Click here for animated tutorial and explanation
Part 2 of 2 - Parallel & Sequential Forking with 1st & 3rd-Party Call Control with RCC
First Party Call Control - traditional telephony POTS, SIP and OCS are designed to provide for first party or first person call control.
Third Party Call Control - 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.
NOTE: The animated tutorial explains the SIP signaling methods and potential "early media" clipping failures.
- Parallel Forking - the proxy forwards copies of the request to multiple destinations simultaneously.
- Sequential Forking - 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.
Key Point - Forking is critical to advanced SIP features such as "find-me follow-me." Critical to this process is ringing to let the caller alert the callee or (called party) of the call. In traditional PSTN-Public Switched Telephone Network communications "early media" refers to ringing and announcements to indicate the status of the call - ringing, busy, fast busy, call redirection, status "you have 12 callers ahead of you." In SIP, the forking process provides "early media" sending specialized ring-tones, audio announcements (e.g. call center status announcements), images or video before SIP session is accepted. However, there is NO common means of providing signaling to the receiver because of different types of hardware, softphones, UI-user interfaces, ringing devices and many other factors. In addition, early media may be onmidirectional or sequential forking, bidirectional or parallel-dual forking.
Early media failures can occur when the callee picks up and the UAS sends a 200 (OK) response with an answer, in parallel with the first media packets. If the first "early media" packets are received by the caller - UAC-User Agent Client, "media clipping (at the beginning of the media sequence)" or "media dipping" (at the end of the media sequence) can occur. This can occur is that the UAC cannot send media until the 200 (OK) response from the UAS arrives. Causes for clipping can be manyfold, however, UAC signaling, packet arrival delays), bandwidth, different SIP "methods" and commands between SIP systems and other factors. In addition, SIP signaling can typically take a different routing path than the media (communications) transmission which can be one of the factors causing media clipping.
"Late media" announcements (such as "will you take a survey" or (click for special offer") occur after the BYE and may have the same problems. Details can be found by reading RFC 3260.
In summary, incompatibility between SIP systems can result in communications chaos.
RCC-Remote Call Control also known as third-party call control is provided by CSTA-Computer Supported Telephony Applications. CSTA was developed by the European Computer Manufacturers Association (ECMA) and subsequently was formally standardized by the ITU-T, incorporating the Switch-to-Computer Applications Interface (SCAI).
CSTA is an OSI protocol stack that provides an open system interface to a PBX-Private Branch eXchange, ACD-Automatic Call Distributor or CO-Centrex central office switching. CSTA uses, among other technologies, SALT-Speech Application Language Tags specification and its SMEX-Simple Messaging Exchange element, telephony call control capabilities in MSS-Microsoft Speech Server to allow a developer to create sophisticated telephony-based speech applications that can exploit both basic call control services such as ANI-Automatic Number Identification (caller ID) and DNIS-Dialed Number Identification Service (800), using the included basic call controls, or extended call control services, to create custom call controls.



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