This blog is the sequel of 'Bluetooth technology' articles covering the Introduction to Bluetooth Technology, the Physical layer of Bluetooth, Network to Application layer in Bluetooth etc. In the last blog 'Bluetooth Profiles', we discussed in detail about the different profiles and their Bluetooth applications. We covered how a single device can act as different profiles simultaneously. In this article, we will see how a single Bluetooth device can get connected with multiple other devices — the foundation of multipoint Bluetooth — and the Bluetooth applications around it. A Bluetooth network is called a Bluetooth Piconet, and a group of interconnected Bluetooth Piconet networks is called a scatternet. Understanding multipoint Bluetooth requires understanding the Bluetooth Piconet structure and multi channel communication mechanisms that make it possible. Embien's cross-domain embedded services include multipoint Bluetooth implementation for automotive infotainment, consumer audio, and industrial IoT Bluetooth applications.
Bluetooth Piconet
The Bluetooth specification defines that one master may communicate with up to seven slaves by forming a Bluetooth Piconet. All devices within a given Bluetooth Piconet use the clock provided by the master as the base for packet exchange. The master clock ticks with a period of 312.5 μs, two clock ticks make up a slot of 625 μs, and two slots make up a slot pair of 1250 μs. In the simple case of single-slot packets, the master transmits in even slots and receives in odd slots. The slave, conversely, receives in even slots and transmits in odd slots. Packets may be 1, 3, or 5 slots long, but in all cases, the master's transmission begins in even slots and the slaves in odd slots. In the below diagram, device A, C, and E act as master for Bluetooth Piconet P1, P2, and P3. In a special use case, C acts as master for P2 network and slave for P1 network — demonstrating the scatternet concept that extends multi channel communication across multiple Bluetooth Piconet instances.
P-P and P-M
The Bluetooth system offers two types of connections: point-to-point and point-to-multipoint, as shown in the diagram below.
P-P and P-M
In a point-to-point setup, the physical channel is shared between two Bluetooth devices. In a point-to-multipoint setup — the basis of multipoint Bluetooth — the physical channel is shared among several Bluetooth devices within a Bluetooth Piconet. Each Bluetooth device can only use one physical channel at a time, which is why multi channel communication in multipoint Bluetooth requires careful time-division management.
HCI Baseband Controller
To support multiple operations at once, devices use time-division multiplexing between the channels. This multi channel communication approach allows a multipoint Bluetooth device to seem like it is operating in multiple Bluetooth Piconet networks simultaneously while still being discoverable and connectable. The Host Controller Interface (HCI) layer plays a big role in managing connections in multipoint Bluetooth with multiple devices. Baseband controller enables the actual physical connection between devices. It concerns connection establishment within the Bluetooth Piconet, addressing, packet format, timing, and power control. Multi channel communication for multipoint Bluetooth is coordinated at this baseband controller level.
Higher level Bluetooth Protocols
The Baseband Control layer provides two basic types of physical links that can be established between a master device and a slave device in a Bluetooth Piconet — Synchronous Connection Oriented (SCO) and Asynchronous Connection-Less (ACL).
Synchronous connection oriented (SCO)
SCO provides a circuit switched connection, where a dedicated point-to-point link is established between the master device and the slave device before communication starts. SCO is a symmetric link with fixed slots allocated for each direction. SCO radio links are used for time-critical data transfer, mainly voice data — one of the primary Bluetooth applications for multipoint Bluetooth headsets. A master device can support three SCO links with the same or different slaves. A slave device can have a maximum of three SCO links with its master device. The focus is minimization of time latency. The maximum data rate of an SCO link is 64,000 bps. Four types of packets are allowed on the SCO logical transport: HV1, HV2, HV3, and DV.
HV1 packetThe HV1 packet has 10 information bytes. The bytes are protected at a rate of 1/3 FEC. No MIC (Message Integrity Check) is present. No CRC is present. The payload length is fixed at 240 bits. There is no payload header present.
HV2 packetThe HV2 packet has 20 information bytes. The bytes are protected with a rate 2/3 FEC. No MIC is present. No CRC is present. The payload length is fixed at 240 bits. There is no payload header present.
HV3 packetThe HV3 packet has 30 information bytes. The bytes are not protected by FEC. No MIC is present. No CRC is present. The payload length is fixed at 240 bits. There is no payload header present.
DV packetThe DV packet is a combined data-voice packet. The DV packet shall only be used in place of an HV1 packet.
Asynchronous Connection-Less (ACL)
ACL has two frame types: Data Medium rate (DM) (which provides Forward Error Correction: FEC) and DH (Data high rate) (which does not provide FEC). ACL is the standard data transport for most Bluetooth applications in a multipoint Bluetooth environment.
| Number of slots | 1 | 3 | 5 |
|---|---|---|---|
| BDR w/ FEC | DM1 1-18 byte |
DM3 2 – 123 bytes |
DM5 2 – 226 bytes |
| BDR w/0 FEC | DH1 1-28 byte |
DH3 2-185 byte |
DH5 2-341 byte |
| EDR 2Mbps | 2 -DH1 2-56 byte |
2 -DH3 2 – 369 byte |
2 -DH5 2 – 681 byte |
| EDR 3Mbps | 3-DH1 2 - 85 byte |
3-DH3 2 – 554 bytes |
3-DH5 2 – 1023 bytes |
Multipoint Bluetooth
Multipoint Bluetooth is a handy feature that allows a single Bluetooth slave device to connect to two different master devices simultaneously (for example, a laptop and a smartphone). It was introduced with the release of Bluetooth 4.0. With multipoint Bluetooth, users can easily switch between their various devices without needing to constantly disconnect and reconnect. Multipoint Bluetooth is a convenient way to stay connected across multiple Bluetooth devices, enabling true multi channel communication at the user experience level.
Multipoint Bluetooth comes in different flavors, including simple multipoint, advanced multipoint, and triple connectivity. Manufacturers also have custom solutions that mimic multipoint Bluetooth functionality. The multi profile specification governs how a device manages simultaneous connections across these multipoint Bluetooth variants.
Simple Multipoint BluetoothThis is the most basic type of multipoint Bluetooth found in every headset device. It lets you connect to two audio sources. The downside is that if you are on a call and get another call on a different device, it cuts off the first call to answer the second one. Simple multipoint Bluetooth is the baseline implementation of the multi profile specification.
Advanced Multipoint BluetoothAdvanced multipoint Bluetooth works differently from the basic version. Instead of dropping the first call when you get a second one, it puts the first call on hold while you answer the new one. Advanced multipoint Bluetooth requires a more sophisticated implementation of the multi profile specification and multi channel communication state management.
Triple ConnectivityAs the name suggests, triple connectivity is a form of multipoint Bluetooth that allows you to connect three audio source devices simultaneously instead of the standard two. This requires advanced multi channel communication scheduling within the Bluetooth Piconet.
Multipoint Bluetooth Use cases
Let us see a working principle of multipoint Bluetooth with an example. A slave device needs to be connected with 2 masters, so that the user can leverage the same device for either master on a need basis. This is also called a multipoint Bluetooth environment. These are common Bluetooth applications in enterprise and consumer contexts. Multimedia on embedded systems platforms frequently implement multipoint Bluetooth to enable seamless audio switching across connected devices.
Two Device Pairing to one slaveWhile pairing the headset in a multipoint Bluetooth setup, Bluetooth shall be turned ON and the device found and paired from both the laptop and smartphone. After the first device/master is connected, disconnect the headset and pair the second device/master. Once both masters are paired, the headset will maintain the Bluetooth Piconet connections and enable multi channel communication between both masters.
Bluetooth Pairing
Accepting Incoming call
After successfully pairing both masters in the multipoint Bluetooth setup, power off/on the headset Bluetooth. Now the headset will connect with both masters — laptop and smartphone. During an incoming call, an HV1 SCO connection will be enabled to transfer audio call to the headset. Multi channel communication ensures that the multipoint Bluetooth connection to the idle master is preserved during the active call.
Bluetooth Incoming call
Dual Incoming Call
This is a special case in multipoint Bluetooth where the headset is already servicing an incoming call and another call is received from the other master. In this case, the headset will take an intelligent call based on the multi profile specification implementation in headset firmware. Based on the priority of the device, the second call might be addressed and the first call Bluetooth disconnected as per simple multipoint Bluetooth. As per advanced multipoint Bluetooth, the headset can put the first call on hold and accept the second call from the second master.
Bluetooth Dual Incoming call
Conclusion
The robustness and reliability of multipoint Bluetooth connectivity ensure that users can enjoy a seamless experience, whether managing calls between a smartphone and a laptop, streaming music from a tablet while receiving navigation prompts from a car infotainment system, or seamlessly transitioning between various audio sources. Multipoint Bluetooth leverages the Bluetooth Piconet architecture and multi channel communication mechanisms to deliver these seamless experiences. The multi profile specification governs how slave devices manage concurrent master connections across different multipoint Bluetooth configurations. These are core Bluetooth applications that are driving continued innovation in wireless audio and IoT connectivity. The same multipoint Bluetooth concepts can be extended to different use cases across many Bluetooth applications.