In the previous blog "Introduction to Bluetooth technology" we described a brief on technology advancement and its details. In this blog, we will discuss in detail the Bluetooth architecture OSI Model mapping, physical layer, data link layer, and BT Wi-Fi coexistence support in the physical layer. Engineers working on edge computing and IoT products will find the Bluetooth FHSS radio layer design and testing concepts covered here especially relevant to real-world deployments.
Bluetooth Technology Introduction
Bluetooth is a wireless technology standard using short-wavelength Ultra High Frequency radio waves in the license-free Industrial, Scientific and Medical (ISM) frequency band. Bluetooth is managed by Bluetooth SIG (Special Interest Group) originally formed by five companies: Ericsson, Nokia, IBM, Toshiba and Intel. The Bluetooth SIG publishes and updates Bluetooth specifications. The IEEE standardized Bluetooth as IEEE 802.15.1, but no longer maintains the standard. The BLE Bluetooth low energy variant — introduced in Bluetooth 4.0 — reduced power consumption significantly, expanding IoT use cases across technology consulting domains.
Bluetooth Technology OSI Layer Mapping
Bluetooth protocol can be mapped to the OSI (Open Systems Interconnection) model as per the below picture.
Bluetooth OSI Mapping
| Physical Layer | Bluetooth operates in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band. It uses frequency-hopping spread spectrum (FHSS) for transmission, dividing the band into channels. Adaptive frequency hopping is applied at this layer to avoid interference. |
| Data Link Layer | Bluetooth uses the L2CAP (Logical Link Control and Adaptation Protocol) protocol, which operates above the baseband layer. L2CAP provides both connection-oriented and connectionless data services to upper-layer protocols. It handles segmentation and reassembly of packets, as well as flow control and error correction. |
| Network Layer | Not applicable |
| Transport Layer | Bluetooth does not directly correspond to the OSI transport layer. Instead, it relies on L2CAP to provide transport services. |
| Session Layer | Bluetooth does not have a dedicated session layer like the OSI model. However, certain Bluetooth profiles define sessions for specific applications (e.g., Audio/Video Control Transport Protocol (AVCTP) for controlling audio/video sessions). |
| Presentation Layer | Data formatting and presentation are handled by upper-layer protocols. |
| Application Layer | Bluetooth profiles such as SPP (Serial Port Profile), HFP (Hands-Free Profile), A2DP (Advanced Audio Distribution Profile), and many others operate at the application layer. These profiles define specific use cases and protocols for various applications such as file transfer, audio streaming, and device control. A BLE scanner and profile manager tool at this layer allows developers to inspect active profiles and advertisement data. |
In this article, we will look in to Physical and Link layer.
Bluetooth Frequency Hopping Technique
Bluetooth frequencies are in the 2.4 GHz ISM band. The frequency in ISM bands ranges from 2400 MHz to 2483.5 MHz. There is 1 MHz space between each Bluetooth channel starting from 2402 MHz and ending at 2480 MHz. This can be calculated as 2401 + n, where n ranges from 1 to 79. This channel arrangement gives a guard band of 2 MHz at the bottom end and 3.5 MHz at the top, preventing interference. Bluetooth employs Frequency Hopping Spread Spectrum (FHSS) by which the radio signals are transmitted by rapidly switching the carrier signal among various frequency channels. The transmitted data is divided into packets and sent on one of 79 designated channels switched randomly at a rate of 1600 times per second. The main drawback of this technique is collision with another wireless device such as Wi-Fi — which is exactly what adaptive frequency hopping was designed to solve.
Bluetooth Physical Layer Design
Bluetooth physical layer consists of baseband and radio specifications as defined in IEEE 802.15.1. It contains the analog communications circuitry responsible for the translation of digital symbols over the air. It is the lowest layer of the protocol stack and provides its services to the link layer. Using this layer, BLE offers data rates of 1 Mbps (Bluetooth v4.2) / 2 Mbps (Bluetooth v5.0). The BLE Bluetooth low energy physical layer is designed for minimal power draw while maintaining adequate range. Some of the main aspects it covers include:
- Frequency band
- Gaussian frequency shift keying (GFSK) modulation scheme
- Transmission speeds
- Power
- Receiver sensitivity
- Time division
Bluetooth LE defines three physical layer variants known as PHYs.
- LE 1M
- LE 2M
- LE Coded
The LE 1M PHY uses a symbol rate of 1 Msym/s with a required frequency deviation of at least 185 kHz. All devices must support the LE 1M PHY.
The LE 2M PHY is like LE 1M but uses a symbol rate of 2 Msym/s and has a required frequency deviation of at least 370 kHz. Support for the LE 2M PHY is optional.
The LE Coded PHY uses a symbol rate of 1 Msym/s. Packets are subject to a coding called Forward Error Correction (FEC) and, depending on configuration, a pattern mapping. This increases the effective range of transmissions but reduces the application data rate. The BLE Bluetooth low energy Coded PHY is particularly useful for long-range sensor applications.
Bluetooth Data Link Layer
The Link Manager (LM) is the system that manages establishing the connection between devices. It is responsible for the establishment, authentication and configuration of the link. The Link Manager locates other managers and communicates with them via the management protocol of the LMP link. To perform its function as a service provider, the LM uses the services included in the Link Controller (LC).
Link Manager Protocol (LMP) — LMP establishes logical links between Bluetooth devices and maintains the links for enabling communications. The other main functions of LMP are device authentication, message encryption, and negotiation of packet sizes.
Logical Link Control and Adaptation Protocol (L2CAP) — L2CAP provides adaptation between upper layer frame and baseband layer frame format. L2CAP provides support for both connection-oriented and connectionless services.
The Link Manager Protocol basically consists of several PDUs (Protocol Data Units) that are sent from one device to another. The following is a list of supported services:
- Transmission and reception of data
- Name request
- Request of the link addresses
- Establishment of the connection
- Authentication
- Negotiation of link mode and connection establishment
The Logical Link Control and Adaptation Protocol (L2CAP) is used to multiplex multiple logical connections between two devices using different higher-level protocols. It provides segmentation and reassembly of on-air packets.
In Basic mode, L2CAP provides packets with a payload configurable up to 64 kB, with 672 bytes as the default MTU, and 48 bytes as the minimum mandatory supported MTU.
In Retransmission and Flow Control modes, L2CAP can be configured either for isochronous data or reliable data per channel by performing retransmissions and CRC checks.
BT Wi-Fi Coexistence with Adaptive Frequency Hopping
Engineers and researchers embarked on a quest to find solutions that would allow Bluetooth and Wi-Fi to coexist harmoniously. BLE Bluetooth low energy introduced low power consumption and advancements in frequency hopping techniques, allowing Bluetooth devices to dynamically switch between channels, reducing the likelihood of interference with Wi-Fi signals. Achieving effective BT Wi-Fi coexistence is one of the most important design goals for any modern dual-radio product.
Bluetooth Wi-Fi Collision
The adaptive frequency hopping technique allows Bluetooth to adapt to the environment by excluding fixed sources of interference (i.e., bad channels) from the available channel list. This re-mapping technique reduces the number of channels used by Bluetooth. The following figure illustrates how adaptive frequency hopping avoids collisions by adapting to the environment.
Bluetooth Wi-Fi Collision Avoidance with AFH
First-generation Bluetooth devices use 79 of the available channels in the 2.4 GHz band, hopping across these channels in a random fashion at 1600 times per second. As soon as another wireless device is introduced into the environment, this type of hopping results in occasional collisions. Adaptive frequency hopping solves this by identifying fixed sources of interference and excluding them from the available channel list.
The Bluetooth Specification requires a minimum set of at least twenty channels for adaptive frequency hopping to remain effective. Identification of bad channels is done initially with RSSI and PER. Post identifying the weak or bad channels, those channels are kept away from use. This complete Bluetooth FHSS radio layer design and testing approach ensures that BT Wi-Fi coexistence is maintained even in dense wireless environments.
AFH for Bluetooth consists of four main sections:
Channel Classification —A method of detecting interference on a channel-by-channel basis, or using a pre-defined channel mask.
Link Management —Coordinates and distributes the adaptive frequency hopping information to the rest of the Bluetooth network.
Hop Sequence Modification —Avoids interference by selectively reducing the number of hopping channels.
Channel Maintenance —A method for periodically re-evaluating the channels.
While the clash between Bluetooth and Wi-Fi signals posed significant challenges in the past, adaptive frequency hopping and other technological innovations have paved the way for effective BT Wi-Fi coexistence solutions.
Conclusion
From the introduction of BLE Bluetooth low energy to the evolution of Wi-Fi co-existence, continuous advancements have transformed Bluetooth as the most popular short-range wireless standard. The adaptive frequency hopping mechanism is central to reliable BT Wi-Fi coexistence and is one of the most important aspects of Bluetooth FHSS radio layer design and testing. With a solid understanding of the physical and data link layers, we will look into the higher layers of the Bluetooth stack in upcoming articles.