CAN XL (extended data-field length)

The third generation of the CAN data link layer supports all three protocol types (CAN CC, CAN FD, and CAN eXtended data-field Length (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 pulse-width modulation (PWM) coding at the attachment unit interface as an alternative to the non-return-to-zero (NRZ) coding of CAN. The PWM coding allows bit rates of 10 Mbit/s and higher (depends on the physical network design).

CAN XL is intended for backbone and sub-backbone network applications. It is designed for an easy integration into transmission control protocol and internet protocol (TCP/IP) network systems.

CAN XL is standardized in ISO 11898-1:2024. Since December 2018, the CiA Special Interest Group (SIG) 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

The CAN XL data link layer (DLL) supports data fields in range 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 phase 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 also provides higher-layer management information, and improved reliability by means of two cyclic redundancy check (CRC) fields.

The data frame (DF) type XL frame format (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 11898-1:2024.

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.

In CAN CC 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 acceptance field).

• 11-bit Priority: This field provides the uniquely assigned priority of the CAN XL DF.
• 32-bit acceptance field (AF): 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.

The CAN XL DF includes two CRC fields: the 13-bit preface CRC (PCRC) in control field and the 32-bit frame CRC (FCRC) 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 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, which means that the CRC polynomials are double-checked.

The 8-bit ISO OSI service data unit (SDU) type (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;
• CAN CC and CAN FD mapped tunneling;
• Nodes tunneling of Ethernet frames.

The 8-bit virtual CAN network ID (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.

The CANsec data link layer security protocol is under development in CiA 613-2. The SEC bit in the control field indicates if the CAN XL DF uses the CANsec protocol. The CANsec 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 CANsec specifies control and data plane whereas key management is one of the priority topics. The CANsec protocol is an optional add-on functionality, which is a part of the measures against cyber-attacks.

The LLC frame fragmentation is under development in CiA 613-3. The goal is to provide a mechanism preventing a CAN XL long frame (e.g. 128 byte <= frame size <= 2048 bytes) from blocking the network thus allowing high priority control information to get through without sacrificing the integrity of a long frame. For that reason, the long frames get fragmented. This mechanism allows to interrupt the ongoing long-frame transmission and to resume the transmission from the fragment as it was interrupted.

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 need 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 protocol 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 phase 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 phase bit rate to the nominal bit rate at the boundary between the FCP(0) 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.