Bandwidth
🎬 This chapter has a 7:57 video: Data Communication →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. How wide a channel is, is its bandwidth.
1What bandwidth is
Bandwidth = highest frequency − lowest frequency
It is measured in hertz (Hz). The higher the bandwidth, the higher the data transfer rate.
A channel is the path the data travels along — a cable, or a slice of the airwaves used by Wi-Fi. That definition 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
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.
2A wider band carries more data
The channels in this lab 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. Make one channel wider and watch how much more gets through each second.
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.
Why a wider band carries more data
Data travels on waves, and one channel can carry many waves at the same time, each at a different frequency. Each wave carries its own share of the data.
But two waves whose frequencies are too close would mix up and the receiver could not tell them apart (this is called crosstalk). So every wave needs its own range of frequencies, far enough from its neighbours.
A higher bandwidth means a wider range of frequencies is allowed — just as a broader highway has more lanes. More lanes fit more waves travelling side by side without mixing, and since every wave carries data, more data goes through the channel at the same time.
Change the bandwidth and count the waves. Then squeeze in more — it is clearest at 2000 Hz. Drag sideways to turn.
A teaching model: 1000 Hz of room per wave and 100 bits per second each. Real Wi-Fi and 4G work the same way with hundreds of closely spaced waves (subcarriers) and far bigger numbers.
Think of a water pipe
So what is “100 Mbps”?
How much data moves each second is the data transfer rate, measured in bits per second: bps, Kbps, Mbps or Gbps. (A bit is the smallest piece of data, a single 0 or 1.) When an internet plan says “up to 100 Mbps”, that is a data transfer rate — the most data the connection will carry each second. The next lesson is all about it.
3From the board papers
▶Show the answer
Bandwidth is the range of frequencies available for sending data through a channel — the highest frequency minus the lowest, measured in hertz (Hz). The higher the bandwidth, the more data the channel can carry each second.
▶Show the answer
(C) Hz — by your NCERT book, bandwidth is a range of frequencies, measured in hertz.
KB (kilobytes) is the size of a file and Km is a distance. Some guides answer (B) Bit instead, reading “network bandwidth” as a data rate, the way CBSE’s 2023-24 sample marking scheme does. CBSE’s official 2024 marking scheme could not be checked (its site was unreachable), so the answer here follows NCERT.
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?