Technical data

Managing Peripheral Devices
8.7 Managing Modems
• Troubleshooting modems
8.7.1 Understanding Modems
A modem converts a digital signal to an analog signal by modulating the digital
information on a carrier signal; a modem converts analog to digital signals by
demodulating—or extracting—digital information from analog signals on an
analog transmission facility such as a telephone line. The two words MOdulator
and DEModulator form the basis for the device name: modem.
Figure 8–1 represents communications between a terminal and a remote
computer system, but the principles apply equally to communications between
two computer systems. One modem converts digital to analog signals on the local
end of the analog telephone connection, and another modem converts analog to
digital signals on the remote end of the connection.
Figure 8–1 Basic Modem Configuration
Computer
ZK−9057A−GE
Terminal
Modem Modem
Digital
signal
Phone line
Digital
signal
Analog signal
Modems are always used in pairs; each one of the pair can act as both a
transmitter and a receiver.
When configuring modems, you must check that:
• The receiving and transmitting modems are wired correctly.
• The modems support compatible analog data formats and speeds.
• Each modem supports a digital format compatible with the attached terminal
or computer.
Once a modem connection has been established, you can layer data
communications over the connection. You can layer at least one, and sometimes
more, of a wide variety of communications protocols on the basic asynchronous
serial ASCII protocol that most modems provide. Point-to-Point Protocol (PPP)
and asynchronous DECnet are examples of protocols that can operate over a
modem link.
Table 8–3 lists references to OpenVMS documentation that discuss other
communications protocols and topics relevant to the use of modems:
Managing Peripheral Devices 8–25