Subject: RE: Capability terminology

From: "Paul Smeulders" <[email-masked]>
Date: Fri, 13 Nov 1998 16:36:17 -0500

On Friday, November 13, 1998 2:50 PM, Dan Wing [SMTP:[email-masked]] wrote:
> On Fri, 13 Nov 1998, Graham Klyne wrote:
> 
> > I do not see the sender disclosing any of its capabilities to the
recipient
> > in G3 fax.  
> 
> Doesn't the DIS go one directiona and the DCS the other direction
> (sender->recipient and recipient->sender)?  I'm not a fax person and my
> copy of T.30 is at home (of course) and I'm at work.  I've certainly
> been wrong before and am likely to be wrong again.

No need to be so hard on yourself...I think you are partly right, but the
whole picture isn't there. Perhaps "capabilities exchange" is a bit
misleading to those who use a strict definition of the latter term. 

Hopefully this hasn't been mentioned before...

I have always understood the DIS/DCS "exchange" as follows:

In the case of calling terminal wishes to transmit:

DIS is issued by the callED terminal, and expresses its capabilities. The
callER listens, and selects those capabilities which it has in common with
the called terminal. The caller decides on the capabilities it wishes to use
(or CAN use), and encodes these in the DCS reply. Now both "know" which
features will be used. The called side is only aware of the capabilities of
the caller which match its own advertised capabilities. If the sender is
"more capable", the receiver is unaware of it. Thus, it is not a complete
"exchange" in the strictest sense of the term.

Take it further, if both advertised their full capability set, there would
need to be a decision phase to select which features would be used, or a
standard algorithm where both sides are guaranteed to select the same
outcome. That doesn't happen, thankfully. As described, the decision is made
by the sender, knowing the constraints placed on it by the receiver, so it
always scores. The last bit of trouble is deciding if the channel can handle
the first selected baud rate (encoded in the DCS), and the training follows.

Now, there is a case where a full "exchange" occurs, but part of it is
ignored. 

When the caller wishes to receive a fax:
As usual, the called side gives DIS, but the caller ignores it, and replies
with DTC, saying "I want to receive, and here are MY capabilities". Now the
called side makes the decision based on the DTC-encoded capabilities, and
issues the DCS, selecting the features for the upcoming transmission. From
then on, the call looks like it was originated by the called side (issues
training etc etc.).

Does my description clear it up?
Do I need to be cleared up?

> 
> [Graham Klyne}
> > Anyway, do you wish to propose an alternative definition for "capability
> > exchange"?
> 
> Your original definition is fine, but I'd just like an additional sentence
> or embedded phrase which clarifies that an "exchange" it isn't necessarily
> a bi-directional exchange (per the dictionary definition of the word
> "exchange").
> 
> -Dan Wing

"Capabilities agreement" - not descriptive enough
"Capabilities negotiation" - inaccurate, perhaps will meet with same
resistance as "exchange" has.
"Capabilities-matching agreement" - descriptive but cumbersome
"Receiver constrained, transmitter-centric capabilities decision" - out of
control

"Capabilities-matching phase (or transaction)" - I actually like that one! 

Paul Smeulders
Network Edge Technologies
Nortel Networks
Ottawa, Canada.

---

From: Dan Wing <[email-masked]>
Date: Fri, 13 Nov 1998 14:56:00 -0800 (PST)

On Fri, 13 Nov 1998, Paul Smeulders wrote:

[...]
> Does my description clear it up?

Yes, thanks.

> "Capabilities-matching phase (or transaction)" - I actually like that one! 

"Remote Capabilities Aquisition" ?

-Dan Wing