Subject: why one level of signalling

From: Sanjay Nayak <[email-masked]>
Date: Fri, 28 May 1999 09:23:24 -0700

I am interested in know why there is one-level of signalling between the
SIP clients .In the PSTN there is two level of signalling. One is
between the end-terminal to the local switch and othere one is between
the switch-to -switch.

-sanjay

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From: Mark Handley <[email-masked]>
Date: Fri, 28 May 1999 10:41:34 -0700

>between the end-terminal to the local switch and othere one is between

Actually a better question is why do you need more than one level of
signalling in the PSTN?  The primary answer is that phones are dumb
and switches are smart.  With IP end-systems, this smart/dumb
distinction is reversed - end-systems are smart and routers are
comparatively dumb.

Thus to make a regular IP-based call, you don't need any smarts from
the network.  One end system directly calls another end-system, and
all the routers do is move packet around.  There's only fate-sharing
between the two end-systems - any of the intervening routers can lose
state, and (pending route convergence) the call stays up.

In practice though, sometimes you want intervening proxies because you
want to do access control, resource reservation, user location, and
other such services.  However you don't want to design an architecture
that mandates a distinction between calling a proxy and calling an
end-system.  An example is where my IP telephone can redirect calls to
my cellphone when I'm not home, and perform access control on these
redirected calls.  In this case it's a proxy, but if I was home it'd
be a telephone again.

Does this help explain a little of the thinking behind the SIP
architecture?  

Cheers,
	Mark

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From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 01 Jun 1999 12:49:10 +0300

Hi,

> redirected calls.  In this case it's a proxy, but if I was home it'd

Let's take this example:
A is PSTN phone, B is SIP Phone, C is PSTN phone.

A calls B. B's phone must ring and there must be some time supervision
somewhere (typically 30 seconds). After timeout, the call must be set up
from B to C. When C answers, charging must take place - A should be
charged for the A-B leg, and B charged for the B-C leg.

Should your SIP phone take care of this?
Shouldn't the SIP server (a network entity) take care of all these
things? If this was the case, we would have a situation similar to
"local switches" today, where to provide certain services, you require
intelligence in the network.

Best regards,

Gonzalo
-- 
Gonzalo Camarillo         Phone :  +358  9 299 33 71
Oy L M Ericsson Ab        Mobile:  +358 40 702 35 35
Telecom R&D               Fax   :  +358  9 299 31 18
FIN-02420 Jorvas          Email :  [email-masked]
Finland

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From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 01 Jun 1999 10:11:40 -0400

> charged for the A-B leg, and B charged for the B-C leg.

I'm not sure what you are trying to accomplish here. Are you talking
about a call forward no answer service? That is, B forwards the call to
C when B is not there? If so, there wouldn't be a leg from B to C,
rather, B would have A contact C directly.

-Jonathan R.

-- 
Jonathan D. Rosenberg                       Lucent Technologies
Member of Technical Staff                   101 Crawfords Corner Rd.
High Speed Networks Research                Holmdel, NJ 07733
FAX:   (732) 834-5379                       Rm. 4C-526
EMAIL: [email-masked]
URL: http://www.cs.columbia.edu/~jdrosen

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From: Gonzalo Camarillo <[email-masked]>
Date: Wed, 02 Jun 1999 09:59:57 +0300

Hi,

[email-masked] wrote:
> about a call forward no answer service?

Yes...

> That is, B forwards the call to
> C when B is not there?

Yes...

> If so, there wouldn't be a leg from B to C,

Yes, that's the way SIP works, and it sounds very sensible from a
technological point of view. I was trying to highlight the charging
problem.

If somebody is calling me to my fixed phone in Finland, he knows that he
will pay a call to Finland (the phone begins with +358... ). It does not
matter if the phone is diverted to my mobile and I am in Spain. I will
pay the leg Finland-Spain since I am receiving that call.

If I had a SIP phone, as you pointed out, the caller would pay a call to
Spain, when he thought that he was going to pay a call to Finland.

How does this kind of charging have to be implemented using SIP?

Henry, how does MCIWorldcom intend to do this?

Thanks,

Gonzalo