Editing RapidIO (section)

=== Physical layer ===
The RapidIO electrical specifications are based on industry-standard Ethernet and Optical Interconnect Forum standards:
* [[XAUI]] for lane speeds of 1.25, 2.5, and 3.125 GBd (1, 2, and 2.5&nbsp;Gbit/s)
* [[Optical Internetworking Forum|OIF CEI]] 6+ Gbit/s for lane speeds of 5.0 and 6.25 GBd (4 and 5&nbsp;Gbit/s)
* [[10 Gigabit Ethernet|10GBASE-KR]] 802.3-ap (long reach) and 802.3-ba (short reach) for lane speeds of 10.3125 GBd (9.85&nbsp;Gbit/s)

The RapidIO PCS/PMA layer supports two forms of encoding/framing:
* [[8b/10b encoding|8b/10b]] for lane speeds up to 6.25 GBd
* 64b/67b, similar to that used by [[Interlaken (networking)|Interlaken]] for lane speeds over 6.25 GBd

Every RapidIO processing element transmits and receives three kinds of information: Packets, control symbols, and an idle sequence.

==== Packets ====
Every packet has two values that control the physical layer exchange of that packet. The first is an acknowledge ID (ackID), which is the link-specific, unique, 5-, 6-, or 12-bit value that is used to track packets exchanged on a link. Packets are transmitted with serially increasing ackID values. Because the ackID is specific to a link, the ackID is not covered by CRC, but by protocol. This allows the ackID to change with each link it passes over, while the packet CRC can remain a constant end-to-end integrity check of the packet. When a packet is successfully received, it is acknowledged using the ackID of the packet. A transmitter must retain a packet until it has been successfully acknowledged by the link partner.

The second value is the packet's physical priority.  The physical priority is composed of the Virtual Channel (VC) identifier bit, the Priority bits, and the Critical Request Flow (CRF) bit.  The VC bit determines if the Priority and CRF bits identify a Virtual Channel from 1 to 8, or are used as the priority within Virtual Channel 0.  Virtual Channels are assigned guaranteed minimum bandwidths.  Within Virtual Channel 0, packets of higher priority can pass packets of lower priority.  Response packets must have a physical priority higher than requests in order to avoid deadlock.

The physical layer contribution to RapidIO packets is a 2-byte header at the beginning of each packet that includes the ackID and physical priority, and a final 2-byte CRC value to check the integrity of the packet.  Packets larger than 80 bytes also have an intermediate CRC after the first 80 bytes.  With one exception a packet's CRC value(s) acts as an end-to-end integrity check.

==== Control symbols ====
RapidIO control symbols can be sent at any time, including within a packet.  This gives RapidIO the lowest possible in-band control path latency, enabling the protocol to achieve high throughput with smaller buffers than other protocols.

Control symbols are used to delimit packets (Start of Packet, End of Packet, Stomp), to acknowledge packets (Packet Acknowledge, Packet Not Acknowledged), reset (Reset Device, Reset Port) and to distribute events within the RapidIO system (Multicast Event Control Symbol).  Control symbols are also used for flow control (Retry, Buffer Status, Virtual Output Queue Backpressure) and for error recovery.

The error recovery procedure is very fast.  When a receiver detects a transmission error in the received data stream, the receiver causes its associated transmitter to send a Packet Not Accepted control symbol.  When the link partner receives a Packet Not Accepted control symbol, it stops transmitting new packets and sends a Link Request/Port Status control symbol.  The Link Response control symbol indicates the ackID that should be used for the next packet transmitted.  Packet transmission then resumes.

==== IDLE sequence ====
The IDLE sequence is used during link initialization for signal quality optimization.  It is also transmitted when the link does not have any control symbols or packets to send.