Part 6 - Serial and Parallel Transmission

Serial and parallel transmissions are another way that computers communicate and are usually used in peripheral communication.  With serial communication bits are sent one at a time over the medium, but since processors handle data in parallel, communication controllers are used for conversion.  This conversion is done using a shift register.  The way a shift register works is it takes bits in parallel chunks and with the use of a clock sends each bit out one at a time to the serial device.  The same is done in reverse with serial data coming back from the device.  The shift register takes one bit at a time until it fills up the register then sends the parallel chunk to the processor [1].  Serial communication is used between your computer and monitor.  That somewhat rectangular port with nine pins that you plug your monitor into on the back of your computer is a serial port.  Another type of serial port you might use are the small round ones with six pins that you plug your keyboard and mouse into.  These are called PS/2 ports.  However, some of you might use a USB mouse, but guess what?  That's serial communication too.  USB stands for Universal Serial Bus.  Anything you plug into a USB port is using serial communication.

Parallel transmissions are used to communicate with devices capable of parallel communication.  With this mode of transmission a communication controller is not needed.  Instead, the processor transmits bits simultaneously through a parallel medium like a ribbon cable.  This mode of transmission is obviously faster, but it is subject to interference because the wires run in parallel and signal leakage (crosstalk) can cause corruption of the data especially at higher speeds [1].  If you are as old as me you might remember the old serial printers with the big fat ribbon cable.  These aren't used too much any more as USB printers have become more common, but you probably still have a serial port on the back of your computer.  It's that long, somewhat rectangular one with 25 pins.  Aside from a few specialized devices, printers are about the only things that use that port, but that does not mean that your computer doesn't use parallel communication if don't have anything plugged into that port.  Inside your computer you may find at least one parallel device; your hard drive or CD/DVD drive.  Many internal hard drives use a 40-pin Parallel Advanced Technology Attachment (PATA) connection.  Some optical drives also uses PATA connections, but I think Serial ATA (SATA) connections are becoming the standard for these devices.  That said, there are still a lot of older computers out there using the PATA drives.  I see them all the time.  In fact, someone reading this right now probably has one in the computer they are reading it on.

There is also a form of parallel transmission that is done through a single line.  This is similar to broadband in that the line’s bandwidth is divide into channels of differing frequency and each bit is sent down a channel [1].  This type of parallel transmission is often used in fiber optics, which is a huge topic on its own, so I will leave that for discussion another day.

To control data flow in serial or parallel communication Xon/Xoff protocol is used.  To illustrate how Xon/Xoff works, computer/printer communication can be used as an example.  When a computer sends data to a printer, the data often comes faster than the printer can print it.  Therefore, most printers have a small buffer to store incoming data until it can be printed.  When this buffer fills up, the printer sends an Xoff signal to the computer which tells it to stop transmitting data.  When the printer catches up and clears some of the buffer it sends an Xon signal telling the computer to resume sending data.  Modems may also use Xon/Xoff in a similar fashion when communicating with a computer’s operating system ("Xon/Xoff", 2010).

This concludes Part 6 of the the Data Transmission and Protocol series.  Part 7 begins the protocol section of the series.  It seems fitting that the first set of protocols I will discuss were be the earliest protocols defined for Internet transmission; Transmissions Control Protocol (TCP) and Internet Protocol (IP).  I have run into some that say, "Don't you mean TCP/IP?"  My answer?  "No."  TCP, IP, and TCP/IP are three completely different things.  If you don't understand, you will after reading Part 7.

[1] Kioskea. (2008). Data transmission - Transmission modes. Retrieved from http://bit.ly/pu9GeN

[2] Xon/Xoff. (2010). In Computer, Telephony & Electronics Industry Glossary. Retrieved from http://bit.ly/nTRyCj