A modem is a device that converts electronic
signals from one data format to another. Modems usually perform
conversions bidirectionally, that is, they can convert the local
data into another data format and transmit the results; modems can
also receive and convert data back to the local data format. Most
modems convert data from digital format to analog format, and from
analog format back to digital format.
With a pair of modems, you can transmit digital communications
over analog media such as telephone lines, and then convert the
communications back into digital signals at a remote location. Pairs
of modems are used to connect a terminal or a local computer to
a remote computer system, and to connect two remote computers to
each other.
The following sections discuss these topics:
Understanding modems
Setting up modems
Troubleshooting modems
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.
Basic Modem Configuration 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 1 Basic Modem Configuration
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.
Related Modem Documentation lists references
to OpenVMS documentation that discuss other communications protocols
and topics relevant to the use of modems:
Table 3 Related Modem Documentation
Reference
Description
Decnet-Plus for OpenVMS Network Management Guide
Explains the use of modems
to establish a dynamic asynchronous DECnet connection between two
nodes. Asynchronous DECnet is a protocol that can operate over a
modem datalink.
HP TCP/IP Services for OpenVMS Management
Explains the use of modems
to establish a serial connection using the PPP (Alpha only) and
SLIP protocols and TCP/IP Services.
HP OpenVMS Guide to System Security
Discusses how to maintain
the security of DECnet modem connections and dial-in modem lines.
TCP/IP
Networking on OpenVMS Systems
Explains the use of PPP
on OpenVMS Alpha and OpenVMS VAX to communicate with remote systems.
HP OpenVMS System Management Utilities Reference Manual: M--Z
Describes the PPP utiltity
and associated commands.
Explains how to configure
and manage virtual terminals.
HP OpenVMS DCL Dictionary and online help
The DCL command SET HOST/DTE discusses
the use of modems to connect to a remote system. The DCL commands
CONNECT and DISCONNECT explain how to set up and disconnect virtual terminals.
Part of the job of configuring a modem to a computer or a
terminal is to decide what type of access the modem will have to
your computing environment and which serial communications ports
best meet your requirements.
You can choose to connect a modem directly to a host system,
or you can connect the modem indirectly to an intermediate network
server device such as a DECserver. Explanations of these two types
of connections follow.
Direct connection A direct connection dedicates the modem to a particular host
system. This reduces the amount of access available to the modem
caller, and can reduce to one the number of systems that you must
protect against unauthorized access through this modem. This is often the configuration of choice for smaller computing
environments, or for connecting a modem to a single computer or
terminal.
Indirect connection An indirect connection creates a pool of modems for a variety
of computer systems on the local area network, including servers
that communicate with the host computers using protocols such as
LAT and Telnet. This type of connection makes better use of the
available telephone lines but increases security requirements. This is often the configuration of choice for larger computing
environments. An indirect connection is commonly used when you use
LAT or Telnet protocols to configure a number of modems, called
a modem pool, to share access to a number
of computer systems.
With either type of connection, you cannot use the modem if
the host or the server the modem is connected to is not operational.
Direct and Indirect Modem Configurations depicts direct
and indirect modem configurations. The remote devices T1 and T2
are indirectly connected to both Host1 and Host2 host computers
using the DECserver and the LAT protocol; T3 is connected directly
to Host2.
Figure 2 Direct and Indirect Modem Configurations
Once you decide which serial communications port to use, either
on a host or a terminal server, you need to determine the connectors
and the pinouts for the port and how to wire the modem to the port.
Refer to the documentation for the modem and for the port; also
see
Setting Up Modems.
Setting Up Modems Follow these steps to set up modems:
Determine connections and wiring pinouts. The connector and pinout determine the specific wiring adapters
and cables you need to connect the modem to the port. To determine
the pinout and the connector on the modem, and the pinout and connector
on the port you are connecting the modem to, refer to the modem
and the port documentation. Two common pinouts found on the EIA-232 DB25 connection are
shown in
Common Pinouts on the EIA-232 DB23 Connection.
Table 4 Common Pinouts on the EIA-232 DB23 Connection
Pinout
Description
Data Terminal
Equipment (DTE)
Transmit information through
pin 2, and receive information through pin 3, among other standardized
pin assignments.
Data Communications Equipment
(DCE)
Transmit information through pin 3, and
receive information through pin 2, among other EIA-232 pin assignments.
DCE devices communicate straight-through with
DTE devices: the transmit pin on each end of the cable is wired
to the corresponding receive pin on the other end. Pin 2 on one
cable is connected to pin 2 on the other cable, and pin 3 on one
cable is connected to pin 3 on the other cable.
Equipment wired with a DCE pinout requires a cross-over to
communicate with another connector wired DCE; pins 2 and 3 on one
cable are connected to pins 3 and 2 on the other cable, respectively.
A cross-over is required in certain situations, because two transmit
pins or two receive pins cannot be wired together. As a specific
example, you need a cross-over to wire two DTE devices together,
or to wire two DCE devices together. A cable with cross-over wiring is sometimes referred to as
a null modem cable, because a null modem
cable of an appropriate length could logically replace all components
of a modem-based communications connection; that is, it could replace
the local serial cable, the local modem, the intervening telephone
circuit, the remote modem, and the remote serial cable.
Connectors describes the most
common connectors used to wire a modem.
A 9-pin connector, containing
a row of four pins, and a row of five pins. The DB9 can have the
EIA-574 commonly used on PC systems or an older standard connection used
on MicroVAX consoles.
DB25
A 25-pin connector, with
a row of twelve pins and a row of thirteen pins. The DB25 typically
uses the EIA-232 pinout and can be wired as Data Terminal Equipment (DTE) or
as Data Communications Equipment (DCE).
MMJ
A 6-pin modular jack, which uses DEC-423
signaling, commonly referred to as DECconnect wiring. DECconnect
wiring greatly simplifies wiring devices, as one need consider only
the appropriate adapter for the device connection; the associated
BC16E cabling is wired consistently.
The pinouts and applications for the common connectors are
shown in
Connector Applications.
A DB9 9-pin
connector with an EIA-574 PC-compatible pinout
The DB9 connectors
found on most PC, AlphaStation, and AlphaServer systems
Use the H8571-J or compatible
MMJ adapter.
A DB9 9-pin
connector that predates the EIA-574 pinout
The console
connector on various MicroVAX systems uses a pinout that predates the
EIA-574 pinout
Use the H8575-B or compatible
MMJ adapter.
A DB25 25-pin
connector with the EIA-232 wiring
The communications
ports on many terminals
Use the appropriate adapters
from the following list 3, or contact an HP sales representative
or HP reseller for information on adapters not listed below:
If your application does not use one of the serial wiring
connections shown in the table, you need to determine the specific
requirements of the device, as well as the specific pinout. You
also need to determine the cabling appropriate for the application.
Contact your hardware support organization, your HP support
representative, or your local HP reseller.
DEC-423 (based on EIA-423)
MMJ office cable, available in various lengths.
H8571-C
25-pin male EIA-232 to DEC-423
DECconnect adapter.
H8571-E
DEC-423 DECconnect 25-pin
adapter with jack screws.
H8571-J
9-pin MMJ adapter. Used
with the PC-compatible EIA-574 DB9 wiring.
H8572-00
MMJ cable extender. Allows
the direct connection of two BC16E cables.
H8575-A
Female 25-pin DEC-423 DECconnect
MMJ to EIA-232 general-purpose adapter.
H8575-B
Female 9-pin DEC-423 DECconnect
to printer adapter. Also used with the DB9 wiring found on some
MicroVAX console ports.
H8584-AB
8-pin DIN to DEC-423 DECconnect adapter.
Most commonly used with various Apple computers.
Choose a type of modem control. As part of connecting a modem to a device, you can add wires
to the host port and the modem. These wires are used to pass signals
called the modem control signals. When you connect to a local terminal for dial out, modem control
is not particularly significant: either the modem is wired or configured
to ignore modem control, or the wiring is set up to pass the modem
control signals from the terminal to the modem. When you connect a modem to a computer, modem control is far
more significant, because the host uses the modem control signals
to direct the modem to accept incoming telephone calls. The modem
control signals also enable the modem to signal the host that a
call has been received or that a call has ended. These signals allow
the host and the modem to take the appropriate actions for a particular
event.
In addition to their use by modems, modem control signals
are also often used to communicate device status between the host
and other serial devices such as serial printers. Various serial
printers use modem control signals as modems do: to indicate to
the host that the printer is powered up and ready to accept output,
or that the printer is powered down or otherwise unable to process
output.
Table 8 Types of Modem Control that Devices Support
Type of Modem Control
Description
No modem control
The host and the modem cannot
intercommunicate the status of the host or the modem. It is possible
to use a modem on this port; however, this type of port is not recommended
for a modem.
Without modem control, the modem cannot
signal the host that the telephone call has been disconnected and
that the host must take appropriate action: suspend or log out the
associated user process. (See Step 5 for the associated security
implications.)
Furthermore, without modem control, you
must set or wire the modem so that it always answers incoming calls,
because the modem cannot know if the host is able to respond. (This
too has security and modem control implications.)
Limited modem
control
The host and the modem can
intercommunicate and can take actions based on the status of the
other device. Limited modem control is the best choice for most
applications.
Full modem control
The host and the modem can intercommunicate
and can pass an extensive amount of control and status information.
Both the host and the modem can take actions based on the status
of the other device.
Limited modem control, which
has similar capabilities, has largely superseded this configuration.
Limited modem control also requires fewer wires on the connection,
making it the more economical choice.
Refer to the device documentation to determine the type of
modem control signal that the device and modem support. This determines
the number of wires and the wiring connections needed for communications.
The following examples show types of modem control and the wires
they require:
DECconnect supports limited modem
control, which requires two of the six wires in the DECconnect cabling.
The other four wires are used for the following purposes:
Transmitting data
Receiving data
The transmit ground
The receive ground
Full modem control requires more than two wires
dedicated to the modem control signaling.
Devices that do not support modem control require
no wires dedicated to modem signaling.
With modem commands or custom-wired cabling, you can force
a modem to operate with a device that does not support modem control.
However, this is not recommended for general use on a host system, because
this wiring can potentially result in security problems.
Determine the command set
used by the modem. The command set includes the commands used to request that
the modem place a telephone call, the telephone number to be called,
and the commands used to configure the modem. Examples of command sets follow:
AT command set:
ATDT phone-number
where:
AT indicates "attention" --
to get the attention of the modem
DT indicates "dial
tone"; (PT would indicate "pulse
tone" ).
DMCL command set:
Ctrl/B [Return]
Ready
DIAL T phone-number
where:
T represents "tone";
(P would represent "pulse" ).
phone-number represents the
phone number you are dialing.
The command set is used to communicate with the modem to request
that the modem perform some action, such as dialing a telephone
number and connecting to a remote modem. You can enter direct modem
commands at a terminal directly connected to a modem, or you can
communicate indirectly with the modem using DCL commands such as
SET HOST/DTE.
Configure the port. After wiring the modem to the connector on the OpenVMS computer
or DECserver, you must configure the port to recognize and properly
operate the modem, and to enable autobaud speed detection.
The autobaud operation detects the speed -- the baud
rate -- of the communications. Including the /AUTOBAUD qualifier
is not required; however, if autobaud detection is disabled, you
must configure both the host terminal or DECserver port, and the
modem, for the same baud rate.
The commands you give depend on whether you are using an OpenVMS
host system or a DECserver:
On an OpenVMS host system, execute
the following command interactively, and also place this command
in the system-wide startup file, SYS$MANAGER:SYSTARTUP_VMS.COM:
$ SET TERMINAL /MODEM /AUTOBAUD /PERMANENT TTAO:
where TTA0: is the name of the terminal
device the modem is wired to. This command requires privileges.
On a DECserver, configure the port using the following
commands:
DECserver> SET PORT n MODEM ENABLE
DECserver> SET PORT n FLOW CONTROL XON ENABLE
DECserver> SET PORT n AUTOBAUD ENABLE
where n is the port number. The commands enable the modem, XON, and autobaud. These commands
require privileges on the DECserver.
Ensure security with your
modems. Dial-in lines allow remote, unauthorized users access to your
system. You need to maintain consistent security and good system
and user password management to keep your system secure from unauthorized users. The following list contains some ways to increase security
on your system:
You can configure a DECserver with
a password to prevent a modem from accessing any other feature.
This password prevents an unauthorized user from accessing or seeing
any information about the local network configuration until after
the user enters the password. You can enable this password on specific
ports.
With OpenVMS, you can establish a system-wide password
requiring the user to specify a password before the system prompts
for a password. This additional password helps reduce the security
risk caused by users with poor passwords. You can enable a system-wide
password on specific host ports.
With OpenVMS, you can establish minimum password
lengths, and you can enable system-generated passwords. These measures
can help reduce the security risk caused by users with poor passwords.
Always use and configure some form of modem control.
Without modem control, a telephone connection that is disconnected
for any reason might be left logged into the host, and a subsequent modem
caller will receive the logged-in session without specifying a password.
Also, without modem control, the host cannot request that a modem
session be dropped when certain system events such as a process
logout occur.
These and other techniques for protecting your system from
unauthorized access are discussed in detail in the HP OpenVMS Guide to System Security.
Troubleshooting
Modems In troubleshooting any serial communications problems, particularly
those problems with a modem, attempt to isolate the problem as much
as possible, testing one component, wire, or device at a time.
Troubleshooting Communications Problems contains some general
troubleshooting suggestions, but it is not a complete list. Basic
serial communications test equipment such as a serial-line break-out
box, can often help you locate communications and wiring problems.
For further assistance, contact your local hardware support organization.
Table 9 Troubleshooting Communications Problems
Problem
Considerations
Modem does
not answer
Check that the telephone
number being called is correct.
Check that the modem
has power.
Check that the host system or device has
power and is operating.
If possible, directly connect
a standard terminal in place of the modem, and test the operation
directly.
Check that the host modem control signals
are present, and correctly wired.
Check that the host
device is configured correctly for a modem by using a SET TERMINAL,
SET PORT, or other appropriate host command.
Check the
wiring. Look for a broken, miswired, or disconnected wire.
Look
for a disconnected connector or a broken, missing, or bent connector
pin.
Telephone malfunction
Using a standard telephone
handset, test that voice calls can be established on the telephone
line.
Is static or other interference noticeable on
the telephone line?
No modem indicator
lights
Check the power connection.
Check
to see that the modem is turned on.
Check to see that
the modem has passed applicable self-tests.
Try swapping
the modem for another device.
No response, or garbled
response to typing
Check that the modem status lights indicate
received data on the transmit line, and transmitted data on the
receive line. This can point to the miswiring of the transmitted
and received data. You can wire serial cables and adapters straight-through
or with a cross-over.
Check for crossed signals.
Check
for incorrect speed detection. Autobaud detection sometimes sets
the speed incorrectly. On lines that are not enabled for autobaud
detection, check that the line is set for the correct speed. On
ports that support it, check the speeds for both the transmitted and
the received data.
Make sure that the port has autobaud
enabled, and that the port and the modem are configured for the
same data rate.
Check for interference or a disconnection
in the wiring.
Check the wiring for any problems.
Check
for any adjacent wiring, power, or video signals that might interfere
with the serial communications.
This table contains
only a subset of the DECconnect adapters available. The adapters
listed in this table might not be suitable for your particular application
requirements; additional DECconnect adapters are available from HP.
HP OpenVMS System Manager's Manual, Volume 1:... 
Managing Peripheral Devices
