ASB logical transport that is implied by the topology and is not preserved. The role switch procedure does not affect AMP physical channels. After the role switch, the original piconet physical channel may cease to exist or may be continued if the original master had other slaves that are still connected to the channel.

The procedure only copies the default ACL logical links and supporting layers to the new physical channel. Any additional logical transports are not copied by this procedure, and if required this must be carried out by higher layers. The LT_ADDRs of any affected transports will be reassigned on the new physical channel and, therefore, may change.

If there are any QoS commitments on the original logical transports, then these are not preserved after a role switch. These must be renegotiated after the role switch has completed.


Enhanced Data Rate

Enhanced Data Rate is a method of extending the capacity and types of Bluetooth packets for the purposes of increasing the maximum throughput, providing better support for multiple connections, and lowering power consumption, while the remainder of the architecture is unchanged.

Enhanced Data Rate may be selected as a mode that operates independently on each logical transport. Once enabled, the packet type bits in the packet header are interpreted differently from their meaning in Basic Rate mode. This different interpretation is clarified in conjunction with the logical transport address field in the header. The result of this interpretation allows the packet payload header and payload to be received and demodulated according to the packet type. Enhanced Data Rate can be enabled only for the ACL and eSCO logical transports and cannot be enabled for the SCO and broadcast logical transports.


Connectionless Slave Broadcast mode

Connectionless Slave Broadcast mode allows a piconet master to transmit profile broadcast data to any number of connected slave devices using the BR/EDR adapted piconet physical channel. To enter this mode, the master reserves a specific logical transport as the dedicated CSB logical transport and starts broadcasting data using the Connectionless Slave Broadcast physical link and the synchronization train procedure. A single Profile Broadcast Data logical link is defined, which carries profile broadcast data using the Connectionless Slave Broadcast logical transport. The profile broadcast data is unframed and bypasses L2CAP.

To receive the Connectionless Slave Broadcast packets, a device must connect with the Connectionless Slave Broadcast Transmitter which has already established a CSB logical transport. To connect, a device follows the Synchronization Scan procedure to obtain the time schedule of the physical link and then starts receiving the Connectionless Slave Broadcast packets. Once connected, Connectionless Slave Broadcast receivers can receive profile broadcast data on the dedicated CSB logical transport and PBD logical link.