When Do You Consider Using A Managed Switch?

Many applications will operate properly when using Plug and Play (PnP) switches. Consider the PnP switches as “smart” terminal blocks that you supply power to, plug in RJ-45 connectors with CAT5 cable and devices begin to communicate without any intervention by you. Remember, with a PnP switch, “What you see is what you get.” Meaning, you get “generic,” “plain vanilla,” Ethernet. Port parameters such as data rate, duplex and full-duplex flow control are automatically set when port connections are made. You have no control over this process. But the time will come to achieve more than basic communications. And a managed switch will allow you to create a fully optimized network by being able to configure the network for performance and to monitor the network for status.

With a managed switch, you can choose the optimal operating parameters for your network ports instead of using auto-negotiated settings.

A managed switch is a switch that supports SNMP (Simple Network Management Protocol). Within the managed switch are data locations that supply a wealth of knowledge about how the system is operating. For example, by using SNMP, you can examine how many messages and message types are being received and sent from each port. You can also identify the number of dropped or faulty packets in order to check the system for devices that may be causing communication problems.

Since you can examine traffic flow, you may decide to take action to control the traffic flow. How? This example illustrates such a point.

You may have a work cell that contains PLCs connected to operator interfaces, vision systems, motion control and also a data collection computer. Remember that the PLCs, OIs, vision and motion systems require the maximum bandwidth for communicating to each other in order to maintain their real-time performance. The data collection computer can get information at a slower rate and still supply the information in a timely manner to a management information system. To accommodate these differing communication requirements, Quality of Service (QoS) is used to prioritize messages. The data collection computer would be configured for low priority; meaning, that many more PLC, OI, vision, and motion messages would be handled before a data collection computer message.

Similarly, it may be advantageous to restrict which messages are sent to connected devices. In our work cell example, there is no reason that the OI, vision, or motion systems need to communicate with each other. They each need to communicate only to the PLC. By using the Virtual Local Area Network (LAN) function of a managed switch, messages may be restricted such that only those devices that must be party to the messages will actually hear the messages while those that are not involved will not. Because of the selective VLAN messaging, there is no communication clutter of useless messages between the OIs, vision, and motion systems; thereby greatly improving the overall bandwidth of the network.

Occasionally, there is a need to build a backbone in a system. A backbone is a connection that allows a large quantity of messages to flow between a group of switches. You may think of it as a superhighway that exists between a series of switches. Each switch is an entrance or exit point for the messages. Typically, the messages will be sent to other switches in the backbone. The simplest backbone is to connect a single port of one switch to a single port of the second, and the second to the third, etc. Keep in mind that normally only one connection can exist between two switches. However, this approach creates a bottleneck and the message traffic may exceed the 100 Mbps bandwidth available on the switches. When requirements exceed 100 Mbps, the trunking capability of a managed switch can be used to create multiple parallel paths between switches, allowing the trunked ports to act as a single port with expanded bandwidth. Trunking can also provide redundancy. You can use trunking to achieve more bandwidth between two switches and to provide redundancy in case one of the parallel paths fails. Communication will simply continue over the remaining path or paths. Trunking provides the quickest recovery from a single fault with 10 ms or less recovery time.

It is also possible for a managed switch to support ring redundancy. In this configuration, switches are connected in a ring topology using two ports on all switches. Assume the first switch in the ring is the master. The last switch in the ring would then be connected to the master using a redundant connection. This connection, called the backup connection, is not active during normal operation, but is activated if a single break is detected around the ring. By invoking the backup connection, communication continues as before and remains that way until the original break is repaired. Contemporary Controls’ RapidRing™ can self heal in less than 300 ms. Your Ring redundancy can improve your network uptime and a relay can alert the user to the fact that the ring is broken.

By utilizing managed switches, you can take advantage of network diagnostic tools that can query the status of managed switches through SNMP. Special SNMP management software can generate a network map of the network in question. Unmanaged switches could be visible during the network mapping operation but unidentified.

Plug and play switches certainly have their place in industrial and building automation. They are simple to install and use. However, by choosing managed switches, they offer the user much more flexibility and improved network performance.


This article was provided by Contemporary Controls. Contemporary Controls® (CC) is recognized for their industrial networking products in these technologies: Industrial Ethernet, Controller Area Network (CAN) including DeviceNet and ARCNET. Their repeating hubs, switching hubs, media converters and interface modules are designed for industrial applications. For more information on Contemporary Controls, please visit their website at: www.ccontrols.com

About The Author

Joe Stasiek is a Sales Manager of Contemporary Controls

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