Re: Details of ppp & modem shut-down
<firstname.lastname@example.org> wrote in message
> > [email]email@example.com[/email] writes:
> > ]Netters !
> > ] I get the impression that few people really know the detailed
> > ]shut-down procedure that happens when one 'exits ppp and
> > ]disconnects the modem from the telco-line'.
> > ]By using "ATM2D 3407501" to my modem ( M2 for sound on),
> > ]I can hear when the carrier stops. Perhaps the CD led would
> > ]show this. But carrier TX and Rx are different.
> > ]There is 2 to 3 seconds delay from the 'ppp-off' command until
> > ]the carrier sound stops.
> > ]Apparently the modem must exit the talking-ppp-mode
> > ]into the command-mode, to drop-out the line-relay ?
> > ]Q1. what are the ppp steps/states during exit ?
> > ]Q2. what are the *details* of the modem changing from
> > ]'talking ppp' to command mode ?
> > ]Q3. Are talking-ppp-mode and command-mode 7 or 8 bit ?
> > ]Q4. going the other direction: what tells the modem to exit
> > ]command-mode into talking-ppp-mode ?
> Bill Unruh wrote:[color=green]
> > Most of this is in your modem manual.
> > Usually +++ with a 1 sec guard time before and 1 sec afterwards is used
> > to put the modem into command mode. Then an ATH hangs up the phone.
> > In addition ppp does some housecleaning while it is being shut down ( eg
> > running ip-down script, removing lock files, etc). The modem is never in
> > "talk ppp" mode. It knows nothing about ppp.
> OK, so it's called "active mode".[/color]
The modem is in that mode once it leaves command mode. And it leaves command
mode at the end of the dial command unless the dial command is terminated
with a semi-colon. It will stay in active mode until either DTR is lowered
(hardware control) or the escape sequence is sent by the modem's host
> > It goes into active mode either after it receives the CONNECT signal
> > after training with a modem on the far side
> > or receives and ATA.
> The char-string "ATA" from the local serial port is unambigious, except
> for the speed. Hence logically, at this stage the modem must know
> what speed it's ser-port is talking.
> So the modem must first determine the ser-port's Tx speed ?
> How ?[/color]
The modem expects an "AT" at initialization point. That is used by the modem
to determine port speed, character length and parity. I do not know the
"nuts and bolts" of how that is done, only that it is done. Once the DTE
rate is determined, the modem will expect that speed to remain constant if
it is set to "fixed DTE rate", a common setting for all error correcting
modems. It is possible to set the modem to "float" DTE rate according to DCE
rate but there's little reason to do that anymore.
> > ppp requires 8 bit .
> > You must set up your modem to do 8 bit communication.
> OK, so changing between 7 and 8 bit can't be used to
> distinguish the 2 modes: command and active ?[/color]
> How does the local machine know that its modem has "trained with a
> modem on the far side", so that it can send the 7 char "CONNECT"
> string (plus <CR>) ?[/color]
The CD (Carrier Detect) lead goes high and the modem sends the "CONNECT"
report to its host system... if the modem is set to verbose mode. It also
may include other data about the connect if those parameters are set.
> Apparently the 'negotiated' speed between the 2 modems, does not
> affect the ser-port speed speed(s) ?[/color]
It should not, under most applications.
> This is handled by the modem(s) data buffers and flow-control ?[/color]
It is handled by your modem's configuration. The commands to do this are
slightly different depending upon chipset. For instance, USR's use &B1 to
set a fixed DTE rate and the rate of the port determines the actual speed
that speed will be, providing the rate is within the range of speeds
supported by the modem.
> The complexity of ppp is exposed via the FSM state diagrams or
> tables, but the 'modem-automata' seems to be glossed over
> in the literature.[/color]
Because the modem is transparent to PPP.
"The only thing I do immediately is procrastinate."