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Understanding the CBA850 Deployment Model


This section covers the following topics:

Deployment Model Overview

The CBA850 3G/4G/LTE wireless WAN bridge ships with a default configuration that accommodates most deployment scenarios. The deployment model assumes that the CBA850 is connected to a DHCP-enabled interface.

Figure 1 shows the model for the CBA850 deployment scenarios.

Figure 1: Deployment Model for the CBA850
Deployment Model for
the CBA850

The CBA850 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 CBA850 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 CBA850 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 CBA850 at a time. Otherwise, multiple devices will reject the address passed to the CBA850. The CBA850 operates in passthrough mode to relay all traffic from the wireless network to the DHCP client.

CBA850 Management Interface

The CBA850 provides a Web-based management interface, which can be accessed through the normal data channel. To access the management interface, open a browser window and type in the address bar and press Enter.

Power over Ethernet

When available, Power over Ethernet (PoE) can be used to power the CBA850. If the CBA850 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).


PoE is enabled only on the LAN2 port of the CBA850.

Dial Modes

The CBA850 can be configured in two modes:

  • Always on—the CBA850 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 CBA850 initiates a connection when it receives traffic from the interface connecting the CBA850 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 CBA850 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 CBA850’s management interface.

When shipped, the CBA850 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 CBA850. If interesting traffic is observed by the CBA850, 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 the Internet connection is reestablished.