1. Signals Transmission of data across network connections: – Collection numerical statistics from another computer – Sending animated pictures from a design workstation – Causing a bell to ring at distant control center Data usable to a person or computer are not in the form for transmission over a network – Photograph can not be transmitted as it is – Encoded description of the photograph (1s and 0s) that tells the receiving device how to reconstruct the image of photograph can be transmitted – Data stream of 1s and 0s must be turned into energy in the form of electromagnetic signals

2. Signals To be transmitted, data must be transformed to electromagnetic signals. Note

3. ANALOG AND DIGITAL Data can be analog or digital. The term analog data refers to information that is continuous; – An analog clock has hour, minute and second hands: The movements of the hands are continuous. – Human voice Digital data refers to information that has discrete states. – Data stored in the memory of a computer – A digital clock: Hours and minutes will change suddenly from 8:05 to 8:06. Analog signals can have an infinite number of values in a range; Digital signals can have only a limited number of values as simple as 1 and 0 Conclusion: – Analog data take on continuous values. – Digital data take on discrete values. Can be modulated into an analog signal for transmission across a medium.

4. Comparison of analog and digital signals

5. Periodic and Aperiodic Signals Analog and digital signals can be in the form periodic and nonperiodic (aperiodic) Periodic signal – Completes a pattern within a measurable time frame called period – Repeats that pattern over subsequent identical periods – Completion of one full pattern is called a cycle Aperiodic signal changes without exhibiting a pattern or cycle that repeats over time In data communications, we commonly use – periodic analog signals (because they need less bandwidth) – Nonperiodic digital signals because they can represent variation in data.