Data link layer
CAN in Automation (CiA) members are developing the third generation of the CAN data link layer protocol – also know as CAN XL. Additionally, CAN FD light, a master/slave protocol, is under consideration.
CAN XL provides more than 2 048 byte of payload including a 1-byte protocol type field indicating the content of the 4-byte address field and the 2 048 byte data field. One of the features is the separation of frame priority and address information. New is also a cascaded CRC (cyclic redundancy check) featuring a Hamming distance of 6 meaning five randomly distributed bit-errors are detected under all conditions. CiA is going to charge a university to double-check the performance of the error detection capability of the CAN XL data link layer protocol.
The CAN XL protocol embeds some OSI management information such as the virtual CAN (VCAN) field. This 1-byte field enables to run up to 256 channels on a single network segment. In order to filter in hardware some of the CAN XL protocol fields, it is recommended to implement a 64-bit acceptance filter covering the 4-byte address field, the protocol type field, etc.
In addition, an embedded data link layer security protocol is under development. This CADsec protocol features a node-to-node protection comprising a 4-byte header with cipher-control information, the secure channel ID, and the freshness value as well as the 128-bit authentication tag. The payload can be encrypted optionally.
CAN XL has as CAN FD two bit-rate phases. In the arbitration phase the bit-rate is limited to 1 Mbit/s as in Classical CAN and CAN FD. In the data-phase, the bit-rate can be increased, because just one node is transmitting. CiA members also develop a CAN physical layer, which supports bit-rates up to 10 Mbit/s and above. This dual-mode approach has two modes: slow and fast. In both modes, bus biasing is active. In the slow mode, there are dominant and recessive bits (as known from Classical CAN). In the fast mode, there are level-1 and level-0 signals. Dual-mode transceivers are connected to the CAN XL protocol controller by means of Mici, the medium-independent CAN interface.
CiA members have also started to develop a master/slave protocol based on CAN FD. It is intended for simple sensor and actuator communication. A typical example is modern LED lamps in passenger cars, which are somehow price sensitive. The CAN FD master node synchronizes the slave nodes with hundreds of LEDs. This simplifies the implementation of the CAN FD slave nodes, because there is no arbitration needed, because the just react on the master requests. It is intended to run such networks with one bit-rate and is therefore limited to 1 Mbit/s, which is fast enough for such applications.
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