Serial vs Parallel Communications
The communication can be broadly divided into serial and parallel communications. In the basic scheme of serial communication, only one wire, in addition to the ground wire, is used to transmit the bits from the sender to the receiver. Every bit is transmitted one by one in a sequential manner. There are two types of serial communications: synchronous and asynchronous. In the synchronous serial communication, timing metadata (clock) is used to synchronize the transmitted data. In the asynchronous serial communication, there is no timing information (clock) in the transmitted bits. However, every character (which consists of several bits) is sent with start and stop bits to indicate the start and end of the character. A start bit is added at the beginning of the character, and a stop bit is added to the end of the character. For example, if a character consisting of 8 bits are to be transmitted, then a total of 10 bits, which are the 8 bits, the start bit, and the stop bit, should be transmitted.
In a parallel communication, every bit of a character is transmitted in a separate wire at the same time, i.e. in a parallel manner. As a result, parallel communication is typically faster than serial communication since all the bits are transmitted in parallel, not in a sequential manner. However, it needs more cables. Excluding the ground wire, an 8-bit data transmission needs 8 wires, a 16-bit data transmission need 16 wires, a 32-bit data transmission needs 32 wires, a 64-bit data transmission needs 64 wires, and so on. This will become expensive as the number of bits increases as more wires are required. Not only expensive, more wires also mean more space required. Therefore, parallel communication is expensive and impractical.
Furthermore, parallel communication for high speed transmission may also result in the so-called clock skew, which means that the transmitted bits do not arrive at the receiver end at the same time, and therefore a waiting period is required until the whole bits are received before transmitting another set of bits. Parallel communication may also cause the so-called crosstalk, which is interference between parallel wires. This effect will become worse with the increase of the cable length. A well known example of parallel communication is Peripheral Component Interconnect (PCI).
Due to the high cost and impracticality of the parallel communication, most of communications are serial. There are so many types of serial communications, including PS/2 for old keyboard and mouse, RS232, RS485, RS422, VGA, DVI, HDMI, Serial ATA (SATA), and Ethernet.
Since the serial communication is typically slower than parallel communication by nature, several newer serial technologies such as PCI Express (PCIe) and Universal Serial Bus (USB) have been made to increase the transmission speed of serial communication. Although PCI is parallel, PCI Express is serial. USB is now widely used everywhere because of its universality, practicality, and high speed as well as because it also transmits power beside data.