Subject: Re: SUGGESTED ARQ TEXT Advice for Internet Subnetwork Designers ID

From: Phil Karn <[email-masked]>
Date: Thu, 5 Jul 2001 14:07:02 -0700 (PDT)

>Decoding may utilize some approximation of "physical" quantities.
>Soft decision, for example, may approximate received information using
>Q levels, with Q > 2, which is then fed to a decoder. The decoder output
>would be binary as usual.

>Thus, soft decisions result in a representation that no
>"upper" layer would want to use. So in your terminology (if I am
>interpreting your text above correctly), the decoder would belong
>to the physical layer.

One important form of soft decoding is erasure decoding, where your
bits (or more often, groups of bits) either arrive reliably or not at
all.

The erasure indications usually come from lower layer codes that are
strong enough to detect channel errors but not strong enough to
actually correct them. So they signal erasures to an upper layer code
that fixes them.

CDs work this way, with two layers of RS codes. While the lower layer
code (the "inner code" in coding parlance) can fix a few errors, by
itself it is pretty weak. Its main job is to mark erasures to the
upper ("outer") code. Thanks to the interleaving between the layers,
the combination is considerably stronger than the individual pieces.

TCP can also be seen to operate in this way. The lower layer code is
the TCP checksum and the upper layer is the TCP ARQ mechanism.

ARQ is TCP's method of choice to recover from these erasures, but it
is not the only way to do it; see the various FEC-based multicast
protocols that have been proposed.

Phil