[Asterisk-Users] Re: Grandstream transfer into outer space

Stephen R. Besch
Thu, 26 Feb 2004 14:14:13 -0000

Olle E. Johansson wrote:
> Going back to the subject, what does the grandstream really do,
> SIP-wise, when you press
> the transfer button?
>

Olle,

The following is an exact transcription of the description given in the
BT101 manual for Blind Transfers:


4.3.7 Call Transfer The user can transfer an active call to a third
phone by using the ?Transfer? button. The sequence is like this: The
user presses the ?Transfer? button and if the other voice channel is
available (i.e., there is no other active conversation besides the
current one), he/she will hear a dial tone. He/She can then dial the 3rd
phone and then hangs up his own phone. 2 kinds of blind call transfers
are supported: using REFER and using BYE/Also. The SIP message flow
based on SIP REFER method looks something like this:

Call Flow Diagram For Blind Call Transfer:

  From Transferee to Transferor

	 INVITE ->
	<-100/180/200
	 ACK ->
	<- RTP Media ->
	<- REFER
	 202 ->
	 NOTIFY ->
	<- 200
	<- BYE
	 200 ->

 From Transferee to Recipient

	INVITE ->
	<-  100/180/200
	 ACK ->
	<- RTP Media ->


The SIP message flow based on BYE/Also  method looks something like this:

 From Transferee to Transferor

  	INVITE ->
	<- 100/180/200
	ACK ->
	<- RTP Media ->
	<- REFER
	501 Not Implemented ->
	<- BYE with ?Also:?
	200 ->

 From Transferee to Recipient
	INVITE ->
	<- 100/180/200
	ACK ->
	<- RTP Media ->

I have no idea if this is accurate, I just copied it and replaced the
arrows indicating direction with "->" and "<-". You
can download the
manual itself from the GS web site.

Stephen R. Besch