Re: BER, FEC, channel access and packet sizes
- To: email@example.com
- Subject: Re: BER, FEC, channel access and packet sizes
- From: Phil Karn <firstname.lastname@example.org>
- Date: Sat, 18 Mar 1995 15:25:41 -0800
- Cc: email@example.com
- In-reply-to: <199503161705.AA165183501@hpsadr2.sr.hp.com> (message from Glenn El
> 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
(.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.