At first glance, it would seem that parallel ports should be able to send data much faster than serial ports. At first glance, it would seem that parallel ports should be able to send data much faster than serial ports.
However, parallel ports suffer extremely from inter-symbol interference (ISI) and noise, and therefore the data can be corrupted over long distances. Also, because the wires in a parallel system have small amounts of capacitance and mutual inductance, the bandwidth of parallel wires is much lower than the bandwidth of serial wires. We all know by now that an increased bandwidth leads to a better bit rate. We also know that less noise in the channel means we can successfully transmit data reliably with a lower Signal-to-Noise Ratio, SNR.
If, however, we bump up the power in a serial connection by using a differential signal with 2 wires (one with a positive voltage, and one with a negative voltage), we can use the same amount of power, have twice the SNR, and reach an even higher bitrate without suffering the effects of noise. The serial communication may be faster than the parallel one, provided the bits leave the transmitting device at a much higher speed. An example of such is the ATA 2 Serial port that however serial can reach a transfer rate of up to 300 MB/s, while the traditional IDE port [P-ATA] goes to 133 MB/s at the most. USB cables, for instance, use shielded, differential serial communications, and the USB 2.0 standard is capable of data transmission rates of 480Mbits/sec. USB 3.0 has transmission speeds of up to 5 Gbit/s, which is 10 times faster than USB2.0 (480 Mbit/s).!!!
In addition, because of the increased potential for noise and interference, parallel wires need to be far shorter than serial wires. Consider the standard parallel port wire to connect the PC to a printer: those wires are between 3 and 4 feet long, and the longest commercially available is typically 25 feet. Now consider ethernet wires (which are serial, and typically unshielded twisted pair): they can be bought in lengths of 100 feet, and a 300 foot run is not uncommon!
The complications of using Parallel data transfer is that that many wires are required as the width of bus. E.g : A 32 bit PCI bus would have 32 wires and the controls are required for each. In serial communication, just two wires will do the mojo. Also, as the speed of data transfer increases, the e.m disturbances also increases, making parallel data transfer very difficult.
Another problem with parallel communication is that the bits may not reach the destination at the same time. The various 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.
Finally, parallel data tx. is semi duplex or half duplex, whereas serial data tx. is full duplex. In the serial communication, since it only uses two wires, the manufacturers usually make four wires available, two for the transmission and two for the reception of data. That makes it possible for the simultaneous transmission and reception of data. Such architectural difference alone makes the serial communication about twice as fast as the parallel communication.