2.12. LINK PROTOCOL
An important feature of RapidIO technology is that packet transmission is managed on a link-by-link basis. In the past, with synchronous buses, a mastering device had explicit handshake signals with the target device. These signals indicated whether a transaction was acknowledged and accepted by the target device.
With a source synchronous interface like the RapidIO specification defines, it is not practical to rely on a synchronous handshake since the target port of a transaction is decoupled from the sending port. Sometimes this decoupling, meaning the distance measured in device count between the sending and target port, can be quite large. Many interconnects have ignored this issue and continue to rely on an end-to-end handshake to guarantee delivery. This has the disadvantage of not allowing precise detection and recovery of errors and also forces far longer feedback loops for flow control.
To address this issue, RapidIO technology uses control symbols for link level communication between devices. Packets are explicitly tagged between each link with a sequence number otherwise known as the AckID. The AckID is independent of the end-to-end transaction ID. Using control symbols, the receiving device indicates for each packet whether it has been received or not. The receiver will also provide buffer status information. Receiving devices can immediately detect a lost packet, and through control symbols, can re-synchronize with the sender and recover the lost ...
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