On Thursday 09 September 2004 23:38, Rony Bill wrote:
Hello All,
Modulation is used to transport a signal on a carrier. In the earlier days frequencies were low and for sat. transmission they had to be mounted on a carrier. But today the freq. that is used for communication is so high that it is a carrier in itself. Eg. a gigabit LAN passes data in gbytes per second.
The moment you switch some component of a (phase, amplitude, frequency) signal you get side bands which depends on the amount of shift of the components. The center freq is the carrier and the info in the side bands. The info size is fixed by natural principles and named after a gent called Nyquist several decades ago. Now this carrier and side band has to be transported from point a to b thru some physical medium like Cu cable, fibre, or space. The ability of the medium to allow the signal+sideband to pass without distortion determines the final throughput. Incase of satellites, it is atmospheric characteristics, solar flares, antenna size, available power, etc that determines the data throughput. In general the higher the signal bandwidth, the higher the SNR required to extract info. Hence more complex and expensive electronics.
Someone told me, in China they use terabit LAN. Sat. freqs. are also in GHz. Nowadays digital formats in broadcasting have narrowed bandwidth utilisation yet provide high volume content. They are able to cut down on side bands.
Can we have a satt. carrier signal that is a fixed frequency but is made up of a constant data train?
You can. Infact all known wireless data transmissions use exactly this method.
As of now, what is the highest download speed achieved by satt. internet? What is the highest speed currently available in the satt. 'pipes' that big corporates use for international inter-office links ?
Afaik 384MBPS with a 6mtr dish. point to point.That is data from private networks. Dont know if anyone is interested in sending mundane webpages using this extremely expensive channel.
Anyway, this is the technical part which will be looked after by satt. designers and other techies.
What I want to know mainly is whether the concept of having such a ***high speed-high volume-repeatitive-serial file download facility*** is acceptable and worth implementing.
DAB on FM is already here. 250Kbps. Just get rid of the sattellite and things become practical. And adhoc wifi networks are even better when coupled with peer-to-peer stuff like bittorrent. With newer modulation techniques like UWB (1Gbps bw and relatively "simple" recieving frontends) becoming commercially viable "unlimited" bw for the masses is only an antenna away.
Many have mistaken this for another television broadcast. It will be similar but not the same. TV is only live entertainment. This is a normal serious ultra high speed file download system for the common man at
HEHE. all the computers and nerds are working hard on the serious stuff so that everbody can have free time for entertainment. At least for me computers are serious entertainment.
a very low monthly rental, for an *infinite number of users* within the footprint. That is the whole issue. How good and usefull is this idea?
If the Govt. opens up the airwaves to private net operators then the entire 58 (60 - existing 2) terrestrial TV channel spectrum could be utilised for digital serial file broadcasting in a similar
Teletext. Another flop. Currently used in Britian (Afaik). 120bps.
way but at a lower download speed than a satt.. Major companies like Red Hat, Novel, Microsoft and many many others can provide their heavy software files on the airwaves to be downloaded in our homes and offices in a fast and economical way. No wiring, no cable.
Carrier pigeon would be better. The bit error rates at BW more than a few 100 kbps over even short distance of 2 kms is so high that it would take forever to download one iso. And cable internet is what (supposed to) works better.
the higher the signal bandwidth, the higher the SNR required to extract info.
actually it is the other way around - you can trade off SNR for bandwidth and vice versa - Higher the bandwidth and you can go with very low SNR. The SNR of the signal from the Mars Rover is just slightly above white noise - so they use a terahertz bandwidth pipe for communication.
Someone told me, in China they use terabit LAN. Sat. freqs. are also in GHz. Nowadays digital formats in broadcasting have narrowed bandwidth utilisation yet provide high volume content. They are able to cut down on side bands.
terabit lan is actually commonplace in research labs all over the world - but they have yet to reach practical use because of the difficulty involved in the electronic switching at those data rates. Its not a limitation of the transmission channel. Imagine having to buffer about 10 seconds of data at those rates ? what would you write it to ?
cut down on side bands ? they can cut down on the time they need to broadcast each slice. in a typical sharing system, if you cut down the time slice of each broadcast, you will widen the frequency band required to transmit it. with digital broadcast, you can send a lot more information in the 4.7mhz channel space of a typical PAL broadcast channel, and thats the only real advantage - you need to approach shanon's limit for transmission channels, and so far digital broadcast is the best way to get there - even though it is a long way off from the ideal. Some ideas have also been proposed where authors think of using turbo codes for txmission,
What I want to know mainly is whether the concept of having such a ***high speed-high volume-repeatitive-serial file download facility*** is acceptable and worth implementing.
it has been implemented. check out moviebeam - its a relatively unsuccessful service based on the same idea. Recently tivo and netflix teamed up to offer similar service.
to-peer stuff like bittorrent. With newer modulation techniques like
UWB (1Gbps >bw and relatively "simple" recieving frontends) becoming commercially >viable "unlimited" bw for the masses is only an antenna away.
ultrawideband is actually severely limited by distance. UWB is actually the perfect example of trading off bandwidth for SNR. UWB uses a spectrum that is many Ghz wide (not 1Gbps as data rate is not bandwidth) and therefore can support an SNR almost at the level of the surrounding noise. One of the features that most uwb proponents talk about (incorrectly) is the anonymity that this low SNR guarantees. UWB is therefore a very good replacement for the 802.11g that is used to solve in-home connectivity issues - but is hardly something that can be used for public broadcast. data rates are then defined according to shanon limit on how much information you can send in this slice of the spectrum.
As a general comment - with the population density in India, and the amount of fiber put in the ground by reliance, wouldnt fiber based services be the way to go ? nothing can ever beat the data rate capability of fiber. Fiber can support a spectrum width of about 30Thz and that is only a limit imposed by the switching speed of the laser, and not the fiber. In any case, even with 30 thz, you can theoretically transmit all of mumbai's internet, cable and telephone traffic through a single fiber pair, provided it is fully utilized. so if youve laid fiber all over the city, the last mile problem is the only thing youve got to address with video delivery networks. satellites are really overkill for anything today.
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Adil Kodian -
sherlock＠vsnl.com