Bandwidth
A two-lane road and an eight-lane highway can both take you to the same city. But in one hour, far more cars get through on the highway. Communication media are the same: some can carry much more data than others. Bandwidth is how we measure that.
1What bandwidth is
A channel is the path the data travels along — a cable, or a slice of the airwaves used by Wi-Fi. Bandwidth says how much that path can carry, the way the number of lanes says how much a road can carry. The higher the bandwidth, the more data can go through each second.
In networks, bandwidth is given in bits per second: bps, Kbps, Mbps or Gbps. When an internet plan says “100 Mbps”, that is its bandwidth. (A bit is the smallest piece of data, a single 0 or 1. The next lesson is all about these units.)
Choose a bandwidth for each channel and compare what reaches the other end. Drag sideways to turn.
The frequencies are real 5 GHz Wi-Fi channels. The bundles are a teaching model: they show that doubling the bandwidth doubles the data on the same quality of line, not a real speed in Mbps.
Think of a water pipe
2Bandwidth as a range of frequencies
Your NCERT book describes bandwidth in a second way, using frequencies. It sounds harder than it is. Take it one step at a time.
Step 1: what a frequency is
Data travels along a channel as waves, going up and down. The frequency of a wave is simply how many times it goes up and down in one second.
- A slow wave, a few ups and downs each second, has a low frequency.
- A quick wave, many ups and downs each second, has a high frequency.
- Frequency is measured in hertz (Hz). One up-and-down each second is 1 Hz. 1 kHz = 1000 Hz, 1 MHz = 1000 kHz, 1 GHz = 1000 MHz.
Step 2: every channel has a lowest and a highest frequency
No channel can carry every wave. Each one carries only the waves that lie between a lowest frequency and a highest frequency. Waves slower than the lowest, or quicker than the highest, do not get through. Think of a door that only lets in people between 120 cm and 190 cm tall.
Step 3: bandwidth is the gap between them
It is the width of the range of frequencies a channel can carry, so it is measured in hertz (Hz).
An old telephone line carries sound from about 300 Hz to 3400 Hz. Its bandwidth is 3400 − 300 = 3100 Hz. That is all there is to it: find the two ends, and subtract.
Pick a channel, or choose your own lowest and highest frequency. Count the ups and downs: more of them means a higher frequency.
The channels in the 3D lab above are real Wi-Fi channels, and the numbers at their edges are their lowest and highest frequencies. The 20 MHz channel runs from 5170 MHz to 5190 MHz: 5190 − 5170 = 20 MHz.
So why does a bigger gap carry more data? A wider range has room for more different waves at the same time, and more waves can carry more bits every second. That is how the two meanings of bandwidth fit together.
3From the board papers
▶Show the answer
Bandwidth is the maximum rate of data transfer over a given transmission medium — the amount of information that can be transmitted over a network in a given time. That is the answer CBSE’s own marking scheme gives.
▶Show the answer
(B) Bit. The question says network bandwidth, which is the amount of data carried each second, measured in bits per second.
KB (kilobytes) measures the size of a file, not a speed, and Km is a distance. Hz is the unit for bandwidth as a range of frequencies, which is why it is there to tempt you — read the word “network”.
4Check yourself
In the lab, Channel 2 is changed from 40 MHz to 160 MHz. How much data does it now carry each second?
A channel carries frequencies from 2000 Hz to 8000 Hz. What is its bandwidth?