USB systems

What is a USB?

The Universal Serial Bus was invented and standardized by a group of computer and peripherals manufactures in 1995.

The idea was to take the whole area of serial port and serial bus and update it with the twenty-first century technology.

It is true that there were many standards of communication between host computers and peripherals, but the goal was to create a technology that combines low speed and high speed bus activity. The technology enables shared access for both speeds, a technology which provides robust protocol, automatic configuring of devices and a serial bus which is simplified and easy to plug into. All those requirements were met with the USB standards.

The USB has become a very popular expansion to the personal computer. It is important to remember the USB isn?t a serial port it is a serial bus, a fact that enables a single port on the computer to be a link for a myriad of devices, (up to 127 devices in a USB system). We can easily chain one device to another and use one port as a connecting point of many devices by using a hub. All these enables us to look at the USB system as a small network of devices.

The plug and play capability of the USB is one of its advantages over other serial buses. This capability enables automatic detection of a new device, which is attached into the system, an automatic configuration of it by the host, and an automatic detection of it's detachment from the system. The flexible attachment and detachment of devices to and from the system allows mobility on the bus and adjustment of the system to new devices without the need of restart the whole system each time a new device is detected.

Another important aspect of the USB is it's mid and high speed flexibility. This feature refers to the ability of the USB to support simultaneously medium-speed devices, (which work in 1.5Mbps), and high-speed devices, (which work in 12Mbps).

The simultaneously work of the USB system finds expression also in the dual support in both isochronous and asynchronous bandwidth allocation methods. Isochronous means that the necessary bandwidth is guaranteed, whenever the device requires it ? it will be available. Asynchronous on the other hand means that there is no guarantee ? the data will be sent whenever it will be possible to send it. Devices, such as video and audio multimedia, that use stream transfer, will use the isochronous method while devices that use bulk transfer, such as printers and scanners will use the asynchronous method.

The USB is robust. Through all the different protocol layers there is an error detection and recovery mechanism, which guarantees low error rate. The USB provides detection of faulty devices and flow control mechanism, which is built in the protocol.

A typical USB system

A typical USB system consists of:

	One host ? there is only one host in the USB system, which is responsible to the whole complexity of the protocol (simplifies the designing of USB devices). The host controls the media access ? no one can access the bus unless it got an approval required from the host.
	Hub ? like the hubs used for computer network. The hub provides an interconnect point, which enables many devices to connect to a single USB port. The logical topology of the USB is a star structure, all the devices are connected (logically) directly to the host. It is totally transparent to the device what is its' hub tier (the number of hubs the data has to flow through). The hub is connected to the USB host in the upstream direction (data flows ?up? to the host) and is connected to the USB device in the downstream direction (data flows ?down? from the host to the device). The hubs' main functionality is the responsibility of detecting an attachment and detachment of devices, handling the power management for devices that are bus-powered (get power from the bus), and responsibility for bus error detection and recovery. Another important role of the hub is to manage both full and low speed devices. When a device is attached to the system the hub detects the speed, which the device operates in, and through the whole communication on the bus prevents from full speed traffic to reach low seed device and vice versa ? prevent from low speed traffic to reach full speed device.
	Device ? everything in the USB system, which is not a host, is a device (including hubs). A device provides one or more USB functions. Most of the devices provide only one function but there may be some, which provides more than one and called compound devices. We refer to two kinds of devices - self powered or bus powered devices. A device that gets its power from the bus is called bus powered and on the other hand a device which supplies its own power is called self powered. As was mentioned before there are two kinds of devices:      Full-speed devices - operates in 12Mbps     Low-speed devices that work in 1.5Mbpsec.