11.4 Modem Connection
By using a terminal-modem combination (without a computer) one may dial out to other computers. Up to the mid 1990s in the US, there were many "bulletin boards" one could dial out to. Some even provided connections to the Internet. But bulletin boards lost out in favor of the Internet.
Dialing out from a terminal
Instead of connecting a terminal (or computer emulating a terminal) directly to a host computer using a cable it may be connected to the host via a telephone line (or dedicated private line) with a modem at each end of the line. The terminal (or computer) will usually dial out on a phone line to a host computer.
Most people use a PC and modem for dialing out. The PC could have a terminal connected to a serial port and the person at the terminal may dial out using the PC. Connecting a real terminal directly to an external modem is more difficult since the real terminal isn't very intelligent and doesn't give as much feedback to the user. For dialing out, many terminals can store one or more telephone numbers as messages which may be "set-up" into them and are sent out to the modem by pressing certain function keys. Many modems can also store phone numbers. The modem initiation sequence must precede the telephone number. When the outgoing call is answered by another modem at the other end of the phone line, the host computer on this modem may run a getty program to enable you to log in.
Terminal gets dialed into
It's common for a computer running Linux to get dialed into. The caller gets a login prompt and logs in. At first glance, it may seem strange how a dumb terminal (not connected to any computer) could accept an incoming call, but it can. One possible reason for doing this is to save on phone bills where rates are not symmetric. Your terminal needs to be set up for dial-in: Set the modem at your terminal for automatic answer (Register S0 set to 2 will answer on the 2nd ring). You turn on the terminal and modem before you expect a call and when the call comes in you get a login prompt and log in.
The host computer that dials out to your terminal needs to do something quite unusual. As soon as your modem answers, it needs to run login (getty). A host may do this by running the Linux program "callback" sometimes named "cb". Callback is for having computer A call computer B, and then B hangs up and calls A back. This is what you want if you are using computer A to emulate a terminal. For the case of a real terminal this may be too complex a task so the host may utilize only the "back" part of the callback program. The setup file for callback must be properly configured at the host. Callback makes the call to the terminal and then has mgetty run a login on that port. Mgetty by itself (as of early 1998) is only for dial-in calls but there is work being done to incorporate callback features into it and thus make it able to dial-out. As of early 1999 it didn't seem to have been done.
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