Why Serial Communication ?

All technologies that exist today are migrating from parallel communication to serial communication.

The new IDE standard for hard disks is serial (ATA Serial).  The Parallel PCI bus is now PCI Express which is serial, with the release of its new version, the PCI Express.  The SCSI interface is also being transformed into serial (SAS - Serial Attached SCSI).

The serial communication differs from the parallel one for only transmitting a bit at a time, while in the parallel communication several bits are transmitted per time.  That makes the parallel communication faster than the serial one.

The above statement is not true with latest peripherals as parallel communication has its own limitation.  

There are several reasons to make the devices migrate from the parallel communication to the serial one.

More number of wires adds complexity and increases cost.

In the parallel communication, since several bits are transmitted per time, a wire is required per each bit adds to the complexity in connectors and controllers. 

Electromagnetic interference

The higher the transfer rate, the bigger the problem with the electromagnetic interference.  Each wire becomes an antenna in potential, capturing a lot of noise from the environment, which may corrupt the data transmitted.  In the parallel communication, since many wires are used, the problem of the electromagnetic interference is a serious one.  In the serial communication, on the other hand, since only two wires are used, that problem is much more easily solved, by simply protecting the two wires used. 

Synchronization

There is yet another problem, a not much discussed one.  Even though we say that in the parallel communication all the bits are transmitted at the same time, the bits do not get to the receiver exactly at the same time.  If in low performance devices the small time difference in the reception of the several bits of data is not important, in high-speed devices that difference in the reception time of the several bits makes the device wastes time having to wait for all the bits to arrive, which may represent a significant fall in performance, since the data transmission operation happens in very short times.

Duplex 

Another difference between the parallel communication and the serial one is that the parallel communication is half-duplex, while the serial one is full-duplex.