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Bert Manfredi, a frequent poster on the OpenDTV mailing list, recently posted the following datacasting Q&A to the list. I am copying it here with his permission. I thought it was very good. Thanks Bert.



Q. What is data broadcast?


A. It is a means of transferring data files (maybe executible files,

maybe even video or audio content in non-real-time) through the one-way,

broadcast channel. A good example is the way software updates are sent

to satellite TV receivers. They are transmitted in the background, one

way only, and install themselves automatically in the receiver. The same

can be achieved with terrestrial TV.


Q. Is data broadcast interactive?


A. No. However, it can be used to transmit ineractive _files_ to a

storage device, such as a hard drive on a PC or a PVR.


But the sort of interactive experience the consumer would have cannot be

compared with browsing the web. It is instead comparable to browsing an

interactive DVD, or playing an interactive game on a CD-ROM. One is

limited to interacting with content in that file. People have plenty of

experience now with interactive CDs and DVDs, so pretending that this

interactivity is more than what it is can only be counterproductive.


Q. Can data broadcast be compared with a very fast broadband Internet

connection?


A. No. Because with a broadband ISP connection, the fast pipe belongs to

the user. The content is content the user requested, potentially from

sites anywhere in the world. In data broadcast, the content is selected

by someone else. You can take it or leave it, just like watching TV. The

user's choice is limited to switching channels among a restricted set of

possibilities, just like watching TV.


Q. Can the DTV spectrum be used for real interactive sessions? Why can't

I just add "a backchannel" and turn this into a two-way connection?


A. Well, sure. But that's a bit like asking whether a regular old Chevy

Cavalier can be raced at Indianapolis. In principle, yes. In practice,

(a) the rules will probably not allow this, and even more importantly

(b) the end result would be very unimpressive.


You could allow broadcasters to transmit packets that are addressed to

individual hosts. These are called "unicast" packets, and are mandatory

for "real" interactive IP sessions. Also mandatory is a return path,

which in principle could be provided by a telephone connection.


This sort of link has been hyped up beyond reason or belief, so let's

discuss it briefly.


Let's compare a DTV channel used as downlink, plus phone uplink, with

ADSL. Assume the entire 20 Mb/s of the DTV channel is dedicated to

individual two-way sessions. This DTV channel can support only 2 1/2

full speed ADSL users, or 20 lower speed ADSL users (about 1 Mb/s

downstream each). Surely, no one can find this to be good use of a

public resource. When one considers that the DTV station will not

dedicate 100 percent of its spectrum to individual interactive accounts,

the absurdity becomes even more plain.


In principle, of course, the entire model can be changed, to make this

idea work. You can create very small cells, with very low power

transmitters, where perhaps a 20 user limit makes some sense.


Hey, guess what, that's called cellular telephony. Sure, you can assign

the TV spectrum to cellular services. But then it's not DTV anymore.
 
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