ProfiNet Myths Debunked

With GE Intelligent Platforms recently releasing their ProfiNet offering, I wanted to take a couple of minutes to go over some of the comments, feedback, and misconceptions surrounding this protocol.

It appears that for many, this is a very new protocol and there are a lot of myths and mis-information floating around, surrounding this protocol.

Myth 1: Profinet Isn’t Ethernet

I have now had several people mention this to me while discussing ProfiNet.  I believe this myth to be propagated due to the protocol name not having “Ethernet” or “TCP” in it’s name.  This myth is completely false.

To counter this myth, we look to the OSI Model.  Ethernet (also referred to as IEEE 802.3), is part of the Physical, and Data Link Layers of the OSI Model.  What this means is that Ethernet is nothing more than a means of physically hooking two devices together and allowing them to talk to each other.  Profinet is a protocol, much like Modbus TCP, TCP/IP or EthernetIP, that runs on top of this physical/data link layer.

As Profinet was designed to run on an Ethernet based media, we have the benefit of using standard cabling, in the case of the GE Intelligent Platforms products, this can be standard copper (100/1000), or fiber.

Myth 2: Profinet is Proprietary

This particular myth typically involves Siemens.  Although Siemens is a big proponent of Profinet, they do not own it, nor is it proprietary.

Profinet is in fact, an open protocol handled by PI.  PI is an open standard working body who’s sole responsibility is to develop and test against the Profinet specification.  Their board of directors and advisory boards contain individuals from many different companies, many of whom are considered to be competitors to each other.

Myth 3: Profinet is new Technology

New is a rather subjective term.  That being said, a technology that made it’s debut in 2003 is hardly new.  Although newer to some manufacturers, the specification and protocol have been around for a very long time.

The benefit of this, is we have an established mature protocol, that has rigorous standards that each and every piece of equipment has to be tested against in order to be Profinet certified.  Unlike most other protocols where a manufacturer only implements a small subset of the functionality and you as a user are left wondering if this will talk to widget “A”,  Profinet requires a manufacturer to put their equipment through a testing facility in order to call it Profinet capable, meaning you should know when buying this equipment that it will in fact work with widget “A”.

Myth 4: Profinet isn’t Deterministic

In order to debunk this particular myth, we first need to look at the word “deterministic”.  While many people like to look at a network in terms of determinism, a better metric to use is that of jitter.

As mentioned in the Wikipedia article, jitter is:

the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications

A deterministic network is not necessarily the best metric.  We can guarantee that every device is given the opportunity to talk every 10ms, however if the jitter on that network is so high that we deviate significantly, we could be recieving information in 10ms, then 14ms, then 11ms, then 22ms; not very reliable at all.

Having a network with low jitter, means we can effectively guarantee when we receive the information, and when talking about I/O this is a much more effective metric.  Taking a look at the graph below you will see a sample application measuring the jitter of a Profinet network, the overall standard deviation was a miniscule 17 nanoseconds!  Compare that with EthernetIP (considered to be solid and suitable for I/O level communications by most) that has a standard deviation of 1.89 milliseconds, and Profinet is looking to be quite reliable.

Profinet Jitter Analysis

EthernetIP Jitter Analysis

So how exactly do they get this jitter out?  The secret lies in the Profinet protocol itself.  Most “industrial ethernet” protocols utilize the TCP and IP layers of the OSI model to handle the communications between devices.  The TCP and IP layers were designed to be as robust as possible, this is why so many devices and protocols can easily use these layers.  The problem with this, is in order to be this robust they have to have a lot of elements in them, that slow these layers down.  Normally this isn’t a problem (surfing the internet for example, you don’t really care if there is a 50-100ms lag on a connection), however when we start talking about industrial I/O that type of delay could be catastrophic.  This is why Profinet does not use the TCP and IP stacks.  They have been replaced by a custom stack that allows the protocol to effectively eliminate jitter and become a rock solid protocol.

Myth 5: Profinet is Profibus on Ethernet

I hear this from people a lot.  Because everyone is familiar with Profibus, they assume that Profinet is Profibus on Ethernet (much like Modbus TCP is Modbus RTU encapsulated in a TCP frame).  This is most definitely not true.  Earlier we talked about the custom stack that replaced the TCP and IP layer, this effectively debunks the myth that Profinet is Profibus on Ethernet, but this story goes a lot farther than just that.

Profinet is so much more than Profibus ever was.  Taking a look through the specification, you will see that the PI group has thought of all of the different uses for an industrial Ethernet network, and has designed the specification around all of these thing.  You need a network to control synchronized motion?  Profinet can do that.  Peer-to-peer communications?  Profinet can do that too.  Safety?  Check out ProfiSAFE.

While Profibus and Profinet share a governing body, Profinet is a next-generation protocol that was designed from the ground up to be open, extensible, and fast.  Regardless of your application, Profinet can help you.

Summary

Having had a chance to work with Profinet, I’m quite excited with what this network will bring to customers in terms of new and exciting architectures, functionality, speed and reliability.

If you are interested, GE will be running a series of workshops in late September on Profinet.  Should you miss these, please give us a call and we would be happy to stop by and discuss Profinet and how it can benefit you.