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SIP Forks to Provide Remote Call Control


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SIP Featured Article

August 19, 2008

SIP Forks to Provide Remote Call Control

By Tom Cross, Technology Columnist

See the animated tutorial at:
Here’s how it might work: using RCC-Remote Call Control, an OC-Office Communicator client is utilized for Presence/IM and the OC softphone is used to control an existing PBX (News - Alert) phone. For example, a user checks the presence for someone via Office Communicator and then clicking on that user to call — but then having their PBX desk-phone call the number (and use that device).   Remote Call Control can also be deployed in conjunction with Dual-Forking using the “dual” calling of simultaneous or sequential ringing feature of “forking” to call the phone(s).

First Party Call Control
Traditional telephony POTS, SIP and OCS are designed to provide for first party or first person call control. Examples of first party call control are:
  • Make/receive or accept/answer incoming or outgoing calls
  • Make another call to another user
  • Conference both the callers
Third Party Call Control
Third party or third person call control is where another element, endpoint, server, telephone or device is involved in the call. Third party call control may mean that the endpoints share call control with another device such as a PBX, ACD-Automatic Call Distributor, CO-Central Office Switch or other device. The third party device such as a server may direct, redirect (fork) or disconnect the call.
SIP Forking Explained
See the animated tutorial for details.
Parallel Forking is where the proxy forwards copies of the request to multiple destinations simultaneously. This is simultaneous ringing where the phone rings your work, home, cell and any other phone at the same time. Forking is the process of processing multiple requests such as desktop, softphone and cell phones or finding the first-available or other call center rules and the proxy is called a Forking Proxy. A stateless SIP proxy will act as a simple forwarding process - ex: Ethernet switch. A stateful SIP proxy will review, route, fork, and modify the SIP header thus staying involved with the SIP dialog until BYE - ex: a Firewall.
Sequential Forking is where the proxy forwards copies of the request to one target at a time and waits for a final response (or failure) before moving to the next address. This is sequential ringing where your work phone rings three times, then forks to your cell phone for three rings, then your home phone for three times and then forks to voice mail.
This presentation is included in online/onsite courses SIP Planning Guide and for OCS-101 Office Communications Server per person (volume and site license discounts available).  For more information, go to:
For customizing, special discounts, website animations, technical/sales training, technical writing and other services, go to or please call Tom Cross (News - Alert) at 303-594-1694 or [email protected] 
This tutorial is also included in TMC (News - Alert) University special course on Microsoft OCS-Office Communications Server and Response Point at For more go here:

Mark your calendars for Internet Telephony Conference & EXPO — the biggest and most comprehensive IP communications event of the year. ITEXPO (News - Alert) will take place in Los Angeles, California, September 16-18, 2008, featuring three valuable days of exhibits, conferences, and networking opportunities you can’t afford to miss. Register now!

Tom Cross, who has three decades of startup and consulting experience, writes the CrossTalk column for TMCnet. To read more of Tom�s articles, please visit his columnist page.

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