Signals and Transmission
What is the difference between an analog and a digital signal?
- Distinguish analog and digital signals
- Compare serial and parallel transmission
- Explain why digital signals dominate modern networks
Overview
A signal is the physical thing that carries a message — a change in voltage, a burst of light, a radio wave. Signals come in two flavours: analog (continuous, like a wave on the sea) and digital (discrete steps, like a light switch on/off). How the bits are lined up on the wire — one after the other or many at once — is another choice that affects speed and cost.
Analog vs digital
Analog signals vary smoothly and can take any value between two limits — think of the needle on an old radio dial or a hand-drawn wave. Digital signals only have a small set of values, usually two: 0 and 1. Digital wins in modern computing because it is easier to copy without losing quality, easier to store, and easier to check for errors.
Serial vs parallel
In serial transmission the bits go down a single wire one after the other — like a single-file queue. In parallel transmission 8, 16, or 32 wires carry bits side by side at the same instant. Parallel sounds faster, but at high speed the wires interfere with each other (crosstalk). That is why USB, Ethernet and SATA all replaced older parallel cables — one clean high-speed serial channel now beats many slower parallel ones.
Bit rate and baud
Bit rate is how many bits are sent per second (bps). Baud rate is how many signal changes per second. If each signal change carries one bit, they are equal. If each change carries 2 bits (four possible voltages) the bit rate is twice the baud rate. Modern modems and Wi-Fi radios encode many bits per symbol to squeeze more data through the same channel.
Signal Sketch
- Sketch an analog and a digital signal on the same axes.
- Label two amplitude values on the analog wave and the 0/1 levels on the digital wave.
- Below the graphs, list one device that uses each type of signal.
- Which type of signal has only two levels, 0 and 1?
Reveal answer
Digital.
- Give one reason serial transmission has replaced parallel for high-speed use.
Reveal answer
Crosstalk between parallel wires limits speed; a single clean serial channel is easier to run fast.
- If a modem sends 2400 baud with 4 bits per symbol, what is the bit rate?
Reveal answer
9600 bps.
Open a USB cable and count the visible wires. Only two carry data (D+ and D-). Research why USB is called a serial bus.