Difference Between Serial and Parallel Transmission

Serial vs Parallel Transmission
 

The primary difference between serial and parallel transmission is in the way the data is transmitted. In serial transmission it is sequential whereas, in parallel transmission, it its simultaneous. In the computer world, data is transmitted digitally using bits. In serial transmission, data is sent sequentially where one bit after the other is sent through a single wire. In parallel transmission, data is sent parallel where several bits are simultaneously transmitted using multiple wires. Due to various reasons, which we discuss below, serial transmission has more advantages than parallel transmission and hence today serial transmission is followed in most used interfaces such as USB, SATA and PCI Express.

What is Serial Transmission?

Serial transmission refers to transmission of one bit at a time where the transmission is sequential. Say we have a byte of data “10101010” to be sent over a serial transmission channel. It sends bit by bit one after the other. First “1” is sent and then “0” is sent, again “1” and so on. So, essentially, only one data line/wire is needed for transmission and it is an advantage when cost is considered. Today, many transmission technologies use serial transmission as it has several advantages. One important advantage is the fact that because there are no parallel bits there is no need for synchronization. In that case, clock speed can be increased up to a very high level that a great baud rate can be achieved. Also, due to the same reason,it is possible to use serial transmission for long distance without any issue. Also, since there are no nearby parallel lines, the signal is not affected by phenomena such as cross talk and interference from the neighboring lines, as what happens in parallel transmission.

The term serial transmission is very much linked with RS-232, which is a serial communication standard introduced in IBM PCs long time ago. It uses serial transmission and it is also known as the serial port. USB (Universal Serial Bus), which is the most widely used interface today in the computer industry, is also serial. Ethernet, which we use for connecting networks, also follows serial communication. SATA (Serial Advanced Technology Attachment), which is used to fix hard disks and optical disk readers, is also serial as the name itself suggests. Other well-known serial transmission technologies include Fire wire, RS-485, I²C, SPI (Serial Peripheral Interface), MIDI (Musical Instrument Digital Interface). Moreover, PS/2, which was used for connecting mouses and keyboards, was also serial. Most importantly, PCI Express, which is used to connect modern graphics cards to the PC also follows serial transmission.

What is Parallel Transmission?

Parallel transmission refers to transmission of parallel data bits simultaneously. Say we have a parallel transmission system which sends 8 bits at a time. It should consist of 8 separate lines/wires. Imagine we want to transmit the data byte “10101010” over parallel transmission. Here, the first line sends “1”, second line sends “0”, and so on simultaneously. Each line sends the bit corresponding to it at the same time. The disadvantage is that there should be multiple wires and hence the cost is high. Also, since there should be more pins, the ports and slots become larger making it not suitable for small embedded devices. When talking of parallel transmission, the first thing that comes to mind is that the parallel transmission should be faster because several bits are transmitted simultaneously. Theoretically this must be so but, due to practical reasons, parallel transmission is even slower than serial transmission. The reason is all parallel data bits must be received at the receiver’s end before the next data set is sent. However, the signal on different wires can take different times and hence all bits are not received at the same time and therefore for synchronization there should be a waiting period. Because of this the clock speed cannot be increased as high as in serial transmission and hence the speed of parallel transmission is slower. Another disadvantage of parallel transmission is that the neighboring wires introduce problems such as cross-talk and interference to each other degrading the signals. Because of these reasons, parallel transmission is used for short distances.

The most famous parallel transmission is the printer port, which is also known as IEEE 1284. This is the port that is also known as the parallel port. This was used for printers, but today, it is not widely used. In the past, hard disks and optical disks readers were connected to the PC using PATA (Parallel Advanced Technology Attachment). As we know, these ports are no longer in use as they have been replaced with serial transmission technologies. SCSI (Small Computer System Interface) and GPIB (General Purpose Interface Bus) are also notable interfaces used in the industry that used parallel transmission.

However, it is very important to know that the fastest bus in the computer, which is the front side bus that connects the CPU and the RAM, is a parallel transmission.

What is the difference between Serial and Parallel Transmission?

• In serial transmission, data is transmitted one bit after the other. Transmission is sequential. In parallel transmission, several bits are transmitted at the same time and hence it is simultaneous.

• Serial transmission needs only one wire, but parallel transmission requires several wires.

• The size of serial buses are generally smaller than parallel buses as the number of pins is less.

• Serial transmission lines do not face interference and cross talk issues as there are no nearby lines but parallel transmission faces such problems due to its nearby lines.

• Serial transmission can be made faster by increasing the clock rate to very high values. However, in parallel transmission, in order to synchronize the complete receive of all bits, clock rate must be kept slower and hence parallel transmission is generally slower than serial transmission.

• Serial transmission lines can transmit data to a very long distance while it is not so in parallel transmission.

• Today most widely used transmission technique is serial transmission.

Summary:

Parallel vs Serial Transmission

Today serial transmission is used much more than parallel transmission in the computer industry. The reason is serial transmission can transmit to a long distance, with a very faster rate at a very low cost. Important difference is that the serial transmission involves sending only one bit at a time while parallel transmission involves sending several bits simultaneously. Serial transmission hence needs only one wire while parallel transmission requires multiple lines. USB, Ethernet, SATA, PCI Express are examples for using serial transmission. Parallel transmission is not widely used today but was used in the past in Printer port and PATA.

Images Courtesy: 

1. Serial cable via Wikicommons (Public Domain)
2. IEEE 1284 via Wikicommons (Public Domain)