Subject: RE: [Sipping] Re: SIP/SDP dictionary for SigComp

From: [email-masked]
Date: Tue, 29 Oct 2002 17:17:33 -0600

> My misreading was largely based on cues I picked up from the
> following passage:
> 
>  "...if the static dictionary is not mandatory, some form of
>   dictionary needs to be uploaded to boost compression ratio.
>   That introduce overhead on the wire (or wireless)."
> 
> For most compression algorithms, this "dictionary" is built out
> of a history of past messages. There is no need for an explicit
> upload of strings that you expect to use in future messages.
> Your contention that building such a dictionary imposes additional
> over-the-air load  would imply that you think otherwise. Is this
> just another case of my misunderstanding what you're saying?

No, your understanding is correct this time. Re the "explicitly upload 
of strings", I'm not sure it is not needed. You may want to upload
a dictionary that is not based on current message but contains some
strings expected to appear in future messages. That said, I do agree 
with you that the history approach does not introduce overhead on the 
wire, assuming that the compressor does not rely on explicit ACKs from 
decompressor to synchronize history.

However, a mandatory static dictionary is handy for a simple 
implementation. There is no synchronization issues at all.

Now back to the memory issue. I guess your concern is the cost of ROM, 
not RAM, since either history or explicit upload approach costs RAM anyway.
I'm wondering in your implementation, how much ROM is needed to store 
SigComp code itself, and why storing static dictionary push it over the
limit.

BR,
Zhigang
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---

From: "Miguel A. Garcia" <[email-masked]>
Date: Wed, 30 Oct 2002 08:56:56 +0200

I agree with Zhigang. Considering the size of the dictionary, 
which is below 8 kb, I wonder how 8kb will decide whether the 
dictionary is implemented or not, considering that you will need
quite a lot of more memory for the SIP stack, the SigComp stack,
and other similar stacks.

I think that the dictionary should be used in the first compressed
message. Actually, this is the reason why we created the dictionary,
because further messages will most likely use previous ones as
reference states to compress. Therefore, I would not agree with a
solution that implies that the dictionary is not used in the first
SIP compressed message.

Having said that, I would like to understand, and if deemed document,
the procedures that Sigcomp offers to guess if a dictionary is 
implemented in the decompressor prior for the compressor to use it.
Help from the Sigcomp guys would be benefitial.

So, providing that Sigcomp can advertise the availability of the 
dictionary in the decompressor, and providing that we document 
(perhasp through a reference) the procedures to find it out, I would
not have a problem to relax the "MUST implement the dictionary" to
a "SHOULD implement the dictionary".

/Miguel

-- 
Miguel-Angel Garcia                     Oy LM Ericsson AB
                                        Jorvas, Finland
mailto:[email-masked]     Phone:  +358 9 299 3553
mailto:[email-masked]        Mobile: +358 40 5140002
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