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87 Infotainment systems | Focus media-independent interface (RGMII). There is also a 10 Gbit/s media- independent interface (XGMII) for designs of even higher speeds. The RMII standard reduces the pin- count by reducing the number of signals required. It therefore reduces the cost and complexity of the PHY and MAC hardware, with RMII using about half the number of signals compared to MII. The two clocks, TXCLK and RXCLK, are replaced by a single clock that is used as an input to the PHY rather than an output. That allows the clock signal to be shared among all PHYs in a multi-port device. GMII operates at speeds of up to 1000 Mbit/s, and is implemented using a data interface clocked at 125 MHz with separate 8-bit data paths for receive and transmit. It is backwards-compatible with the MII specification and can operate at fall-back speeds of 10 or 100 Mbit/s. The GMII interface was first defined for 1000Base-X in IEEE 802.3z-1998. RGMII specifies a reduced interface that is simpler to use. It uses half as many data pins as GMII by clocking data on both the rising and falling edges of the clock in 1000 Mbit/s operation, and by eliminating non-essential signals such as carrier sense and collision indication. That means RGMII has only 12 pins, rather than the 24 in GMII. XGMII is a standard defined in IEEE 802.3 for connecting full-duplex 10 Gigabit Ethernet (10GbE). It uses two 32- bit data paths for receive and transmit, and two 4-bit control flows, operating at 156.25 MHz. The next stage will be for a cost- effective 25 Gbit/s standard for the link between the PHY and MAC chips. To address the latency challenges of Ethernet, Time-Triggered Ethernet (TTEthernet) is a scalable networking technology that uses time scheduling to deliver deterministic real-time comms. It is specifically designed for real-time, fault-tolerant designs. It adds redundancy management and fault-tolerant clock synchronisation into the network layer, and is specified in the SAE AS6802 standard. The packets can be partitioned into different traffic classes, allowing a range of protocols to be combined on one high-speed physical network. This is already being used in autonomous vehicle designs for the safety systems, and has the potential to extend to the infotainment systems by combining the real-time audio and the video channels. Optical fibre Fibre optic technology has been a potential networking technology for vehicles for many years but has yet to break through. The development of a 1 Gbit/s technology on plastic fibre led to more cost-effective modulation and signal processing technology. These techniques have been used with glass fibres to deliver 1, 5, 25, 50 and even 100 Gbit/s speeds, all with the same transceivers and connectors. This has been enabled by fibre makers who have been developing specialist coatings for the glass fibres that can perform well in the more hostile environment of an automotive design. The developers of the technology believe a scalability from 1 to 25 Gbit/s with the same chips and connectors will drive economies of scale to push down the cost of the components and the fibre itself to challenge copper systems that are currently on 100 Mbit/s and 1 Gbit/s speeds. Aligning the fibre within the connectors is one of the biggest challenges with the wiring harness. Previously this had to be done by hand in an expensive process that has been difficult to automate. Any tiny misalignment in the connector compromises the performance of the link, reducing the data rates. However, developers are working on how to ensure that the construction of the optical connectors can be cost-effective. Power consumption is also a potential issue. In other applications, fibre optic links consume a lot of current and generate a lot of heat, which has to be removed. For automotive designs, the challenge has been to reduce the current to achieve the thermal and power requirements for reliability while still achieving high speed data rates. That challenge relates to the noise and signal distortion, which needs complex signal processing and new signal modulation and coding of the signal. The aim is to standardise this Unmanned Systems Technology | April/May 2020 This fibre optic transceiver board for automotive designs can deliver 1-25 Gbit/s with the same transceivers and connectors (Courtesy of KDPOF)