ISTM that it should be acceptable for a UA that only does fax to offer
only T.38. And a UA that supports T.38 ought to accept such a call. And
or course an SBC should not disrupt such legitimate usage.
But of course there is a difference between what *should* be done and
what is actually observed in the wild. :-(
Thanks,
Paul
On 8/6/12 8:21 AM, Kevin P. Fleming wrote:
> On 08/06/2012 05:03 AM, Saurav Sarkar (sausarka) wrote:
>
>> 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?)
>
> In the Asterisk community we have come across some occasionally, but
> we've usually convinced the vendor to change their requirements.
> However, in these cases they have required *both* T.38 and audio in the
> initial INVITE. I've never seen an endpoint that supported *only* T.38,
> and in the latest T.38 recommendation it is suggested that such
> endpoints should still support INVITEs with audio offers, even if they
> won't generate any audio.
>
>> 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).
>
> It would seem wise for an SBC to not break a session that is setup
> initially with T.38.
>
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