The Language of CDNs


Lots of follow-on questions in the mail about content-delivery networks, starting with this plum from reader Dan: “I’ve seen my company put in fiber rings, regional fiber rings and, more recently, a fiber backbone. How is that different than a CDN?”

Also this, from reader Chris: “Ingest, multicast, transcoding, adaptive streaming, MPEG DASH - the CDN jargon is intense. Help.”

Welcome to the trove of terms that describe what it takes to get video content to Internetconnected screens, beyond and including the television. Laptops, tablets, game consoles, PCs, smart phones.

Let’s start with you, Dan. If fiber is the physical conduit over which IP (Internet protocol) video packets flow, CDN is everything else: how those packets are collected, stored and packaged for receipt by all the screens we’re watching.

A brief history of CDNs: In one sense, they’re the older sibling of the technologies of video-ondemand. Remember back when VOD meant a few thousand hours of storage, mostly movies? These days, operators are gearing up for 20,000 or more hours of storage, for episodic TV and movies.

That storage happens hierarchically, in CDNs - one or two big library servers (think “long tail”), with caching servers closer to consumers for popularly viewed titles.

Remember “pitchers and catchers” as the way to move video assets from source to destination? CDNs change that. Instead of pitching up to satellite and catching down on the ground, CDNs use fiber backbones, linked to regional fiber rings and linked to hybrid-fiber coax, to move content.

CDNs are in vogue right now because of the desire to use them for live and linear content, too, especially for channels that are nationally available (as in, not local broadcasters).

Which brings us to your laundry list of CDN curiosities, Chris. “Ingest,” as the name implies, is the process of feeding titles into the hierarchical storage. “Multicast” optimizes bandwidth for delivery - you want to see something, you put the flag up on your mailbox, so to speak. So does everyone else who wants to see it. The show moves down the CDN once, then you join the stream.

“Transcoding” formats video streams for receipt by the varying screen sizes and resolutions available - what goes to tablet doesn’t need to be as large as what goes to an HDTV, for instance.

Adaptive streaming, or “fragmented MPEG-4,” is the slicing of a piece of content into different sizes. It’s a way to suit what’s best for the end screen, as a function of available bandwidth - if there’s not enough, then downshift to a smaller slice.

The “DASH” part of “MPEG DASH” stands for “Dynamic Adaptive Streaming over HTTP,” and is a hopefully harmonized way for content owners and service providers to stripe content for display on different screens.

That’s a quick look at CDN lingo. It’s a big part of the whole transition to IP (Internet protocol) video, and likely a big topic of engine-room talk for the foreseeable future.

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