What is USB?

1
What is USB?

• 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.

2
USB Features

• 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.

3
How USB Ports Work

• 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.

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• 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.

5
USB and the Operating System

Windows device manager stack in Windows XP for 2
USB devices
6
Hubs, Cables and Plugs

• 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

The Hub
Typical A connection
Typical B connection
7
What is FireWire?

• FireWire 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.
• FireWire features
• Fast data transfer (up to 400Mbps)
• Lots of devices on the bus
• Ease of use
• Hot pluggable
• Plug-and-play
• Low cabling cost
• Low implementation cost

8
FireWire vs. USB

• While USB employs hubs in the connections,
  FireWire uses daisy-chaining
• Data can be sent through up to 16 hops for a
  total maximum of about 72 meters.

hop
Sample daisy-chain
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• USB 2.0, released in 2001 is primarily
  host-based, where a device must be connected to a
  host for USB functioning whereas FireWire has
  peer-to-peer properties where two devices can
  communicate with each other without going through
  a computer.

Summary of FireWire and USB features
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• Implementing FireWire 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.
• FireWire is more suitable for digital and audio
  devices and deployments that need long-haul
  cabling such as sound stages and studios.
• Both FireWire and USB are backward compatible.

11
FireWire and USB Releases