You've now learned about latency (how fast one request is) and throughput (how many requests a system handles). But there's a third piece of the puzzle that ties them together:

How much data can your connection actually carry?

Think about a water pipe. You can have the highest water pressure in the world (low latency) and multiple pipes running in parallel (high throughput), but if each pipe is only 1 centimeter wide, there's a hard limit to how much water can flow through it. No matter what you optimize, you can't push more water through a narrow pipe.

That pipe width? That's bandwidth.

Bandwidth matters in everyday life more than you might think:

• Your home internet plan is defined by bandwidth: "100 Mbps download speed" means the pipe to your house can carry 100 megabits of data per second
• Netflix recommends 25 Mbps for streaming 4K video. If your bandwidth is 10 Mbps, no amount of optimization will get you a smooth 4K stream
• Cloud providers charge for bandwidth. AWS charges per gigabyte transferred out of their data centers. Companies like Netflix spend millions on bandwidth costs every month
• Mobile networks have significantly lower bandwidth than wired connections, which is why apps feel slower on cellular data

Understanding bandwidth helps you make critical decisions: How much data should your API responses contain? Should you compress images? Do you need a CDN? Can your architecture handle video streaming?

Bandwidth is the maximum amount of data that can be transferred over a network connection in a given time period. It's the theoretical upper limit, the size of the pipe.

It's measured in:

• bps, bits per second
• Kbps, kilobits per second (1,000 bits)
• Mbps, megabits per second (1,000,000 bits)
• Gbps, gigabits per second (1,000,000,000 bits)

Important: Note that bandwidth uses bits (lowercase b), not bytes (uppercase B). There are 8 bits in a byte. So a 100 Mbps connection can transfer at most 12.5 MB (megabytes) per second. This trips up a lot of beginners!

The Pipe Analogy

Here's the critical insight: Throughput can never exceed bandwidth. Just like the actual water delivered per hour can never exceed what the pipe's width allows, the actual data transferred can never exceed the connection's bandwidth.

But throughput is often less than bandwidth because of:

• Network congestion (too many users sharing the pipe)
• Protocol overhead (TCP headers, encryption, etc. Take up space)
• Packet loss (some data needs to be re-sent)
• Server limitations (the server can't send data fast enough to fill the pipe)

Real-World Bandwidth Ranges

Notice the massive difference: a data center link is roughly 1,000x faster than a home connection. This is why the same app feels instant on a server but sluggish on a phone.

Let's build up an intuition for bandwidth with a series of scenarios. Watch how the same data takes wildly different amounts of time depending on the pipe it flows through.

Now that you've learned all three concepts, let's see how they relate. This is one of the most important mental models in system design.

The Highway Analogy. Revisited

Imagine a highway connecting two cities:

Why CDNs Exist

Content Delivery Networks (CDNs) solve a fundamental bandwidth problem. If Netflix served all video from a single data center in Virginia, the bandwidth required at that one location would be astronomical, and users in Tokyo would experience terrible latency.

Instead, CDNs copy content to hundreds of locations worldwide. Each edge server needs only enough bandwidth to serve its local users. This distributes the bandwidth requirement across many locations.

Mobile-First Design

When building applications, you need to consider bandwidth constraints of mobile users:

• A typical 4G connection is 10-50 Mbps, sounds fast, but shared among all apps