6.4. FIELD ALIGNMENT AND DEFINITION

The RapidIO Common Transport Specification adds a transport type (tt) field to the logical specification packet that allows four different transport packet types to be specified. The tt field indicates which type of additional transport fields are added to the packet.

The three fields (tt, destinationID, and sourceID) added to the logical packets allow for two different sizes of the device ID fields, a large (16-bit), and a small (8-bit), as shown in Table 6.1. The two sizes of device ID fields allow two different system scalability points to optimize packet header overhead, and affix additional transport field overhead only if the additional addressing is required. The small device ID fields allow a maximum of 256 devices to be attached to the fabric. The large device ID fields allow systems with up to 65 536 devices.

Figure 6.3 shows the transport header definition bit stream. The shaded fields are the bits associated with the logical packet definition that are related to the transport bits. Specifically, the field labeled 'Logical ftype' is the format type field defined in the logical specifications. This field comprises the first four bits of the logical packet. The second logical field shown 'Remainder of logical packet' is the remainder of the logical packet of a size determined by the logical specifications, not including the logical ftype field which has already been included in the combined bit stream. The unshaded fields (tt=0b00 or tt=0b01 and destinationID and sourceID fields) are the transport fields added to the logical packet by the RapidIO Common Transport Specification.

Figure 6.3. Destination-source transport bit stream