The USB standard has become the universal means of connection peripheral devices and equipment to the computer. Computers have either two or four USB port connectors. Each of these can support a large number of connections but to be able to attach more than two or four devices requires a multi-connection hub. There are many different types of hubs and this article seeks to explain some of the important considerations in selecting the best choice of hub for your system.

The USB bus system in computers was introduced in 1996 and has gone through a number of developments and enhancements. It started out as USB 1 which was very slow and had many bugs. Version 1.1 brought a higher bandwidth of 12 MBits/second. The latest version of 2.0 gives a bandwidth of 480MBits/second and is fully backward compatible with USB 1 and 1.1. Link to a table showing differences between USB speeds (http://www.usb-products.com/usb-speed.aspx)

Port Connections and Power
All USB standards support up to 127 connections (or devices) per computer port. It is unlikely that this would be achievable as the port can only supply 0.5 Amps maximum. Some devices consume a full 0.5 Amps, so are powered from a power adaptor from the mains supply.

Bandwidth Sharing
Generally, the bandwidth of the port (http://www.usb-products.com/usb-port.aspx) is shared between all the devices that are attached through a hub. This means that if you want to add two USB 2 devices with a USB 1.1 device, the bandwidth is shared between all three devices. However, there is an extra complication. Most hubs (http://www.usb-products.com/usb-hub.aspx) have a single chip controlling the bus. This means that the hub will work at the lowest bandwidth determined by the connected devices resulting in the USB 2 devices working at much lower transfer speeds. Nearly all USB hubs on the market today (passive and powered) operate like this.

Multiple Chip Hubs
The transaction translator (TT) is the name for the controlling chip that assigns bandwidth to each connected device. In hubs that have one TT all devices share the same bandwidth and operate at the speed of the slowest. In hubs with a TT per socket, each receives a share of the bandwidth, but this is not restricted to the slowest device. Examples of this type of hub is the Belkin Tetra hub (http://www.usb-products.com/Products/Hi-Speed-USB-2-0-4-Port-TetraHub.aspx) This has 4 ports each with its own TT.

What this means in reality is that a multiple TT hub will provide much higher bandwidth per port when several devices are connected from one hub.

Maximum Performance and choice of Where to connect a device If you are after maximum performance for each of your USB 2 devices, each device should be plugged into its own USB port in the computer. Additional plug in cards can provide a vast number of USB ports for most needs. However, this situation might be going too far except for the most demanding of situations. Although several USB devices might be connected at the same time it is unlikely that they will all be in use together.

A useful compromise is to plug small bandwidth devices together into a single hub. These would include: mice; keyboard; Speakers; little used devices.

For devices that require a high bandwidth and are used often, direct connection in to a USB port is the best option. Devices would include: Flash memory drives; external disks; Web cams; Other frequently used devices.

If you dont have sufficient ports then either more ports should be added with a plug-in card or a good quality hub with multiple transaction translators should be used. The devices placed on the hub are those that are used less often. Those devices placed directly on the port are used most often.

Link to a selection flow chart that helps in selecting a hub (http://www.usb-products.com/USB-Hub-Selection.aspx)

Access Considerations
What type of configuration and mix of port and hubs will depend on the number of devices that you wish to attach. Desktop computer cases usually have ports in the front of the case for easy access. These can be used for devices that are often plugged / unplugged and require the highest bandwidth. These are flash memory drives and portable external drives and MP3 players.

USB hubs tend to clutter up the area with cable connections and power lead. These naturally go out of sight if possible or behind the monitor. For mobile computers, a small compact hub can be obtained. These may have there own wind-in or tidy away cables.

Powered and Passive Hubs
Hubs may be restricted in their use without a power connection as passive hubs rely on the power of the USB bus. Powered hubs have a mains power adaptor to provide additional power for devices. These devices range in the power they require. It is possible to run a number of different devices all from a single passive hub running of a USB port as long as they have low power requirements. With only 0.5 Amps current it only takes one of the devices requiring the full power requirement for the hub to be overwhelmed. Nothing serious is going to happen though except that the single will degrade and many or all the devices will loose connection with the computer. To avoid this situation a powered hub should be used.