Real-time behaviour of Ethernet on the example of
PROFINET
Heitzer Bernhard#1
1
bernhard.heitzer@stud.hs-regensburg.de
#
Maschinenfabrik Reinhausen
Falkensteinstrasse 8
93059 Regensburg
Jürgen Mottok#2
2
Juergen.Mottok@hs-regensburg.de
#
Laboratory for Save and Secure Systems, HS-Regensburg
Seybothstrasse 11
93059 Regensburg
Abstract: Ethernet is used more and more in industrial
environment. In these applications it is a basic
requirement to provide real-time communication. This
paper describes the limitations why standard Ethernet
cannot be used as a real-time system and which
additions have to be implemented to make it real-time
capable. As example the real-time Ethernet system
PROFINET is described in this paper along with an
illustration of some performance measures.
Keywords: Ethernet, real-time, PROFINET, automation,
performance
1. INTRODUCTION
The usage of Ethernet in automation has grown
continuously in the last years, where Real-time Ethernet
systems represent the main factor for its strong use.
Ethernet is not only used because of its real-time ability,
but rather because of the possibility for an easy
integration in an existing IT-network.
In automation many different requirements have to be
met for real-time. Also, there are various basic approaches
to accomplish real-time ability for Ethernet. The first part
of this paper describes the most important requirements in
a real-time Ethernet application. Furthermore fundamental
problems are considered which must be addressed to
create a real-time Ethernet system which meets the given
requirements.
The last chapter shows the principles of the real-time
Ethernet system PROFINET and which measures are
taken to meet the real-time requirements. Additionally
some theoretical performance examples illustrate
transmission times of real-time Ethernet frames and
update times in a PROFINET system.
2. REQUIREMENTS FOR A REAL-TIME ETHERNET
SYSTEM
A function in a system is executed within a defined
interval, this is the specification for real-time. This
interval is defined between R (Release), the earliest start
time of the function, and D (Deadline), the latest end time
for the function. If the function is not finished within this
interval the processed data is corrupted. Real-time is
given when the demands of punctuality and simultaneity
are met. Punctuality means the function is finished before
D and simultaneity describes the cyclic occurrence of an
event in a given interval.
The update time of the process data is divided in three
classes (Table 1. Quality of Service (QoS)). These
definitions are based on existing application-classes from
fieldbus-systems.
Table 1. Quality of Service (QoS)
QoS Class
Application
QoS Latency
1
Controller-toController
Distributed I/ODevices
Motion Control
100ms
2
3
10ms
<=1ms
3. FACTORS WHICH INFLUENCE A REAL-TIME
ETHERNET SYSTEM
An entire switched Ethernet network could be seen as a
system of queuing devices which influence the quality of
communication. The scheduling strategies to be used in
these devices are defined in the IEEE Standard. Possible
strategies are first-come first-serve (FCFS) and nonpreemptive priority queuing (PQ) algorithms. FCFS
schedules all frames in an equal way. The forwarding
time is short because of the fact that this algorithm is very
simple. The PQ algorithm allows to prefer specific frames,
where frames with a high priority are forwarded first.
However this could lead to longer forwarding times for
some frames, especially in huge Networks with Line
topology. Also protocol runtimes and the time for frame
generation have to be taken into consideration for the
transmission time.
Category A Systems are based on the standard Ethernet
TCP/IP protocol and use standard HW. They can be used
in normal IT-Networks and allow a simple integration in a
corporate network. The communication is not
deterministic and mainly influenced by the TCP/IPProtocol runtimes.
Devices are identified by the IP-Address and for this it
is also possible to transmit frames beyond the local
network.
4. IMPLEMENTATION OF REAL-TIME ETHERNET
In a standard Ethernet system the CSMA-CD protocol
is used, which can lead to unpredictable behavior because
of the collision detection. In modern Ethernet systems
only full-duplex switched-Ethernet is used and for this
there are no frame-collisions on the wire.
Priority based scheduling like PQ could be
implemented by using IEEE802.1D/Q. So it is possible to
separate time critical frames and non-critical frames from
each other to fulfill the QoS class 2 requirements.
In a switched network the transmission time for time
critical frames increases with the amount of switches.
This is why a standard Ethernet system cannot be used for
time-critical applications like motion control (QoS Class
3).
To use Ethernet in QoS Class 3 applications a method
like TDMA has to be used. With TDMA the bus cycle is
divided in different time-slots. The advantage of this
method is that the transmission time can be calculated
from the beginning on.
A bus cycle has to start at the same time on every
device to coordinate the time critical and non-critical
time-slots. In order to achieve that synchronization
between the devices is necessary. A common standard for
time synchronization is the Precision Time Protocol (PTP).
PROFINET for example uses its own synchronization
protocol which is called PTCP. This algorithm is nearly
identical to PTP. By using PTCP in a switched Ethernet
system a jitter <1µs is possible.
Common available real-time Ethernet systems can be
assigned into three categories. Distinguishing features are
the grade of compliance to the Ethernet standard (which
includes the possible use of standard Ethernet HW) and
the performance (Figure 1).
Figure 1. Categorization of real-time Ethernet systems
In category B time-critical frames are not sent with the
IP-Protocol anymore. This results in shorter protocol
runtimes in the devices. Devices are now identified by the
MAC-Address. Non-time critical frames can be sent in
addition with IP-based services also beyond the local
network.
The category A and B systems both use prioritization of
the time-critical frames within the VLAN-tag which is
defined in IEEE802.1D/Q.
Sending time-critical frames in category C is also
performed without the IP-Protocol. Besides that TDMA is
used in this category for scheduling and therefore
standard Ethernet HW cannot fulfill these requirements
anymore. Here both facts shorter protocol-runtime and a
deterministic access lead to meet the requirements of QoS
Class 3. Identical to category B it is also possible to send
non-critical frames.
5. REAL-TIME ETHERNET SYSTEM PROFINET
PROFINET is a Master-Slave system. The Master (IOMaster) can send frames to all devices in the network as
well as receive frames from all devices. A Slave (IODevice) has the possibility to send frames to the IO-
Master only. Cross traffic to other IO-Devices is only
possible non-cyclically.
The communication concept of PROFINET is designed
modular. Functionalities in communication differs in the
way of data transfer to meet the different performance
requirements.
PROFINET IO specifies the transfer of I/O-Data for
peripheral devices such as sensor or actor. PROFINET IO
includes real-time (PROFINET RT) and isochronous realtime (PROFINET IRT) communication with the periphery.
For a simpler use of PROFINET IO in applications
three application classes are specified. Each application
class has its own minimum requirements. Their categories
are:
• Applications
with
unsynchronized
communication.
• Bus-synchronization
with
unsynchronized
applications.
• Synchronous applications.
In addition to the application requirements there are
additional determinations for the devices, method of
communication (RT, IRT), transmission medium and
redundancy.
A. Conformance Class A
Conformance Class A allows the usage of standard
Ethernet Hardware. CC-A includes cyclic RTcommunication and acyclic TCP/IP-communication.
RT-frames are priorized as described in IEEE802.1D/Q
and transmitted on base of the MAC-address. These
frames are preferred against non time-critical TCP/IPframes. The cycle for the transmission of an RT-frame has
to be defined by the user. There is no synchronization
between the devices. In PROFINET this method of
sending RT-frames is called RT-Class 1 (RTC1). Further
PROFINET features are:
• Alarms/Diagnosis
• Automatic address-resolution
• Neighborhood detection
• 100Base Tx/Fx
• Wireless-communication
B. Conformance Class B
CC-B includes the functionalities from CC-A plus the
additional features:
• IO-Device cross transmission
• Device swap-out without engineering tool
• Media-redundancy-protocol MRP
C. Conformance Class C
Conformance Class C extends the functionalites of CCB with the fundamental difference of the synchronization
between the IO-Devices. In PROFINET this method of
communication is called isochronous real-time (IRT).
Cyclic and acyclic communication is separated by
TDMA as it is shown in Figure 2. The first time-slot is
reserved for the cyclic IRT-communication. Every IODevice has to send his cyclic RT-frames within the IRTinterval. For the acyclic TCP/IP communication there is a
reserved second time-slot. Using CC-C it is possible to
reach bus-cycles of 250µs.
Figure 2. PROFINET TDMA process
Synchronization of the IO-Devices in CC-C is
implemented with the PTCP and so all IO-Devices work
with the identical time-base. Using PTCP it is possible to
synchronize the IO-Devices with max. Jitter of <1µs.
The IRT-communication uses two different methods for
sending RT-frames, RTC2 and RTC3, which differ in
performance.
D. RT-Class-2
For the transmission in the IRT-interval all switches
create an address-table on start-up. With this table it is
possible to route RT-frames on base of their source- and
destination-MAC-address. The routing mechanism on
base of the MAC-address allows to change the topology
of the network, but it must be ensured that the
transmission of an RT-frame is finished within the IRTinterval. This is the reason why in RTC2 the IRT-interval
has an additional time reserve as it is shown in Figure 3.
The capacity of the available bandwidth is not optimal
amongst others in RTC2 because of the throughput time
(ca. 10µs) in the switch. This period of time is needed to
evaluate the source and destination MAC-address.
This duration is called Propagation Delay pmax . The
Propagation Delay can be determined exactly by using the
PTCP.
G. Bridge Delay: q
The throughput time for a frame from one port to
another port in a switch is called Bridge Delay q . An
estimated value for the Bridge Delay in PROFINET
Figure 3. PROFINET IRT RTC2
switches is q max = 10 µs for RTC1/RTC2 and ca.
qmin = 3µs for RTC3 (the exact value is given in the
E. RT-Class-3
In RTC3 there is no time reserve left in the IRT-interval.
This will only work under the prerequisite that the
topology of the network is known before startup. On base
of this information the communication plan hast to be
calculated before. The calculation with an engineeringtool, that also requires throughput times of the switches.
In RTC3 this throughput time is approximately 3, 5µs.
The exact throughput times for RTC1/RTC2/RTC3 are
available in the PROFINET GSD-file which can be
processed by the engineering-tool. Every PROFINET
manufacturer has to offer a GSD-file for the produced
Device, so throughput times for every Device are known
by the engineering-tool.
Forwarding of RT-frames in RTC3 is not based on
MAC-addresses anymore. The network topology is
planned with the engineering-tool before startup and with
this information the engineering-tool creates a table for
the RT-frame routing. This table is stored on every
involved switch. Now it is possible to make accurate
predictions when and where a specific RT-frame is
forwarded. This means that the communication is
deterministic.
Worst-case scenario for IRT-communication is a line
topology. In this case the defined send order causes that
the RT-frame for the last IO-Device in the line-topology
to be sent first. When the first RT-frame is on its way, the
second RT-frame is sent to the second to the last IODevice.
6. PERFORMANCE CALCULATION
F. Propagation Delay: p
The Ethernet standard specifies a maximal section
length of 100m between switches for cooper wire.
Electrical signals spread at a speed of ca. 200000km/s in
cooper. The maximal duration to send one bit within one
section is:
100 ÷ 200000 km / s = 0 ,5 µs
GSD-file). For further calculation also a variable d is
specified which represents the number of IO-Devices in
the network.
H. Transmission Delay: t
One Ethernet-frame has a specified header-length of 34
Bytes and has an additional CRC-value with 4 Bytes. The
data-field has a minimum size of 46 Byte and therefore
the minimum size of one Ethernet-frame is 84 Byte. If the
data-field is completely filled the maximal size of one
Ethernet-frame is 1538 Byte. The PROFINET system
runs with a speed of 100Mbit/s. The calculated
transmission time for one Ethernet-frame with minimum
size t min is:
84 Byte ÷ 100 MBit / s = 6 , 72 µs
and for one Ethernet-frame with maximum size t max :
1538 Byte ÷ 100 MBit / s = 123 ,04 µs
I. Transmission Time without waiting Time:
Tmin
The example in Figure 4 shows a calculation for the
transmission time of one PROFINET frame (minimal size
84 Byte) from the IO-Master to the IO-Device 4. The
Mathematical formula for the calculation in Figure 4 is:
Tmin = d * (qmax + p ) + t min
Since the bridge delay is 10µs and because of this a
RTC1 or RTC2 communication must be used.
Nevertheless, it could happen that a RTC frame has to
wait until another frame is transmitted on the wire, as
shown in the next example.
RTC-frame. The IO-Master transmits four RTC3-frames
(minimum size 84 Byte) one for each IO-Device. The
Mathematical formula for the calculation in Figure 6 is:
U = d * t min + q RTC 3 + p
4 * (10 µs + 0 ,5 µs ) + 6 ,72 µs = 48 ,72 µs
Figure 4. Min. Transmission Time
J. Transmission Time with waiting Time: Tmax
The example in Figure 5 shows a calculation for the
transmission time of one PROFINET frame (minimal size
84 Byte) from the IO-Master to the IO-Device 4.
The Mathematical formula for the calculation in Figure
5 is:
Since IO-Device 2, 3 and 4 receive the frames before
the IO-Master transmission is finished, q and p are
considered only once.
The previous examples calculate how long it takes to
transmit one PROFINET-frame. In practice it is common
that more than one frame is sent in one bus-cycle. Every
time more than one frame shall be sent the second and
further frames are queued until the previous frame has
been forwarded. In Figure 6 RTC3 communication with
the TDMA method is used and, additionally the IOMaster has knowledge of the network topology. As
described earlier the IO-Master builds a send order. The
frame for the last IO-Device in the line is transmitted first.
This method saves time, because the forwarding time is
optimized.
Tmax = d * (q max + p ) + t min + (d − 1) * (t max + q max )
The calculation is influenced by the fact that the RTC
frame has to wait before it can be forwarded. Again a
bridge delay of 10µs is used. In this worst case the
transmitted frame has to wait at every IO-Device until a
maximum UDP-frame (size 1538 Byte) is transmitted on
the wire. The UDP-frame was sent just before the
PROFINET frame has arrived. This leads to a waiting
time for the PROFINET frame of 123, 04µs at every IODevice.
4 * 6,72 µs + 0 ,5 µs + 3 µs = 29 ,93 µs
Figure 6. Updating time
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& !"!' (
4 * (10 µs + 0,5µs ) + 6,72 µs + ( 4 − 1) * (123,04 µs + 10µs ) = 447,84µs
Figure 5. Max. Transmission Time
K. Updating Time with RTC3: U
The example in Figure 6 shows a calculation for the
Update time of the process data. This is the time how long
it takes that every IO-Device in the network gets one
The Update time also gives a reference for the duration
of a bus-cycle for a specific number of IO-Devices. In
opposite it is also possible to calculate how many IODevices can be connected to a PROFINET network with a
given bus cycle time. The Mathematical formula for the
maximal number of IO-Devices for a given bus-cycle is:
d = (t cycle − p − q min/ max ) ÷ t min/ max
E.g. how many IO-Devices can be updated with a buscycle time of 250µs if every IO-Device is updated with a
minimum RTC3 frame?
(250 µs − 0,5µs − 3µs ) ÷ 6,72 µs = 36,68 ≈ 36
Solution: With RTC3 it is possible to update 36 IODevices with a bus-cycle time of 250µs (the result has to
adjust downward).
CONCLUSIONES
As described Ethernet especially real-time Ethernet,
can be considered as an adequate communication system
for automation and it is suitable even for applications with
hard real-time requirements a short cycle-times (e.g.
motion control).For automation applications the
advantage to integrate the communication in existing ITnetworks without losing the real-time characteristic gives
additional benefit.
The real-time Ethernet system PROFINET was
explained in some of the most important details. The final
practice view from the application side covers especially
how it is implemented and the performance of the realtime communication.
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2007
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Heitzer,
„Evaluierung
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EchtzeitKommunikationssystemen für die Automatisierung
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