Re: [rohc] RFC4815 no TS bits

Carl Knutsson <carl.knutsson@effnet.com> Thu, 04 February 2010 09:02 UTC

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Date: Thu, 04 Feb 2010 09:57:11 +0100
From: Carl Knutsson <carl.knutsson@effnet.com>
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To: Jer-ming Lin <cs202050@csie.ncu.edu.tw>
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Cc: rohc@ietf.org
Subject: Re: [rohc] RFC4815 no TS bits
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Jer-ming,

Jer-ming Lin wrote:
> Dear ROHCer,
> 
>     In the RFC4815 4.2 "When no TS bits are received in the compressed
> header, the scaled TS value is reconstructed assuming a linear
> extrapolation from the SN, i.e., delta_TS = delta_SN * default-slope,
> where delta_SN and delta_TS are both signed integers."
> 
>     Does that mean the decompressor must receive at least one scaled TS
> before get no-TS bits? How about absolute TS or unscaled TS? 

No, at minimum it must have received TS_STRIDE (default-slope in this
text) and 32 bits (unscaled) TS, for example an in IR packet.

>                                                               Since the
> decompressor can derive the scaled TS directly when it gets the unscaled
> TS, the compressor can send several unscaled TS to establish the
> TS_STRIDE and the relationship of delta_TS. Is that right?
> 

No, the compressor must first establish the TS_STRIDE by sending it
as-is together with unscaled TS. From those two values, TS_SCALED and
TS_OFFSET can be derived and used for later for compression.

cheers,

/Calle

[rohc] RFC4815 no TS bits Jer-ming Lin
Re: [rohc] RFC4815 no TS bits Carl Knutsson
Re: [rohc] RFC4815 no TS bits Jer-ming Lin