Think About 'Linear IP Video'9/11/2012 12:54 PM Eastern
Language gets weird. Even what seems obvious can quickly muddle. I’m reminded of a panel about over-the-top video, where a representative of that community (OK, it was Roku) made this polite, explanatory remark about the types of video the device can stream: “We call them ‘channels.’ ”
The audience was nearly 100% cable people. I couldn’t resist this retort: “We do too!”
Language gets especially weird around seemingly obvious stuff in tech. Here’s a recent example: “Linear IP video,” to describe what we nowadays call live, broadcast television. Channel by channel, show by show, linear TV — but delivered via Internet protocol. So, in that sense, the term “linear” was teased out a bit, to mean “broadcast.”
So far, so good. Except for one important technical distinction: By definition, video delivered via a cable modem — in IP — isn’t broadcast. It’s switched.
Broadcast is one to many. One channel, one “stream,” sent over the air or over a wire, to millions of receiving TVs.
Stuff that goes through the headend part of the cable modem — the CMTS, for cable-modem termination system — is inherently switched. Session-based. Clicking on a link to watch a YouTube instantiates a stream between them and your screen. One to one.
The tech name for this (perhaps predictably) is “unicast.” Unicast is fine, but a bear on bandwidth. Imagine if we all streamed the Super Bowl as a unicast session. Hundreds of millions of people, all asking for the same thing — shipped stream by stream, not in bulk. Tough on the pipes.
Happily, people are already tackling this conundrum. They call it “multicast,” which is essentially the IP version of what we now call “broadcast.”
With multicast, you do the streaming equivalent of flipping up the flag on your (physical) mailbox. Then you “join” that stream, along with anyone else in your node who may be streaming it.
Switching is not a new one on cable. VOD was the first example — you request a movie, the VOD server sets up a session with you and only you. Next, switched digital video (SDV) technologies emerged as a way to recycle available digital bandwidth.
Does this linguistic gymnastics matter? Not really. Television will continue to steamroll onto different screens, served by different networks in different ways. But the next time you hear “linear IP video,” you can at the least make a snappy remark about how it’s technically not linear, because it’s switched. So there.