Signals
The physical representation of bits: analog vs digital, encoding, and noise.
Level: intermediate
What is it?
In communications, a signal is the physical representation of information over a medium. Signals may vary continuously (analog) or represent discrete symbol levels (digital), but real digital systems still use physical waveforms with finite noise margins.
Why does it matter?
Network links must encode bits into electrical, optical, or radio signals. Noise, bandwidth limits, attenuation, interference, and timing determine how reliably a receiver can distinguish transmitted symbols.
Mental model
The sender maps bits to symbols; the channel alters them; the receiver estimates which symbols were sent. Encoding and modulation are choices about how symbols use the available channel.
How it works
- Analog signals vary continuously; digital communication uses a finite symbol alphabet and decision thresholds.
- Noise and attenuation reduce the receiver's ability to distinguish symbols, affecting bit-error rate.
- Line coding and modulation are not identical: line coding maps bits into signal patterns, while modulation changes a carrier's properties.
- Sampling and quantization matter when converting analog signals into digital representations.
- Error detection/correction adds redundancy to detect or recover corrupted information.
Steps
- The transmitter maps a bit sequence into symbols.
- The physical transmitter produces a waveform over copper, fiber, or radio.
- The channel changes the waveform through noise, attenuation, dispersion, or interference.
- The receiver estimates symbols and reconstructs bits; error detection identifies some corrupt frames.
Real-world usage
- Understanding line coding, modulation, bandwidth, and link reliability.
- Explaining why link errors increase with poor signal conditions.
Common mistakes
- Assuming digital signals are perfectly square or immune to noise.
- Using bandwidth and bit rate as synonyms without stating the model and units.
- Confusing modulation, multiplexing, and error correction.
Interview questions
Why can a digital link suffer errors?
Its physical waveform is affected by noise, attenuation, timing, and interference; the receiver can misclassify a symbol.
Does higher signal amplitude always solve transmission errors?
No. Receiver saturation, noise, distortion, interference, and channel limitations still matter; link budgets and modulation/coding choices must be considered.