Solution for mixing industrial, standard-office Ethernet
Mixing industrial Ethernet and standard office Ethernet on the same cable has its risks. With no sense of priority, an industrial telegram, which may be critical for operating some machine, might be delayed, sitting in some switch’s store-and-forward buffer waiting for an IT telegram to pass through.
Mixing industrial Ethernet and standard office Ethernet on the same cable has its risks. With no sense of priority, an industrial telegram, which may be critical for operating some machine, might be delayed, sitting in some switch’s store-and-forward buffer waiting for an IT telegram to pass through. Others have addressed the potential problem (see box below), but Harting launched its solution at Hannover Fair 2009. Its Ha-VIS “Fast Track Switching” technology was one of the five finalists for the Hermes Award, a 100,000 Euro prize for technology innovation presented at Hannover Fair. Fully compatible with standard Ethernet, Harting’s Fast Track Switches guarantee deterministic data transmission at the field level.
The store and forward buffers in Ethernet switches are the source of the problem for industrial telegrams, according to Harting engineers. They identified two basic problems. First, as the telegram passes through the switch, it gets stored in a buffer until the entire message is received; only then it is forwarded out of the switch. Second, if the industrial message arrives at the switch at the same time the switch is processing an IT telegram, it must wait until the entire IT telegram passes through the switch before it can move on.
Because a frame with a higher priority is always given precedence, automation frames are delayed if the queue buffers are filled with frames that have the same or higher priority. And, automation frames can be delayed for an unpredictable amount of time. There may also be congestion at the switch’s output port: When the output port is filled with frames, high-priority frames have to wait until the port is available again. If a lower priority IT frame with a 1,500 byte payload is being sent from the output port, an automation frame that has a higher priority has to wait for up to 125 microseconds until the port is available. Moving a message through a hierarchy or series of switches compounds the problem.
Harting’s solution is to, first of all, pass all automation telegrams straight through the switch, bypassing the store-and-forward mechanism. It’s as if the industrial telegrams are treated like first-class passengers on a train or plane. Secondly, if an industrial telegram arrives at a switch while an IT telegram is being processed, that processing stops immediately and the industrial telegram is given clearance to pass through. After it has cleared the switch, the IT telegram is re-transmitted through the switch. This is called a “cut through” method.
Hannover Fair demonstration
In the Harting stand in Hall 11, 16 Ethernet switches were lined up and connected in series to pass Profinet messages from one to another, across a total cable distance of about 3 meters. Also connected to the same Ethernet cable was a PC that would simulate standard “office” IT traffic on the line, ranging from about 10-60 MBits/sec. With standard “store and forward” switching, the delay for the Profinet messages was over 2,000 microseconds, but when the Fast Track Switching was turned on the maximum delay was about 50 microseconds. When there was no traffic on the network, the data rate (100 MBit/sec), frame length (64 byte minimum) and switch latency determined the frame delay time. The minimum transmission delay in this example was 160 microseconds. When the network load increased, delays occurred at the input ports, and there was congestion at the output ports.
When a very long frame (1,500 bytes) is sent from an output port on the transmission path—and if a high-priority automation frame is waiting to be sent at the same time from the same port—the automation frame may have to wait up to 125 microseconds until the port is available. It is statistically possible that this effect can be repeated on the transmission path, and the resulting delay can be as high as several milliseconds. The probability grows that the automation frame will be delayed as the network load increases. In this Hannover Messe example with 16 switches, delays of up to 2.1 milliseconds could have occurred.
Michael Babb is editor of Control Engineering Europe, firstname.lastname@example.org
Others are aware of the potential problem of mixing industrial and standard-office Ethernet on the same cable. Siemens has designed its ERTEC (Enhanced Real-Time Ethernet Controller) Profinet chips to deal with the situation. These industrial Ethernet ASICs are high-performance Ethernet controllers (based on 32-bit ARM 946 processors) that have integrated real-time switches especially developed for industrial use. The chips prioritize the isochronous real-time motion control telegrams and make sure they get to their destination in a timely fashion.
Phoenix Contact has also announced a Profinet enhancement to speed up industrial frames, and will release its DFP (Dynamic Frame Packing) chip later this year.
In general, as far as real-time Ethernets are concerned, neither Profinet IRT nor EtherCAT — two big winners — will need Harting’s Fast Switch technology.