Cost

From a silicon standpoint, it might seem that the often touted high-volume cost economies of Ethernet would give Ethernet a significant advantage over RapidIO. While this might be true for 4- 8 port GE switches used in LANs, Ethernet switches for use in many embedded applications require more specialized functionality such as VLAN QoS and SERDES PHYs, significantly reducing the number of accommodating vendors, overall shipping volume and, therefore, cost economies. Additionally, RapidIO technology in general assumes a maximum backplane or board-level channel of 100 cm using copper traces on FR4-like materials. Ethernet PHYs for twisted-copper pair must support channels 100 times longer than RapidIO technology’s and, since Ethernet cabling assumes bundling with many other similar pairs, it must tolerate significantly more crosstalk. Together, these result in significantly higher PHY complexity than is actually required for backplane applications. As a result, some switches provide a non-standard SERDES PHY for these applications.

Computing true PHY cost must also be done carefully. For example, one commercially available RapidIO endpoint supporting messaging was only 25% larger than a straightforward Gigabit Ethernet controller without full TOE capabilities. Likewise, a four-lane RapidIO SERDES is about 50% larger than a single XAUI lane. This suggests that the silicon complexity required for endpoints is comparable.

For its part, RapidIO technology offers 2.5 times more effective bandwidth per link than GE. Yet, the cost per port of a 16-port RapidIO switch is competitive with, or better, than similar GE switches. For applications requiring more than 1 Gbps, the only alternative for Ethernet is 10 Gbps. Today, RapidIO technology offers higher effective bandwidth for payloads less than 1024 bytes at lower cost, even without taking into consideration the cost imposed on Ethernet endpoints to process protocol stacks at 10 Gbps. Such processing also has a significant impact on power consumption as a GHz-class processor to terminate each GE link increases power by watts.

Virtually any application layer service can be supported by either Ethernet or RapidIO. The difference between the two technologies, however, resides in their individual inefficiencies and the level of hardware processing supported. Ethernet has a long history in the LAN which, because of backwards compatibility, header and protocol inefficiencies, software dependence, complex and proprietary offloading mechanisms, and lack of implementation standards, makes it a less than ideal choice for backplane applications. The RapidIO standard was specifically designed to provide optimized performance for embedded applications.

This article was written by Greg Shippen, System Architect, Freescale Semiconductor’s Digital Systems Division (Austin, TX). For more information, contact Mr. Shippen at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , or click here.