[hylafax-devel] Re: Getting Fax over the GSM network

[Dmitry Bely <dbely@mail.ru>]

Eric Huaume <eric.huaume@art.alcatel.fr> writes:

> I am involved in a Fax Class 1 implementation developpment over the GSM
> network.
> 
> The configuration is as follows :
> 
> - a linux PC with hylax4.1beta2 connected to a mobile phone with a
> serial line,
> - the hylafax software is in charge of the Fax Class 2 protocol (T.30
> recommendation),
> - the DCE is a mobile phone in charge of the Fax Class 1 protocol over
> the GSM network (3.45 recommendation),
> - the communication between these two entities is performed by the way
> of AT commands (T.31 recommendation).

T.31 defines Class 1.0 standard, while Hylafax supports only Class 1
(TIA/EIA-578). These standards are *different*.

> So, hylafax sends the CSI hdlc frame (23 bytes) and then the DIS hdlc
> frame (10 bytes) to the mobile phone for network emission.

Apparently you are describing receiving session. Wouldn't it be better to
show the logs?

> The mobile phone returns back CONNECT after the CSI emission and OK
> after the DIS emission (as required by the T.31 recommendation).
> 
> The hylafax application then sends an AT+FRH command so as to get the
> TSI frame and DCS frame response (25 bytes length and 9 bytes length
> respectively).
> 
> The time delay between the reception of the AT+FRH command and the
> CONNECT response is 1.5 second (the T.31 recommendation specifies that
> this response is returned by the DCE when receiving flags from the
> network).
> 
> The whole TSI frame reception, including the preamble flags, takes 1.71
> second.
> 
> So , the whole delay between the AT+FRH command reception and the
> sending back of the TSI frame received is 3.21 seconds while the hylafax
> application is only waiting for 3.1 seconds.

BTW, T4 timer is adjustable (FaxT4Timer parameter). T.30 defines it as
4.5s +/- 15% for manual units
3.0s +/- 15% for automatic units

> The question I am wondering is about this hylafax timer of 3.1 seconds.
> It starts when emitting the AT+FRH command and stops when receiving the
> first byte of the TSI HDLC frame (0xFF), while the T.30 recommendation
> specifies a T4 timer starting at the command emission and stopping when
> receiving flags (another timer of 3 seconds has then to be started for
> the whole frame reception).

Hylafax does exactly that -- it stops T4 timer when all flags have been
received and the address field (0xFF) have been identified.

Note that in Class 1.0 mode HDLC flags are stripped by the DCE, while in
Class 1 they are passed to DTE, and Hylafax is trying to strip them itself.

> Due to the kind of communication we have on the serial line (by the way
> of the T.31 recommendation), the mobile phone does not return the
> received flags but the CONNECT response instead.

In Class 1 flags are returned *after* CONNECT message from DCE and have
nothing to do with "the kind of communication we have on the serial
line". Possibly your device is working in Class 1.0 mode? Then be ready for 
trouble :-)) 

> So, is the 3.1 second timer really conformant to the T.30 recommendation
> or would it be better conformant if it stopped when receiving the
> CONNECT response instead of stopping when receiving the first effective
> byte of the HDLC frame (0xFF byte) ?

Hope to hear from you soon,
Dmitry



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