The physical layer describes the device level interface specifics such as packet transport mechanisms, flow control, electrical characteristics, and low-level error management.
This partitioning provides the flexibility to add new transaction types to the logical specification without requiring modification to the transport or physical layer specifications. The RapidIO standard supports a number of capabilities that enable RapidIO to serve as a communications fabric, such as the extensions defined in the Data Streaming logical layer specification and the Multicast specification. Quality of service is an inherent part of the RapidIO specification, implemented directly in hardware and enabling traffic to be classified into as many as six prioritized logical flows.
While the mechanism for forward progress in the fabric relies upon ordering rules at the physical layer to give responses higher priority, the degree to which prioritization results in lower average latency or jitter for a particular flow is specific to the actual implementation.
QoS also affected by specific fabric arbitration policies. While the specification explicitly defines prioritized flows, developers are free to choose the particular arbitration policies to put into place to prevent starvation of lower-priority flows, such as the well-known leaky-bucket scheme.
As even the least aggressive design must support these mechanisms, higher-priority flows are guaranteed to demonstrate better lower-average latency. For applications requiring even more aggressive and effective QoS, advanced flow control and data plane capabilities are available. The RapidIO protocol defines multiple flow control mechanisms at the physical and logical layers. By managing physical layer flow control at the link layer, short-term congestion events are effectively managed for serial and parallel applications using both receiver- and transmitter-controlled flow control.
Longer-term congestion is controlled at the logical layer using XOFF and XON messages which enable the receiver to stop the flow of packets when congestion is detected along a particular flow. Receiver-only flow control, where the transmitter does not know the state of receiver buffers and the receiver alone determines whether packets are accepted or rejected based on receiver buffer availability, results in packets being resent, creating wasted link bandwidth.
Additionally, ordering rules require a switch to send higher-priority packets before resending any packets associated with a retry, aggravating worst-case latency for lower priority packets. Transmitter-based flow control avoids bandwidth wasting retries by enabling the transmitter to decide whether to transmit a packet based on receiver buffer status.
Through receiver buffer status messages sent to the transmitter using normal control symbols, the transmitter is able to limit transmissions within the maximum number of buffers available at the receiver. In general, priority watermarks at the various buffer levels are used to determine when the transmitter can transfer packets with a given priority.
The RapidIO specification achieves further efficiency and higher throughput through the use of data plane extensions. Since data plane fabrics can carry multiple data protocols, these extensions enable the encapsulation of virtually any protocol using a data streaming transaction type with a payload up to 64 Kbytes.
Also describes standard maintenance transactions for reporting error conditions to host devices. RapidIO Specification Revision 4. RapidIO Specification Revision 3. RapidIO specification Revision 3. RapidIO Specification Revision 2. Specifies the header information added to a RapidIO logical packet and the way the header information is interpreted by a switching fabric. Describes the standard approach for providing directory based coherent memory in a RapidIO-based multiprocessor system.
Describes the optional error management extensions, including additional register definitions for collecting device state.
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