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TCP-group 1995


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Re: BER, FEC, channel access and packet sizes



>  Current wisdom among some of the wireless communications professionals
>seems to be that expecting requiring significantly better than .1% - 1%
>*bit* errors from the physical layer for mobile links is not effective

That's a high error rate all right. Let's see...at .1% BER, the
probability of getting a 256-byte packet through would be

(.999)^(256*8) = .129 or 12.9%. So I guess we're already doing pretty good.

The usual rule of thumb for ARQ protocols is that the packet loss rate
should be kept below 1%, i.e., 99% of the packets should get
through. For a 256-byte packet, that means the bit success rate must
exceed

        (.99) ^ (1/2048) = .99999509

which corresponds to a bit error rate < 4.9e-6.

>From one of the references I have, a (64,40) Reed-Solomon code over
GF(64) could provide this performance on a hard-decision channel with
1% BER. That is, even at this high error rate you'd need only a third more
bits for error correcting overhead. That's a lot better than hammering
away with uncoded frames, hoping you'll get lucky.

Phil






Document URL : http://www.a00.de/tcpgroup/1995/msg00218.php
Ralf D. Kloth, Ludwigsburg, DE (QRQ.software). < hostmaster at a00.de > [don't send spam]
Created 2005-01-02. Last modified 2005-01-02. Your visit 2020-10-21 13:02.35. Page created in 0.0215 sec.
 
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