In order to establish a PPP link between two computers using the telephone line as medium for data transmission, you need to install and configure one Modem device in each computer. On the other hand, if you're planning to connect the same computer to different networks simultaneously (e.g., to build a proxy between them), you will need to install and configure one Modem device for each different network you plan to establish such simultaneous connection on. To install a Modem device in a computer you need to attach the Modem hardware to the computer and later the telephone line to the Modem hardware. To attach the Modem hardware to your computer, you need to connect the serial or USB cable that comes from the Modem hardware to the appropriate input on your computer. To connect the Modem hardware to the telephone line system, you need to unplug the cable that connects your telephone device and plug it on the Modem device, specifically in the port reserved for data transmission. Later, using a similar cable, you could connect your telephone device to the Modem's telephone port, so you can realize telephone calls when no data transmition take place, as well. To be on the safe side, do all the installation with the computer turned off. Then, when everthing has been put in place, turn the computer on. Once the system is up, you can verify the Modem hardware using either the lsusb or lspci commands. These commands need to be run with administrative privileges, so probably need to sudo them or login as root user in order to execute them. For example, assuming you are logged in as root user, and you are using an USB Modem like that one we mentioned before, the output of lsusb command would look like the following: Bus 003 Device 001: ID 0000:0000 Bus 001 Device 001: ID 0000:0000 Bus 001 Device 002: ID 058f:6366 Alcor Micro Corp. Multi Flash Reader Bus 002 Device 001: ID 0000:0000 Bus 005 Device 003: ID 06e0:f104 Multi-Tech Systems, Inc. MT5634ZBA-USB MultiModemUSB (new firmware) Bus 005 Device 001: ID 0000:0000 Bus 005 Device 002: ID 046d:c018 Logitech, Inc. Optical Wheel Mouse Bus 004 Device 001: ID 0000:0000 The relevant line here is that one mentioning the existence of a Multi-Tech System, Inc. MT5634ZBA-USB MultiModemUSB (new firmware) I want to thank my friend Brians Suarez Alonso for bringing this Modem to me and for his paitient, resisting my repetitive calls at night to realize connection tests. device. This line confirms that your Modem hardware is supported by &TCD; and it is possible to transmit data through it. Otherwise, if the Modem you installed doesn't appear in this list, it is probably because such hardware is not supported by &TCD;, yet. Once you have confirmed the Modem hardware has been installed in the computer (either client or server), you need to determine the device name the operating system assigned to it. This information is required by programs like mgetty and wvdial, so they can know what device to talk to. Assuming you've connected your Modem device through an USB port, the operating system might use the the /dev/ttyACM0 file to talk to it. On the other hand, assuming you've connected your Modem device through a serial port, the operating system might use the /dev/ttyS0 file to talk to it. To be absolutly sure about what device name the operating system assigned to your Modem hardware, you can use the wvdialconf program from wvdial package or the lshal command from hal package. Modem devices are configured through system-config-network command. This command produces Modem configuration files under the /etc/sysconfig/network-scripts directory. Configuration related to Modem devices can take different file names, but they can be clearly identified by the value set on the DEVICE variable, inside the configuration file. This variable can take names like ppp0 for the first Modem device, ppp1 for the second Modem device, and so on for other Modem devices. The Modem configuration files are used by pppd and wvdial programs when a PPP connection is established between two computers. If you are runing mgetty and pppd to attend incoming calls, it is very important that you also specify the IP address and the network mask in the configuration file of the Modem device you are using to establish the connection. Otherwise, if the remote peer doesn't set the IP information for you, it would be difficult for both computer to establish a permanent interchange of data across the link created. # Please read /usr/share/doc/initscripts-*/sysconfig.txt # for the documentation of these parameters. TYPE=Modem DEVICE=ppp0 BOOTPROTO=none ONBOOT=no USERCTL=yes PEERDNS=yes AC=off BSDCOMP=off VJCCOMP=off CCP=off PC=off VJ=off LINESPEED=115200 MODEMPORT=/dev/ttyACM0 PROVIDER=ProviderName DEFROUTE=yes PERSIST=no PAPNAME=a WVDIALSECT=ProviderName MODEMNAME=Modem0 DEMAND=no IPV6INIT=no IDLETIMEOUT=600 NETMASK=255.255.255.0 IPADDR=192.168.1.1