2.6 Modifying the medium to carry the message
Earlier we said that we can think of a signal as a deliberate variation in some property of the medium used to convey the data. Such variation needs to be done in a meaningful way. For example, think of the way pulses of light could be used to convey Morse code. (Morse code is a code in which letters and numbers are represented by groups of short dots and long dashes.) The light could be switched on and off so that a short light pulse could represent a dot and a longer pulse a dash. The same principle can be used with an electrical voltage applied to a copper wire. The sequence of on and off periods could be used to represent data, say, a stream of 1s and 0s. These can be detected and decoded at the receiving end in a communication system.
In radio signals, some property of the electromagnetic wave could be varied in a meaningful way – its frequency, for example. This is shown in Figure 7. Here we have indicated a number of equal time intervals. In the first interval, the wave completes three whole cycles, but during the second, third and fourth intervals, the wave completes only one and a half cycles per interval. So in these intervals the frequency of the wave is half the frequency of the first interval. Similarly, during periods 5, 6 and 8 the wave completes three whole cycles per interval, and only half that in intervals 7, 9 and 10. These changes of frequency can be detected and interpreted as data – say a 1 or a 0 depending on the frequency of wave during the measured interval.
The electromagnetic wave can be described as a carrier because it carries the data. That is, some property has been modified to represent the data. The process of modifying the carrier in this way is called modulation. A transmitter takes the data from the sender, modifies the carrier, and then sends the resulting signal through the communication link. At the receiving end, a receiver takes the signal, and extracts the data by a process known as demodulation, then passes the data to the recipient. The process is shown in Figure 8.
Figure 8 is in many ways similar to the diagrams used earlier in Figures 1 and 2, but it is slightly more abstract in some parts. For example, you are no longer shown, nor are named computing devices like a printer or computer. Instead the end points are rectangles labelled 'sender' and 'recipient'. These rectangles could be computers or printers, but could also be other devices. There may well be human users on the other side of them, but we are not concerned with those in this model so we have omitted them.
Another abstraction is the cloud that represents the network. This has also been done to simplify the diagram. We've chosen to focus on the parts that are most relevant to our discussion (which concerns the transmitter and receiver) and for this purpose we're not concerned with what is happening in the network links. The cloud indicates that there is something going on there but doesn't give any detail of what it is.
Notice in Figure 8 how we've shown the communication to be flowing in one direction only. This is because the transmitter receives data only from the sender and sends it only to the receiver. In this model there is no communication in the opposite direction. Some devices, known as transceivers, perform both the sending and receiving of signals so they could replace both the transmitter and receiver shown in the figure.
Activity 10: exploratory
How would you modify Figure 8 to model the use of transceivers in place of both the transmitter and receiver?
The modifications needed would be:
the end points both become sender/recipient because both can send or receive;
the transmitter and receiver are both replaced by transceivers;
the arrows showing the communication flow now run in both directions.
These modifications are shown in Figure 9.