I’m Johnny Cache

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generate so many groanertwists
on clichés: The
cache cow. Cache in your
chips. His mouth is writing
checks that his body can’t
cache. (OK, OK, I’ll stop.)

Nonetheless, in the
language set of the
content-delivery network
— the CDN — “caches”
are a big part of the scene.

The purpose of a cache, in a CDN, is
to temporarily store the most popularly
viewed content so that it’s readily available
to lots of people using lots of different
devices (not just the television). It’s
all part of this shift toward distributed
storage — gigantic servers in the center,
linked to regional servers, linked to caching
serves at the edge (headend or hub).

Caching is buffering. You’ve seen it
dozens of times, especially when video
streaming first began. It’s the little animated
circle that spins around on the
screen when you’re watching something
over the Internet. Caching reloads more
bits to your screen. As cable operators
continue architecting their CDNs this summer,
caches matter for local content —
stuff that can’t easily be encoded nationally,
like cable channels — HGTV, ESPN,
Discovery. Big cities can host more than
100 local TV stations. The MSO encoding
500 national channels at a centralized
facility (the “origin server,” in CDN speak),
and 100 channels locally, may need
to cache a million or more file chunks,
switched out several times per hour.

An HDTV title, for instance, may get
compressed and encoded into eight different
stream sizes, ranging from 1 Megabit
per second up to 10 Megabits per second,
to be adaptively streamed to suit the
different screen sizes at the end points —
the more bandwidth, the bigger the chunk.

As a direct result, caches can fill up
real fast, especially if you’re a service
provider tasked with serving up movies
and TV at scale, to millions of viewers on
dozens of different screen sizes.

Part of the caching equation is figuring
out what can and can’t be cached. Some
stuff doesn’t lend itself well to caching,
like file segments that are “stateful” —
meaning they’re tied to something you’re
doing, like pausing to resume in another
room on a different screen.

As a European cable technologist reminded
me last week, all operators delivering
VOD already have a CDN, or the beginnings
of it. The difference: When VOD
began, it was cheaper to store the same
title once, at hundreds of edge points,
than it was to store everything centrally,
then ship it out over satellite or fiber.

These days, both storage and transport
is cheap, comparatively. So why
not store the hot stuff at the edge,
as needed, and keep everything else
deeper in the network, to stream as

If you’re a cable operator planning your
CDN, chances are high that a big part
of the discussion is edge caching: How
many servers, at what size, where, with
what local encoding to handle off-airs, encryption
and ad insertion. It’s the perennial
tradeoff between the economics of
storage and transport.

Cache out.

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