in serial ports serial transmission mode is ued i.e bits 
travel one by one though it is slow but it requires only 
one communication line for transmission whereas in parallel 
transmission many bits are sent simultaneously. And each 
bit requires one communication line hence there are more 
number of communication lines used which is expensive.

Hi shweta, 

But can i know how an USB data transfer occurs, is it serial
or parallel.,

if it is an serial data transmission, how it is faster??

In computing, a serial port is a serial communication 
physical interface through which information transfers in 
or out one bit at a time (contrast parallel port). 
Throughout most of the history of personal computers, data 
transfer through serial ports connected the computer to 
devices such as terminals and various peripherals.

While such interfaces as Ethernet, FireWire, and USB all 
send data as a serial stream, the term "serial port" 
usually identifies hardware more or less compliant to the 
RS-232 standard, intended to interface with a modem or with 
a similar communication device.

For many computer peripheral devices the USB interface has 
replaced the serial port — as of 2007, most modern 
computers are connected to devices through a USB 
connection. While nearly every server has a serial port, 
many modern low-end workstations and laptops often don't 
have one. The serial port is omitted for cost savings, and 
is considered to be a legacy port

Introduction to USB
USB, which stands for Universal Serial Bus, is a high-speed 
serial standard that, along with FireWire, has almost 
totally replaced the ordinary serial and parallel standards 
that have been used to connect peripheral devices to PCs 
since the 1980s.

Click here! to go to the information on FireWire (IEEE1394) 
on Page 2 of this article. You can use your browser's Back 
button to backtrack.

In theory, up to 127 devices can be connected to a single 
USB port on a computer's motherboard, using only one of a 
computer's 16 or 23 Interrupt Requests (IRQs), either by 
daisy-chaining them together, or by using a USB hub, which 
itself has a number of USB ports.

Enter Interrupt Requests in the Google search box at the 
top of this page (with its Web radio button enabled) to 
find links that provide information about them.

Before going any further into this article, read this Q&A 
on this site: I pushed the FireWire connector into the 
FireWire port on my PC the wrong way around and fried the 
iLink board on my camcorder. Use your browser's Back button 
to backtrack.

Visit the Desktop PCs section of this site for information 
on how to buy, build, repair, fix, and protect the various 
types of desktop PCs.

Visit the Laptop/Notebook PCs section of this site for 
information on buying and protecting them, and fixing 
problems with them. 

A USB hub can have its own power source, or draw its power 
through the computer to which it is attached. To avoid 
power problems, a powered hub is the best choice. Seven 
peripherals can be attached to each USB hub. One of these 
peripherals can be a second hub to which up to another 
seven peripherals can be connected, and the second hub can 
be attached to a third hub, etc..

To reduce complexcity in data transmission , 
for long distance communication.
Universal Serial Bus has only 4 data lines for 
communication ie. datain,dataout,Vcc,Gnd.where pararllel 
port has 25 lines which is more complex .