Signals &
Encoding
Understanding how data travels as signals, how we shape it, and how we measure performance.
Basics of Signals
Data is transmitted in the form of electromagnetic signals. These signals can be:
Analog Signals
Continuous wave form that changes smoothly over time. Represents infinite values within a range.
Example: Human Voice, Radio Waves
Digital Signals
Discrete values (0s and 1s). Changes essentially instantly between levels.
Example: Computer Data
Periodic Signals: Repeat a pattern over identical periods (e.g., Sine wave).
Aperiodic Signals: Changes without a repeating pattern (e.g., Microphone input).
Network Performance
Core concepts used to measure the performance of a network.
Bandwidth
The maximum rate of data transfer across a given path.
Throughput
The actual rate of data transfer at a specific time.
Throughput is often less than bandwidth due to:
- Network Congestion
- Packet Loss
- Protocol Overhead
Analogy: Bandwidth is the width of the highway (lanes). Throughput is the actual flow of cars.
Latency (Delay)
The time it takes for a message to travel from sender to receiver. It has 4 components:
Distance / Speed
Time to travel the wire
Size / Bandwidth
Time to push bits onto wire
Congestion
Waiting in router buffers
Router CPU
Time to check headers/errors
Jitter
The variation in the packet arrival time. Crucial for real-time apps like VoIP or Gaming.
Bandwidth-Delay Product
Defines the number of bits that can fill the link.
Volume = Bandwidth × RTTTransmission Impairments
Ideally, a signal at the start and end of a medium should be identical. In reality, imperfections cause impairments.
Attenuation
Loss of energy (signal strength) as it travels. Overcome by Amplifiers.
Distortion
Alteration of the signal's shape or form. Occurs in composite signals when different frequencies travel at different speeds.
Noise
Random/unwanted signals added to the data (Thermal noise, Crosstalk, Impulse noise).
Data Rate Limits
How fast can we send data?
Nyquist Bit Rate
For a Noiseless Channel.
BitRate = 2 × Bandwidth × log₂LL = Number of signal levels.
Shannon Capacity
For a Noisy Channel.
Capacity = B × log₂(1 + SNR)B = Bandwidth, SNR = Signal-to-Noise Ratio.
Digital Transmission
Converting digital data into digital signals (Line Coding) or analog signals into digital data (PCM).
PCM (Pulse Code Modulation)
The standard method for digitizing analog audio (used in CDs, Telephony).
Line Coding
Converting digital data to digital signals.
Types: Unipolar (NRZ), Polar (NRZ-L, NRZ-I, RZ, Manchester), Bipolar (AMI).
Block Coding
Adding redundancy to ensure synchronization and error detection. (e.g., 4B/5B coding).
Analog Transmission (Modulation)
Converting digital/analog data into analog signals (Carrier signals).