[e2e] Stupid Question: Why are missing ACKs not considered as indicator for congestion?
detlef.bosau at web.de
Thu Feb 1 12:54:29 PST 2007
Francesco Vacirca wrote:
> Some link layers use a strongest FEC to protect header.
I heard about this and it is somewhat confusing. Particularly as I find
it difficult to imagine that a sender switches between different
convolutional codes within an IP packet. However, your post perhaps
gives me a clue:
> E.g. in some UMTS coding scheme the link layer employs a 1/3
> codification for RLC header, whereas the payload can use a different
> scheme (e.g. from 4/5 to 1/3)...
Are these really different coding schemes? Or is it the same
convolutional code but differently punctured? So in fact, you start with
a hardly punctured frame, thus a Viterbi decoder would hopefully produce
only little bit errors, and afterwards (after the header) your frame is
punctured more severely?
Or do you, an alternate approach, use differently punctured RLP frames
for an IP packet´s header and tail?
> Maybe it could be applied also to TCP. Note that this can decrease the
> goodput in case of non lossy links... obviously it depends on the
> ratio between useful bits and transmitted bits.
I heard of it in the context of VoIP over mobile wireless networks.
(To tell my honest opinion on this one: That´s a hoax even not worth
wasting a word on it.)
> In the 802.11 standard some part of the packet (MAC header) is sent
> with a different rate to be more protected against channel impairments
> and also for compatibility purposes. A cross layer approach could
> adopt low rate also for TCP header (also IP obviously)... but I do not
> think that the benefits are more than disadvantages.
I think the question is: What´s the problem for this solution?
WRT VoIP the mess is clear: For a voice stream, a media stream in
general, you need three parts of information.
You need to know
- what to be played out
- where and
In TDM, "where" and "when" are cared for by the scheduler and so you´re
even free to accept errors in the "what".
In VoIP over packet switching the "where" and the "when" suffers the
same errors like all other data.
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