The USB (Universal Serial Bus) is a set of specifications for establish a way of communication between different devices and a coordinator (usually a PC). The USB port was created for replacing the old serial and parallel ports. It is used for connecting external devices as: keyboards, mouses, digital camera, printers, storage devices, etc.

The USB communication is based on logical channels. One logical channel realise the communication between the coordinator (host) and a logical entity (endpoint). An external device could incorporate one or multiple logical entities.

There are 2 types of communication channels (pipes): stream pipe and message pipe, depending on the communication type.

The transfer of data could be done in 4 different ways:

  • isochronous transfer: guarantee data rate transfer but possible with data loss;
  • interrupt transfer: for devices which need to response very quickly;
  • bulk transfer: random transfer of huge quantity of data (no bandwidth or latency could be guarantee)
  • control transfer: used for  short messages between the devices (commands).
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