Monday, 7 April 2008

A brief explanation of a headend and why its so important

Most cable companies have a DMC that stores all the movies, up and down links content and provides monitoring on input and out put. Each geographical region they have a headend. The old analogue headends were quite simple affairs, now they are massive complex installations.

The number of channels, number of concurrent VOD streams and number of internet connections that a cable network can provide is dependent on the number of multiplexers. The multiplexers live in the headend (a bit simplistic, but generally true). The Conditional Access (CA) system also lives in the headend.

The headend is the most expensive part of the network, it is often more expensive than all the boxes that it serves.


Steve Nimmons said...

I’ve built many headends in my time, even one for the Colombian government. If we ignore the actual network and uplinks (e.g. if satellite based), my breakdown would be as follows. Maybe you could expand a post on each?

First we need video source whether as analogue feed or digital feed (e.g. in terms of pre-digitised VoD content)
Next we need compression and digitisation (assuming we start with analogue source). I’ve seen everything from decent 6Mb/s compressed video to some shocking services running 256Kb audio (in stereo taking 512)
So out pops, (what was in old money MPEG-2) compressed content with iFrames and pFrames
From there we take multiple feeds into the MUX where a transport stream is composed of what I consider to be spliced frames from each MPEG stream
Next the SI Generator feeds in the specific tables required by the decoders, e.g. NIT, EIT, CAT, SDT (a discussion on each would be interesting)
EMMs and ECMs go into the MUX and hence transport stream (generated by the Conditional Access system). They are passing keys to the encryptor to ensure elements of the transport stream go out encrypted
Once we have the transport stream ready to go it is a matter of modulating this onto the delivery network. The main types are of course QPSK and QAM, the former associated with satellite and the latter with cable. It would be interesting to expand and contrast the differences in headend technology for Cable, Satellite, MMDS, LMDS, Internet, Terrestrial etc.
I’ll not go into it here, but a discussion on what happens at the decoder (e.g. set top box) would also be interesting (e.g. how it reads the tables in the transport stream to locate service information and build EPG)
As you say, the headend is a very complex part of a DTV network, what technologies would you use (or have used) for monitoring, alerting, recovery etc.). Have you ever looked at dynamic multiplexing for efficient bandwidth usage?
My knowledge on VoD is about 8 years old, I played around with nCube servers and it would be really interesting to hear more about the current state of the art and market
I could talk for hours about DVB Simulcrypt and maybe it would be worth expanding on how Conditional Access systems can be run ‘in parallel’
A complex topic with low end set top boxes was always over the air code downloads. To what extent do you think this has improved with more modern box designs with greater memory and processing capacity?

Jeremy Chu said...

Interestingly Steve, almost everything that you describe (from your 'old' knowledge of 8 years ago) is completely relevant today!
The main difference would be that the headends are now becoming IP-based towards the middle and only at the ends come out in the broadcast formats that you know and love (for example, ASI).

The real question that needs to be asked is: Yes, the STB/broadcast world moves slowly but why?

As a sidenote as well: DVB isn't the only broadcast standard - don't forget ATSC and DSS...

Elliott - a little more

London, United Kingdom
I am an architect with shed loads of familiarity in providing high profile consumer media, products and services. I conceive ideas, design and lead projects to create new consumer products. I love brainstorming ideas with marketing counterparts and creating future facing and innovative solutions. I have been responsible for high volume mass consumer market features where scale, reliability and the ability to quickly respond are of crucial importance.