Subject: Re: CPL Features

From: Tom Gray <[email-masked]>
Date: Thu, 13 May 1999 14:51:55 -0700 (PDT)

--- Jonathan Lennox <[email-masked]> wrote:
> On Wed, May 12 1999, "Tom Gray" wrote to
> "[email-masked]" saying:
> 
> > Has anyone considered what would be the practical 
> maximum feature set

...

> > Since the point of the CPL is to allow features to
> be assembled out of CPL
> scripts, I don't think that a "maximum feature set"
> is really possible; the
> whole point of the CPL is that it's extensible.  The
> feature interaction
> problem is discussed somewhat in the CPL
> requirements I-D.  (This has
> expired from the I-D archive, by now, but it's
> available off my web page --
> <http://www.cs.columbia.edu/~lennox/>).  I'm
> planning on issuing an updated
> version of this document soon, so any comments on it
> are appreciated.
>  

The issue as I see it is the practical maximum extensibility of the
system. This would be the number and type of features that could be
made to work in a determinsitic manner. This is the feature
ineteraction problem and any call control system should be assessed
against its ability to handle this problem. This is especially true in
a system in which it is intended that the end user will be able to add
features at will. I suppose what I am saying is that the feature set
which will be enabled or made possible by CPL  should be identified. It
is not clear to me that some useful features of current systems are
possible with the SIP/CPL combination. This may be only the refelction
of  my lack of familiarity with this system. I have read the
specifications for CPL but the connection to SIP and the rationale of
how features are created do not seem to be spelled out clearly in these
documents.

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---

From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 13 May 1999 21:42:23 -0400

Tom Gray wrote:
> possible with the SIP/CPL combination. 

CPL definitely does not handle certain features - call waiting, for
example. In particular, features which require user input after the call
is established, or features which require call state, are not possible.
In an IP telephony context, many, if not all, of these features are most
easily supported in end systems. CPL is best at providing call routing
related services; the infinite flavors of forwarding, follow-me,
screening, and so on. These are services which require network support,
often since the user may not be logged in or otherwise available. I
don't think we can enumerate them all. If we could, may as well number
them, do away with CPL, and have users select a service by number. But,
after all, CPL is a scripting language of sorts, and the point is we can
use it to develop new services (at least those supportable within the
primitives CPL provides. Its not Turing complete, so everything is not
possible).

This more restricted scope helps mitigate many feature interaction
problems. There are some we know of (the call forward/call screening
interaction), and these are documented in the framework. If you know of
any other interactions you see, please post them.

It would also be helpful if you could point out particular features you
feel are useful yet not supported by CPL.

-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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