# A Perspective on Ethernet in Automotive Communications--Current Status and Future Trends
## 2 Current In-Car Network Architectures and Technologies
### Legacy In-Car Networking Technologies
Compared to CAN, the higher speed allows CAN FD to offer higher throughput and faster software downloads at the end-of-production line or during maintenance in garages for software updates [15]. CAN FD controllers also support Classical CAN frames, i.e., CAN FD controllers can send and receive classical CAN frames as well as CAN FD frames. Both protocols (Classical CAN and CAN FD) are standardized in ISO 11898-1:2015. _CAN XL_[16] represents the next step in CAN evolution [17; 18]. The \"XL\" part of the name is because CAN XL provides \"extra large\" payloads of up 2048 bytes and a data rate of up to 20 Mbps. Specified by CiA 610-1 (CAN in Automation) and currently standardized as part of ISO11898-1, CAN XL is a CAN data link layer that fills the gap between CAN FD and Ethernet 100 BASE-T1. CAN XL protocol controllers are also backward-compatible, i.e., they are able to perform Classical CAN and CAN FD communication. CAN XL also enables, among other things, transparent tunneling of Ethernet frames and the use of the TCP/IP stack ([[https://www.bosch-semiconductors.com/ip-modules/can-protocols/can-xl/](https://www.bosch-semiconductors.com/ip-modules/can-protocols/can-xl/)]([https://www.bosch-semiconductors.com/ip-modules/can-protocols/can-xl/](https://www.bosch-semiconductors.com/ip-modules/can-protocols/can-xl/)) (accessed on 23 December 2022)). * _FlexRay_[19]. The FlexRay protocol implements deterministic and fault-tolerant communications for time- and safety-critical applications, applying Time Division Multiple Access (TDMA) and using two channels for redundancy. The transmission speed is 10 Mbps and can reach 20 Mbps if the two redundant channels are used to transmit different data. The communication follows an offline-defined schedule based on the _network cycle_ that consists of a communication cycle, during which transmissions occur, and of a network idle time that is used for clock synchronization. The communication cycle is split into a static segment (pure TDMA transmissions), a dynamic segment (based on minisolotting), and a symbol window. FlexRay offers payload lengths up to 64 bytes, multiple topologies (i.e., bus, star, multistar), a bus guardian that prevents faulty nodes from transmitting within time slots not assigned to them to improve fault-tolerance, and support for power management, i.e., sleep modes and fast wake-up times. FlexRay was mainly intended for the chassis and the x-by-wire domain [20,21]. However, the limited flexibility of the TDMA communications, the complexity of building a schedule for the dynamic segment, the higher cost of the FlexRay technology compared with other in-car network technologies, and the crisis of the automotive sector in the years in which the FlexRay standard was published have contributed to its limited adoption in commercial cars. In the future, it is very likely that FlexRay will be replaced by Automotive Ethernet. * _MOST_[22]. The Media Oriented Serial Transport (MOST) is the current automotive infotainment backbone for high-quality audio, video, data and real-time control over a single medium. MOST is able to network up to 64 devices, such as radio, amplifier, DVD, telephone, microphone, e-Call, navigation, displays, and headphones, in a ring topology. Different transmission speeds are available, i.e., 25 Mbps on Polymeric Optical Fiber (POF), 50 Mbps on Unshielded Twisted Pairs (UTP), and 150 Mbps (POF). MOST is a synchronous network driven by a single Timing Master, which is able to stream audio and video data with different possible data rates. A MOST system covers all the layers of the ISO-OSI model and organizes the network traffic in a continuous sequence of frames that are generated by a Timing Master (typically, the Head Unit of the infotainment system) with a fixed frequency. Two frequencies are used: 48 kHz (professional audio) and 44.1 kHz (audio CD). The communication around the ring is one-directional.