Dialogic Global Call IP Benutzerhandbuch

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Dialogic Corporation

The Dialogic

®

 Global Call API functions that supports IP call transfer are described in the Global 

Call API Library Reference; protocol-specific information about the individual call transfer APIs 
can be found in the subsections of 

. General information on implementing call transfer can be found in 

, on page 310, and SIP-specific details can be found in 

The following topics describe the call transfer capabilities provided when using the SIP call 
transfer supplementary service: 

 

 

 

SIP call transfer uses the REFER method (with NOTIFY support) to reroute a call (a SIP dialog) 
after the call has been established; in other words, after two endpoints have an established media 
path. 

There are two fundamental types of call transfer:

Unattended transfer, which is referred to as “blind transfer” in most other technologies and 
protocols. In this type of transfer the transferring party (called the Transferor in SIP) has a call 
(or SIP dialog) with the transferred party (called the Transferee in SIP) but not with the 
transferred-to party (called the Transfer Target in SIP).

Attended transfer, which is referred to as “supervised transfer” in most other technologies and 
protocols. In this type of transfer, the Transferor has a dialog with both the Transferee and the 
Transfer Target. 

In its simplest terms, a SIP call transfer involves the Transferor issuing a REFER to the Transferee 
to cause the Transferee to issue an INVITE to the Transfer Target. The Transferee and Transfer 
Target negotiate the media without regard to the media that had been negotiated between the 
Transferor and the Transferee, just as if the Transferee had initiated the INVITE on its own. 

Once a transfer request is accepted by the Transferee, the Transferor is not allowed to send another 
transfer request to the Transferee. Only if a transfer request is rejected or fails is the Transferor 
allowed to attempt another transfer request to Transferee. 

The disposition of the media streams between the Transferor and the Transferee is not altered by 
the REFER method. A successful REFER transaction does not terminate the session between the 
Transferor and the Transferee; if those parties wish to terminate their session, they must do so with 
a subsequent BYE request.