Controller Area Network Extra Long (CAN XL)

The third generation of the CAN data link layer supports all three protocol types (Classical CAN, CAN FD, and CAN XL). Similar to CAN FD, there are two bit-timing settings specified. The data field length is 1 byte to 2048 byte. New is the separation of the CAN-ID field into the 11-bit-priority field and the 32-bit acceptance field. CAN XL provides some new protocol-embedded configuration possibilities regarding higher layers. Additionally, there are optional configuration features such as switching-off the error signaling and enabling the PWM (pulse-width modulation) coding at the attachment unit interface as an alternative to the normal NRZ (no-return to zero) coding. The PWM coding allows bit-rates of 10 Mbit/s and more depending on the physical network design.

CAN XL is intended for backbone and sub-backbone network applications. It is designed for an easy integration into TCP/IP network systems.

CAN XL specifications

CAN XL is based on the concepts as specified in ISO 11898-1:2015. Since December 2018, the CiA SIG (Special Interest Group) CAN XL is specifying the CAN XL protocol features. The following document series include the CAN XL related specifications and recommendations:

  • 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 data link layer (DLL) and physical coding sub‑layer (PCS)

The CAN XL DLL supports data fields from 1 byte to 2048 byte. For the XL data phase, the CAN XL DLL provides the information how to switch from the Nominal bit rate to the XL Data bit rate and vice versa, and how to switch the CAN transceiver mode from Arbitration mode to Data TX Mode/Data RX Mode or vice versa (PWM coding). Whether this CAN transceiver mode switching is supported is done by means of local configuration. It can be enabled when an appropriate CAN transceiver supporting mode switching is connected. The CAN XL data link layer provides also higher-layer management information, and improved reliability by means of two CRC fields.

Please note that, there are different terms in CiA 610-1 at the DLL and in CiA 610-3 at the PMA (physical medium attachment) sub-layer used as follows.

Table of CAN transceiver modes signaled
CAN transceiver modes signaled to the PMA sub-layer in CiA 610-1 PMA operating modes for CAN SIC XL transceivers as specified in CiA 610-3
Arbitration Mode SIC Mode
Data TX Mode FAST TX Mode
Data RX Mode FAST RX Mode

The CiA 610-1 document specifies the CAN XL DLL and the CAN XL PCS.

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 shall contain all content needed for all CAN frame formats and types, including the selection of a specific CAN frame format. In the interaction between LLC and MAC, the content of that parts of the LLC frame that are not used for the selected CAN frame format are ignored. Only the LLC frames as specified in CiA 610-1 can be transferred to the LLC sub-layer.

LLC frame fields

LEGEND
FTYP: frame type
BRS: bit-rate switch
ESI: error state indicator
SDT: service data unit type
SEC: simple extended content
DLC: data length code
VCID: virtual CAN network ID
AF: acceptance field
LLC data: logical link layer data

MAC Data Frame (DF) in XL Frame Format (XLFF)

The DF type XLFF of variable length performs and controls data transmission and reception between CAN XL nodes. The XL DF can transmit 1 byte to 2048 byte in the data field, while the data length can change in one-byte steps. CAN XL nodes are able to transmit and to receive all frame formats specified in ISO 11898-1:2015.

On transmission, an LLC frame is converted into a MAC frame. On reception, a MAC frame is converted into an LLC frame. MAC DFs in XLFF are composed of the following seven different bit fields.

MAC Data Frame in XLFF

Arbitration Field
Control Field

Priority and addressing separated

In Classical CAN and CAN FD, the CAN-ID field (11 bit or 29 bit) is used for both arbitration and addressing purposes. In CAN XL these functions are separated. The CAN XL protocol separates the priority functions (11-bit Priority ID) and the addressing (32-bit AF).

  • 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.

True Hamming distance of 6

The CAN XL DF includes two CRC (cyclic redundancy check) fields: the 13-bit PCRC (Preamble CRC) in Control Field and the 32-bit FCRC (Frame CRC) in CRC Field. The CRCs are cascaded, which means the FCRC protects the whole frame, including the PCRC. Both CRCs are able to detect any five randomly distributed bit-errors. This corresponds to a Hamming distance of 6. The university of the Stuttgart proposed the CRC polynomials for PCRC and FCRC, and they published their argumentation in iCC 2020 proceedings. The University of Kassel evaluated the error detection capabilities of the CAN XL MAC layer, what means that the CRC polynomials are double-checked.

SDT (SDU type)

The 8-bit SDT indicates the used next OSI layer protocol. It is an embedded OSI layer configuration information as described in ISO 7498-4:1998. The SDT is similar to the EtherType function indicating the used next higher layer protocol.

CiA 611-1 specifies the SDT values and the corresponding usage to unfold the power of this field. The first version of CiA 611-1 specifies SDT values for:

  • content based addressing (i.e., use of message IDs)
  • node addressing
  • Classical CAN and CAN FD mapped tunneling
  • nodes tunneling of Ethernet frames

VCID (Virtual CAN network ID)

The 8-bit VCID field allows running up to 256 logical networks on one single CAN XL physical network segment. This enables the implementation of multiple homogeneous networks determined by the same SDT. That means, CAN XL is able to run several logical (virtual) network applications on the same cable using the same SDT. This field is also an OSI layer management information as described in ISO 7498-4:1998.

Optional DLL security

The CADsec data link layer security protocol is under development. The SEC bit in the Control Field indicates, if the CAN XL DF uses the CADsec protocol. The CADsec protocol features a 4-byte header with cipher control information, the CAN secure channel ID, and a freshness value. The 16-byte trailer contains the authentication tag. The CADsec protocol is an optional feature, which improves the measures against cyber-attacks.

CRC FieldACK FieldEOF Field

Bit rate switching and CAN transceiver mode switching

CAN XL is scalable regarding bit rates and the PMA sub-layer. CAN XL nodes can use CAN high-speed transceivers as specified in ISO 11898-2 and CAN SIC transceivers as specified in CiA 601-4. To support bit rates of 10 Mbit/s and beyond, CAN XL nodes needs to use CAN SIC XL transceivers as specified in CiA 610-3. The CAN SIC XL transceivers have three modes and can be switched from Arbitration Mode into the dedicated operating mode (Data TX Mode and Data RX Mode) for the XL Data Phase. This mechanism is called “transceiver mode switching”. To maintain the transceiver mode switching, CAN XL protocols provides the optional PWM encoding function as specified in CiA 610-1, to be linked to the PWM decoding function provided by the CAN SIC XL transceiver as specified in CiA 610-3.

During the Data TX Mode, the push-pull PMA bus driver characteristic (level_0 and level_1) is activated instead of the dominant and recessive characteristic. The receiver thresholds are changed accordingly.

The bit rate is switched from the Nominal bit rate to the XL Data bit rate at the boundary between the ADH bit and the DH1 bit in the ADS field. The bit rate is switched back from the XL Data bit rate to the Nominal bit rate at the boundary between the FCP0 bit and the DAH bit in the DAS field.

When the CAN transceiver mode switching is enabled, the time duration of the ADH bit and the DAH bit is used to signal the CAN transceiver to switch its operating modes.