In this age of science and technology, everything is done electronically and with electric power. It has become next to impossible to think of a communication system which would not involve electronics or electricity in some way or the other. If you were thinking of arguing that some devices use batteries and not electricity, remember that batteries operate devices on charge, a form of electricity.
Since all these work on electricity and principles of electronics, it is important to know about the factors which would affect the efficient working of the systems. This is because, there are times and situations in which you would experience a bad connection or a weak link. You should know the reason for such a situation so that you can be aware why this happens and try to avoid it.
The main reason for interference between communications links is a phenomenon called Electro-Magnetic Interference. This is mainly caused due to a phenomenon which occurs when a source is nearby the communication device which emits or radiates electric or magnetic signals, or a combination of both.
One of the easiest and most common examples is that of the phone. Suppose you are using your landline phone and talking to your friend. If your cell phone is nearby and it suddenly rings or if you get a message, you would know it before the ringtone becomes active, through a disturbance in the landline. This is a very good example of EMI.
The cell phone emits electromagnetic signals when it receives something, which interferes with the electromagnetic signals being transmitted in your landline. You can see a disturbance in the CRT tube televisions when lightning strikes, another example of EMI. Hence, there are many sources from which communication disturbances can occur, leading to a weak or disturbed link in the network.