[SIPForum-discussion] REFER in a call transfer ...

Thu Aug 26 12:12:37 UTC 2010

Murli;

Murali - Got your point. Need a small clarification further. B calls C, C is
in some other call and B goes on hook immediately.  Would A be getting busy
tone ?? And as B has already gone on hook there is no point of retrieving
the call back. A can terminate the dialog with C. Pls. clarify me.

Vivek - I think, its just mistype, B does not call C, in you case B is
calling A and A calls C.

Anyway yes, if transferor (B) goes onhook immediately after performing
transfer activity and dialog between A & B has been terminated, A should
honor the provisional and final responses received from C, and hence tones
should be played by A accordingly.

There is no point to terminate the dialog between A and C since this dialog
would never have been existed due to receipt of final error response (486
Busy). So A should play Busy Tone in that case.

Murali - As far as I know  B receives a NOTIFY only after the transfer is
completed(i.e, after the speech path is established b/n A and C). I don't
see any NOTIFY being sent for alerting. Either it would be Notify with 100
trying or Notify with 200OK. My only concern here is how come B gets Ring
back without receiving the alert messages.

Vivek - I couldn't point any RFC which can give you better idea for NOTIFY
with 180 Ringing or other messages, but what I can surely say that the
protocols just provide the frame-work and it is upto us to innovate ensuring
they are still compliant with the recommendations of protocol.

In our implementation and mapped with your example, B receives NOTIFY with
180 Ringing and hence play RBT. I think this is the best practice and
implemented by other vendors too.

Murali : I guess there are a few phones that are connected trough PAP2 which
still doesn't support REFER.

Vivek -  Are you referring Linksys PAP2 ATA? If yes, I have worked a lot on
this device and it also support REFER method. You need to check the settings
again.

Best Regards,

Vivek Batra

  _____  

From: Murali Vuppandla [mailto:mvuppand at cisco.com] 
Sent: Thursday, August 26, 2010 5:23 PM
To: 'Vivek Batra'; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] REFER in a call transfer ...

Hi Vivek,

For further analysis, here we go:

*     Murli]: There was a typo:-). I do agree with you that it's the
transferor that should be getting the busy tone. But if we observe the
standard call flows, it's transferee( A in our case) that receives 486 busy
response. I'm stuck exactly here... I don't have any manual setup to test
the scenario .. I'm totally relying on automated scripts .. that's the
reason why I'm so concerned about this .. what does 486 busy sent to
Transferee convey (If we go by call flows defined in docs) ?

[Vivek] - Yes, its correct that A receives 486 Busy, but A does not need to
play Busy Tone for this. A should only send NOTIFY (with 486 Busy) to B and
B should immediately retrieve the call back on              receipt of
NOTIFY (with 486 Busy).

            As I mentioned earlier, Playing bust tone to A would lead to
hangup the call by A and B would never be able to retrieve the call back.

Murali - Got your point. Need a small clarification further. B calls C, C is
in some other call and B goes on hook immediately.  Would A be getting busy
tone ?? And as B has already gone on hook there is no point of retrieving
the call back. A can terminate the dialog with C. Pls. clarify me.

*     [Murli]: But if you see the message flow, it's only A that would be
receiving all the alerting messages though you hear the ring back tone at
B(obviously B has to be receiving alerting messages to get a ringback) .I'm
stuck here again. Just need to confirm whether or not B receives any
alerting messages.

             [Vivek]: Yes, again only A receives all provisional/final
response from C, but at the same time A will send each alert in NOTIFY
message to B and hence, B will play tones locally if required.

Murali - As far as I know  B receives a NOTIFY only after the transfer is
completed(i.e, after the speech path is established b/n A and C). I don't
see any NOTIFY being sent for alerting. Either it would be
Notify with 100 trying or Notify with 200OK. My only concern here is how
come B gets Ring back without receiving the alert messages.

*     [Murli]: I was referring to the SIP  phones that don't support REFER
method.

             [Vivek]: Without support of REFER method, how they support Call
Transfer?

             Murali : I guess there are a few phones that are connected
trough PAP2 which still doesn't support REFER.

Regards,

Murali.

From: Vivek Batra [mailto:Vivek.Batra at Matrixcomsec.com] 
Sent: Thursday, August 26, 2010 12:38 PM
To: 'Murali Vuppandla'; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] REFER in a call transfer ...

Murli;

[Murli]: There was a typo:-). I do agree with you that it's the transferor
that should be getting the busy tone. But if we observe the standard call
flows, it's transferee( A in our case) that receives 486 busy response. I'm
stuck exactly here... I don't have any manual setup to test the scenario ..
I'm totally relying on automated scripts .. that's the reason why I'm so
concerned about this .. what does 486 busy sent to Transferee convey (If we
go by call flows defined in docs) ?

[Vivek] - Yes, its correct that A receives 486 Busy, but A does not need to
play Busy Tone for this. A should only send NOTIFY (with 486 Busy) to B and
B should immediately retrieve the call back on receipt of NOTIFY (with 486
Busy).

As I mentioned earlier, Playing bust tone to A would lead to hangup the call
by A and B would never be able to retrieve the call back.

[Murli]: But if you see the message flow, it's only A that would be
receiving all the alerting messages though you hear the ring back tone at
B(obviously B has to be receiving alerting messages to get a ringback) .I'm
stuck here again. Just need to confirm whether or not B receives any
alerting messages.

[Vivek]: Yes, again only A receives all provisional/final response from C,
but at the same time A will send each alert in NOTIFY message to B and
hence, B will play tones locally if required.

[Murli]: Again a typo here. My questions here is if B goes onhook and say C
is not reachable . Does A hear any REORDER tone or any announcement.

[Vivek]: You haven't any choice other than to play reorder tone. You can not
play blank anytime.

[Murli]: I was referring to the SIP  phones that don't support REFER method.

[Vivek]: Without support of REFER method, how they support Call Transfer?

Best Regards,

Vivek Batra 

  _____  

From: Murali Vuppandla [mailto:mvuppand at cisco.com] 
Sent: Thursday, August 26, 2010 11:24 AM
To: 'Vivek Batra'; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] REFER in a call transfer ...

Vivek,

Thanks for your points. Pls. check below my comments in blue. 

Regards,

Murali.

From: Vivek Batra [mailto:Vivek.Batra at Matrixcomsec.com] 
Sent: Thursday, August 26, 2010 10:28 AM
To: 'Murali Vuppandla'; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] REFER in a call transfer ...

Murli,

Comments inline in RED.

Best Regards,

Vivek Batra

  _____  

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Murali Vuppandla
Sent: Wednesday, August 25, 2010 11:55 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] REFER in a call transfer ...

Hi All,

Just need some basic clarifications on SIP REFER in a transfer scenario.

Say there are three SIP phones that support REFER method.

A calls B and B answers the call. B hook flashes and blind transfers the
call to C. Ideally B would  be supplied with the ring back tone, but at the
same time some docs say that when C is in ringing state, A would be hearing
ring back tone.

What is wrong in it?

 I'm in a state of confusion here if it's  A or B that is supplied with Ring
Back tone since that practical implementation of Transfer seems to be a bit
different from the one described there in docs.

If you are talking about traditional PBX, then its more complex. In
traditional legacy PBX, only transferor gets RBT as long as transferor is
off-hook, and transferee gets MoH. As soon as transferor goes on-hook before
transfer target answers the call, transferee now starts getting RBT instead
of MoH.

That is what most of the legacy PBX does.

To add on this, Say when B blind transfers the call to  C and C is busy .
who should be hearing the busy tone(A or B) . if you go by the practical
scenario .. when B blind transfers the call it hears the busy tone . but if
we go by the standards it's A that should be getting busy tone ..

Nops, If u give busy tone to A, A would probably hangup the call. Only B
should get Busy Tone as a part of transfer notification and after short busy
tone, A and B should be connected again.

To summarize the above points .

1)       A calls B.. B blind transfers the call to C . If C is busy with
another call .. who should be getting the busy tone(A or C).

Neither A nor C :-) its B who should get Busy Tone else the Transferor (in
this case its B) will never become to know that Transfer Target was Busy.
You can play Busy Tone for short period of time and then again connects the
call between A & B.

There was a typo:-). I do agree with you that it's the transferor that
should be getting the busy tone. But if we observe the standard call flows,
it's transferee( A in our case) that receives 486 busy response. I'm stuck
exactly here... I don't have any manual setup to test the scenario .. I'm
totally relying on automated scripts .. that's the reason why I'm so
concerned about this .. what does 486 busy sent to Transferee convey (If we
go by call flows defined in docs) ?

2)       A calls B .. B blind transfers the call to C . if C is ringing who
should be getting the ring back tone .

A and B, both should get Ringing Tone. In result, B will become to know that
the transfer activity performed by him is in ringing state. For A, that will
be indication that he has been transferred to some other phone and that
phone is ringing now.

But if you see the message flow, it's only A that would be receiving all the
alerting messages though you hear the ring back tone at B(obviously B has to
be receiving alerting messages to get a ringback) .I'm stuck here again.
Just need to confirm whether or not B receives any alerting messages.

3)       A calls B .. B blind transfers the call to C ..A goes on hook
immediately and If REFER fails .. what has to be the ideal behavior.

If A goes on-hook and has been triggered BYE, dialog at both side viz A and
B should be terminated. 

Again a typo here. My questions here is if B goes onhook and say C is not
reachable . Does A hear any REORDER tone or any announcement.

4)       And does the call transfer in case of Normal phones differ from the
ones that supports REFER .

What you are referring as normal phones?

I was referring to the SIP  phones that don't support REFER method.

Awaiting your responses on this.

Regards,

Murali.

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