What is Data Communications?

Data Communications is the transfer of data from a sender to a receiver over some medium (channel). It is a simple model:

Note the terminology for the channel. Simplex is one-way (like this lecture). Half-duplex is both ways, but one way at a time (like the airplane radios with a transmit button). Full-duplex is both ways at the same time (like a telephone).

A sender and a receiver (or transmitter/receiver) are typically computers in data communications. The medium is usually some sort of wire, although wireless communications is also very common. Both bounded media (wires) and unbounded media (wireless such as radio waves) use the same basic principles for data transmission given below. You will get a lot more details on this in the later course. Electrical and light energy are really just different wavelengths in the Electromagnetic spectrum:

You can see that there is a continuum of energy between electrical signals (lower wavelength) and light (higher wavelength). UTP uses wavelength frequencies (waves/second) between 10-10^8 and coax uses 10^3-10^9. Optical fiber uses 10^14-10^15. That range includes visible light. You can see that there is a continuum between electrical and light energy. All electromagnetic energy is analog in that it is continuous and not at discrete levels.

Since the senders and receivers are computers, the data is in digital form. Digital information must be discrete which means at a certain number of levels. Computers are also binary, which means that there are only 2 levels, usually represented as 1 and 0. Therefore any message from a computer can be represented as a string such as 101100011100...

Remember that computers use codes such as ASCII to map letters to a binary code. For example, The letter "A" is encoded as "01000001." You can see an ASCII table from the link in this folder. So you can see that any string of binary information could be any message.

Now we need to figure out how this string of binary data gets from the sender to the receiver. It has to take the form of a signal put on a medium. A signal is an electrical or light pulse or frequency. A medium is the material that the signal travels over. Bear in mind that the air is a medium (that's how your radio or television work). A typical example involves an electrical signal traveling over a copper wire. In this case, we must decide how to encode our digital data on the electrical signal. One way to do this would be to use voltage levels. If we choose 0 volts to represent "0" and +V volts to represent a "1," we get the following for the string "0111111100."

This is called an NRZ or "Non Return to Zero" encoding. There are many more digital encoding methods that we will see in the later course. If we put this digital signal on that copper wire, we can see how the message gets from the sender to the receiver (we find out later how they "understand" the message).

In general for data networking, we need to have:

senders/receivers

media protocols services

Senders/receivers are usually computers--we will call them nodes. Media are bounded: wires or optical fibers or unbounded: the airwaves. Protocols are the "rules of the road" that determine how nodes "understand" the strings of ones and zeros. Services are programs that are offered over the network--they are reason for data communications! An example is what you are using right now--the web. We will discuss all of these in the context of a scenario of Internet access.