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Out of band access to RTUs using serial and LAN connections (FAQ)

Out of Band Access is one of 8 Reasons to Choose RTU vs Embedded Monitoring.

What is "out of band access"?

Out of band access is an important tool that allows you to see your equipment without network connections. It essentially provides you with a backup path in case of network communication failure.

Your individual devices can have out of band access if they're really critical, but it's an especially important feature for remote management. Many problems with your primary transport can be fixed with simple commands. It might be as simple as a reboot. The only requirement is that you have some other way to send the command while your transport is offline. Out of band access provides this other way to receive data and send commands.

What is an example of out-of-band access?

An example of out of band access is a serial port connected to a modem that provides dial-up access to the router. It might be an overhead channel on a microwave system. Your normal payloads are carried in the main channel, but you get an overhead channel that is outside of the normal band (out of band). Other examples include T1 when you use LAN as your primary method. It's also becoming more common to use cellular connections as a backup for LAN.

Why is it important to have out-of-band access?

If you rely on any one data path to connect to your remote sites, you're at risk for losing communications. You'll have to send a tech to the site. This is why it is critical for your remote monitoring equipment to support out-of-band access. Two possible choices are LAN with dial-up backup or LAN with serial backup.

Having a backup communications path is especially important in a remote monitoring and control context. If your primary communications go down, you have a problem. With a backup channel available, you can access your alarm remote at the site and execute a fix remotely. For more serious trouble (equipment failure, overheating), at least you can send your tech out to the site with a good idea of what's happening. Knowing what replacement parts to bring is key.

If I am using a serial connection or LAN as my main alarm-reporting channel, do I still need a dial-up modem?

Even if you're using a serial connection or LAN as your main alarm-reporting channel, a dial-up modem can still be a useful backup data path. If anything goes wrong with the serial connection, you can still receive alarms via dial-up reporting.

What is a downfall of using in-band access instead of out-of-band access for SNMP management?

SNMP management information travels the same network path as your data. It uses the same WAN and LAN routers, hubs, and communications links. And that is the problem. While the network is operating normally, SNMP packets flow between the managed devices and the management workstation with no problem. Its TRAPs, SETs, and GETs all flow with the same priority as regular traffic on the LAN/WAN and provide management information to the workstation or commands to the controlled devices. However, when the network goes down or is severely disrupted, SNMP traffic has no way to get between the managed device and the management workstation. Telnet is usually used along with an SNMP workstation. Telnet packets are also unable to move between the management workstation and managed devices during those network issues.

NetGuardian 832A's dial-up connection can be used as a main or backup path for reporting SNMP traps

How would out of band access provide a solution in communicating across an SNMP network?

With out-of-band access, a second path is open to the managed devices that do not depend on the LAN/WAN. It is usually an RS-232 access switch connected to the management port of each device. Every network device that supports SNMP also contains a RS-232 management port. Although it doesn't provide the fancy GUI interface of most SNMP workstations, this method provides the native interface for each device being controlled.

Do the two methods (in-band SNMP and Out of Band access) compete with each other?

No, they work with each other. System cost increases only slightly when adding out of band access management, while functionality goes up quite a lot. The SNMP GUI can be used for normal network monitoring and metrics, alarm reports, and data reduction. Then, when problems surface, you can connect to the remote access switch for direct control of your remote equipment. Since there is direct access to the management port, troubleshooting doesn't have the added complexity of a network coming between the device and the technician. And when the network is down, the same connectivity is still there. This provides a direct link between the technician and the device causing the problems.


Being able to access your remote monitoring system is an absolute requirement at all times. It's the lifeline of all of your revenue-generating equipment. That's why your monitoring gear must always be the last thing to fail. To this end, good alarm remotes and masters are designed with redundant power supplies, redundant fuses, and redundant storage devies. Similarly, they should have more than one reporting channels to enable instant failover if one channel goes down.

See how one telecommunications company fulfilled their requirement of Out of Band Access.
Then, take a look at dedicated monitoring devices.


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