RapidIO Generation 2 – Part II

By Tom Cox, RapidIO Trade Association

The embedded industry looks to RapidIO® to meet the needs of mobile users and increase quality of service. The Gen2 specification provides more usable bits per milliwatt while remaining backward compatible with Gen1. This is done not just by increasing raw bandwidth, but also by providing flow control facilities to manage short- and medium-term congestion and to avoid long-term congestion.

Physical Layer Enhancements

The physical layer now supports 5.0 Gbaud and 6.25 Gbaud lane rates, allowing a 4x RapidIO port to have up to 20 Gbps bandwidth. The link width options now include 2x, 8x, and 16x. With five lane speeds and five lane widths, system designers can tailor a RapidIO port’s raw bandwidth to anything between 1 and 80 Gbps.

The 8B/10B encoding scheme used in Gen 1 has been maintained. However, with higher lane frequencies, the protocol must support additional functionality necessary to maintain signal integrity despite increased attenuation. The Gen2 physical layer now enables automatic transmit emphasis adjustment between link partners and the use of differential feedback equalization (DFE). The result is an increase in maximum reach to 100 cm of FR4 with two connectors. Also the required bit error rate for RapidIO 5 Gbaud and 6.25 Gbaud lanes has improved to 1 in 1015 bits, matching industry norms for Gen1 parts.

The Gen2 physical layer protocol adds eight virtual channels to Gen1 prioritized flows for a total of nine. Virtual channels are associated with output bandwidth allocations on a per-port basis, which guarantee minimum throughput for traffic with different quality-of-service requirements. Such guarantees help system designers faced with complex quality-of-service challenges that pure priority-based systems cannot meet.

One significant quality-of-service challenge is information exchange so time-sensitive that discarding packets is preferable to waiting for error recovery. The typical scenario is voice or video traffic where loss of some packets has little effect on the quality of user experience, but a time lag due to waiting for packet retries could cause significant degradation. RapidIO Gen2 supports this traffic using virtual channels. Each one can be configured to support either the “Reliable Traffic” paradigm for guaranteed packet delivery or the new “Continuous Traffic” mode for time-sensitive traffic. Virtual channels configured to support Continuous Traffic will never retransmit a packet, ensuring time-critical data always makes predictable forward progress with guaranteed system latency.

The most efficient networks optimize quality of service while maximizing network utilization. The Gen2 specification defines a “Virtual Output Queue Backpressure” mechanism that enables high network utilization while preserving quality of service. Information about network congestion is communicated using control symbols. It can be used to reallocate flows depending on where congestion is occurring. The comprehensive RapidIO flow control mechanisms ensure that the raw bandwidth available is utilized most efficiently.

Typically, the Virtual Output Queue Backpressure mechanism is used to manage medium-term congestion, which lasts from tens to hundreds of microseconds.

Tom Cox is Executive Director of the RapidIO Trade Association.  You can reach him at tom.cox@rapidio.org.