System Administration Guide, Volume 2

Terminals, Modems, Ports, and Services

Terminals and modems provide both local and remote access to system and network resources. Setting up terminals and modem access is an important responsibility of a system administrator. This section explains some of the concepts behind modem and terminal management in the Solaris environment.


Your system's bit-mapped graphics display is not the same as an alphanumeric terminal, which connects to a serial port and displays only text. You don't have to perform any special steps to administer the graphics display.


Modems can be set up in three basic configurations:

A modem connected to your home computer might be set up to provide dial-out service, meaning you can access other computers from your own home, but nobody outside can gain access to your machine.

Dial-in service is just the opposite. It allows people to access a system from remote sites, but it does not permit calls to the outside world.

Bidirectional access, as the name implies, provides both dial-in and dial-out capabilities.


A port is a channel through which a device communicates with the operating system. From a hardware perspective, a port is a "receptacle" into which a terminal or modem cable might be plugged.

However, a port is not strictly a physical receptacle, but an entity with hardware (pins and connectors) and software (a device driver) components. A single physical receptacle often provides multiple ports, allowing connection of two or more devices.

Common types of ports include serial, parallel, small computer systems interface (SCSI), and Ethernet.

A serial port, using a standard communications protocol, transmits a byte of information bit-by-bit over a single line.

Devices that have been designed according to RS-232-C or RS-423 standards (this includes most modems, alphanumeric terminals, plotters, and some printers) can be plugged interchangeably (using standard cables) into serial ports of computers that have been similarly designed.

When many serial port devices must be connected to a single computer, it might be necessary to add an adapter board to the system. The adapter board, with its driver software, provides additional serial ports for connecting more devices than could otherwise be accommodated.


Modems and terminals gain access to computing resources via the serial port software. The serial port software must be set up to provide a particular "service" for the device attached to the port. For example, you can set up a serial port to provide bidirectional service for a modem.

Port Monitors

The main mechanism for gaining access to a service is through a port monitor. A port monitor is a program that continuously monitors for requests to log in or access printers or files.

When a port monitor detects a request, it sets whatever parameters are required to establish communication between the operating system and the device requesting service. Then the port monitor transfers control to other processes that provide the services needed.

The table below describes the two types of port monitors included in the Solaris environment.

Table 12-1 Port Monitor Types

Port Monitor 



Controls access to network services, such as handling remote print requests prior to the Solaris 2.6 release. The default Solaris operating environment no longer uses this port monitor type.


Provides access to the login services needed by modems and alphanumeric terminals. Admintool automatically sets up a ttymon port monitor to process login requests from these devices.

You might be familiar with an older port monitor called getty(1M). The new ttymon is more powerful; a single ttymon can replace multiple occurrences of getty. Otherwise, these two programs serve the same function.