[VOIPSEC] 4G Issue Map: signaling complexity - more

[Geoff Devine]

Try doing true 1A2 key system emulation, the full ACD feature set, and a true attendant console for a large-ish corporation rather than a toy receptionist workstation.  You invariably end up with sets of features that work with groups of users and features that put multiple line apperances on a single device.  If you fully distribute it to the endpoint devices, the complexity and scaling problems explode on you.  Invariably, you end up with nasty race conditions and avalanches of distributed status messages.  I've gone down this path a couple of times in the 80's and once in the 90's.  It's possible to build it but it's hard to keep it stable as you add features to it.  As I said, every new feature ends up being a protocol design exercise with nasty consequences if you get it wrong.  I think a model where these kinds of group features are handled by common control is a lot easier to build and manage.  There are a number of IP PBX implementations that take this approach where the end device doesn't do most of the group-oriented features.  That way, you don't need an architect-level person to design every feature.

Geoff


-----Original Message-----
From: Henry Sinnreich [mailto:hsinnrei at adobe.com]
Sent: Thu 8/24/2006 8:34 PM
To: Geoff Devine; Paul E. Jones; Brian Rosen; bill at flanagan-consulting.com
Cc: Voipsec at voipsa.org
Subject: RE: [VOIPSEC] 4G Issue Map:  signaling complexity - more
 
Geoff Divine writes:

> I don´t see that the fully distributed call processing model 
> is workable in the general case.

This is an interesting point, since the most useful telephony applications can be implemented with SIP call control in the endpoints:
- IVR 
- Auto-attendant
- Receptionist workstation
- Contact center agent workstation.
So even for core telephony, all this maze of servers is not required.

What do you think?

Henry

-----Original Message-----
From: Geoff Devine [mailto:gdevine at cedarpointcom.com] 
Sent: Thursday, August 24, 2006 6:25 PM
To: Henry Sinnreich; Paul E. Jones; Brian Rosen; bill at flanagan-consulting.com
Cc: Voipsec at voipsa.org
Subject: RE: [VOIPSEC] 4G Issue Map: signaling complexity - more

Henry Sinnreich writes:
> Such complexity is better placed in the endpoints, the only
> ones that understand the application and that can be
> developed in a controlled environment.
 
Right.  ...and what that means is any time you need to add a feature, you need to extend the protocol. When you extend the protocol, you need to ensure interoperability with potentially dozens of different client implementations and potentially dozens of different software revisions of each implementation.  Go look at what TISPAN or the PacketCable Residential SIP Telephony Spec (after the IPR review period expires next month) have done.  Unless you vendor lock on one client implementation, it will be wildly difficult to ever make the network stable.  The cell phone service providers can limit the damage by controlling the number of implementations and testing the heck out of everything.  They´ve also adopted a model where as many features as possible are done by the core network.  I don´t see that the fully distributed call processing model is workable in the general case.
 
Geoff