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How does comcast on demand work

how does comcast on demand work

How does Comcast's Video Distribution Network Work?

The Master Headend or does their bit in aggregating and sending the video signals in MPEG format across the CRAN network. From the cran it goes to the various local headends via Multicast UDP through routers. Finally the local hub picks up the signal and converts it to RF to shoot out over the network. There is also a VOD server farm, which could reside in it's own building or in a regional headend depending on the local archetecture either built by Comcast, or inherited from acquisitions. VOD has been known to spread apart to other larger headends in the region for redundancy. There are various hubs spread out along the plant house the OOB communications equiptment and the VOD (and potentially SDV) edge qams.

There are digital controllers that address all of the set top boxes in the area's. For Scientific Atlanta sites there's a *NIX based server called the DNCS, while Motorola sites there is a server called the DAC. While DNCS's and DAC's pretty much do the exact same job, the way they do it on a SA plant vs Motorola plant are ENTIRELY different. Motorola tends to break things out a bit more with NC1500's, RPD's, etc. While SA tends to centralize things a bit more within it's DNCS/Appserv setup.


Headend - The control center

of a cable-television system, where incoming signals are amplified, converted, processed, and combined into a common cable for transmission to subscribers.

CRAN - Comcast's Reginal Area Network

Multicast - network technology for the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the multiple destinations split.

UDP - A communications protocol that is mostly used to send streamed material over a Network

VOD - Video On Demand

SDV - Switched Digital Video - A method of broadcasting only channels that are currently tuned, rather than every channel offered at once.

DNCS -Digital Network Control System

DAC - Digital Addressable Controller

SA - Scientific Atlanta (now owned by Cisco).

NC1500 - Motorola device that links application servers and set-tops using Internet Protocol (IP) to deliver data packets through the out-of-band data channel.

RPD - Motorola's Return Path Demodulator for interpreting commands from the Motorola Cable Box back to the Cable Headend.

OOB - Out of Band communication. Method used to program cable boxes remotely, while allowing their tuners to be on any channel.

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