USB System Architecture

High Bandwidth Throughput

Table 12-4 lists the maximum capability of the high-speed bus for supporting high-bandwidth isochronous and interrupt endpoints. The numbers in Table 12- 4 include the maximum packet size, but smaller packet sizes can be used. Also, note that the percentage of bus bandwidth required to transfer three 1024-byte transactions is listed as 41% for isochronous and 42% for interrupt (shown in parentheses). This difference is due to the handshake packet that is included in interrupt transactions. Notice that the number of bytes remaining in column 4 is less for interrupt transactions because of their greater overhead

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Table 12-4: High-Bandwidth Capability for Isochronous/Interrupt Transactions

Non-Periodic Transfers

Control transfers receive a 20% bandwidth reservation during each microframe. However, a control transfer may not complete within a single microframe. Bulk transfers receive no bandwidth reservation and are performed when bandwidth is left over after all periodic transactions and control transfers have completed.

Most of the characteristics of bulk and control transfers are identical to the fullspeed implementations. The obvious differences relate to the high-speed transmission rate and the related data throughput that can be achieved. One new feature added to the non-periodic transfers is the required support for ping transactions that are used in conjunction with OUT transactions. See “Ping Transactions” on page 260 for details.

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