USB uses a four-wire cable. Two wires handle data transmission, a third car-
ries five volts of power to peripherals, and the fourth is an electrical ground.
Data is sent in serial form, but USB is closer in technology to an Ethernet
network or the Internet in the way information is gathered into packets; in
addition to start and stop bits that identify the computer words, the packets
include a destination and return address. This is key to the way that an entire
chain of devices links to a single USB port.Under USB, the computer takes an active role in the management of the port.
This begins when the system identifies all devices attached at bootup or
plugged in later; the computer manages how much of the pipeline it assigns
to each device depending on their need for speed. For example, hard drives
and CD or DVD drives need a bigger pipe, while keyboards and mice make
minimal demands on the system. Every time a device is added to the bus or
removed, the computer reconfigures bandwidth allocation and adjusts identi-
fication codes used to label packets of data.USB devices are hot swappable,meaning they can be plugged in to a laptop
that is already running, or removed without shutting down the system. And
the icing on the cake: Support for the USB is fully integrated into current ver-
sions of Microsoft Windows (Windows 98SE, 2000, ME, and XP) so that you
can plug and play.The first time a new device is plugged into a laptop, the
system will search for necessary drivers and other software; if it finds what
it needs already on the computer, you’re good to go. If not, you’re asked to
direct the laptop to the location of any needed drivers or software — on a
CD, a floppy disk, or at an Internet address.The USB standard provides electrical current along with data channels. The
power is sufficient for many devices such as WiFi, modems, and NICs. But
you’ll likely have to provide power from an AC adapter for devices with
motors, including hard drives and CD/DVD players and recorders.The data side of the USB chain can be split off many times, allowing as many
as 127 devices to connect to a single port. However, each of the devices on
the chain is also sharing the same low-power electrical feed. I recommend
you use a powered hub once a port is split into more than four lines. Powered
hubs use an AC adapter to boost the electrical amperage to a sufficient level
for multiple devices. And, some USB devices require their own power source,
especially hard drives and CD/DVD drives with motors.Although you may not contemplate adding 127 devices to your laptop — after
all, the idea is to get smaller and more compact — it is important to understand
that USB does allow you to replicate all of the functions of a full-sized computer
while at your desk and then take just the core laptop when you are on the road.244 Part IV: Failing to Communicate