Proven 1394 technology trumps Ethernet AVB potential for the vehicle network

Let's take a closer look at the requirements of an automotive network. The network must connect a variety of passenger-entertainment and driver-information systems. For example, the network might need to carry video from a central DVD or Blu-ray player to video screens located at each passenger seating location. The same network might need to carry real-time video from a rear-facing camera to a driver display so that the driver can see approaching traffic or move in reverse more safely. The network might also link to storage devices such as Flash memory or hard drives that might store navigation data. With wireless WAN data networks such as WiMax emerging, the network might carry Internet traffic. And of course the network will carry streaming music.

When considering a network for the above scenario, the evaluation should include the types of media a network can use as well as the supported PHY layer performance and topologies. The evaluation should also consider the higher-layer protocols that ensure that devices interoperate seamlessly. For example, a network-connected display must be compatible with a source such as a DVD player. Unlike in the home, the DVD player won't connect directly to the display. Instead, both devices must connect and interoperate via a network. Finally, the evaluation should consider the standards body behind the network, and the roadmap for the future. See figure 1 below for examples of automotive networks.

The 1394 standard is promulgated by the 1394 Trade Association (TA). The 1394 Automotive standard includes 1394 technology developed for PC and consumer electronics applications, as well as additional auto-centric technology. For example, 1394 Automotive includes all of the auto-centric standards development originally handled in the IDB Forum, and includes a direct link to AMI-C, ISO, IEC, IEEE, SAE and USCAR standards. Consider a comparison of the 1394 Automotive standard and Ethernet Audio/Video Bridging, AVB (see Table 1).

Ethernet AVB is being promulgated by the AVnu Alliance. The alliance will promote A/V transmission using the established Ethernet PHY and Link layers combined with the AVB addition developed in the IEEE 802.1 standards body. Specifically, Ethernet AVB will use the new 802.1Qat Stream Reservation Protocol and 802.1AS Precision Time Protocol to add multimedia capabilities to the Ethernet LAN.

Let's start the detailed comparison with the maximum data rates supported by 1394 and Ethernet. Today, 100-Mbps and 1-Gbps Ethernet ICs designed for IT applications are readily available. In the case of 1394, 400-Mbps (S400) and 800-Mbps (S800) automotive grade ICs are readily available and the technology is broadly deployed in computer products. 800-Mbps 1394 is essentially equal to1-Gbps Ethernet in usable bandwidth due to the fact that Ethernet uses 8b/10b data encoding and incurs what's essentially a 20-percent performance hit in terms of actual data rate. While both Ethernet and 1394 offer plenty of raw bandwidth for automobile multimedia network needs, the evaluation must go beyond data rate to topology considerations and cost.

Topology and weight in the automotive network
It's fair to question why we need an automotive network. Why wouldn't point-to-point connections like the ones most broadly used in consumer A/V systems work perfectly well? Automakers in recent years have used point-to-point connections such as LVDS to carry video. The answer to why a network is better lies in the weight of the cable. Automakers are constantly seeking to reduce vehicle weight to increase both fuel efficiency and performance.

Wiring harnesses contribute significantly to vehicle weight. Lower-speed networks (like CAN), which connect driveline systems such as braking and engine control, came about largely based on the desire to minimize the wiring harness. Now, automakers seek a similar solution for multimedia data. The 1394 Automotive technology has demonstrated that a network can carry the multimedia data that in many cases today still traverses point-to-point connections.

In fact, 1394 offers far more flexibility in topology than does Ethernet, and that advantage translates into simpler, more flexible and lighter-weight wiring harnesses. The 1394 Automotive standard inherently supports star, tree, ring, or daisy-chained topologies. In most automotive cases, the daisy chain choice delivers the desired simplicity in the wiring harness. Table 2 below illustrates some of the basic topology differences.