[e2e] Stupid Question: Why are missing ACKs not considered as indicator for congestion?

Detlef Bosau detlef.bosau at web.de
Thu Feb 1 12:54:29 PST 2007

Hi Francesco,

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
- when.

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.

More information about the end2end-interest mailing list