Do I
Need a Managed Switch?
By
Marty Huff
Director of Computing Solutions
MSI Tec, Inc.
As the trend to implement Ethernet on the factory floor and
into control networks continues to grow, more and more is being
said about managed switches. The simple question of “Do I need
a managed switch?” can be given the simple answer of “Maybe -
Maybe not.” This article will provide a brief overview of the
differences between managed and unmanaged switches and give some
examples of situations where a managed switch should be a
consideration.
Managed vs. Unmanaged Switches
Unmanaged switches have traditionally provided the backbone for
basic network connectivity. Unmanaged switches are available
with features such as redundant power inputs, extended operating
temperature, reduced noise susceptibility, and other hardware
features that can be crucial to industrial networks.
Many control networks have specific needs such as;
deterministic or real time communications. Managed switches
provide additional built-in protocols and software tools that
can increase determinism, and provide optimized constant and
consistent data flow. If a business network is down for ten
minutes during which time users cannot get e-mail, it’s merely
an inconvenience. If certain industrial networks are down for
ten minutes, the results could be costly or even catastrophic.
Some applications such as motion control often demand response
times of a few milliseconds or less. In order to achieve
consistent communications times and deterministic communications
it’s often necessary to implement a managed network
Managed Network Components
Managed switches allow us to take control of communications
over an Ethernet network. There are three types of control:
Bandwidth Control, Data Control, and Traffic Control.
Depending upon the type and number of devices on a network, one,
two or all of the above can be applied.
Bandwidth Control can be achieved by using the IGMP Snooping Protocol and
tools such as Rate Limiting and Port Trunking. IGMP Snooping
allows intelligent routing of Multicast traffic. There are
three types of transmissions over an IP network - Broadcast,
Unicast, and Multicast. Broadcast traffic is data that is sent
out to all devices on the network. Unicast messages are
messages intended for one device, whereas multicast messages are
packets sent to a predefined group of devices. Routers are
aware of multicast messages being passed to and from the
internet or separate networks making multicast messaging more
efficient than Unicast messaging. However, LANs are not aware
of multicast traffic unless the IGMP snooping protocol is
implemented. Without IGMP snooping, these messages become
broadcast messages which can decrease network performances, or
possibly bring down the network altogether. Since Multicast
messaging is more efficient, many popular industrial protocols
such as Ethernet/IP use it for real time or near real time
communications. Rate limiting insures that a malfunctioning or
misconfigured device does not use more than a predetermined
amount of bandwidth for sending or receiving which could degrade
network performance. Port Trunking expands the bandwidth
between managed switches by aggregating multiple ports.
Data control refers to guaranteeing that the most important
data is delivered first, and not affected by other traffic on
the network. Most managed switches offer Quality of Service
(QoS), which provides the ability to prioritize network
traffic. QoS increases determinism by insuring that
high-priority traffic is passed through first. For example,
frames to and from a motion controller would be processed before
traffic to and from an e-mail server. Manufacturers with
devices that use the Profinet protocol highly recommend the use
of QoS with their products.
Traffic Control can be accomplished by implementing Virtual
Local Area Networks (VLANs). VLAN configuration allows a switch
to logically group devices and to isolate traffic between these
groups even if all the devices share a common physical media.
For example, if the switch was being used for both office
communications and factory communications, two VLANs could be
created to isolate the office traffic from the factory traffic.
Some switches also allow devices to be located on multiple
VLANs. This is sometimes called overlapping VLANs. If one
device, a SCADA system for example, needs to communicate with
both the office and the factory, then this device would exist in
both the office VLAN and the factory VLAN. This would isolate
traffic between the remaining office and factory devices, but
allow the SCADA system to communicate on both networks.
Another important feature of a managed switch is Network
Redundancy. Redundancy institutes a “back-up plan”, and reduces
or eliminates potential downtime. Managed switches can provide
protocols such as the Rapid Spanning Tree Protocol (RSTP). RSTP
allows for alternate cabling paths while preventing loop
situations that can cause a network to stop functioning.
The above-mentioned features are just a sample of common
tools and protocols found in most managed switches. There are
other features that may be available depending upon the
particular switch being used.
With or Without Managed Switches
Now, back to the question of “Do I need a managed switch?”
There are only two disadvantage of using a managed switch. The
first one is that they are physically larger. This can be a
consideration when control panel real estate is limited. The
second disadvantage is the price. Managed switches are
generally two to three times the price of unmanaged switches.
So how can you tell which type of switch is the right one for
your application? One method of defining the requirements of
your network is to monitor it. A protocol analyzer can be
employed to detect the types and amount of traffic by port or on
an aggregated basis. Monitoring allows us to optimize
performance by defining which of the above mentioned tools
should be used and where. It also allows us to take a
“snapshot” of the network for benchmark testing. A popular
protocol analyzer is Ethereal which can be downloaded at no
charge (http://www.ethereal.com/).
Most managed switches also provide built in traffic monitoring
capabilities.
Another helpful source in defining your network’s
requirements is to ask the device manufacturer. There is no
manufacturer who can specifically quantify how much traffic will
be generated by their devices in every situation. They can,
however, tell you what type of traffic will be used and make
recommendations as to what protocols and features, if any, will
increase network performance.
So what does this all tell us? It tells us, “maybe you need a managed switch”. It seems absurd that in this
high-tech world where networking is a precise science that
someone might suggest good old fashioned trial and error when it
comes to designing an industrial network. The truth is that you
can listen to your hardware vendors, you can do all of the
research you possibly can, you can perform calculations, and you
can hire consultants, but until you actually have a real world
functional test, the rest is meaningless. Consultants and
manufacturers have the tendency to suggest a “safe” solution,
and since the additional cost is not coming from their pocket,
why not recommend managed switches? Most network hardware
vendors will offer “consignment” terms, or a free trial period
which will give you an opportunity to get live results as to
whether a managed switch is truly necessary.
In conclusion, chances are an unmanaged switch will work just
fine for a small or isolated control network. Managed switches
offer several methods of communications control and should be
considered for mixed networks where the control network shares a
common infrastructure, as well as, high traffic networks,
devices that use multicast messaging, mission critical data, and
real-time communications. If you are not sure what the demands
of your application actually are, I recommend monitoring and
benchmark testing in your real world environment.
Marty Huff is MSI Tec's Director of Computing Solutions and an expert in industrial networking. For further information on any of the above, or to discuss your own networking issues with Marty and our team, call us at (720) 875-9835.
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