On 10/30/14 2:11 AM, Mahudeswaran A wrote:
Hello All,
Is it possible to transfer a call without using SIP REFER.
Call path:
[uac]-------[sip proxy]--------[uas]
The SIP REFER is not supported in the above sip proxy. Is there a way to
achieve call transfer without using sip refer...
It depends on what you mean. There are some ways that work in restricted
situations, and with limitations.
For the moment, consider only attended transfers - Alice first calls
Bob, then Alice calls Charlie, then Alice wants to get out of the call
and leave Bob talking to Charlie. Some possibilities:
1) Alice does reinvites to Bob and Charlie to broker an offer/answer
between them so that they end up sending media directly to one another.
Alice stays in the signaling path but not the media path.
2) Alice stays in the signaling path and media, relaying the media
between Bob & Charlie.
3) There is an intermediary in the call path (you called it a proxy, but
it is really a B2BUA.). Alice uses REFER, but the intermediary
intercepts it, changes it into reinvites. This isn't totally without REFER.
4) similar to (3), but REFER isn't used. In this case there must be some
form of non-standard signaling between Alice and the intermediary to
indicate the desire to transfer.
5) a variant of (4): the UA representing Alice isn't really the device
on her desk. Rather it is a B2BUA (what is somethings called a SIP PBX.)
The device on Alice's desk may be a SIP UA, but it is treated as a dumb
slave device controlled by the B2BUA. Feature signaling from Alice's
device to the B2BUA may be propretary, or might be DTMF over SIP. The
B2BUA can then use reinvite to handle transfers.
Thanks,
Paul
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