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Understanding the CBA750B Deployment Model


This section covers the following topics:

Deployment Model Overview

The CBA750B 3G/4G wireless WAN bridge ships with a default configuration that accommodates most deployment scenarios. The deployment model assumes that the CBA750B is connected to a DHCP-enabled interface.

Figure 1 shows the model for the CBA750B deployment scenarios.

Figure 1: Deployment Model for the CBA750B
Deployment Model for
the CBA750B

The CBA750B maintains the wireless modem (or modems, if more than one modem is used) in a disconnected state, triggering a new connection as soon as the SRX Series Services Gateway requests a new lease. The modem(s) are disconnected as soon as the lease expires and are reconnected only when the services gateway requires another new lease.

When you are using the 3G link as the primary connection, long lease times can be used, as generally there will not be a need to constantly connect and disconnect the line. On the other hand, if the CBA750B is used to provide a backup connection, short lease times (approximately 1 minute) are commonly used when the primary link is active. In the worst-case scenario, when the lease expires, the backup link can be disabled, triggering a disconnection.

The CBA750B assigns the address received from the wireless service provider to the services gateway (normally a public address). Only a single device can be connected to the CBA750B at a time. Otherwise, multiple devices will reject the address passed to the CBA750B. The CBA750B operates in passthrough mode to relay all traffic from the wireless network to the DHCP client.


The Juniper Networks CBA750B 3G/4G wireless WAN bridge supports passthrough mode only and does not support router mode. Ensure that the Router/Passthrough switch on the CBA750B right side panel is always set to the 1 position.

CBA750B Management Interface

The CBA750B provides a Web-based management interface that is accessible even when 3G modems are not used. Because passthrough mode is used instead of a routed connection bridge, which does not perform Network Address Translation (NAT), the management interface cannot be accessed through the normal data channel.

The management interface is still accessible through the Ethernet port. Table 1 lists the parameters that VLAN tagging uses to separate management traffic from data traffic.

Table 1: Management Network Parameters

Card Model

Wireless Technology

Management subnet

Management address



Power over Ethernet

When available, Power over Ethernet (PoE) can be used to power the CBA750B. If the CBA750B is connected through a switch or a gateway that does not support PoE, an external power supply can be used (provided with the basic install kit).

When PoE is used, the device will require about 3.5 W of power per modem connected, so plan your power budget accordingly.

Dial Modes

The CBA750B can be configured in two modes:

  • Always on—the CBA750B connects to the 3G network after booting. The connection is always maintained, as long as there are no network or connectivity problems.

  • Dial-on-demand—the CBA750B initiates a connection when it receives traffic from the interface connecting the CBA750B and services gateway. DHCP request messages trigger a connection, and the connection is dropped after a configurable inactivity timeout has elapsed.

Regardless of the mode, the CBA750B can accept multiple cards simultaneously. In the event of a failure or other inability to connect, the remaining card(s) are used. You can configure the connection priority through the CBA750B’s management interface.

When shipped, the CBA750B is configured for dial-on-demand mode and set at 20 minutes idle timeout by default. Most carriers prefer the modem to disconnect if there is no interesting traffic. After the modem times out, DHCP requests from the SRX Series Services Gateway will result in a 192.168.30.x/24 response from the CBA750B. If interesting traffic is observed by the CBA750B, the modem attempts to connect again. Modem connection takes about 15 to 20 seconds generally. After that, the next DHCP request from the SRX Series Services Gateway will fetch the actual 3G IP address, and Internet connection is reestablished.