The Language of CDNs

8/01/2011 12:01 AM Eastern

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

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

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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