Document revision date: 30 March 2001

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

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:

Table 8-3 Related Modem Documentation

Reference	Description
DECnet-Plus for OpenVMS Network Management	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.
Compaq 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.
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.
OpenVMS System Management Utilities Reference Manual: M--Z	Describes the PPP utiltity and associated commands.
Section 8.6.2	Explains how to configure and manage virtual terminals.
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.

Direct and Indirect Connections

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.

Figure 8-2 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 8-2 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 Section 8.7.2.

8.7.2 Setting Up Modems

Follow these steps to set up modems:

1. 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 Table 8-4.

   Table 8-4 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.

   Straight-Through and Cross-Over Wiring
   
   Descriptions of straight-through and cross-over wiring follow:

   • 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.
   
   Table 8-5 describes the most common connectors used to wire a modem.

   Table 8-5 Connectors

   Connector1	Description
   DB9	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.

   ---

   ¹All connectors in this table are available in both male and female genders.
   

   ---

   
   The pinouts and applications for the common connectors are shown in Table 8-6.

   Table 8-6 Connector Applications

   Connector and Pinout	Example	Adapter1
   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, 2 or contact a Compaq sales representative or Compaq reseller for information on adapters not listed below: H8575-A DB25 female to MMJ adapter, straight-through 3 H8571-C DB25 male to MMJ adapter, cross-over 4 H8575-E DB25 male to MMJ, straight-through 3 A compatible adapter
   An 8-pin DIN (round) connector		Use the H8584-AB or compatible MMJ adapter.
   A Modified Modular Jack (MMJ) DECconnect socket

   ---

   ¹This 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 Compaq.
   ²The genders listed are those of the connector on the adapter.
   ³Straight-through indicates that the EIA-232 Transmit Data signal is wired to the DEC-423 Transmit Data signal, and so on.
   ⁴Cross-over indicates the EIA-232 Transmit Data is connected to the DEC-423 Receive Data, and vice versa, and that DTR and DSR are similarly connected.
   

   ---

   
   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 Compaq support representative, or your local Compaq reseller.

   MMJ Accessories
   
   Table 8-7 lists order numbers and descriptions of some DECconnect accessories available from Compaq.

   Table 8-7 DECconnect Accessories

   Order Number	Description
   BC16E-02 BC16E-10 BC16E-25 BC16E-50 BC16E-A0	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.

2. 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.

   Note 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 8-8 contains descriptions of types of modem control that devices can support.

   Table 8-8 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.

3. 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.
4. 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.

   Note 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.

5. 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 OpenVMS Guide to System Security.

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