Hi. I'm Lucas, and I work at Cherry Heaven Internet Shops. I've been asked to tell you about USB products: why you need them, how they work, and how to use them.

Before the introduction of USB technologies, connecting up your computer system was simple. The monitor, keyboard, mouse, printer, games joystick, and loudspeaker, each had a cable with a plug shaped to fit the appropriate socket.

The computer could control all the devices and, for the future, there were a few spare internal connectors for expansion interfaces to support extra devices, such as memory card readers, touch screens, and musical keyboards.

However, as more devices were added, the computer used up all its available resources, and avoiding conflicts became increasingly difficult or impossible.

The Universal Serial Bus, USB, is a plug-and-play digital connection between peripherals and the computer, designed to replace most of the old plugs, sockets, cables, and expansion interfaces. Up to 127 fast-access devices can be set up automatically and quickly, ready to use, and hot-swapped without turning the computer off.

Universal means that it's internationally accepted and supported. A bus is the computer's two-way connection which carries data between the processor, the memory, and the peripherals. Serial means that data is sent one bit after another, as a data stream.

Similarly, a road bus carries passengers between work, home, and the shops. And a teevee serial shows the episodes one after another, in the right sequence.

Most computers support USB, simplifying the connection of peripherals such as disk drives, memory sticks, flash cards, digital cameras, card readers, GPS systems, webcams, and mice. The system is easily extended, using a multi-port hub.

There are three standards: USB 1.0 has a maximum data transfer rate of 1.5 Mb/s. USB 1.1 has a maximum data transfer rate of 12 Mb/s. USB 2.0 has a maximum data transfer rate of 480 Mb/s: 480 million bits a second. USB 2.0 is backwardly compatible with USB 1.1 and USB 1.0 devices.

USB is supported by Windows ME, 2000, XP, Vista, MAC 9.0 and higher, and Linux 2.4 and higher. USB flash memory devices can also improve the general performance of Windows Vista. A feature called ReadyBoost allows Vista to use the flash memory as extra system RAM.

Many USB peripherals, such as disk drives, flash memory sticks, and mice, need no installation or set up. Just plug them in and they work.

Other USB peripherals, such as webcams and GPS, come with a CD which you use, at the beginning, to install software that allows the devices to be more useful. Once set up, just plug them in and they work.

You don't have to leave the device connected, so it can go in the cupboard. However, when you want to use it, just plug it in: the computer searches its USB list, recognises the device, and sets it up ready for you to use. When you unplug it, the computer knows it's gone, but doesn't interfere with anything else you're doing. The simplicity of USB makes it a true plug-and-play system.

The USB bus can provide up to 500mA at each port, so low-power devices that would normally require a separate mains adapter can be powered through the computer's USB cable.

Some USB hubs can take their power from the computer's USB bus, but most have a mains adapter. To maximise battery life, notebook computers usually have just two or three USB ports.

Most devices use a USB A to USB B cable. The A plug connects to the computer's A socket, and the B plug connects to the device's B socket.

Smaller devices usually use a USB A to USB Mini-B cable, although a few use non-standard connectors and come with their own special cable.

They use four-pin connectors: two pins provide the power and two carry the serial data stream. If you cut through the shielded cable you'll find the four wires: a red for +5V power, a brown common, and a twisted pair, yellow and blue, for the data.

The signal integrity decreases as the cable length increases. However, a USB Hub, as well as providing more sockets, can clean up the data stream and send it down a second cable, increasing the transmission distance.

The maximum cable length, about 5.0m, is set by the USB delay-specification of 26ns. This allows reflections to settle at the transmitter before the next bit is sent. This doesn't mean that the line voltage has fully settled by the end of the bit, but there's been enough damping so that the reflection amplitude has been reduced to an acceptable level. So, now you know.

You can use any mix of cables, extensions, hubs, and up to 127 devices, providing single-run cables do not exceed 5.0m.

A flash memory chip is a very large grid of very tiny transistors. Each transistor-pair acts as a switch by storing or not storing electrons. When data is written to the chip, a section of the grid is used to store a pattern of ons and offs which can be read later as a sequence of ones and zeros, representing binary data.

Each transistor-pair is surrounded by silicon dioxide to prevent electrons escaping. It's so effective that the chip needs no power supply to retain data for 10 years.

The chip changes its storage patterns every time data is written to it. Data can be written over a million times before errors begin to occur.

The way in which memory is measured, in KB, MB, or GB, is not an exact measure of the amount of available memory. Although a USB device is sold as, for example 256MB, the available memory may only be 244MB. This is broadly true for all memory devices, not just flash memory.