Very Computer

Yes, you are communicating through the computer's RS232 port in
asynchronous mode.

Quote:

>                                                        If so, does
>it mean that each octet in the PPP frame is further encapsulated in
>those start/stop bits ?

Yes, you are using 10 bits to transmit each octet.  One start bit,
8 data (octet) bits, and one stop bit.

Quote:

>                      Can you use the synchronous mode of the modem
>such as HDLC when using PPP ?

Not with the Sun asppp.  The reason that software package is called
"Async PPP" is because it only supports asynchronous operation of
the serial port.  Sun's Solstice PPP software can support synchronous
operation of the serial port as well as asynchronous.

However, other factors will probably prevent you from using synchronous
mode, even if you use the Solstice PPP software.  Your modem must support
synchronous operation (many inexpensive ones do not), and the site that
you're talking to must also support synchornous operation.

Also, since there is no standard defined (as far as I know) that will
let a modem perform data compression in synchronous mode, your throughput
would probably be limited to the highest speed that the modem-to-modem
rate that matches the modem-to-computer rate:  19200 bps.  Because of
this, you can get far better performance by using asynchronous mode,
despite the extra framing bits on each octet.

Quote:

>Can you use PPP on ethernet (is that considered a tunnelling ) ?

Yes, you can, but what would be the use?  If you have a network connection
that supports TCP/IP directly, why add the extra overhead of PPP?

  -Greg
--

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You should not worry about this if you are using any modern async
modems, as the data compression and error correcting protocols
actually squash out the start and stop bits between the modems,
and construct frames of many tens to hundreds of octets and send
these synchronously.

The start and stop bits will only be present between the serial
port and the modem. This normally runs at a significantly higher
bit rate than the modem-to-modem effective bit rate, so the extra
overhead of start and stop bits there don't really matter.

--
Andrew Gabriel
Consultant Software Engineer