Unit 6 · The Internet & Computing Systems · Lesson 6.8

Bandwidth and Latency

A truck full of hard drives driving across the country carries an enormous amount of data — and takes three days to deliver the first byte. “Fast” means two different things. Today you separate them.

Big Idea 4 — Computer Systems & NetworksCED 4.1 · digital divide, first naming

Before this lesson

No reading — this page is the reading. Unit 1 arithmetic returns: 1 byte = 8 bits, and a 12-megapixel photo at 3 bytes per pixel is 36 MB.

Where We Are

People→ Bits→ Programs→ Internet→ Security→ Big Data→ Impact

Internet. The path works (6.6–6.7). Now: how fast is it — and which kind of fast?

The Hook

The truck vs. the video call. A truck carries 100 hard drives of 20 TB each across the country in 3 days. Averaged over the trip, that’s about 62 gigabits per second — far more than most home internet. Would you use the truck for a video call?

Huge amount, terrible delay. Those are two different measurements.

The Idea

	Bandwidth	Latency
What it is	the maximum amount of data sent per unit of time	the delay before data starts arriving
Units	bits per second (Mbps = millions)	milliseconds (ms)
Analogy	how wide the pipe is	how long the pipe is
Who cares most	streaming 4K, downloading big files, backups	gaming, video calls — anything live
Fix	faster connection, more capacity	shorter or more direct path, fewer hops

Throughput is what you actually get — usually less than the bandwidth, because the connection is shared and some packets get resent.

Transfer time ≈ latency + (size in bits ÷ bandwidth). Convert bytes to bits first (× 8). A 36 MB photo is 288 megabits; at 12 Mbps that’s 288 ÷ 12 = 24 seconds. For a big file, bandwidth dominates. For a tiny message, latency does.

Convention on this page: 1 MB = 1,000,000 bytes = 8 megabits, the way network speeds are advertised.

Try It — How Long Will It Take?

file:

connection:

SIZE (MB)

BANDWIDTH (Mbps)

LATENCY (ms)

Try the text message on satellite vs. fiber, then the movie on both. Which number mattered each time? (The connection speeds are typical examples, not measurements.)

Vocabulary

bandwidth Internet: The maximum amount of data that can be sent in a fixed amount of time, usually measured in bits per second.
latency Internet: The time delay between sending data and its arrival, usually measured in milliseconds.
throughput Internet: The amount of data actually transferred per second — usually below the bandwidth.

Check

Five questions. Convert bytes to bits before you divide.

0 of 5 answered

Impact Check

The digital divide is a bandwidth gap

If your home has 3 Mbps, a 4 GB school video takes almost three hours to download. Uploading homework, joining a video class, or applying for a job online all get harder. Rural and low-income areas are more likely to have slow connections or none — that gap is called the digital divide. Unit 8 takes it on fully; Lesson 1.9 already hinted at it with compression.

Connections

Came from 1.5 — the 36 MB photo: 12 million pixels × 3 bytes. Now you know how long it takes to send.

Came from 1.9 — compression makes files smaller, so the same bandwidth sends them faster.

Next in 6.9 — another kind of speed: doing many things at once. Parallel computing.

Returns in 6.12 — you measure your own latency to three continents.