RapidIO: The Next Generation Communication Fabric For Embedded Application

8.9. PACKET PACING

If a device cannot transmit a packet in a contiguous stream, perhaps because the internal transfer of data is delayed or cannot keep up with the RapidIO transfer rate, it may insert idle control symbols, called pacing idles, into the packet stream. As with the other control symbols, the pacing idle control symbols are always followed by a bit-wise inverted copy. Any number of pacing idle control symbols can be inserted, up to some implementation-defined limit, at which point the sender should instead send a stomp control symbol and cancel the packet in order to attempt to transmit a different packet. Figure 8.12 shows an example of packet pacing. The receiving device ignores the idle control symbols. More data is sent when it becomes available. Pacing idle control symbols can be embedded anywhere in a packet where they can be legally delineated by the framing signal.

The receiver of a packet may request that the sender insert pacing idle control symbols on its behalf by sending a throttle control symbol specifying the number of pacing idle control symbols to delay. The packet sender then inserts that number of pacing idles into the packet stream. If additional delay is needed, the receiver can send another throttle control symbol.

Note that, when calculating and testing the CRC values for a packet, the pacing idle control symbols that might appear in the symbol stream are not included in the calculation.

Figure 8.12. Pacing idle insertion in packet (8-bit port) ...