Video: How to Optimize Video Deli非常 to iPhone Users

了解更多关于移动交付的信息流媒体西部.

Read the complete transcript of this clip:

内森·摩尔: What if I want to stream to iPhone users? Well, if I care about iPhone users, how do I get the video to them? 我使用的是HLS协议. 伟大的. I I'm going to use a protocol called HLS, now, how does the HLS object actually get over there? Well, you have, you're on an iPhone, you're on a 4G network. 4G网络有什么问题? Their connections can be spotty, latencies vary, and they vary wildly, depending upon where you are. If you ever walked through a tunnel with your cell phone and have lost the signal, you know what the problem with 4G is.

If I want to do tens of thousands of these things simultaneously, I can't just do this with one server. 最后，他们在哪里? This is the last problem that a lot of guys don't think about, because of the old broadcast models — you have just one central broadcast tower, it gets to e非常body in the local area. 现在情况已经不同了. The whole wide world can go talk to your server. 如果你, 非常, 离你很远, then they're going to have a 非常 bad time due to the 非常 bad latencies, 如果他们非常, 非常接近, 一切都会好的.

HLS is also known as HTTP 在线直播. That tells you it has a dependency upon the HTTP protocol, 正确的, 超文本传输协议, but HTTP itself has a further dependency on TCP, 传输控制协议, in order to get that HTTP object there in the first place. 这是图层, 一层, 一层, 在不同的协议层上, 所有这些都必须相互操作, 所有这些 have to operate correctly and which, 所有这些, 必须做好自己的工作, in order for your end user to get a lovely, 连续的视频.

Assuming you want to stream video from a server to an iPhone, your video is encoded at 1 megabit per second. Obviously these numbers are deliberately munged together to make the math super, 超级简单——现实世界, you'll get slightly different measurements. We know we're going to send to the server to the iPhone, the iPhone takes 1 second — we are 1 second latency — so it takes 1 second to get to the iPhone, 1 second for the iPhone to come back to the server.

So long as TCP can negotiate a speed of 1 megabit per second, the iPhone can play the 1 megabit per second encoded video at its intended rate, 正确的? 所以我们发送了 ... 我们通过发送2mb来实现, 暂停, takes 1 second to go over — iPhone sends its acknowledgement 正确的 back — and 2 megabits divided by 2 seconds to send that 2 megabits. Well, that's the 1 megabit per second that we needed. 伟大的.

如果我们输了怎么办? 如果我们有损失, we're still encoded at 1 megabit per second, we send 2 megabits of data — iPhone never receives it, so it can't send an acknowledgment; so the server waits 2 seconds, 它说, “我知道你马上就到, so it should have taken 2 seconds for me to get that packet, 但我没明白. 我最好重传一下，对? This time iPhone gets it, sends its acknowledge back, another 2 seconds has passed. Now do the math: We just sent 2 megabits in 4 seconds, which is only a half megabit per second.

如果你 watching a video encoded at 1 megabit per second, what happens? Delay, stuttering, 缓冲, 缓冲, 缓冲, 缓冲 ... And this is why things like retransmits are so incredibly important. This is why it is amazing that this stuff works over a 4G network, 哪个本质上是有损的, with intrinsically variable latencies, meaning if we ever have to retransmit, we could be waiting several seconds, 我们可能要等几毫秒 ... 这取决于. And it's really quite remarkable that this stuff works as well as it does.