Latency

Latency is the amount of time between when you start sending data and when the recipient starts receiving it. Latency is often measured in milliseconds. When you visit a web site, your computer sends a request to the web server that hosts the site. The web server responds with a web page. Latency, not bandwidth, determines how quickly your request gets to the web server and how quickly you start to see the web page.

Bandwidth

Bandwidth is the amount of data that can be transmitted per unit of time. Bandwidth is often measured in megabits per second. While the request that your computer sends to a web server is very small, the actual web page that the web server sends back is much larger. Bandwidth determines how long it will take for the entire web page to appear.

Bandwidth vs latency

Both measurements are important. When browsing typical web sites, checking email, and playing online games, low latency is far more important than high bandwidth. When downloading music or sending emails with large attachments, high bandwidth is more important.

Example 1: High bandwidth, high latency

Consider a situation in which you bring your collection of 500 DVDs to your friend’s house, which is 5 minutes from your house. You just transferred 18,800,000 Mb in 300 seconds for a bandwidth of 62,667 Mbps. Since the first piece of data arrived 5 minutes after you started, your latency is 300 second. This “connection” has a very high bandwidth but also a very high latency.

You could have sent the data on the DVDs to your friend electronically. Consider a situation in which you upload the data over your 5 Mbps Internet connection. Perhaps it takes 50 ms for your friend to start receiving the data, so your latency is 50 ms. Unfortunately, the bandwidth is only 5 Mbps, so it takes you 44 days to transfer all of the data.

In this situation you had a very large amount of data to transfer, so very high bandwidth was important.

Example 2: Low latency

Consider playing a real time action game online against your friend. Each time you move, jump, or shoot, you need your friend to see it as quickly as possible. You don’t need to transmit much information, you just need to transmit it quickly. In this case, low latency is far more important than high bandwidth.