2.3. RAPIDIO PROTOCOL OVERVIEW

2.3.1. Packets and Control Symbols

RapidIO operations are based on request and response transactions. Packets are the communication element between end point devices in the system. A master or initiator generates a request transaction, which is transmitted to a target. The target then generates a response transaction back to the initiator to complete the operation. The RapidIO transactions are encapsulated into packets, which include all of the necessary bit fields to ensure reliable delivery to the targeted end point. RapidIO end points are typically not connected directly to each other, but instead will have an intervening fabric. We use the word fabric to mean a collection of one or more switch devices which provides the system connectivity. Control symbols are used to manage the flow of transactions in the RapidIO physical interconnect. Control symbols are used for packet acknowledgement, flow control information, and maintenance functions. Figure 2.2 shows how transactions progress through a RapidIO system.

In the example, shown in Figure 2.2, the initiator begins an operation in the system by generating a request transaction. The request packet is sent to a fabric device, typically a switch, which is acknowledged with a control symbol. The packet is forwarded to the target through the fabric device. This completes the request phase of the operation. The target completes the request transaction and generates a response transaction. The response packet associated with this transaction returns through the fabric, using control symbols to acknowledge each hop. Once the response packet reaches the initiator and is acknowledged, the operation is considered complete.

Figure 2.2. Transaction progress through a RapidIO fabric