What is USB n n Universal Serial Bus

What is USB? n n Universal Serial Bus – pertaining to a way of connecting computers and devices in such a way that information is shared quickly and easily USB tries to solve particular problems in the past such as, Printers being connected to parallel printer ports where most computers only had one, with zip drives needing high speed access to the computer also through the parallel port. Modems using serial ports along with other devices such as palm pilots or digital cameras while most computers had at most two serial ports. Devices that needed faster connections had to come up with their own cards and card slots were limited - software for these cards was also difficult to install.

USB Features n n n Allows connection of 127 devices. Transfers data at a rate of 12 Mbps (megabits per second). Implements plug-and-play. - device can be attached to computer without turning computer off. Uses hubs for multiple connections and bus-powered devices. Involves isochronous devices - devices that stream data between the computer and them with guaranteed bandwidth and without error correction.

How USB Ports Work n n n The computer acts as host. When host powers up, it performs enumeration, meaning, it queries all devices and assigns each one an address. It then determines what type of data transfer to perform under 3 categories. Interrupt – devices like a mouse or keyboard which will be sending little data, would choose this mode. Bulk – devices that receive data in one big packet uses bulk transfer. A block of data is sent from the host and verified if correct. Isochronous – streaming devices such as speakers require data transfer in streams and without error correction.

Host can also send commands or query parameters with control packets. While host enumerates, it also keeps track of the bandwidth of the isochronous and interrupt devices. These devices can use up to 90 percent of the available bandwidth. What is left over (at least 10 percent) is used up by control packets and packets for bulk transfer. USB also divides the available bandwidth into frames, and the host controls the frames. Frames contain 1, 500 bytes, and a new frame starts every millisecond. During a frame, isochronous and interrupt devices get a slot so they are guaranteed the bandwidth they need.

USB and the Operating System Windows device manager stack in Windows XP for 2 USB devices

Hubs, Cables and Plugs n The Hub Inside a USB cable: There are two wires for power -- +5 volts (red) and ground (brown) -and a twisted pair (yellow and blue) of wires to carry the data. The cable is also shielded Typical A connection Typical B connection

What is Fire. Wire? n n Fire. Wire is a less technical term for IEEE 1394, a system very similar to USB, in that it also aims at transmitting information between devices and computers efficiently. Fire. Wire features: Fast data transfer (up to 400 Mbps) Lots of devices on the bus Ease of use Hot pluggable Plug-and-play Low cabling cost Low implementation cost

Fire. Wire vs. USB n While USB employs hubs in the connections, Fire. Wire uses “daisychaining” hop Sample “daisy-chain” n Data can be sent through up to 16 hops for a total maximum of about 72 meters.

n USB 2. 0, released in 2001 is primarily host-based, where a device must be connected to a host for USB functioning whereas Fire. Wire has peer-to-peer properties where two devices can communicate with each other without going through a computer. Summary of Fire. Wire and USB features

n n n Implementing Fire. Wire costs a little more than USB leading to the adoption of USB as the standard for connecting peripherals that do not require a high-speed bus. Fire. Wire is more suitable for digital and audio devices and deployments that need long-haul cabling such as sound stages and studios. Both Fire. Wire and USB are backward compatible.

Fire. Wire and USB Releases