Introduction to Communication Systems

Fundamental Model of Communication Systems

Electronic (and even non-electronic) communication systems can be represented by the following block diagram:

Block diagram of general communication system
Fundamental Model of Communications
  • Message Source: The originator of the message
  • Input Message: The message/data/info that is to be communicated
  • Input Transducer: Converts the input message into electrical form
  • Input Signal: The data in electrical form (this is a baseband signal)
  • Transmitter: Modifies the signal for transmission
  • Channel: The medium over which the transmitted signal is sent (e.g., wire, air, optical fiber, free space)
  • Distortion/Noise: External signals/features that affect the signal
  • Receiver: Modifies the received signal, undoing the modifications done by the transmitter
  • Output Transducer: Converts message from electrical signal back into its original form
  • Output Message: The message/data/info that has been communicated
  • Message Destination: Who/what the message/data/info was intended for

Example System #1: FM Radio

A radio DJ (message source) speaks (input message) into a microphone (input transducer) which converts the sound into an electrical signal (input signal). The electrical signal is then modified by an FM modulator to convert it to FM radio frequencies, and then transmitted from the radio station antenna (transmitter). The FM signal travels over the air (channel) where it may be modified by environmental disturbances (noise). The signal is then received at the antenna of your radio (receiver) where it is modified again to bring the electrical signal back to the baseband frequency. The electrical signal runs through the radio’s speakers (transducer) which vibrate and you (message destination) can hear what the DJ said (output message).

Example System #2: Sending a Text Message

You (message source) have a message you want to send to a friend (input message). You type the message into your cell phone (input transducer) , and your message get turned into 1s and 0s of electronic data (input message).  The 1s and 0s are modulated by the transmitter/antenna (transmitter) on your phone and turned into an electromagnetic signal (transmitted signal). The electromagnetic signal is received at the antenna of the cellular network base station (receiver), and converted back into 1s and 0s (output signal) The output signal then gets sent over the cellular network to the base station nearest your friend (this whole system could also be modeled using the fundamental model too) and the process is reversed to send the message from the base station to your friend.

Electronic Communication Systems

Electronic communications use the properties of electricity, and electromagnetism to send information from one point to another.  It is a wide field of study because of the different kinds of signals used for sending the information, the different media (systems) the information is sent through, and because of the many complex ways of representing information.

Signals

Signals are the means by which we transfer information.  Signals can come in many forms, but in electronic communications, the signals we are interested in are electrical signals (voltage) that vary with time.  By properly manipulating the signals, we can encode information onto them which can be transmitted to someone/something that requires the information.

A typical signal can be represented like this:

v(t) = V_psin(2\pi f t + \phi)

  • v(t) is the signal
  • V_p is the peak amplitude of the signal
  • f is the frequency of the signal (in Hz)
  • t is time
  • \phi is the phase shift of the signal

The features of this signal that can be manipulated to encode information are the peak amplitude of the signal, the frequency of the signal, and the phase shift of the signal. Although this signal v(t) is a continuous, analog, deterministic, and infinite signal (we’ll learn more about all of these features in the next section), the idea of changing features of the signal to encode information on it is universal across all types of signals.

Systems

Analyzing the signals themselves can be interesting, a discussion of communication systems is not complete without including the systems that the signals travel through.

The part of the electronic communication systems that we are most interested in here are the transmitter, the channel, and the receiver

Transmitter

The transmitter is the means for putting data out onto the channel, and it includes components for encoding, modulating, and transmitting the signal

Channel

This is the medium over which the data is sent.  Examples include:

Twisted Pair: This is a pair of insulated copper wires that are twisted together.  The purpose of twisting is to prevent the pair of wires from acting like an antenna.  Typical uses of a twisted pair are in telephone systems, and Ethernet (i.e. Category 5 cables)

Coaxial Cable: Also known as coax.  This type of cable consists of a copper wire running down the middle that is surrounded by an insulator.  Surrounding this insulator is a cylindrical conductor covered in a plastic protective sheath.  Coaxial cable has excellent noise immunity and can carry high data rates.  Coax is commonly used for cable television and local area networks.

Fibre Optics: A fibre optic cable consists of a very long thin fibre of glass down which light pulses can be sent.  The data rates supported by fibre optic networks are incredibly fast.  So fast in fact that most people involved in fibre optic development now say that in relation to network speeds, computers are hopelessly slow, and so we must try to avoid computation at all costs.

Wireless:  When electrons move, they create electromagnetic waves that propagate through free space (and unlike sound waves they can even propagate through a vacuum).  As we will see later on in this course, these waves can be manipulated to carry information from a transmitter to a receiver.  This “manipulation” is the basis of wireless communications.  Typical examples of wireless include AM and FM radio (as we will see later AM and FM refer to more than just the radio stations you can listen to), cellular networks, satellite communications and WLANS.

Receiver

The receiver consists of the components that extract the signal out of a channel, and may include an antenna, a demodulator, a decoder and filters.

Data Representation in Communication Systems

Data can be represented in a communication system in any manner desired or required. The only necessity is that the sender and the receiver of the message both use the same method. In general, there are two broad classes for representing data in electronic communication systems: analog communication and digital communication

Analog Communication

In analog communication, the communicating signal has continuous values within its valid range. This means that the signal can take any value between the minimum and maximum value it can have. The basic example of an analog form of communications is speech; a speech signal, or waveform has continuous values (amplitudes) over the range of possible values.

Digital Communication

Digital messages are transmitted using a finite set of values.  A classic example is Morse code where a dash can be transmitted by one value/amplitude/voltage and a dot can be transmitted by another.  Basic binary transmission also uses two different values where a 1 is transmitted by one value/amplitude/voltage, and a zero by another.  However digital communication systems aren’t limited to using only two values.  A system can use many different “symbols” or values to pack more data into each symbol.  This kind of system is called an M-ary system.  We will study these in more detail later, but to give a basic example, think of a system in which a 1 is transmitted by a sine wave of one frequency, and a 0 is transmitted by another frequency.  This is called a frequency shift keying (or FSK) system. It is easy to imagine expanding this system to include more frequencies for more values (e.g. four frequencies to represent 00, 01, 10, and 11).  We will study more about FSK and other digital communication systems later.

Data, Information, Knowledge, Wisdom

Let’s take a step back from all of this electronics, electromagnetism and mathematics, and consider what we are trying to do with electronic communication systems. At the most basic level they are used to send data back and forth. But data are just raw symbols (numbers, characters, bits…) that do not have any meaning by themselves. Data must be given a context (that is shared and known by the transmitter and receiver). When data is given a context, then it becomes information. Another way to look at it is that information is the “stuff” that we know, and data is the representation of that “stuff”. But even if we think about a communication system as a means to send information back and forth, I don’t think we have the complete picture yet either.  Consider receiving a text message from a friend that simply says “i’m home”. This is certainly a piece of information, but you need to already have other information such as who “I” is referring to, and where this home is in order to glean anything useful from the message. When you tie a number of pieces of information together in a useful framework, then you have some knowledge. However, even if you know exactly who “I” is and exactly where that home is, you still need something else. You need to be able to take that knowledge and have the ability to interpret and apply it, in order take action based on the message “i’m home” that you received.  When you can do this, then you have wisdom (at least to a small degree).

So, the bottom line is this:  these fantastic communication systems that surround us everyday are there in order that we can quickly send data back and forth in order to build up the amount of information that we know so that we can add this information to our knowledge base, and then (hopefully) act appropriately on it. So let’s try not to get lost in the technology during our study of data communications and try to always keep in mind what their ultimate purpose is.