In the race to develop the most appealing infotainment solutions, car makers can now take advantage of standard 1394 bus technology. Under the names FireWire (Apple), i.Link (Sony) and Lynx (Texas Instruments), 1394 has proven useful for carrying infotainment data in a variety of applications. It now promises to provide the same high performance in automobiles with a version known as the 1394 Automotive network.
The bandwidth requirements for an automotive network become clear with a glance at the list of devices that will connect to the network (Figure 1):
PCs and other devices interfacing with Web pages and e-mail available via cellular and Wi-Fi networks
Fig. 1: Auto network devices (click on image to enlarge). |
These devices need a significant amount of network bandwidth, with throughput requirements likely to grow continuously into the future. The traffic must be handled properly to provide good quality-of-service. The technology used in this network has both hardware and software consequences.
Introduction to 1394 Automotive technology
The 1394 standard covers two types of data communication: isochronous and asynchronous. Isochronous data (video and audio) is guaranteed for timeliness but not for delivery. The very nature of such data makes it useless if it arrives late, but some of the data can be discarded without causing major problems. The 1394 spec has periodic 125-s time slots allocated to send isochronous data.
On the other hand, asynchronous data is guaranteed for delivery, but not for timeliness. This class of data must reach its destination, but the specific timing is not critical. A node resends data if a delivery attempt fails.
Figure 2 shows the 1394 protocol stack, consisting of Transaction, Link and Physical Layers. In a 1394 controller IC, the Transaction Layer is implemented in software, the Link Layer is implemented in software or hardware and the Physical Layer is done in hardware.
Fig. 2: IEEE 1394 Protocol Stack (click on image to enlarge). |
1394 Automotive comparison with competing technologies
The easiest way to get a perspective on the capabilities of 1394 Automotive is to compare it with alternative technologies. MOST150 (the highest-speed version of MOST) and Gigabit Ethernet have significant disadvantages compared to 1394, as the following overview underscores. These disadvantages involve both the networking standards and the IC implementations available on the market.
MOST150 disadvantages
Lack of integrated DTCP
Content protection is a must in automotive digital multimedia networks, and Digital Transmission Content Protection (DTCP) is the leading technology to implement this function. Because MOST150 products lack this feature, MOST networks require additional devices to implement content protection (Figure 3).
Fig. 3 (click on image to enlarge). |
This lack of content protection increases both the hardware cost and overhead of the additional software support required. Moreover, even with the use of an external DTCP device, the multimedia content in a MOST150 system is protected only over the network and not on the storage media (HDD, etc.).
By contrast, 1394 Automotive ICs have built-in DTCP. This approach minimizes both hardware and software cost and ensures complete data protection.
Low bus speed
MOST150 networks are only capable of speeds up to 150 Mbps, a fraction of the 800-Mbps bus speeds available from today's 1394 Automotive devices. Future 1394 devices are expected to double this speed.
Requirement for external data compression
To compensate for low bus speed, MOST150 networks can bring data throughput up to a level that compares with that of 1394 Automotive by using data compression. This strategy requires external codecs to compress and decompress multimedia data, thereby creating two problems.
First, the typical devices required for multimedia compression and decompression (such as for MPEG2-TS) add a significant latency — possibly as high as 400ms. As a result, the driver will see a noticeable delay in the perimeter camera video. The compression/ decompression latency will also cause extensive delays when users fast-forward a DVD (Figure 4).
Fig. 4: Significant latency in MOST150 networks due to external multimedia codecs (click on image to enlarge). |
The second problem resulting from the use of external codecs is cost. Along with codecs, the implementation requires external frame memory to buffer video. In fact, a MOST150 implementation may require as many as eight silicon devices.
Since 1394 Automotive networks offer much higher bus speeds, compression may not be necessary at all. If anticipated bandwidth requirements are extreme, however, implementations can achieve both low latency and low cost by taking advantage of codecs built into Fujitsu 1394 Automotive devices. We maintain that these built-in codecs keep latency to just 4
ms, and the implementation can be accomplished with only two chips.
