Controller Area Network Extra Long (CAN XL) Protocol

Controller Area Network Extra Long (CAN XL) Protocol: Next Step in CAN Evolution

If you have an interest in automotive networking and communication, you may be familiar with the Controller Area Network (CAN) protocol. This standard dictates the exchange of data be

tween various devices within a vehicle, including sensors, actuators, ECUs, and diagnostic tools. Since the 1990s, CAN has been extensively utilized in the automotive industry and has undergone evolution to cater to the growing demands of contemporary vehicles.

However, as vehicles become more complex and connected, the limitations of CAN become more apparent. For example, CAN has a maximum data rate of 1 Mbps, which is not enough to support high-bandwidth applications such as advanced driver assistance systems (ADAS), infotainment, or over-the-air updates. Moreover, CAN has a fixed frame size of 8 bytes, which limits the amount of data that can be transmitted in a single message. Furthermore, CAN does not support encryption or authentication, which raises security and privacy concerns.

To address these challenges, a new protocol called CAN XL has been developed by the CAN in Automation (CiA) organization. CAN XL is an extension of the CAN FD protocol, which itself is an improvement over the classic CAN protocol. CAN XL aims to provide higher data rates, larger payloads, and enhanced security features for automotive networking. In this blog post, we will introduce some of the main features and benefits of CAN XL, and how it differs from CAN FD.

Introduction to CAN-XL Protocol

The third iteration of the CAN data link layer has full support for all three protocol types, namely Classical CAN, CAN FD, and CAN XL Protocol. Similar to CAN FD, two different bit-timing settings are specified. The data field length has been expanded to accommodate 1 byte to 2048 bytes. A novel feature is the division of the CAN-ID field into the 11-bit priority field and the 32-bit acceptance field.

CAN XL offers some additional protocol-embedded configuration options for higher layers. There are also optional configuration features, such as disabling the error signalling and enabling pulse-width modulation (PWM) coding at the attachment unit interface instead of the standard non-return-to-zero (NRZ) coding. With PWM coding, bit rates of up to 10 Mbit/s or higher can be achieved, depending on the physical network design.

CAN XL is engineered for use in backbone and sub-backbone network applications, and it has been specifically designed for seamless integration with TCP/IP network systems.

Features of CAN-XL Protocol

  • Higher Data Rates: CAN XL supports data rates of up to 20 Mbit/s, which is a significant improvement over the 5 Mbit/s offered by the CAN FD protocol. This increased data rate allows for faster communication between devices in complex systems, such as autonomous vehicles and smart factories.
  • Increased Payload Size: The payload size in CAN XL has been increased from the 64 bytes supported by CAN FD to 2048 bytes. This enables the transmission of larger data packets, which is particularly useful for high-bandwidth applications such as sensor data fusion, software updates, and diagnostics.
  • Enhanced Error Detection Mechanisms: CAN XL incorporates advanced error detection mechanisms to ensure the reliability and robustness of data communication. This includes the use of Cyclic Redundancy Check (CRC) and a new mechanism called Forward Error Correction (FEC), which can correct single-bit errors without the need for retransmission.
  • Improved Efficiency: The CAN XL protocol features improved efficiency in data communication through the use of variable data length coding and header compression techniques. This results in a reduction of overhead and an increase in effective data throughput.
  • Backward Compatibility: CAN XL is engineered to maintain backward compatibility with existing CAN FD networks. This implies that CAN XL devices can be seamlessly incorporated into current systems without necessitating substantial hardware or software modifications.
  • Bit Rate: CAN XL protocol targeted for high-speed CAN XL transceivers (up to 20Mbit/s).
  • Incremental upgrade & mixed networks (CAN FD & CAN XL): Co-existence of “cheap” CAN FD and fast CAN XL nodes in same network.
  • Supports Complex Network Topologies: Flexible trade-off between speed and complex networks
  • Price: Expected to be cheaper than 10BASE-T1S.
  • Large payload size + New Functions (SDT, VCID, …):
  • Extreme scalability:
    • Wide range of bit rates configurable [up to 20 Mbit/s].
    • Any transceiver (Classic, FD, SIC, SIC XL) usable.
    • Use Cases:
      • Signal Based Communication (SBC).
      • Service Oriented Communication (SOC via ETH tunnelling).
  • AUTOSAR support: CAN-XL, designed to increase the data rate and payload size for automotive and industrial communication systems, is anticipated to be integrated into AUTOSAR’s layered software architecture, which is widely adopted in the automotive industry.
  • Availability: The practical availability of CAN-XL in terms of supported hardware, software, and tools has been rolling out progressively.

CAN XL Protocol Specifications

The foundation of CAN XL is grounded in the principles outlined in ISO 11898-1:2015. The features of the CAN XL protocol were defined by the CiA SIG (Special Interest Group) as of December 2018. Subsequently, the CAN-XL protocol was officially released by the CAN in Automation (CiA) group in 2020.

  • CiA 610: CAN XL specification and test plans.
  • CiA 611: CAN XL higher-layer services.
  • CiA 612: CAN XL guidelines and application notes.
  • CiA 613: CAN XL add-on services.

CAN-XL Protocol in OSI Layer

The CAN-XL protocol provides some embedded OSI layer management fields, such as the SDU-type field and the VCAN field, which enable the use of different higher OSI layers on the same network segment and multiple instances of the same higher-layer protocol. The CAN-XL protocol can achieve bit rates up to 20 Mbit/s, depending on the physical network design and the transceiver mode.

This preserves the CAN properties of arbitration, robustness, and long stubs, and it improves the reliability by using two CRC fields. It is designed for backbone and sub-backbone network applications, with a focus on seamless integration into TCP/IP network systems. The protocol has undergone submission for ISO standardization, and its availability is anticipated by the end of 2022.

CAN-XL Protocol Physical Layer

The physical layer of the CAN XL protocol is responsible for bit timing and synchronization, signal level, and signal encoding/decoding. CAN XL is designed to be compatible with various physical layer specifications, including CAN FD.

The increase of data baudrate in CAN-XL is impacting the physical chip level changes which is not much difference with backward compatibility for all other CAN protocol physical layers.

You can check the below datarate differences for different CAN Protocols.

  • Classical CAN up to 1Mbit/s
  • CAN FD up to 2Mbit/s
  • CAN FD-SiC up to 5-8Mbit/s
  • CAN XL up to 10 Mbit/s and beyond (in development)

The CAN XL DLL has the capability to handle data fields ranging from 1 byte up to 2048 bytes in size. It provides instructions on how to transition from the nominal bit rate to the XL data phase bit rate and vice versa, as well as how to switch the CAN transceiver mode between Arbitration mode and Data TX Mode/Data RX Mode (using PWM coding). The ability to switch CAN transceiver modes is dependent on local configuration and whether the connected CAN transceiver supports mode switching.

Moreover, the CAN XL data link layer includes higher-layer management information and enhanced reliability through the use of two CRC fields.

CAN-XL Protocol LLC and MAC sub-layers

The ISO 11898 series specifies two data link sub-layers:

  • LLC (Logical Link Control): It acts as a sub-layer between the OSI network layer and the MAC sub-layer.
  • MAC (Medium Access Control): It is responsible for moving frames from the LLC sub-layer to the PMA sub-layer and protects the transmission by means of stuff-bits, CRC fields, etc.

The LLC frame structure is designed to include all essential content for various types and formats of CAN frames, enabling the selection of a specific frame format. In the communication between LLC and MAC, any components of the LLC frame that are not essential for the chosen CAN frame format are disregarded.

It is crucial to note that only the LLC frames specified in CiA 610-1 are allowed to be transferred to the LLC sub-layer.

Priority and Addressing in CAN-XL Protocol

While in Classical CAN and CAN FD protocols, the CAN-ID field (11-bit or 29-bit) serves both arbitration and addressing purposes, the CAN XL protocol separates these functions. In CAN XL, the priority functions are specifically assigned to the 11-bit Priority ID, while addressing is managed by the 32-bit Acceptance Field.

  • 11-bit Priority: This field provides the uniquely assigned priority of the CAN XL DF.
  • 32-bit AF (Acceptance Field): This field is included in the 64-bit hardware acceptance filter of the CAN XL controller. It may contain node address or content-indicating information.

SDT (SDU type) style Addressing in CAN-XL Protocol

The 8-bit SDT is responsible for identifying the next OSI layer protocol being used. It serves as embedded OSI layer configuration information, as defined in ISO 7498-4:1998. The SDT function is comparable to that of EtherType, which identifies the next higher layer protocol in use.

  • Message/Content Based Addressing.
  • Node addressing
  • Classical CAN and CAN FD mapped tunneling
  • Nodes Tunneling of Ethernet Frames.

CAN-XL Protocol Frame Format

The CAN-XL frame format consists of six fields: Start-of-Frame (SOF), Identifier (ID), Payload Length (PL), Payload Data (DATA), Cyclic Redundancy Check (CRC), and End-of-Frame (EOF). The following figure shows the structure of a CAN-XL frame:

Start of Frame (SOF) Field in CAN-XL

A single dominant bit indicating the beginning of a new CAN frame.

Arbitration Field in CAN-XL

The arbitration field in CAN and CAN-XL protocols serves the purpose of controlling access to the bus. It determines the priority of messages, with lower numerical values having higher priority.

In CAN-XL, the arbitration field has undergone extension to accommodate larger frame sizes and higher data rates of the protocol. It comprises several parts:

MAC Data Frame In CAN-XL 11-Bit Frame Format

The CAN-XL protocol introduces the MAC (Medium Access Control) data frame, which allows for significantly larger payloads than the previous CAN or CAN FD protocols.

CAN-XL MAC Data Frame In 11-Bit Frame Format

VCID (Virtual CAN network ID) in CAN-XL Protocol

In the CAN XL protocol, the Virtual CAN network ID (VCID) is an 8-bit field that identifies a logical (virtual) network within a physical network segment. The VCID enables the implementation of multiple homogeneous networks determined by the same SDT (Service Data Type) on the same physical network segment. This means that a single CAN XL physical network segment can support up to 256 logical networks, each identified by a unique VCID value.

Each node on a CAN XL network can configure itself to participate in one or more logical networks identified by different VCID values. This allows for the separation of different types of data or applications into different logical networks, while still using the same physical network infrastructure. The VCID field is an important feature of the CAN XL protocol, as it allows for more efficient and flexible use of the network resources.

With an 8-bit VCID field, a single CAN XL physical network segment can support up to 256 logical networks. This allows the implementation of multiple homogeneous networks that share the same SDT. In other words, CAN XL can run multiple logical (virtual) network applications over the same cable using the same SDT. Additionally, this field serves as OSI layer management information, as defined in ISO 7498-4:1998.

Applications of CAN-XL Protocol

The CAN XL protocol is well-suited for a wide range of applications that require high-speed data communication and large payload sizes. Some potential use cases include:

  1. Automotive: With the increasing complexity of modern vehicles, the need for high-speed communication between various electronic control units (ECUs) is becoming critical. CAN XL can support advanced driver assistance systems (ADAS), in-vehicle infotainment systems, and diagnostics.
  2. Industrial Automation: In smart factories and industrial IoT applications, the need for real-time communication and large data payload sizes is essential. CAN XL can facilitate the exchange of sensor data, machine-to-machine communication, and control systems in these environments.
  3. Medical Devices: Modern medical devices often require high-speed data communication for accurate diagnostics, monitoring, and control. CAN XL can be employed to ensure reliable communication between various devices in medical systems.
  4. Aerospace: In aerospace applications, CAN XL can facilitate communication between avionics systems, offering real-time data exchange and efficient communication between various subsystems.

Future Development and Enhancement of CAN-XL Protocol

  1. Higher Data Rates and Improved Latency: As applications grow more complex and require even faster data transfer rates, there may be a need to further increase the data rates supported by CAN XL. By optimizing the protocol’s underlying physical layer, using advanced modulation schemes, and implementing more efficient error correction techniques, future iterations of CAN XL may offer even higher data rates with improved latency.
  2. Enhanced Security Features: With the increasing importance of cybersecurity, especially in critical infrastructure and automotive systems, adding more robust security features to the CAN XL protocol will be crucial. This may include advanced encryption and authentication methods, secure key management systems, and intrusion detection mechanisms to protect against unauthorized access and tampering.
  3. Time-Sensitive Networking (TSN) Integration: The integration of Time-Sensitive Networking (TSN) with CAN XL can further enhance the real-time communication capabilities of the protocol. TSN is a set of IEEE standards that enables deterministic communication in Ethernet-based networks. Integrating TSN with CAN XL can achieve real-time communication with guaranteed latency and Quality of Service (QoS), making the protocol more suitable for time-critical applications in the automotive, aerospace, and industrial automation sectors.
  4. Support for Multi-Gigabit Speeds: To cater to the increasing demand for bandwidth in applications like autonomous vehicles and aDeveloping new physical layer technologies, implementing more efficient encoding techniques, and employing advanced error correction algorithms could achieve this.
  5. Seamless Integration with Wireless Technologies: As wireless communication becomes more prominent in various industries, it is crucial to enable seamless integration of CAN XL with wireless technologies like 5G, Wi-Fi, and Bluetooth. This will allow CAN XL-based networks to communicate wirelessly with other devices, improving overall system flexibility and interoperability.
  6. Improved Energy Efficiency: Future developments in the CAN XL protocol may also focus on improving energy efficiency, especially for battery-powered devices and electric vehicles. Optimizing the power consumption of CAN XL transceivers, introducing power-saving modes, and implementing advanced error correction techniques that reduce the need for retransmissions can achieve this.

Conclusion of CAN-XL Protocol

Controller Area Network, commonly known as CAN, is a robust communication protocol widely employed in automotive and industrial applications. One significant evolution is the introduction of CAN Flexible Data-rate (CAN-FD), offering enhanced data transfer capabilities. The protocol operates at the Data Link Layer (DLL) and supports various Protocol Versions, such as CAN 2.0A and CAN 2.0B.

Understanding these versions is crucial for implementing effective message arbitration strategies, ensuring seamless communication in a networked environment. This comprehensive overview delves into the intricacies of CAN, CAN-FD, DLL, Protocol Versions (CAN 2.0A, CAN 2.0B), and Message Arbitration, providing valuable insights for engineers, developers, and enthusiasts in the field.

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