Data transmissions are composed of one or more transactions which are implemented using packet. Individual types of data transmissions then correspond to the used packet types.
The purpose and function of the control transmissions are practically identical to the USB 2.0 interface, it enables communication between the host software and terminal device for configuration, control and status.
The control transmissions use message channels, each device must support the default control channel. The control transmissions have the highest priority on the bus, similarly to the USB 2.0 interface cannot request a specific bandwidth. The maximum payload is 512 bytes, and it cannot utilize the bursting.
The bulk transmissions operate like on USB 2.0 interface similarly to control transmissions. They are designed for peripheral devices that need to move relatively large volumes of data. They use any available bandwidth (with low priority), such as copying data to an external drive.
Bulk transmission guarantees delivery of data, but does not guarantee bandwidth and delay. It uses stream channels, so there are no requirements on data structure. Stream channels are unidirectional, bidirectional data flow requires two channels (IN and OUT). Bulk transmission may be divided into more transactions.
This is a completely new type of transmission on USB SuperSpeed interface. Standard channel for bulk transmission represents the ability of transfer of one stream data type FIFO (First In First Out) between the host and the terminal devices via the memory stack of host and terminal devices. SuperSpeed Streams provide support for a multistream model at the protocol level. The streams between the host and terminal device are managed by stream protocol. Stream ID SID is assigned to each stream.
Stream protocol defines status messages, which allow for a host or a terminal device to establish the current stream ID CSID of endpoint. Host uses the CSID to select data buffers of endpoint which will be used for subsequent transmissions on the channel. The terminal device uses the CSID to select data buffers for function.
SuperSpeed streams allow the usage of more host data buffers for the terminal device (by default only one). SuperSpeed streams are interrupted in case of failure of standard bulk channel, which SuperSpeed streams use. SuperSpeed streams extend the possibilities of bulk transmissions with minimal changes in the hardware. This function may be used for example for USB Mass Storage devices.
Similar to USB 2.0 interface transmission with interruption is dedicated for terminal devices that require high reliability transfer of small volume of data in a limited operating range, such as a mouse or keyboard. The host periodically requests data and terminal device according to their possibilities it provides.
Transmissions with interruption guarantee maximum operating range (delay of data). It uses stream channels, so the data may not have a defined format and the channel is unidirectional. Up to 90% of the available bandwidth may be allocated for transmissions with interruption. Endpoint can in its handler operating interval in multiples of 125 µs. It enables retransmission of three packets per operating interval.
Synchronous transmission and its purpose compared to USB 2.0 remains unchanged. It is used for data streaming, i.e. for periodic transmissions with limited service interval and the error margin enabling continuous data flow. USB SuperSpeed interface uses for synchronization timing isochronous packets ITP.
Synchronous transmission guarantees bandwidth for traffic with limited delay and required bandwidth is also guaranteed and it is given in descriptor. Synchronous channels are streams with unidirectional data flow. Host before starting synchronous transmission sends to terminal device a packet transaction (PING) due to possible delays in transfer caused by bus power management and this transaction activate all links on bus to active state. Maximum bandwidth and service intervals are consistent with transmission with interruption. Synchronous transmission allows up to three burst transactions after 16 packets per operating interval.