The benefits from consistent use of standard Ethernet IEEE 802.3 networks

The concept of Automation IT is having a tremendous impact on the market because it has concentrated discussions about the use of Ethernet in industry on the most important factor: the benefits to the customer. These benefits result from the consistent use of standard Ethernet IEEE 802.3 networks. In automation applications, it is essential that features such as deterministic data transport are incorporated: something that cannot be achieved by using the switching technology used in standard Ethernet systems. In reality, the customer benefits of Automation IT can only be realised by adding a new dimension to standard Ethernet: The new Fast Track Switching Technology (FTS) described in this article.

Communication in industrial applications: Ethernet

The high expectations associated with the initial enthusiasm for Ethernet included, amongst other things, the concept of a network for all applications. It quickly became apparent, however, that, although Ethernet was the right technology, it could not fully meet automation requirements in terms of consistent networks.

When applied to automation, the performance of Ethernet proved to be not yet sufficient to fully replace the fieldbus systems used to date, with the result that efforts to further develop Ethernet were stepped up significantly. This led to a number of incompatible industrial Ethernet profiles.

What many of these profiles had in common was the fact that, in accordance with IEEE 802.3, they had changed Ethernet into proprietary solutions, which meant that they was incompatible with devices and applications using standard Ethernet. The performance problem was solved by changing Ethernet on the OSI layer 2. The performance of these profiles is generally good. In other words, their performance in terms of their determinism, speed, topologies and installation is similar to that of today’s fieldbus systems, and this is the benchmark that Ethernet must achieve for automation.

At that time that these approaches were being developed, it was not known that there was an alternative to changing the Ethernet standard. As a result, the lifecycle of industrial Ethernet became uncoupled from that of standard Ethernet – which led to the birth of automation IT. The visionary Automation IT approach came into being in 2006 in order to counteract the deviation from the Ethernet standard and to secure a uniform communication platform for office IT and industrial automation: in other words, to have a single Ethernet standard for both communication platforms.

Automation IT: a platform for all applications

Automation IT is the communication platform for all applications in a manufacturing organisation. The principle involves all applications being interlinked via a uniform Ethernet network. This ensures direct communication between the various applications - ERP and MES, for example - that determine the business process. This approach avoids complex transitions and accelerates processes, leading to improved production efficiency. Networks based on Automation IT deliver a range of customer benefits including reduced costs, simplified installation and increased availability.

Fast track switching (FTS): the key technology for automation IT

When selecting the communication standard for automation IT, there are no alternatives as the standard has already been set for MES and ERP communication. Ethernet has become established in office IT worldwide. In office IT environments, communication is linked with the strict observance of the Ethernet specification IEEE 802.3. Consequently, communication platforms are only possible within the standard IEEE 802.3 Ethernet. However, as appropriate performance is also necessary in automation networks, suitable technologies have always been investigated under this premise.

The breakthrough came in 2008 when HARTING established that it was possible to realise network components that could supply the network with the automation performance required. Fast Track Switching is the key technology used in these components. In fact, standard Ethernet can be applied to automation only by using this technology. The result is a network that works with unchanged Ethernet protocols, recognises automation protocols and accelerates these deterministically.

Status quo: Ethernet and switching technology

The performance of switching technology is greatly enhanced by the use of ‘cut-through’ rather than ‘store and forward’ techniques (Fig.1). However, determinism cannot be achieved with either store-and-forward or cut-through technology, with the result that neither of the two technologies offers sufficient performance for automation applications. Protocol prioritisation in accordance with IEEE 802.1q is also ineffective, as automation protocols compete with all protocols of the same and higher priority.
For this reason, there is a statistical and consequently - for automation – an unacceptable delay. The two key delay mechanisms are as follows:
•    Delays in the input port: If an input port’s queue (memory) is saturated with traffic from other protocols, which are of the same or higher priority than the automation protocols, then the automation messages are delayed (Fig. 2), leading to unpredictable delays for automation protocols.
•    Bottleneck in the output port: If the output port of a switch is saturated with messages, high-priority automation protocols also have to wait for port release (Fig.3).

For example, if a low priority message with a length of 1500 bytes leaves the output port, the high-priority automation message then has to wait up to 125 µs for port release.
If traffic in the network is very low, then only the Ethernet transmission rate, the message length and the switch latency periods determine the message transition delay. In the example given above, the minimum message transition delays are approximately 160 µs. If the load in the Ethernet network increases, this results in delays in the input ports and also bottlenecks in the output ports of the switches. If a very long message leaves an output port on the above route, and if a high-priority automation message is to leave the switch on the same port, the automation message has to wait for the release of the port.

Statistically, this effect can be repeated on the route and culminate in a delay of several milliseconds. In a line, all it takes is for this to occur at one switch while both messages travel along the route. The automation message always follows the long message, and each time it has to wait until it has left the ports; it can no longer overtake on the route. The probability of this undesired effect grows with the load in the network. With just 16 switches, message transition delays of several milliseconds can occur.

The determinism required in automation is therefore not guaranteed with today’s switching technology. IT protocols lead to delays for automation protocols. These delays accumulate in line topologies.

Deterministic Ethernet with Fast Track Switching (FTS)
The principle of Fast Track Switching offers a solution to this problem. The Fast Track Switch detects automation protocols in order to pass these on with priority over all other protocols. In this way, it gives automation priority over other applications on Ethernet. The Fast Track Switch accelerates all detected automation messages using the integrated cut-through method and prevents delays.

Moreover, with Fast Track Switching, automation messages can overtake other messages if the latter are occupying a required port. This means that there are no ‘waiting times’. If an IT message is being sent and the port is occupied by an automation message, the forwarding of the IT message is terminated in a controlled manner so that the automation message can be forwarded directly according to the cut-through method. The buffered IT message is then passed on. Fast Track Switching guarantees message transition delays at a higher performance than today’s fieldbus systems.

Comparison of switching technologies
Fast Track Switching also has to become established in the technological environment. Today’s established store-and-forward switching is the benchmark in terms of universality. Worldwide, there are an immense number of devices with Ethernet interfaces. All these devices can be linked via the store-and-forward mode. Not all of these devices are relevant for automation. However, innovations in automation are generally triggered via new technologies that are integrated into new devices.

For example, the topics of vision and RFID are not derived from classic automation, and devices in these sectors do not generally support automation-specific technologies. However, as a rule, they have Ethernet interfaces. Consequently, openness to standard Ethernet also means openness to innovations.

Another consequence is that Fast Track Switching can also be used for all automation profiles that support standard Ethernet communication: Ethernet IP and PROFINET RT, for example. This has the benefit of facilitating device design, while also allowing users (such as mechanical engineers) who have to support different automation profiles to use traffic-analysing components and uniform designs to create networks.

Store-and-forward switching offers high performance only in ‘flat’ office hierarchies, since QoS (Quality of Service) considerations do not ensure that high-priority messages overtake lower priority ones. However, this effect essentially influences the performance of line topologies and is considerably influenced by network capacity utilisation. This effect can be avoided using Fast Track Switching. Only special processes offer a comparable performance.

Fast Track Switching therefore combines the advantages of today’s IT methods with the special processes. The separate technology lifecycle of industrial Ethernet automation solutions is again coupled to a uniform Ethernet lifecycle, which offers additional advantages. With the dynamic development of Ethernet technology, the automation application can participate in all new developments, for example, in the field of bandwidth or security. With a decoupled lifecycle, the discussion about the substitution of the fieldbus systems would be resumed in five to ten years’ time, albeit on a new level.

The Automation IT system landscape
Automation IT is directly related to network convergence. The IT network and today’s automation network are separate networks, each with an assigned infrastructure. These two networks are interconnected. Consequently, the concept of the platform has a correspondence with the structure of the network itself. Unnecessary redundancies are dispensed with.

This is shown in the Automation IT system landscape (Fig.4), which operates on all network levels with standard Ethernet technology. All devices with an Ethernet interface can thus be integrated. Fast Track Switching significantly increases the performance of automation profiles that are compatible with Ethernet. An Automation IT Ethernet communication platform is now available for all applications at field level, from safety to rapid I/O communication. With Fast Track Switching, the negative effects of IT communication and line topologies on the automation performance are eliminated. Users therefore benefit from maximum freedom when utilising topologies adapted to the respective application. Strict rules for segmenting network areas and also the dedicated planning of transmission performance are no longer necessary.

Ethernet communication can now be deployed as far as the field level because Fast Track Switching guarantees determinism. Automation and IT applications use a joint communication platform and therefore a uniform network infrastructure. Automation IT is now a reality.

by Andreas Huhmann and Stefan Korf, HARTING LTD


Harting Ltd
Brackmills
Northampton

Can be contacted on

Tel: 01604 827500
Fax: 01604 706777