Engineers often discover ways to leverage technology in applications for which it was never intended. Sometimes the fit is "good enough" and the resulting economies of scale are sufficient to make the extended technology implementation successful. But the desire to use a familiar technology runs the risk of stretching the technology beyond its capabilities, complicating designs and creating unforeseen problems.
For example, Ethernet is frequently considered a prime candidate for the convergence protocol in system-level fabrics, yet comparing the familiar Ethernet to the RapidIO specification for board- and chassis-level applications can be surprising.
Ethernet was designed for large networks with many endpoints, each with a powerful processor available for protocol stack processing. RapidIO technology was designed specifically for embedded in-the-box and chassis control plane applications, and emphasizes reliability with minimal latency, limited software dependence, protocol extensibility, and simplified switching that provides effective data rates from 667 Mbps to 30 Gbps. Additionally, RapidIO technology features hardware-based protocol processing, support for read/write operations, messaging, data streaming, HWQoS, data plane extensions, and protocol encapsulation.
Ethernet is supported by a wide range of Layer 3 and higher protocols that implement optional and advanced functionality, expanding Ethernet's scope in the network while introducing various derivative protocol definitions without unified specification. This results in significantly increased stack complexity to accommodate the variations, leading to higher costs for processing resources and higher packet latency.
Another challenge for Ethernet is how and when protocol off-load engines will be standardized in the embedded industry, as there is no standard driver-level interface for hardware off-load. Every off-load implementation is proprietary, each with a proprietary Ethernet stack, which locks OEMs into one option for future designs.
The RapidIO specification outlines the baseline functionality for chassis- and board-level applications, resulting in lower implementation costs and reduced overall complexity. Since most of the RapidIO protocol is implemented in hardware, its software drivers are simpler than the typical Ethernet TCP/IP stack. RapidIO stacks can depend upon the existence and consistency of standardized implementations.