Dear SIP Implementors,
I am aware that today T.38 (for Fax) is negotiated in mid-call, i.e. after an
audio call is established we can have a re-INVITE or an UPDATE (in SIP) to
negotiate Fax capabilities (T.38) depending on the fax tone.
SBC
INVITE (audio only) ----->|
100 Trying <-----|
|-----> INVITE (audio only)
|<----- 100 Trying
|<----- 180 Ringing
|<----- 200 OK (audio only)
|-----> ACK
200 OK (audio only) <-----|
ACK ----->|
<----- RTP Flowing, Fax tone detected ----->
re-INVITE (T.38 fax) ----->|
100 Trying <-----|
|-----> re-INVITE (T.38 fax)
|<----- 100 Trying
|<----- 200 OK (T.38 fax)
|-----> ACK
200 OK (T.38 fax) <-----|
ACK <-----|
<----- Fax Stream ----->
What my query is that if there is any implementation that requires T.38 in the
initial call itself? (Say, for example, any Fax endpoint that supports T.38 in
the initial call?)
Call flow, for example:
SBC
INVITE (T.38 fax) ----->|
|-----> INVITE (T.38 fax)
|<----- 200 OK (T.38 fax)
|-----> ACK
200 OK (T.38 fax) <-----|
ACK ----->|
<----- Fax Stream ----->
I am asking this to know if a support of this sort is required on the Session
Border Controller (SBC). And if yes, then what is the mechanism that is used
(because there cannot be any fax tone to start off with).
Your ideas and any pointers on this will be really helpful.
Thanks and Regards,
Saurav
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