FX3 Programmers Manual, Doc. # 001-64707 Rev. *C 19
Introduction to USB
Inspired by the PCI Express and the OSI 7-layer architecture, the USB 3.0 protocol is also
abstracted into different layers as illustrated in the following sections.
In this document, USB 3.0 implicitly refers to the SuperSpeed portion of USB 3.0.
Figure 2-6. USB Protocol Layers
2.2.2 Protocol Layer
USB 3.0 SuperSpeed inherits the data transfer types from its predecessor retaining the model of
pipes, endpoints and packets. Nonetheless, the type of packets used and some protocols associated
with the bulk, control, isochronous, and control transfers have undergone some changes and
enhancements. These are discussed in the sections to follow.
Link Management packets are sent between links to communicate link level issues such as link
configuration and status and hence travel predominantly between the link layers of the host and the
device. For example, U2 Inactivity Timeout LMP is used to define the timeout from the U1 state to
the U2 state. The structure of a LMP is shown here.
Figure 2-7. Link Management Packet Structure
Transaction packets reproduce the functionality provided by the Token and Handshake packets and
travel between the host and endpoints in the device. They do not carry any data but form the core of
the protocol.
For example, the ACK packet is used to acknowledge a packet received. The structure of a
transaction packet is shown in Figure 2-8.
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