Serial & Parallel Data Transmission

There are two ways to transfer data between computers: Serial Transmission and Parallel Transmission.

Serial Transmission

Data is sent bit by bit from one computer to another in two directions.  Each bit has a clock pulse rate.  Eight bits are transmitted at a time with a start and stop bit known as a parity bit, which is 0 and 1 respectively.  Data cables are used when transmitting data to a longer distance.  The data cable has D-shaped 9 pin cable that connects the data in series.

Categories of Serial Transmission

Asynchronous transmission – an extra bit is added to each byte to alert the receiver on the arrival of new data.  0 is used as a start bit while 1 used as a stop bit.

Synchronous transmission – no extra bit is added to each byte.  Data is transferred in batches which contains multiple bytes.

Parallel Transmission

Several bits are transmitted together simultaneously with one clock pulse rate.  It transmits quickly as it utilizes several input and output lines for sending the data.

It uses a 25-pin port with 17 signal lines and 8 ground lines.  The 17 signal lines are divided as

  • 4 lines – initiates handshaking
  • 5 lines – communicates and notifies errors
  • 8 lines – transfers data


Serial transmission occurs between two computers or from a computer to an external device located some distance away.

Parallel transmission can take place within a computer system, through a computer bus or to an external device located a close distance away.


An example of serial mode transmission include connection between a computer and a modem using the RS-232 protocol.  An RS-232 cable can accommodate 25 wires, but only two of these wires are for data transmission, the rest are for overhead control signaling.  The two data wires run on simple serial transmission in either direction.

In this example, a computer may be far from the modem, making parallel transmission very expensive.  With this, speed of transmission is considered less important compared to the economic advantage of serial transmission.

An example of parallel mode transmission include connection between a computer and a printer.  Most printers are within 6 meters or 20 feet from the transmitting computer and the slight cost for extra wires is offset by the added speed gained through parallel transmission of data.

Comparison between Serial and Parallel Transmission

Basis for Comparison Serial Transmission Parallel Transmission
Definition Data flows in 2 directions, bit by bit Data flows in multiple directions, 8 bits (1 byte) at a time
Cost Economical Expensive
Number of bits transferred per clock pulse 1 bit 8 bits or 1 byte
Speed Slow Fast
Applications Used for long distance communication Used for short distance communication
Example Computer to computer Computer to printer

Differences between Serial and Parallel Transmission

  • Serial transmission requires a single line to send data.  Parallel transmission requires multiple lines to send data.
  • There are less errors and noise in serial transmission since transmission is done one bit at a time.  There are more errors and noise in parallel transmission since transmission is done multiple bits at a time.
  • Serial transmission is slower since data flows through a single line.  Parallel transmission is faster since data flows through multiple lines.
  • Serial transmission is full duplex since the sender can send and receive data.  Parallel transmission is half duplex since the data can be sent or received at a time.
  • The cables used in serial transmission are thinner, longer and economical as compared to the cables used in parallel transmission.
  • Serial transmission is reliable and straightforward.  Parallel transmission is unreliable and complicated.

Both serial and parallel transmissions have advantages and disadvantages.  Parallel transmission is used for shorter distance and provides greater speed.  While, serial transmission is reliable for transferring data to longer distance.  Both serial and parallel transmissions are individually essential for transferring data.