Bluetooth Architecture from Scratch

The Bluetooth structure determines which capabilities ought to be operational in an implementation and the way they need to be organised

Bluetooth consists of a number of applied sciences, protocols, and components. Their relationship and utilization are complicated, presenting a barrier to entry when beginning to research this expertise. This text goals to function an introduction to the structure of a Bluetooth communication system and make clear the position every of its elements performs.

The structure described within the intensive Bluetooth commonplace dictates which capabilities should exist in an implementation and the way they need to be organized, so understanding it significantly facilitates understanding the operation of a tool, a codebase, or Bluetooth as a expertise normally.

Nonetheless, as it's structured, the usual will not be one of the best research documentation for simply and easily beginning in Bluetooth expertise, so introductory documentation with a didactic intention on this matter is critical.

This text will give attention to explaining three basic factors:

• The separation of the structure into two essential elements: host and controller.
• The protocol stack.
• The Bluetooth structure intimately and the place every layer of the protocol stack is carried out.

However first, it's essential to speak about BR/EDR and LE.

Bluetooth Variations: BR/EDR and LE

Earlier than delving into the architectural elements of Bluetooth, it's advisable to deal with a differentiation that runs via the whole structure since two variations of Bluetooth can be found:

• BR/EDR: Additionally known as Bluetooth Traditional because it was the primary to seem.
• LE: Bluetooth Low Vitality born with the intention of being integrated into low-energy consumption units.

Bluetooth BR (Fundamental Price) is the first model of Bluetooth and was later prolonged with EDR (Prolonged Information Price), which permits for a better most knowledge transmission charge.

Most switch charge in Bluetooth Fundamental Price (BR):

• 1 Mbps (megabit per second)

Most switch charges in Bluetooth Prolonged Information Price (EDR):

• 2 Mbps with EDR 2
• 3 Mbps with EDR 3

BR and EDR are absolutely suitable, so they're thought-about a single system and known as BR/EDR.
Bluetooth LE (Low Vitality) was later developed as a low-power various for less complicated and cheaper units, initially with a decrease knowledge transmission charge than BR/EDR, though within the newest variations of the protocol, they've elevated to nearly match these of BR/EDR
Most switch charges in Bluetooth Low Vitality (BLE):

• Bluetooth 4.0 and 4.1: 1 Mbps
• Bluetooth 4.2: 1 Mbps (enchancment in knowledge effectivity and capability for bigger packets)
• Bluetooth 5.0 and 5.1: 2 Mbps (doubled in comparison with earlier variations)
• Bluetooth 5.2: 2 Mbps (enhancements when it comes to knowledge capability, effectivity, and vary)
• Bluetooth 5.3 and 5.4: 2 Mbps (enhancements that enhance effectivity, safety, and performance)

Though each operate equally when discovering units and establishing connections, the mechanisms they work with are very totally different and incompatible with one another, to allow them to be seen as totally different applied sciences with comparable names.

Separation between Host and Controller

A Bluetooth system consists of two sorts of components: a bunch and a number of controllers.

• Host: Serves as an interface for functions and the working system and implements the protocols of the upper layers. It communicates with the OS functions to carry out duties with Bluetooth.
• Controller: Implements the essential capabilities and protocols of the stack and helps the totally different Bluetooth applied sciences: BR/EDR and LE. A controller can assist BR/EDR, LE, or each mixed.

The host might be seen as the pc working Home windows that receives processed Bluetooth packets and communicates with person functions, whereas the controller can be the Bluetooth USB dongle that runs firmware and performs low-level duties reminiscent of capturing packets, saying the system, channel synchronization, and so forth.Host and controller talk with one another utilizing the HCI (Host Controller Interface) packet transmission protocol, permitting the host to ship actions to a controller and obtain occasions in response, controlling the state of communications at a excessive stage.

Relationship between Host and Controller

A controller can assist BR/EDR, LE, or a mix of each, whereas the host is similar for each applied sciences. It is because the elemental variations are discovered within the lower-level protocols of the stack (bodily and media entry protocols), every implementing its personal set of instructions and packets, whereas the higher-layer protocols carried out within the host should not unique to BR/EDR or LE and are widespread to each. Thus, a bunch can communicate to any controller in the identical language; the controller should implement the assist for BR/EDR and BLE and is agnostic to the host.

An attention-grabbing consequence of the separation between host and controller is that the host doesn't have data on every little thing that occurs within the controller, solely what the controller informs it of via HCI packets. That is particularly related as a result of, in complicated techniques like PCs and smartphones, the host is usually carried out as a part of the working system, whereas the controller is within the firmware of a separate {hardware} peripheral over which the OS doesn't have direct management.

That's, a person interacting solely with the host will usually not have management or data of what occurs within the layers carried out within the controller since it's an remoted ingredient, normally with proprietary firmware from the producer.

Bluetooth Protocol Stack

The usual defines a normal Bluetooth stack divided into layers and sublayers. Every layer implements a set of associated capabilities essential for the layer instantly above and makes use of the capabilities carried out by the straight decrease layer. Thus, the decrease layers implement low-level capabilities like {hardware} interplay, whereas the upper layers implement capabilities nearer to the person software, reminiscent of managing system discovery, making connections, and so forth.

A Bluetooth knowledge packet is obtained by the decrease layers, which interpret the content material and ship the end result to the higher layers till it reaches the person layers.

Generalized Bluetooth Stack

Bodily Bluetooth Layer

The bodily layer in a Bluetooth connection is chargeable for the precise knowledge transmission via the communication medium, i.e., the air. This layer defines points associated to sign modulation, working frequencies, and radiofrequency spectrum administration. Its essential operate is to determine and keep the bodily hyperlink between Bluetooth units, permitting the switch of information bits via radio waves. Merely put, it's the basis that enables units to “hear” and “talk” to one another via wi-fi indicators.

For instance, it determines the frequencies at which it emits, the period of messages, and synchronization in message transmission to keep away from two units transmitting concurrently.

This layer divides its capabilities into three sublayers:

• Bodily Transport: Handles knowledge packaging and transmission by way of radio waves.
• Bodily Channel: Defines frequency and synchronization parameters for packet transmission. There are various kinds of bodily channels, every with totally different parameters.
• Bodily Hyperlink: A communication session established between units utilizing a selected bodily channel.

Logical Bluetooth Layer

The logical layer in a Bluetooth connection manages how knowledge is organized and transmitted between units. This layer handles capabilities reminiscent of hyperlink multiplexing, circulate management, and error dealing with. Primarily, the logical layer ensures that knowledge is shipped and obtained in an orderly and environment friendly method, permitting dependable and coordinated communication between Bluetooth units.

The logical layer ensures communication stability and reliability to the higher layers (L2CAP) and controls points reminiscent of knowledge circulate sort (synchronous or asynchronous), packet numbering, acknowledgment messages (ACK), or retransmission of misplaced packets. It creates “logical links,” that are communication periods that assure particular communication circumstances between units. There are various kinds of logical hyperlinks that present totally different communication necessities.

An instance of the capabilities of the logical layer is the ACL (Asynchronous Connection-oriented Hyperlink), utilized in each BR/EDR and LE, and it offers communication with particular traits:

• Level-to-point communication: Solely two units talk with one another, not in broadcast.
• Dependable communication: The receipt of packets is assured via mechanisms reminiscent of acknowledgment and retransmission of misplaced packets.
• Bidirectional communication: Each ends of the communication can ship and obtain knowledge.
• Connection-oriented communication: There's a technique of session institution and termination, and knowledge can't be despatched outdoors of this.
• Asynchronous communication: Information is shipped in packets separated by arbitrary waits, in contrast to synchronous communication, the place a relentless circulate of information is shipped with out intermediate waits.

The sort of hyperlink is utilized in BR/EDR to transmit management frames and packets, and in LE for about 90% of all visitors.

There are different sorts of hyperlinks for transmitting knowledge in broadcast or steady knowledge streams.

Hyperlinks:

• SCO (Synchronous Connection-Oriented Hyperlink): In Bluetooth BR/EDR for real-time voice knowledge transmission.
• eSCO (Prolonged Synchronous Connection-Oriented Hyperlink): Evolution of SCO, bettering audio high quality.
• LE Coded PHY (Lengthy Vary): In Bluetooth LE to enhance connection vary and robustness.
• Broadcast:
• Isochronous: Launched in Bluetooth 5.2, used for LE audio transmission (LE Audio).

Bluetooth L2CAP Layer

The L2CAP (Logical Hyperlink Management and Adaptation Protocol) layer permits knowledge from totally different functions to be despatched over the identical ACL logical hyperlink. It offers a typical interface for all higher-level protocols within the stack (known as “application protocols”) by creating “L2CAP channels.”

An L2CAP channel (to not be confused with bodily channels) is an abstraction used to transmit knowledge between the 2 ends of a Bluetooth communication with out coping with the points of logical hyperlinks, bodily hyperlinks, and so forth. It may be understood as an application-level tunnel.

It's a basic protocol that encapsulates nearly all sorts of Bluetooth communication aside from stream and broadcast knowledge, which use different logical hyperlinks.

In numerous sections of the usual, the idea of “connection” is referenced, and though there is no such thing as a precise and unequivocal definition of what it means, it may be deduced {that a} connection in Bluetooth refers back to the institution of a communication session between two or extra units, establishing for this function a bodily hyperlink, a logical hyperlink, and an L2CAP channel.

Relationship between Bodily Architecture and Protocol Stack

Analyzing the Bluetooth bodily structure intimately and relating it to the protocol stack reveals that BR/EDR and LE capabilities are carried out in another way inside the identical construction. This implies every has its personal particular traits and mechanisms to handle communication.

Implementation of the Bluetooth Protocol Stack

• The bodily transport layer (PHY) in BR/EDR and LE is chargeable for knowledge packaging and transmission and reception by way of the antenna within the bodily transport sublayer.
• The baseband in BR/EDR teams many Bluetooth capabilities and manages channels and bodily hyperlinks, in addition to the logical layer.
• The LMP (Hyperlink Supervisor Protocol) is a management protocol that establishes and manages the state of connections between two units. This consists of connection institution, authentication, encryption, hyperlink upkeep, and disconnection, protecting each bodily and logical hyperlinks.

By way of HCI, the host can instruct the controller to provoke or cease high-level procedures, however the particulars are managed internally within the controller utilizing LMP. Safety capabilities are additionally delegated to the controller, and the host solely intervenes when cryptographic materials reminiscent of keys or knowledge in regards to the distant system and connection is requested.

The same construction is present in LE, the place the element known as the “Link Layer” manages all points associated to bodily channels and hyperlinks, in addition to logical hyperlinks. On this case, there's additionally a protocol to handle and negotiate the state of a connection, known as LLCP (Hyperlink Layer Management Protocol), however in contrast to LMP, LLCP doesn't management the safety capabilities of the connection.

These safety capabilities in LE are carried out in a protocol above L2CAP, known as SMP (Safety Supervisor Protocol). Then again, LLCP will not be talked about as ceaselessly as LMP, as it's normally referred to extra usually as LL (Hyperlink Layer Protocol).

The remainder of the layers, these carried out within the host, are encapsulated in L2CAP or use among the logical hyperlinks straight.

The HCI interface, which communicates the host with the controller, permits sending instructions (HCI instructions) and receiving occasions (HCI occasions) along with encapsulating L2CAP packets or software protocol knowledge meant for logical hyperlinks, however it isn't strictly essential for the implementation of a Bluetooth system. HCI communicates the host and the controller in complicated techniques the place they're in separate {hardware} components and want a typical communication interface. Nonetheless, each elements might be carried out in the identical {hardware} system (for instance, in the identical SoC), so this communication interface will not be essential and might be changed by an easier programming API.

Conclusion and Closing

The complexity of Bluetooth protocols and components makes learning the expertise considerably difficult, however understanding the elemental elements makes the opposite functionalities and ideas simpler to know.

After this introduction to the essential ideas of the expertise, studying documentation, code, or the Bluetooth commonplace itself can be a lot less complicated, and it is going to be a lot simpler to know every ingredient’s position inside the expertise.