[hylafax-devel] Getting Fax over the GSM network

[Eric Huaume <eric.huaume@art.alcatel.fr>]

Hello,

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).

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

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.

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).

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.

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) ?

I would be grateful for your answer.

Eric Huaume

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