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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 (see fig. 3). 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 μsec for
port release.
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Fast Track
Switching
Store & Forward
Switching
Fig. 3: The effect of a low-priority protocol in the output port on
automation protocols
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 this example, the minimum message transition delays
are approx. 160 μsec. 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 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 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
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
could overtake other messages if the latter are occupying a
required port. This means 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 cutthrough
method. The buffered IT message is then passed
on. Fast track switching guarantees message transition
delays at a higher performance than today’s field bus
systems.
Comparison of switching technologies
Fast track switching also has to become established in
the technological environment. Today’s established storeand-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. In this way, the topics
of vision and RFID are not derived from classic automation.
The devices do not generally support automation-specific
technologies. However, as a rule, they have an Ethernet
interface. Consequently, openness to standard Ethernet
also means openness to innovations.
Another effect is that fast track switching can also be used
for all automation profiles that support standard Ethernet
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