USB 3.0

The protocol layer defines the “end-to-end” communications rules between a host and device .

The SuperSpeed protocol provides for application data information exchanges between a host and a device endpoint. This communications relationship is called a pipe. It is a host-directed protocol, which means the host determines when application data is transferred between the host and device. SuperSpeed is not a polled protocol, as a device is able to asynchronously request service from the host on behalf of a particular endpoint.

All protocol layer communications are accomplished via the exchange of packets. Packets are

sequences of data bytes with specific control sequences which serve as delimiters managed by the link layer. Host transmitted protocol packets are routed through intervening hubs directly to a peripheral device. They do not traverse bus paths that are not part of the direct path between the host and the target peripheral device. A peripheral device expects it has been targeted by any protocol layer packet it receives. Device transmitted protocol packets simply flow upstream through hubs to the host.

Packet headers are the building block of the protocol layer. They are fixed size packets with type and subtype field encodings for specific purposes. A small record within a packet header is utilized by the link layer (port-to-port) to manage the flow of the packet from port to port. Packet headers are delivered through the link layer (port-to-port) reliably. The remaining fields are utilized by the end-to-end protocol.

Application data is transmitted within data packet payloads. Data packet payloads are preceded (in the protocol) by a specifically encoded data packet headers. Data packet payloads are not delivered reliably through the link layer (however, the accompanying data packet headers are delivered reliably). The protocol layer supports reliable delivery of data packets via explicit acknowledgement (header) packets and retransmission of lost or corrupt data. Not all data information exchanges utilize data acknowledgements.

Data may be transmitted in bursts of back-to-back sequences of data packets (depending on the scheduling by the host). The protocol allows efficient bus utilization by concurrently transmitting and receiving over the link. For example, a transmitter (host or device) can burst multiple packets of data back-to-back while the receiver can transmit data acknowledgements without interrupting the burst of data packets. The number of data packets in a specific burst is scheduled by the host. Furthermore, a host may simultaneously schedule multiple OUT bursts to be active at the same time as an IN burst.

The protocol provides flow control support for some transfer types. A device-initiated flow control is signaled by a device via a defined protocol packet. A host-initiated flow control event is realized via the host schedule (host will simply not schedule information flows for a pipe unless it has data or buffering available). On reception of a flow control event, the host will remove the pipe from its schedule. Resumption of scheduling information flows for a pipe may be initiated by the host or device. A device endpoint will notify a host of its readiness (to source or sink data) via an asynchronously transmitted “ready” packet. On reception of the “ready” notification, the host will add the pipe to its schedule, assuming that it still has data or buffering available. Independent information streams can be explicitly delineated and multiplexed on the bulk transfer type. This means through a single pipe instance, more than one data stream can be tagged by the source and identified by the sink. The protocol provides for the device to direct which data stream is active on the pipe.

Devices may asynchronously transmit notifications to the host. These notifications are used to convey a change in the device or function state. A host transmits a special packet header to the bus that includes the host’s timestamp. The value in this packet is used to keep devices (that need to) in synchronization with the host. In contrast to other packet types, the timestamp packet is forwarded down all paths not in a low power state. The timestamp packet transmission is scheduled by the host at a specification determined period.