The plug-and-play requirement has many implications at all levels in the system,
from the hardware to the operating system and the applications software. One of the
primary objectives of the design of the USB has been to provide a plug-and-play
The discussion above points to the need for an interconnection system that
combines low cost, flexibility, and high data-transfer bandwidth. Also, I/O devices may
be located at some distance from the computer to which they are connected. The
requirement for high bandwidth would normally suggest a wide bus that carries 8, 16, or
more bits in parallel. However, a large number of wires increases cost and complexity
and is inconvenient to the user. Also, it is difficult to design a wide bus that carries data
for a long distance because of the data skew problem discussed. The amount of skew
increases with distance and limits the data that can be used.
A serial transmission format has been chosen for the USB because a serial bus
satisfies the low-cost and flexibility requirements. Clock and data information are
encoded together and transmitted as a single signal. Hence, there are no limitations on
clock frequency or distance arising from data skew. Therefore, it is possible to provide a
high data transfer bandwidth by using a high clock frequency. As pointed out earlier, the
USB offers three bit rates, ranging from 1.5 to 480 megabits/s, to suit the needs of
different I/O devices.
Figure 23 Universal Serial Bus tree structure.