USB Modem Overview

A USB modem connects to a services router through modem interfaces that you configure. The device applies its own modem AT commands to initialize the attached modem. Modem setup requires that you connect and configure the USB modem at the device and the modem at the user end of the network.

• USB Modem Interfaces
• How the Device Initializes USB Modems
• USB Modem Connection and Configuration Overview

USB Modem Interfaces

You configure two types of interfaces for USB modem connectivity: a physical interface and a logical interface called the dialer interface:

• The USB modem physical interface uses the naming convention umd0. The services router creates this interface when a USB modem is connected to the USB port.
• The dialer interface, dln, is a logical interface for configuring dialing properties for USB modem connections.

See the interface naming conventions in the Junos OS Interfaces Configuration Guide for Security Devices.

The following rules apply when you configure dialer interfaces for USB modem connections:

• The dialer interface must be configured to use PPP encapsulation. You cannot configure Cisco High-Level Data Link Control (HDLC) or Multilink PPP (MLPPP) encapsulation on dialer interfaces.
• The dialer interface cannot be configured as a constituent link in a multilink bundle.
• If you are using the same dialer interface for ISDN connections and USB modem connections, the dialer interface cannot be configured simultaneously in the following modes:
  • As a backup interface and a dialer filter
  • As a backup interface and dialer watch interface
  • As a dialer watch interface and a dialer filter
  • As a backup interface for more than one primary interface

How the Device Initializes USB Modems

When you connect the USB modem to the USB port on the services router, the device applies the modem AT commands configured in the init-command-string command to the initialization commands on the modem. For more information about configuring modem commands for the init-command-string command, see Modifying USB Modem Initialization Commands.

If you do not configure modem AT commands for the init-command-string command, the device applies the following default sequence of initialization commands to the modem: AT S7=45 S0=0 V1 X4 &C1 E0 Q0 &Q8 %C0. Table 23 describes the commands. For more information about these commands, see the documentation for your modem.

When the services router applies the modem AT commands in the init-command-string command or the default sequence of initialization commands to the modem, it compares them to the initialization commands already configured on the modem and makes the following changes:

• If the commands are the same, the device overrides existing modem values that do not match. For example, if the initialization commands on the modem include S0=0 and the device’s init-command-string command includes S0=2, the services router applies S0=2.
• If the initialization commands on the modem do not include a command in the device’s init-command-string command, the device adds it. For example, if the init-command-string command includes the command L2, but the modem commands do not include it, the device adds L2 to the initialization commands configured on the modem.

USB Modem Connection and Configuration Overview

To use USB modems to remotely manage a services router, you perform the tasks listed in Table 24. For instructions, see the cross-references in the table.