What is an Interface?

By Steve Parvarno

Interfaces work at many different levels. In computing, each level builds on the one below it, adding something more useful but more specific. Just like a pyramid starts wide at the bottom and gets narrow at the top. An example will help: Lets talk about how this web page got from the server at DeepScience.com to your computer.

Physical

First, your computer had to be plugged in to the Telecom phone jack. There are two wires in cable, a red one and a white one. So the first interface is physical: it defines what kind of wires there are, what kind of plug goes on each end of the wire, what pin of the plug each wire attaches to, and sometimes even the colour of the wires. This is the broad base of the pyramid. Wire is useful for all kinds of things: transmitting electricity to your house; holding the exhaust pipe on the car; keeping sheep in paddocks; making braces for teeth. This is a specific use of one type of wire: connecting telephones together.

Electrical

But wait, theres more! The signals on the wire have to be right: it is like the difference between a whisper and a shout to a deaf person. They can hear the shout, but not the whisper. But if you shout really loud in someone elses ear, you can break their ear drum. So you have to speak at the right volume. The same on the wires. This is the electrical layer of the interface pyramid.

Verbal

The next layer up describes the "language", the basic sounds used to transmit information, and how thise sounds fit together. It is no good asking a Japanese person a question in Swahili- so you also have to encode the sounds into words that make sense. Your modem does this job. It takes the binary data from your computer and turns it into sounds that can be sent across the telephone system to another computer and modem. Thats what all the funny screeching is when you first dial into the internet. Its computer language for "Hello! Are you there? Do you speak Swahili?"

Protocol

Now we are at the software level of the interface / pyramid. When you dial in to the internet, your computer starts a conversation with another computer. You usually dial in to an Internet Service Provider (ISP) like ClearNet, IHUG, or XTRA. Lets use ClearNet. We ring them up. They answer. When you enter your password it is like saying "we are friends, we can talk together because we know each other."

We have come a long way... now we know how loud to speak, what language, who to speak to, and whether they want to speak to us. How do we have to ask for the information we want? We have to ask nicely! There is a special way of asking for files. Its an interface in software, called a protocol. It carefully defines how to ask questions so we can get the answer we expected. Otherwise it is like walking into a mechanics workshop and saying "Can you pass me the metal thingy over there on the bench...? No. The other thing, the ring thing with the stick on the end... No. The one next to the gadget... No... Doh!"

Request

The request is passed from the computer at ClearNet to DeepScience.com. It checks on the hard drive to see if it is there, packages it up and sends it to you. You can see this process happening in Internet Explorer when it says "Connecting to site...", then "Loading..." on the status bar at bottom of the window.

Simple huh!

Extra for Experts

Just like peeling the layers off a cake, starting with the chocolate on top and working down...

Layer	Uses	What happens
Display	Browser	You ask for and get this file
Request	URL	Specific question
Protocol	HTTP	Say "Please!"
Verbal	Modem	Speak in English
Electrical	Signal	Sound waves
Physical	Wires	Air molecules

Challenge: find definitions for the words URL, HTTP and Modem (try looking on www.whatis.com)

Note for teachers: this was loosely based on the 7 layers of the ISO OSI model of data communications. See www.whatis.com/osi.htm for more information.

Steve is a freelance computer programmer and web site builder.
He is also currently studying at the Auckland Bible College.
Steve quite enjoys riding a unicycle down Queen Street.

© 2000 DeepScience.com and Steve Parvarno