User's Manual
V34bis Fax-modem - AJ 2885 P
AJ 2885 P - Page 27
Modem speed can be controlled thanks to the following commands.
Asynchronous and Synchronous modes
: modem can provide a synchronous or
asynchronous interface to the terminal
Synchronous mode
: DTE speed is the same as the DCE speed, i.e. the terminal
speed might change upon connection from configuration speed.
Asynchronous mode
: DTE speed does usually not depend on DCE speed, it is a
constant rate interface. The terminal speed does not change from configuration speed
upon connection. The opposite applies on two configuration cases :
(1) in direct mode : *M0 command
(2) in disabling the constant DTE modem : *C0 command
Autobaud
: this features provides :
(1) an automatic detection of the terminal data format and speed when receiving AT
commands,
(2) an automatic adaptation of the connection (DCE) speed to the speed lower or equal
to the terminal one as to avoid its overflow.
V32Bis and V34
: these modes of connection support the same speedfrom 4.8 to 14.4 KBPS.
The @OM command selects the desired mode which is especially suitable for leased line
connection.
Modulation – DCE speed
: the @M command selects the connection (DCE) speed when the
Autobaud feature is off (parameter different from @A1)
Buffering and flow control
: in asynchronous mode and especially when the DCE speed is
lower than the DTE speed, modem can buffer data received from the terminal in order to adapt
the data rate to a lower speed. The AJ2885 modem provides a buffer of about 1000 characters.
Modem can run a flow control process when this buffer is about to get full in order to stop the
sending terminal.. The same buffer and feature is available in the modem to terminal direction.
Buffers and flow control are available in normal mode (*M1) and with error correction and data
compression (*M2 and
M3). On the other hand, modem is a transparent channel in direct
mode (
M0) and in synchronous mode (&M1) : buffering and flow control are then not possible in
both modes.
The flowing picture summarizes the action of these commands depending on their parameters.
This assumes that the last AT commands were sent to the modem at a speed of 38400 BPS,
and were :
AT@An
and then eventually
AT@Mn
:
