Linux – How to replace a hardware modem by virtual modem

I have an embedded device that is connected by an old dial-up modem to
the Internet. I cannot change the embedded device but would like to
connect the embedded device to a Linux machine that has an Internet
connection. The Linux machine has a free RS232 port that I would like to
use. It should offer a virtual modem that behaves just as a normal
dial-up modem including a PPP server that allows the embedded device to
set up a PPP connection.

If I could make a wish, I would prefer a new device pppN that I could
use with normal routing and firewalling.

What is a quick way to do so? What is the best way to do so? Are there
better alternatives using RS232 that might require minor changes to the
embedded device?

Update:
I actually now simply use pppd as described here:
http://www.tldp.org/HOWTO/PPP-HOWTO/direct.html

However, I like tcpser and I am pretty sure I will use it for another project sooner then later.

Thanks for your feedback.