USB Simplified

USB (Universal Serial Bus) establishes communication between devices and a host controller (usually personal computers). The USB format is intended to replace many of the serial and parallel ports previously depended upon. USB is used to connect computer peripherals such as mice, keyboards, digital cameras, printers, personal media players, flash drives, external hard drives and more.

A USB system consists of a host and a multitude of "downstream" USB ports, and multiple peripheral devices. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. A USB host may have multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including the hub devices themselves may be connected to a single host controller.

USB devices are linked in series through hubs. There always exists one hub known as the root hub, which is built into the host controller. There are also sharing hubs, which allow multiple computers to access the same peripheral device(s), also exist and work by switching access between PCs, either automatically or manually. They are popular in small-office environments.

Distance Limitations
Like many digital formats, there are distance limitations with USB. The maximum length of a standard USB cable (for USB 2.0 or earlier) is 5.0 meters (16.4 ft). The primary reason for this limit is the maximum allowed round-trip delay of about 1,500 ns. If USB host commands are no answered by the USB device within the allowed time, the host will consider the command lost. The USB 2.0 specification requires cable delay to be less than 5.2 ns per meter (192,000 km/s, which is close to the maximum achievable speed for any standard copper cable). This is what allows for a 5 meter cable.

The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification. For copper wire cabling, some calculations have suggested a maximum length of perhaps 3m. No fiber optic cable designs are known to be under development as of the release of the specification.

Bandwidth
USB supports following signaling rates:

• A low speed rate of 1.5 Mbit/s is defined by USB 1.0. It is very similar to "full speed" operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks.
• The full speed rate of 12 Mbit/s is the basic USB data rate defined by USB 1.1. All USB hubs support full speed.
• A hi-speed (USB 2.0) rate of 480 Mbit/s was introduced in 2001. All hi-speed devices are capable of falling back to full-speed operation if necessary; they are backward compatible. Connectors are identical.
• A SuperSpeed (USB 3.0) rate of 4.8 Gbit/s. The written USB 3.0 specification was released by Intel and partners in August 2008. The first USB 3 controller chips were sampled by NEC May 2009 and products using the 3.0 specification are expected to arrive beginning in Q3 2009 and 2010. USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation. There is some incompatibility with older connectors.
  
  

 

USB 3.0  
The USB 3.0 is slated to be turning up in products late 2009 early 2010 and promises to be a quantum leap forward for USB. Here are some of the latest features:  USB 3.0 Features

• A new major feature in USB 3.0 is what is called the SuperSpeed bus, which provides a fourth transfer mode at 4.8 Gbit/s-10 times that of 2.0
• When operating in SuperSpeed mode, full-duplex signaling occurs over 2 differential pairs separate from the non-SuperSpeed differential pair. This results in USB 3.0 cables containing 2 wires for power and ground, 2 wires for non-SuperSpeed data, and 4 wires for SuperSpeed data, and a shield (not required in previous specifications).
• To accommodate the additional pins needed for the SuperSpeed mode, the physical form of the  USB 3.0 connectors have been modified from those used in previous versions. Standard-A cables have extended heads where the SuperSpeed connectors extend beyond and slightly above the older, legacy connectors. Similarly, the Standard-A receptacle is deeper to accept these new connectors. On the other end, the SuperSpeed Standard-B connectors are placed on top of the existing. A legacy standard A-to-B cable will work as designed and will never contact any of the SuperSpeed connectors, ensuring backward compatibility. However, SuperSpeed USB cables, with their longer connectors, will not fit into the legacy receptacles.
• SuperSpeed establishes a communications pipe between the host and each device, in a host-directed protocol. In contrast, USB 2.0 broadcasts packet traffic to all devices.
• USB 3.0 extends the bulk transfer type in SuperSpeed with Streams. This extension allows a host and device to create and transfer multiple streams of data through a single bulk pipe.
• New power management features in USB 3.0 include support of idle, sleep and suspend states, as well as Device, Link and Function level power management.
• USB 3.0 does not define cable assembly lengths, except that it can be of any length as long as it meets all the requirements defined in the specification. However, it is estimated that cables will be limited to 3 meters at SuperSpeed.

USB Connectors Types

USB has several types of connections; Type A, Type B, Mini-A, Mini-B, Micro-AB and Micro B. Some hardware manufacturers have propriety versions so check you equipment manual carefully before purchasing connectivity accessories. Be aware that USB 3.0 receptacles are electrically compatible with USB 2.0 device plugs if they can physically match. Most combinations will work, but there are a few physical incompatibilities. However, only USB 3.0 Standard-A receptacles can accept USB 3.0 Standard-A device plugs.

USB v.s. Firewire
A USB network relies on a single host at the top of the “tree” to control the network. In a FireWire network, any capable node can control the network (conditions permitting).

USB networks use what is referred to as tiered-star topology, while FireWire networks use a tree topology. USB 1.0, 1.1 and 2.0 peripherals cannot communicate with the host unless the host specifically requests communication (USB 3.0 will allow for such communication). A FireWire device can theoretically communicate with any node at any time providing conditions allow.

USB runs with a 5 V power line, while Firewire can supply up to 30 V.

USB ports can provide from the typical 500mA[2.5 Watts] of current to USB 3.0 & USB On-The-Go's 1800mA[9.0W] (for dedicated battery charging, 1500mA[7.5W] Full Speed or 900mA[4.5W] High Speed), while FireWire can in theory supply up to 60 watts of power, although 10 to 20 watts is more typical.

These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum transfer rate, FireWire 400 has performance advantage over USB 2.0 Hi-Speed in real-use, especially in high-bandwidth use such as external hard-drives. The newer FireWire 800 standard being twice as fast as FireWire 400 outperforms USB 2.0 Hi-Speed both theoretically and practically.The chipset and drivers used to implement USB and Firewire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world, along with compatibility with peripherals.

How to choose the right USB Cable for your application
Because of all the different connection types, you must be certain that you choose the correct USB cable type for the receptacles on your equipment. USB Cable type "A to B" is the most popular but check your equipment to be sure. You must also be aware of which release version (1.0, 2.0, 3.0) your hardware will support. As with other digital cables, distance will be limited. Keep in mind the medium for transferring this digital data is still over analog copper cabling cable construction will still play some role in final performance. As always, look for a well made, molded cable from a reputable source at a fair market price. Do not waste money on fancy gold connectors or braided jackets as they provide no tangible performance benefit. Since USB cable distances are limited, standard grade USB cables are all that are needed for everyday use. Avoid retail brands that charge exorbitant prices. For more about overall cable construction, click here.

Relevant Products: USB Cables, USB Extenders, USB Hubs, USB Converters, Firewire Cables, Firewire Extenders