Bit rate, bandwidth, and latency

All of the computing devices on the Internet are communicating in binary. Whether they are connected via wired or wireless, they are sending electromagnetic signals that represent streams of 1s and 0s.

Let's explore how those bits are sent and how quickly they can be sent.

When computers need to internally represent the number $5$‍ ($101$‍ in binary), they can use three wires to represent the three bits: one wire on, one wire off, one wire on.

If a computer wants to send the number $5$‍ to another computer, they can't use as many wires as they want. In fact, they may only have a single wire to send information over. Instead, they can send the number $5$‍ over three time periods: first sending an on pulse (and waiting), then sending nothing (and waiting), then sending an on pulse.

As long as the two computers agree on the time period, then they can transfer information to each other, turning binary data into signals and turning the signals back to binary data.

In an electrical connection (such as Ethernet), the signal would be a voltage or current. In an optical connection (such as a fiber-optic cable), the signal would be the intensity of light.

The process of turning binary data into a time-based signal is known as line coding. There are various line coding schemes that can be used based on the needs of the connection.

Network connections can send bits very fast. We measure that speed using the bit rate, the number of bits of data that are sent each second.

The earliest Internet connections were just $75$‍ bps (bits per second). These days, connections are more often measured in Mbps (megabits per second).

A megabit is huge: $1$‍ million bits! A $10$‍ Mbps connection transfers data at $10$‍ million bits per second.

That's one bit every $100$‍ nanoseconds ($0.0000001$‍ seconds).

We also measure bit rate in smaller units like kilobits ($1$‍ thousand bits) or much bigger units like gigabits ($1$‍ billion bits) and even petabits ($1$‍ quadrillion bits).

We use the term bandwidth to describe the maximum bit rate of a system. If a network connection has a bandwidth of $100$‍ Mbps, that means it can't transfer more than $100$‍ megabits per second. Fortunately, that's still a lot!

Ever heard the term "broadband Internet"? That refers to a connection with a minimum bandwidth of $256$‍ Kbps. That's enough bandwidth for basic Internet use like checking emails and reading websites, but not quite enough for watching online videos. As of 2016, only 40% of people in developing nations have access to even broadband Internet.

Another way to measure the speed of a computer network is latency. You might guess what that means from the word itself: latency measures how late the bits arrive. To put it in more formal terms: latency is the time between the sending of a data message and the receiving of that message, measured in milliseconds.

We typically measure the "round-trip" latency of a request. Let's walk through a real example to see what that means.

My computer sends a message to the Google server. $30$‍ milliseconds later, Google receives the message. $40$‍ milliseconds later, my computer gets an acknowledgement from Google that it received the message.

That's a total round-trip latency of $70$‍ ms. The latency depends on a number of physical factors: the type of connection from my computer to Google, the distance from my computer to the Google servers, and the congestion in the network (which may mean my request has to wait in line).

There's a major limiting factor to latency: the speed of light. Nothing can move faster than light, not even our very important Internet requests. The speed of light is $1$‍ foot per nanosecond, which means a trip length of at least $30$‍ ms from Los Angeles to Tokyo. We can't do much about the speed of light, but we can decrease latency by reducing congestion and improving our physical connections.

Speed is a combination of bandwidth and latency. Computers split up messages into packets, and they can't send another message until the first packet is received. Even if a computer is on a connection with high bandwidth, its speed of sending and receiving messages will still be limited by the latency of the connection.

You can measure the current speed of a network using an Internet speed test: a website that downloads and uploads data while tracking how quickly the data is transferred.

Here are the results from an Internet speed test from my home laptop:

The latency (also called the ping rate) was just $18$‍ ms. That's fast enough for most multi-player online games.

The download bit rate is $39$‍ Mbps and the upload bit rate is $5.85$‍ Mbps, significantly less. Actually, that's expected. Internet providers often support a much faster download speed than upload speed, because Internet users spend much more time downloading data (reading articles, watching movies) than uploading data (writing blog posts, submitting forms).

🔍 What's the speed of your Internet connection? What bandwidth does your Internet provider promise? What type of physical connection are you using? This is a great opportunity to get a deeper understanding of the physical infrastructure you use every day.

🙋🏽🙋🏻‍♀️🙋🏿‍♂️Do you have any questions about this topic? We'd love to answer—just ask in the questions area below!