YouTube buffering or slow downloads? Blame the speed of light

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Ars examines the business of bandwidth starting with the forgotten—latency.

Bandwidth—the number of bits per second that a device or connection can transfer every second—is the number that everyone loves to talk about. Whether it be the gigabit per second that your Ethernet card does, boasting about your fancy new FTTP Internet connection at 85 megabits per second, or bemoaning the lousy 128 kilobits per second you get on hotel Wi-Fi, bandwidth gets the headlines.

Bandwidth isn't, however, the only number that's important when it comes to network performance. Latency—the time it takes the message you send to arrive at the other end—is also critically important. Depending on what you're trying to do, high latency can make your network connections crawl, even if your bandwidth is abundant.

Why latency matters

It's easy to understand why bandwidth is important. If a YouTube stream has a bitrate of 1Mb/s, it's obvious that to play it back in real time, without buffering, you'll need at least 1Mb/s of bandwidth. If the game you're installing from Steam is about 3.6GB and your bandwidth is about 8Mb/s, it will take about an hour to download.

Latency issues can be a bit subtler. Some are immediately obvious; others are less so.

Nowadays, almost all international phone calls are typically placed over undersea cables, but not too long ago, satellite routing was common. Anyone who's used one or

will know that the experience is rather odd. Conversation takes on a disjointed character because of the noticeable delay between saying something and getting acknowledgement or a response from the person you're talking to. Free-flowing conversation is impossible. That's latency at work.

There are some applications, such as voice and video chatting, which suffer in exactly the same way as satellite calls of old. The time delay is directly observable, and it disrupts the conversation.

However, this isn't the only way in which latency can make its presence felt; it's merely the most obvious. Just as we acknowledge what someone is telling us in conversation (with the occasional nod of the head, "uh huh," "go on," and similar utterances), most Internet protocols have a similar system of acknowledgement. They don't send a continuous never-ending stream of bytes. Instead, they send a series of discrete packets. When you download a big file from a Web server, for example, the server doesn't simply barrage you with an unending stream of bytes as fast as it can. Instead, it sends a packet of perhaps a few thousand bytes at a time, then waits to hear back that they were received correctly. It doesn't send the next packet until it has received this acknowledgement.

Because of this two-way communication, latency can have a significant impact on a connection's throughput. All the bandwidth in the world doesn't help you if you're not actually sending any data because you're still waiting to hear back if the last bit of data you sent has arrived.

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