[opendtv] Re: WiFi Supplanting Broadcasting? Get Real!
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 18 May 2004 18:15:37 -0400
I had written something that might be misinterpreted.
> Come now. First of all, the 50 to 100 Mb/s are raw bit rates. The
> two-way MAC costs you at least half of that bit rate, making spectral
> efficiency much less than it is for DVB or ATSC.
What I mean by this is *NOT* that the 54 Mb/s of IEEE 802.11g
(for example) is somehow divided up 50 percent for upstream and
50 percent downstream, even if that concept would make any sense
in Wi-Fi. What I meant was that the maximum actual bit rate you
can cram through a Wi-Fi net is no more than half the advertized
"raw" bit rate. Whereas the maximum bit rate you can cram
through an Ethernet LAN is more like 98 percent of the raw
signaling rate. And ATSC or DVB-T are even more efficient, if
you measure the bit rates in the same way.
This is because the medium access control protocol designed
into Wi-Fi had to take into consideration the considerable
vagaries of the wireless links, so that upper layer protocols
(e.g. HTTP over TCP/IP) could operate over Wi-Fi more or less
the same as they operate over wired networks. This does not
come for free. So one ought to wonder, if you want to broadcast
TV content on a different frequency band, why you wouldn't at
least consider doing so with reasonable efficiency.
Bert
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