手机 3G 网络收看流媒体视频缘何如此慢

iz 告诉你：手机 3G 网络收看流媒体视频缘何如此慢

于2010-06-12 20:24:28翻译 | 已有1464人浏览

为什么用手机上网总是这么慢呢？

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The thing about wirelessly streaming video to millions and millions of phones is that it's, like, hard.

对各种各样的手机来说，无线流媒体视频播放起来确实有点……吃力。

Wireless vs. Wired

无线 vs 有线

Why is it, you might be wondering, that wireless speeds can't just zoom zoom, faster faster, the way that Verizon or Comcast seem to press a button and magically, new, faster internet speeds appear. Well, for one, it's not that magical—even cable and fiber optic "wired" broadband costs billions of dollars per year for new internet pipe, with plenty of griping from carriers about videos and torrents and other bandwidth hoggery, hence all the buzz about net neutrality.

你 可能在想，为什么无线传输的速度不能变得更快，就像 Verizon 或者 Comcast 轻轻按下一个按钮，一种全新的、更快的网络传输速度就神奇诞生了。好吧，其中一个原因是真实情况没这么梦幻——即使是电缆和光纤的宽带“有线”网络每年也 会因为大量占用运营商能力的视频和 BT 种子及其他的带宽杀手而花费上亿美元开发新的网络通道，这都是因为关于网络中立性的舆论。

But there are more demanding constraints when it comes to wireless broadband:

但是说到无线宽带就有更多麻烦的约束。

• Speed: No matter what happens, wired technologies will be faster than wireless, because electrical impulses guided upon a wire, or optical impulses running through fiber, are more efficient than radio waves scattering themselves into the air in the hope of getting picked up.

• 速度：不论如何，有线技术总是比无线的快，因为由于无线电波信号为了保证被收到必须分散在空间中，故通过线路传送的电子脉冲或者通过光缆传送的光学脉冲总比比无线电波信号效率更高。

• Reliability: Even when you can consistently pick up wireless signal, its strength may vary, not just because of how close you are to the cell tower or Wi-Fi hotspot, but because radio is blocked by the foliage on the trees, or the water in an aquarium.

• 可靠性：即使你能保证一直收到无线信号，它的强度也有强弱之分，不仅和你离手机信号发射塔或者 Wi-Fi 热点的距离有关，也会受到树上的树叶或者水池中的水的阻碍。

• Cost: Delivering the same bandwidth wirelessly will always be more expensive, because radio waves—due to the above constraints—require massive amounts of power to work well. As we'll see, there's also a matter of paying for the right to use radio waves, a privilege that is only granted after payouts in the billions of dollars.

• 费用：输送相同的带宽时，无线网络总是比较贵，因为无线电信号——就如上面说到的限制——需要更大的能量保证工作。我们都知道，还有一个问题，那就是我们必须付费使用无线电信号，只有花费了上亿美元才能获取某一频率的所有权。

If that sounds a bit remedial, it's supposed to: Wireless always, always lags behind wired. Think of how much faster gigabit ethernet is compared to Wireless N. It's just how the world works. But people want wireless connections, in their pockets, for obvious reasons. What we're talking about is why it's so hard to pull off well.

如果你想听些不那么令人泄气的话，我这样说吧：无线技术一直跟在有线的尾巴后面。想想 G 级的有线网络比任何一款无线网络都快了多少吧。事实就是如此。但是人们因为种种原因，想要他们口袋中的手机能无线上网。我们所谈论的正是难以割舍这根网线的原因。

What's Coming

未来

See that chart up above? That's the growth of data traffic on AT&T's network over the past four years. Despite all the email, photos, music, tweets, apps and voice data traveling across the network, the single largest type of traffic is video. Funny thing is, the true video explosion hasn't happened yet.

看看上面的图表吧。这是 AT&T 的网络近四年来的数据传输量增长曲线。不去管那些在网络中传输的电子邮件、照片、音乐、应用程序和语音数据，占用数据量最大的就是视频了。有趣的是，真正的视频高峰还没有来到呢。

What do I mean by that? Well, take Netflix's Watch Instantly streaming video service, for example. Right now, the only mobile device it's available on is the iPad, with an iPhone app promised by the end of the year. But Netflix's vision is to be on basically every device with a screen. Imagine a world where every phone, millions and millions of them, can stream nearly any movie over the air. Where phones with bigger, better, higher res screens demand serious quality video to take advantage of the extra pixels. Multiply that by apps current or future apps for Hulu, SlingPlayer, ABC, CBS, NBC, HBO, Vimeo, and oh yeah, YouTube.

这 是什么意思呢？好吧，我以 Netfix 的 Watch Instantly 流媒体视频服务为例。现在它支持的移动设备只有 iPad，在年底预计会发布一款支持 iPhone 的应用。但是 Netfix 的目标是支持所有有屏幕的设备。想象一个各种各样的手机都能无线收看几乎任何视频的世界吧。当手机的屏幕越来越大，显示效果越来越好，分辨率越来越高，就 需要高清的视频来使其有用武之地了。看看现在以及将来会发布的支持 Hulu、Slingplayer、ABC、CBS、NBC、HBO、Vimeo 和 oh yeah、YouTube 的应用程序，这一趋势将更明显。

Not to mention streaming video from phones, which are on the verge of universally breaching HD quality recording. Today, Microsoft's Kin phones—running on Verizon's 3G network—automatically upload every 5- and 8-megapixel photo, every 720p video you record, to the cloud. They're just the first, to be sure.

因此就更别提手机的流媒体视频了，很难实现高清视频录制。如今，微软的 Kin 手机——使用 Verizon 的 3G 网络——可以自动把 500 和 800 万像素的照片以及所录制的 720p 视频上传到云中。可以肯定的是，他们是第一家这么做的。

Two years from now, that bar at the far right of the chart may appear as tiny as the one at the beginning, compared to the traffic that's coming.

根据未来的流量来看，2 年后，图表中最右边的条形可能会变得和最左边的那条一样细了。

There are three major constraints on streaming video to a mobile device over the air: wireless spectrum, backhaul, and the device itself.

把流媒体视频无线传送到移动设备存在三大问题：无线带宽、回程，以及设备本身。

Need More Spectrum, Dude

老兄，我需要更大的带宽

Wireless spectrum, while invisible, is not an infinite resource. In fact, it's pretty damn constricted, at least in crowded urban areas.

无线带宽，虽然看不见，但也不是一种无穷无尽的资源。事实上，无线带宽异常之小，至少在拥挤的城市中如此。

To radically simplify it, an easy way to think about spectrum is kind of like a highway, divided into lanes. In the US, the FCC designates who and what's allowed to travel in each lane. (Check out the FCC's spectrum dashboard to see who owns what spectrum where.) The FCC typically divides the spectrum into "blocks" (stick with the mixed metaphor here) that are 10- or 20-MHz wide (so a carrier would get, say, a slice from 700MHZ to 710MHz). A standard configuration is for a carrier is to use half of each block to send a signal, and half to receive (outbound and inbound traffic). Each lane/block can only carry so much traffic. So when you get a ton of people pumping a ton of stuff over the airwaves in a small area, you run into issues.

简 单地说，你可以把带宽看作分了多条车道的高速公路。在美国，由联邦电信委员（FCC）会来决定谁以及什么数据可以在哪条“通道”上“行驶”。（你可以看看 FCC 的带宽记录，了解谁在哪里拥有哪条“通道”。）FCC 一般把带宽分为许多 10 或者 20 兆赫兹（所以一家运营商会获得比如 700 兆赫兹到 710 兆赫兹的范围）的“块”（在这方面 FCC 又一次使用交通作比喻）。运营商的一种标准配置是使用一半的块来发送信号，另一半接收（即下行和上传通道）。每条通道或者块就只能负载这么大的数据量。所 以当一个小范围的无线网络中有一群人发送了一堆数据时，问题就来了。

The solution, though, is not simply to build more cell towers for a given frequency ad infinitum—it doesn't actually create more wireless spectrum in the universe for signals to travel on, and in fact, if you crowd too many towers too close together, you get bunch of noise and interference. Basically, you don't paint extra lines on a freeway in order to make way for more cars.

不过，光是无穷无尽地建造更多频率固定的信号发射塔是不能解决问题的——这并不能真正在空间中创造更大的无线带宽供数据传输，而且实际上，如果你把太多的塔凑得太近，就会产生很大的噪音干扰和冲突。简单地说，你在高速公路上多画几条线也不能让更多的车同时开过。

The best solution, from a carrier perspective, is to get more spectrum allotted from the FCC. Typically the rights have to be purchased for billions of dollars, as you might've noticed during the frenzied devouring of the 700MHz block by AT&T and Verizon for their upcoming 4G LTE networks. The thing about 4G is that is uses really fat channels—really wide lanes—which is why they can transfer data really fast.

从 运营商的角度来说，最好的解决方法是从 FCC 那里分得更多的频率范围。一般这些频率的使用权需要花费上亿美元，所以你可能注意到 AT&T 和 Verizon 为了他们即将发布的 4G LTE 网络正在疯狂地压榨 700 兆赫兹的块。要说 4G 网络那可真是非常宽裕的带宽——通道确实很宽——这就是为什么它们确实可以快速传输数据。

As a side note, not all spectrum is the same: If you remember your high-school physics, lower frequencies travel farther with less energy, and penetrate buildings better too. As such, they are better suited for sharing massive amounts of wireless data, hence the popularity of the 700MHz block, for carriers who generally deal with spectrum from 1700MHz to 2100MHz.

另 外需要提到，并不是所有的带宽都是相同的：如果你还记得你的高中物理的话，其中提到较低频率的电波传播所需的能量更低，也更容易穿过建筑物。正因如此，它 们更适用于传输大量的无线数据，这也导致了通常使用 1700 至 2100 兆赫兹的运营商们如此钟情于 700 兆赫兹的块。

No matter how many FCC auctions, limited spectrum availability for carriers dealing with a data tsunami is going to continue to be an issue—Clearwire says a 120MHz-wide slice of contiguous spectrum is what's needed for legit mobile broadband. The wireless industry association CTIA says the whole industry needs about 800MHz of spectrum total, as opposed to around 400MHz currently allotted. That's why part of the FCC's national broadband plan is to reallocate 300MHz more for mobile broadband in the next five years.

不 管 FCC 拍卖了多少频率，运营商可以使用的带宽是有限的，面对将来的数据海啸还是捉襟见肘——Clearwire 认为 120 兆赫兹的连续带宽才能满足一般移动宽带的需求。无线产业协会 CTIA 认为整个产业总体需要大约 800 兆赫兹的带宽，而现在只有 400 兆。这就是之所以 FCC 的全国宽带计划目标定于在接下来的 5 年内未无线宽带腾出 300 兆赫兹。

Need More Backhaul, Dude

老兄，我需要更好的回程

Next up is backhaul, which is basically the connection between cell towers and the rest of the network. Even if a carrier had a virtually infinite amount of spectrum to carry all of that data back and forth between phones and towers in a fantasy world with exceptional signal strength and no interference, they'd still need fat pipes running from each and every cell tower.

下一个问题是回程，这基本上是指发射塔和网络的其余部分之间的连接。即使在一个信号强度异常强大、没有干扰的理想世界中的运营商拥有理论上无限的带宽来将数据在手机和发射塔之间来回传送，他们仍然需要在每个发射塔之间设置充裕的通道。

Without decent backhaul, cell towers will run into the same kind of congestion problems you run into at home when you're trying to torrent more than your internet connection can handle. Everything slows down, and it sucks. The problem is that a huge portion of the cell towers in the country are still connected using slow copper lines, and running fiber backhaul to them is expensive. (No carrier will reveal how much of their backhaul is actually fiber for competitive reasons.)

如 果没有足够的回程，发射塔就会陷入堵塞问题，就像你在家中用 BT 下载把网络连接的可用带宽撑满了一样。所有的东西都变得缓慢了，这太糟糕了。问题在于整个国家中很大一部分的发射塔仍然使用慢速铜制网线连接，而如果使用 光缆费用就太高了。（由于竞争的关系，没有一家运营商愿意透露他们有多少回程使用了光缆。）

There is also the option of a wireless microwave backhaul, but it requires cell towers that are in the same line of sight, and at some point the data still has to go wired.

我们也可以选择无线微波回程，但这必须保持发射塔在同一条视线上，而且在一些情况下数据必须通过有线网络传输。

The Phones, They Are Puny

手机们，你们太弱了

The final constraint on delivering streaming video over the air? The phones themselves. Sure, the chips inside of them may technically support wireless broadband speeds of 3.6 or 7.2Mbps, or even faster, but actual speeds tend to be about half of their theoretical maximum, in part because running full blaze would kill their batteries that much faster. And remember those phones with the antennas you had to yank out? Tech may have gotten better, but those antennas went away for mostly cosmetic reasons—we'd be better off with big old metal wands sticking up out of our Droids and iPhones.

无 线传输流媒体视频的最后一个问题是什么？是手机本身。当然，手机里的芯片在技术上是支持 3.6 Mbps、7.2 Mbps 甚至更快的无线宽带，但是实际速度趋近于它们理论最大值的一半，部分原因在于全速运行会让电池消耗得更快。你还记得那些需要你把天线猛拽出来的手机吗？技 术虽然是进步了，但这些天线主要是因为美观的考虑才被移除的——如果有一根老式的粗长天线从我们的 Droid 和 iPhone 中伸出来的话，网速会更快些。

Let's Talk About the Video

说说视频吧

Okay, so let's talk a bit about the actual video, and the ways it gets it to you. As it turns out, there are actually special standards in place for mobile video that are a bit different from the more familiar standards for the general internet, since they're designed specifically for phones. But, with phones getting better and better at handling the real web—since they're just very personal computers—a shift is happening, so that mobile standards are more like the real internet.

好 吧，现在谈谈视频本身，以及它被发送给你的过程。实际上，对于移动视频有一些特别的标准，因为它们是专为手机设计的，所以和我们比较熟悉的通用网络标准有 所不同。不过，随着手机发展，能够越来越好地处理通用网络——因为它们和个人电脑越来越相似了——改变正在进行之中，因此移动标准和互联网越来越像了。

Some of the most standard, um, standards are defined by the 3GPP and 3GPP2—the 3rd Generation Partnership Project (roughly, GSM stuff, so in the USA, T-Mobile and AT&T) and 3rd Generation Partnership Project 2 (roughly, CDMA stuff, concerning Verizon and Sprint)—which lays out standards and specifications for telecommunications things, including mobile phone multimedia. In fact, they even have specified container formats for audio and video (the file candy coatings around the codec center, like h.264 or MPEG-4 or MP3) called 3GP (defined by the 3GPP for GSM phones) and 3G2 (designed by the 3GPP2 for CDMA phones) that most 3G phones can play.

大 部分标准中，啊，标准是由 3GPP 和 3GPP2——第三代合作伙伴项目（总的来说就是 GSM，在美国也就是 T-Mobile 和 AT&T）和第三代合作伙伴项目 2（总的来说就是 CDMA，与 Verizon 和 Sprint 有关）所定义——它们发布通讯的标准和规格，包括移动电话多媒体。实际上，它们甚至还规定了音频和视频的格式（包裹在例如 h.264 或者 MPEG-4 及 MP3 的编码核心外的文件外衣），叫作 3GP（由供 GSM 手机使用的 3GPP 定义）和 3G2（由供 CDMA 手机使用的 3GPP2 定义），这些格式大部分 3G 手机都能播放。

More importantly, though, is the 3GPP's specification for a packet-switched streaming service—explained in great detail here (PDF)—and its protocols like Real Time Streaming Protocol and the Real Time Transfer Protocol (RTSP and RTP), which define one of the major frameworks used to stream video to mobile phones. The main thing to know about is that they're designed to be global standards, and they're built to adapt to wildly varying network conditions, adjusting bitrates on the fly. And if a carrier or service is serving 3GPP video to mobile phones, they need special servers to do it.

不 过，更重要的是 3GPP 对封装交换的流媒体服务和它的协议，比如定义了手机流媒体视频的一种主流架构的 Real Time 流媒体协议和 Real Time 传输协议（RTSP 和 RTP），而作出的规格——你可以从这个 PDF 文件中了解详情。其中最重要的一点是它们被设计为国际标准，并且被建立成能够广泛适应各种网络环境、在运行中调节比特率。如果运营商或者服务要为移动电话 提供 3GPP 视频，它们就需要特殊的服务来实现它。

But because of wirelessly connected netbooks and devices like the iPad, more and more video comes over mobile networks in the form of good ol' HTTP. The hyper-text transfer protocol—the basis of all web browsers—comes in two general flavors. There's non-adaptive HTTP streaming, which is incredibly simple, just a stream pumped out at a given bitrate, no matter what the network conditions. Trouble is, it's so simple, you're apt to see plenty of stutters and freeze-ups if your network connection suddenly goes south.

但 是由于上网本和类似 iPad 的设备的无线连接，越来越多的视频以 HTTP 的形式在移动网络中传播。超文本传输协议——所有网络浏览器的基础——主要有两种类型。其中一种是非自适应 HTTP 流媒体，它及其简单，就是不管网络状态如何都以固定的比特率传输的一段流。问题在于，它太简单了，如果你的网络连接突然变差，那视频就常常变得断断续续， 时常停滞。

HTTP adaptive streaming is what it sounds like, a smarter take on HTTP streaming that it adapts in real time to network conditions, switching to different bitrates depending on what the current bandwidth situation is like. It doesn't require a special server, either. It's actually what Apple uses as its standard for streaming video over-the-air to the iPhone and iPad. (They call it HTTP live streaming.) Microsoft has its own spin, called smooth streaming.

HTTP 可适应性流媒体正如其名，是个更有效的 HTTP 流媒体形式，它能实时适应网络环境，按照当前的带宽状态变换不同的比特率。它也不需要一个特别的服务器。苹果在 iPhone 和 iPad 上使用的无线流媒体视频标准正是它。（他们管它叫 HTTP live 流媒体。）微软有他自己的叫法，smooth 流媒体。

A quick word on codecs. Just like much of the internet has anointed h.264 as the standard for internet video, so goes mobile video. Verizon and SlingPlayer, for instance, both currently use WMV for their streams, but without promising anything, Verizon says "h.264 looks like it's got a lot of promise," while Sling says they're definitely moving to h.264 (as they already have on the iPhone), since it's a "tighter codec" that'll help them use a little bit less bandwidth and support hardware accelerated decoding. Of course, Apple's HTTP live streaming only supports h.264, so if you use an app that streams video over 3G, it's coming via HTTP live streaming, and it's encoded in h.264.

简 单说明一下编码。就像大部分因特网都将 h.264 作为网络视频标准一样，移动视频也是如此。例如 Verizon 和 SlingPlayer 当前都是用 WMV 作为他们的流媒体格式，但是这并不能说明什么，Verizon 称“h.264 看上去有不错的前景，”而 Sling 称他们一定会转移到 h.264（就像他们已经在 iPhone 上做的那样），因为它有着“更紧密的编码”，得以节约一点带宽，并且支持硬件加速解码。当然，苹果的 HTTP live 流媒体只支持 h.264，所以如果你的应用程序使用了 3G 的流媒体，那么它就是通过 HTTP live 流媒体，并且以 h.264 编码的。

(In case you were curious, most of the 3GPP video is encoded in h.263 MPEG-4, with AMR audio, but it's gradually shifting to h.264, too.)

（如果你很感兴趣，大部分 3GPP 视频都是以 h.263 MPEG-4 以及 AMR 音频编码的，但是它们现在也逐渐在转移到 h.264 了。）

An Alternative Approach to Streaming

流媒体的两种选择

So, there are two broad approaches to get video to you—unicasting and multicasting. Unicasting is what I mostly described above, and what you're probably most familiar with, actually. When you look at a YouTube video, pull up a Netflix stream, watch a video on a site, or any kind of standard internet video, it's probably unicast—it's going to you on demand, from start to finish. Multicast, on the other hand, is basically broadcasting—it's being pumped out there continuously for any number of people to pick up. It works best for live events, like news or sports, but if you don't jump in at the start of an event, you'll miss something.

那么，现在有两种获取视频的主流方式——单播和多播。单播是指我在上文提到的，实际上也可能是你最熟悉的方式。当你观看 YouTube、Netflix 流媒体、某个网站的视频，或者任何一种网络视频，它都极可能是单播的——它按照从开始到结尾的顺序发送给你。而另一方面，多播基本上是一种广播——它持续 不断地发布给任意数量的用户。它在直播中非常有用，例如新闻或者体育比赛，但是如果你没有从一开始就收看节目，那你就会错过之前的部分。
  

The most prolific of the multicasters in the US is Qualcomm's MediaFLO, which exists as a separate service, and is offered through Verizon's VCAST and AT&T's Mobile TV. It requires specific phones with MediaFLO-support, since they need the MediaFLO receiver and decoder chipset. The basic flow, if you will, is that Qualcomm takes content from a broadcaster, sends it out to its own national network of broadcast towers, and phones can tune in—just like broadcast TV, but beamed using the FLO protocol.

在 美国，多播应用最广泛的是 Qualcomm 的 MediaFLO，它作为一个独立的服务，通过 Verizon 的 VCAST 和 AT&T 的移动电视发布。它需要支持 MediaFLO 的特定手机，因为它们需要 MediaFLO 接收器和解码器芯片。简单的流程是 Qualcomm 从广播中获取内容，发送到它自己的全国广播塔网络中，而手机可以调频接收——就像是广播电视，不过通过的是 FLO 协议。

The advantage of multicasting is that it's extremely scalable: For crazy live events—say, the Super Bowl or World Cup—it's no more demanding to serve video to a million phones than it is to ten thousand phones. And since MediaFLO uses Qualcomm's own setup, it takes strain off of the main cell network for the carriers. That's why Qualcomm sees MediaFLO as complementary to the growing availability of streaming on-demand internet video.

多 播的一大优势在于它极高的可调整性：例如播放超级杯或者世界杯这样的令人疯狂的赛事，给一百万手机播放并不比给一万台播放需要更多的系统资源。而且由于 MediaFLO 使用 Qualcomm 自己的设备，它将为运营商分担手机网络的压力。这就是 Qualcomm 将 MediaFLO 视为不断增长的网络视频流媒体的一大补足的原因。

The Current State of Video

目前的视频状态

What kind of video are we actually getting today, anyway? Does it actually look decent? Well, here's a brief assessment.

归根结底我们现在在观看的视频究竟是什么类型的呢？它的显示效果如何？下面是大体评估。
  

Verizon's VCAST service adapts to the device—meaning they have to encode a single video several times at varying quality levels—so at the top end, a phone like the HTC Incredible would get a stream at around 256kbps and 15fps and Verizon is exploring going higher, up to 400kbps with 30fps. AT&T still prefers 3GPP video, since most of its phones support it, streamed at bitrates between 64-200kbps. Qualcomm's MediaFLO broadcasts a single stream of 320x240 stream. And Apple's HTTP live streaming specs for cell networks—which every video streaming app for iPhone and iPad has to use, from Netflix to Sling—run from 64kbps to 240kbps. All of them are a long, long way from HD.

Verizon 的 VCAST 服务可以适应于不同的设备——这表示他们必须将同一部视频以不同的质量等级多次编码——所以到最后，比如 HTC Incredible 会收到一段约为 256kbps、15fps 的流媒体，而 Verizon 正向着更高的目标前进，高达 400kbps、30fps。AT&T 依然偏好比特率在 64-200kbps 内的 3GPP 流媒体视频，因为它的大部分手机都支持它。Qualcomm 的 MediaFLO 只播放 320x240 的流媒体。而苹果为手机网络特制的 HTTP live 流媒体——每个 iPhone 和 iPad 的应用程序都必须使用它，不管是 Netflix 还是 Sling——使用 64kbps 至 240kbps 的比特率。所有的编码都离高清相去甚远。

Netflix over 3G is actually impressively watchable today. But with 4G networks—LTE and WiMax—and new devices with faster, more energy efficient processors, a near-future where we're all streaming near-HD video anywhere and everywhere isn't so far away, if you squint hard enough.

说实话，3G 的 Netflix 视频的现实效果已经不错了。但是有了 4G 网络——LTE 和 WiMax——和有着更快、更节能的处理器的新设备，在不远的未来，我们就能在任何地点观看接近高清质量的流媒体了。

You know, unless we really do explode the internet.

你懂的，除非我们真的把因特网玩爆了。