difficulty within the UCLA proposal "Generic Uneven Level Protect ion(ULP)"

[Baese Gero]

Dear authors,

While studying your last ITU-proposal APC-1905, "Generic Uneven Level
Protection (ULP)"
for Annex I of H.323, which had been submitted for the Portland meeting in
August, we
discovered a mismatch between one of the major features stated and the
simple example given in Figure 1.

On page 3, you state in section "Protection procedure" that
"In order to achieve more efficient usage of the channel bandwidth, the
protection
levels need to be grouped in such a way that when data in one packet
protected
by level p are recoverable, all the data of the same packet protected by
level below p
are recoverable."

We definitely agree on this feature, since we are both interested in
protecting any type
of progressive or prioritized media stream most efficiently!


However, when considering your example in Figure 1 on the next page, we
found
a simple loss situation, for which the above statement does not seem to hold
anymore:
Let us assume that out of the seven given packets, exactly (information)
packets #2 and #3
are lost, but all the others have been received correctly. Since packets
#1-#3 are only
protected with one single FEC packet, ULP #1, at level L_0, the leftmost
part of the
lost packets #2 and #3 can definitely not be recovered anymore.

If we now consider the protection strategy for level L_1, we see that the
second part of
packets #1, #2, #4, and #5 are protected via the respective part of FEC
packet, ULP #2. But
only the part belonging to packet #2 has been lost (packet #3 is not related
to ULP #2),
which should definitely be recoverable then.
Thus, we would be able to overcome the loss in packet #2 at protection level
L_1, but
not at level L_0, which is a contradiction to the above statement.



Can you please tell us (and possibly all other interested participants), if
this conclusion
is correct, or if we have misunderstood something?

Although Figure 1 only contains a simple example, we think that obeying the
above statement
at each step while constructing a suitable and effective protection strategy
can be
quite difficult. If you think that this is not the case at all for your
proposal, it
would be probably best to specify your coding principle in more detail, such
that all participants of the group can evaluate how complex the choice of a
good
protection scheme can be.



Thanks a lot in advance,
Gero Baese
Guenther Liebl

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