Data Communication Comparing Ethernet and RapidIO

RapidIO, similar to Ethernet, exchanges packets over a variety of media. Recently, the concept of “lossless Ethernet” has been publicized. Lossless Ethernet is also known as “Data Center Ethernet” (DCE). How far can a comparison between these two technologies go?

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RapidIO supports messaging and read/write semantics, enabling control and data plane operations to use the same physical interconnect – a feature that improves efficiency and simplifies system design. Designed from the ground up to minimize latency, RapidIO technology has been optimized to be implemented in hardware.

RapidIO links guarantee the delivery of packets, even in the presence of congestion and transmission errors, through simple flow control and error-recovery mechanisms. These mechanisms are implemented with short, link-specific quantities called “control symbols.” Most control symbols can be embedded within packets, ensuring that the control loop latency for reliable packet exchange is minimized.

Ethernet has traditionally been a “lossy” medium, requiring software support of protocols, such as Transmission Control Protocol (TCP), to provide reliable transport services. Ethernet’s usual flow control mechanisms have been network layer protocols that control transmission rates based on the detection of packet loss, as this is the most effective way to deliver reasonable throughput in a planet-wide network.

Ethernet technology overview

Recently, the concept of “lossless Ethernet” has been publicized. Lossless Ethernet is also known as “Data Center Ethernet” (DCE). Lossless Ethernet reduces packet loss due to congestion in the Ethernet network by establishing link-level flow control. This has the potential to increase the efficiency of the overall network since the amount of packet discarding is greatly diminished. Lossless Ethernet under congestion can perform in a manner similar to RapidIO.

For a detailed comparison of RapidIO and Ethernet technologies, refer to www.rapidio.org, System Interconnect Fabrics: Ethernet Versus RapidIO Technology.

RapidIO guarantees packet delivery on each link. In the case of a 100m 10 Gbps fiber connection, recovery can be accomplished with the exchange of three control symbols, which can occur in approximately 2.5 µs. For a chip-to-chip link, the exchange can be completed in less than 300 ns.

Lossless Ethernet is a partial misnomer, because packets can still be lost due to transmission error. Data Center Ethernet, therefore, continues to require offload engines and/or software stacks, which add latency to message delivery. For example, the Fibre Chanel protocol enables error recovery in FCoE. Error recovery, therefore, has much higher latency than RapidIO. This high latency has significant impact for system resource usage and performance.

Ethernet task groups have been working on incorporating Forward Error Correction (FEC) in the transmission scheme for higher data rates. While this can greatly reduce the chance of an error occurring, it also has the effect of increasing latency for every inter-device transfer throughout the network.

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