ATM NETWORKS FOR FACTORY COMMUNICATION Cseh, C.*, Janßen, M.* and Jasperneite, J.** * Dept. of Computer Science (Informatik IV), University of Technology Aachen (RWTH), Ahornstr. 55, D-52056 Aachen (GERMANY) T.+49241 8021414 F.+49241 8888220 E-mail:
[email protected] ** Phoenix Contact GmbH &Co., Technology Development P.O.B. 1341, D-32819 Blomberg, (GERMANY) T. +495235 341147, F. +495235 341065 E-mail:
[email protected] Abstract - In this paper the authors discuss the suitability of the Asynchronous Transfer Mode (ATM) as a data transfer technology for factory communications. The paper will focus on the special requirements of the sensor/actuator level to transmit real-time control data via an ATM network. A performance analysis of a potential ATM system on fieldbus level is presented by providing simulation results. The simulations study the influence of different network topologies, ATM cell scheduling strategies and the presence of video streams on the transmission of real-time control data.
1. INTRODUCTION In automation technology computer integrated manufacturing (CIM) plays an essential role in the control of production processes. Over the years there has been a development from a centralized approach to a distributed control environment, linking together autonomous systems with local processing power. This development led to more reliable and flexible automation facilities. Consequently the communication scenario on the factory floor shifted from a „vertical“ data exchange between a central controller and the attached devices to „horizontal“ data transmissions between components on the same level [1]. Nevertheless there is still a certain amount of „vertical“ communication between the different layers, because of the need to synchronize and coordinate different subsystems or to control complex processes in production plants. In recent years emerging data transfer technologies like ATM, originating from the office and home environments offer improved capabilities like the integration of multimedia streams on a single network with high bandwidths [2]. ATM enables the deployment of new multimedia applications in distributed control systems and will have great impact on the factory communication systems of the future [3],[4]. Applications like tele service, tele training or the use of audio and video for process surveillance will become more important, as high speed ATM networks, capable of providing quality of service guaranties will be installed. The remainder of this paper is organized as follows. Chapter 2 describes the requirements of factory
communication networks. In chapter 3 the need for an application interface for a distributed communication system is highlighted. In part 4 ATM is introduced as a new data transfer technology. The simulation scenario and set up for the sensor/actuator level are explained in chapter 5. In part 6 of the paper the simulation results with respect to transfer times for control data are presented. The simulations study the influence of different network topologies, ATM cell scheduling strategies and the presence of video streams on the transmission of real-time control data. Finally conclusions are drawn and the suitability of ATM networks for a factory communication system is discussed. 2. REQUIREMENTS In automation systems three layers can be identified with different data transfer requirements with respect to response time, frequency, length and lifetime [5]. In Fig. 1 the different layers and requirements are illustrated. Response time
Length
min
MBytes
ms
Bits
Lifetime hs.
Frequency rare
Factory Control / Process Sensor-Actuator
ms
often
Fig.1 Layer model of Factory Communication
In addition to these classical requirements innovative multi media applications impose new demands on the communication system with respect to real-time transmissions requiring high bandwidths. The integration of factory control data and audio/video traffic on a single network makes the guaranty for
quality of service, regarding bandwidth, delay and jitter, mandatory. The requirements for a distributed communication system in automation technology can be separated into three different aspects: performance requirements, functional aspects and physical characteristcs [6]. 2.1 Performance Requirements Tab. 1 presents the requirements of applications on the different layers and of multi media services with respect to the performance of the communication network. The combination of several real-time streams with different characteristics, like control data and video, is a challenging task, that is not easily fulfilled by contemporary network technology. Application Bandwidth FactoryLayer ControlLayer Sensor/ ActuatorLayer Video (MJPEG) Audio
Delay
Jitter
Frequency
Focusing on the lower two levels, control and sensor/actuator, the robustness of the cabling and the network components plays an important role, as the system may be used in a rough industrial environment. The communication system has to be insensitive against external hazards, like shocks, vibrations, torsion, humidity, dust, temperature, EMI, etc. Furthermore the exchange of components should be possible in full operation (hot swapable), without affecting the other network devices. Wit respect to the operating personnel the physical connectors of the network components have to be simple, intuitive and must have a common design and layout. It should be possible to operate, maintain and configure the network without the need of special knowledge in computer science or data communication technology. Ideally it is not necessary to make any adjustments at the attached end systems themselves (plug & play).
10 MBit/s
Data per station 4 MBit
500 msec
-
-
1 MBit/s
800 Bit
5 ms
1 ms
10 ms
1 MBit/s
16 Bit
1 ms
0,01 ms
1 ms
16 MBit/s
0.5 MBit
200 ms
1 ms
30 ms
Especially on the sensor/actuator level the simplicity of the communication system and the low cost of the equipment are key requirements.
64 kBit/s
8 Bit
200 ms
10 ms
125 µs
3. APPLICATION INTERFACE
Tab.1 Requirements for data transfer
2.2 Functional Aspects In addition to the performance parameters, a distributed factory communication system must provide a set of service primitives that implement certain functional aspects [6]. These include the timely transmission of data according to a given deadline, an acknowledged data transfer service, a prioritized transmission scheme and the addressing of single, multiple or all devices attached. Furthermore the communication system has to offer a gateway to other networks, security features for an authorized access to the devices and has to support the configuration of the network. With regard to the operation and maintenance of the communication system, a fault management for the monitoring of the network components has to be implemented. 2.3 Physical Characteristics As the communication system has to be used in an automation technology environment some physical requirements have to be taken into account. The cabling has to support different network extensions, ranging from 100 meters on the sensor/actuator level to several kilometers for interconnecting buildings on the factory level. The number of connected network end systems varies on the different layers. On the sensor/actuator level up to 100 devices may be attached, whereas on the process layer the data of approx. 10 production cells is collected. Again on the factory level many PCs, work stations and servers can be interconnected in an office environment.
For an application that wants to send data over a factory control network it is desirable to have access to the ATM service primitives via a common interface. Such a middleware abstracts from the actual implementation of the transfer services and hides any ATM specific parameters from the application (user). This common interface enables the deployment of different network technologies in an application transparent manner. For this reason new network techniques, like ATM, with improved performance can be introduced, without changing the application software. Many of the functional requirements, introduced in chapter 2 are not directly supported by the service primitives of an ATM network and need therefore to be implemented inside the middleware layer. Functions like a distributed, system-wide synchronized clock for checking the deadlines of a timed transmission or the management of acknowledgements from multiple receivers have to be provided by the middleware. Specific mechanisms like the retransmission of lost messages, if permitted by the timing constraints, or the maintaining of member information for multicast groups, should be handled by the interface software [6]. Complex functionality that is needed for the operation of the complete network, like a network management center or a security module can be implemented on the application layer, above the middleware, using its services. As mentioned in the previous chapter the guaranty of quality of service (QoS) parameters, like delay bounds
or minimum bandwidth, is crucial for the fulfillment of real-time requirements. Because ATM has its own special mechanisms and functions, introduced in the next chapter, it is the task of the middleware to map the user’s QoS-requests on the appropriate resources. The advantage of this approach is, that the service user does not need a detailed knowledge of the underlying network technology and is freed from configuring any special ATM dependent parameters. The selection of ATM traffic classes, transfer modes, etc. is taken over by the interface software [7]. With respect to all the demands that this middleware has to fulfil, it is clear that the design and implementation are very challenging, taking into account the high performance and the complex functions of the software. In addition all systems equipped with this middleware will need advanced hardware, regarding memory and processing power, to execute the software. 4. ASYNCRONOUS TRANSFER MODE (ATM) In this chapter the Asynchronous Transfer Mode (ATM) as an innovative high speed, real-time data transfer technology is introduced. Broadband networks, like B-ISDN, allow the integration of data and multimedia transmissions, with high bandwidth requirements and real time constraints into one network. The International Telecommunications Union - Telecommunication Standard Sector (ITU-T) and the American National Standards Institute (ANSI) have chosen the Asynchronous Transfer Mode (ATM) as a connection oriented switching and multiplexing technology for B-ISDN [8]. In 1991 the ATM Forum was founded to promote the use of ATM and to define specifications and standards for the deployment of ATM technology [9]. ATM is a connection oriented, cell-based multiplexing and switching technique. It operates on data packets of a fixed size (cells) of 48 byte data payload and a 5 byte header and enables high speed data transfer at constant or variable bit rates. The main advantages of ATM networks are the integration of data, voice and video streams, the guaranty of QoS characteristics for the connection, the scalability of the allocated bandwidth and the seamless transition from local to wide area networks [10]. An important feature of ATM is the support of QoS parameters for a connection. In order to guarantee a certain quality of service, a traffic contract is negotiated between sender, ATM network and receiver [11]. Traffic management functions, like the call admission control (CAC) or the usage parameter control (UPC), assure that the ATM network is able to provide the required performance [10]. Upon call setup a description of the traffic- and QoS-
characteristics is transported via the ATM signalling along the transmission path from the sender through the network to the receiver. All ATM network nodes (switches) along the path will check by their CAC functions, based on the signalling information, for the availability of the resources requested. If possible and if already existing connections are not negatively affected, the resources for the new connection according to its QoS demands are reserved. The UPC operates at the entry point of the ATM network and checks if the traffic stream originating from the sender corresponds to the traffic parameters negotiated at connection setup. A corresponding function is available between network nodes, called the network parameter control (NPC). In order to characterize the different data streams, transmitted over an ATM network, four different service classes, according to bit rate and QoS parameters, have been defined by the ATM Forum [12]. Two of them constant bit rate (CBR) and variable bit rate real-time (VBR-rt) offer a minimum bandwidth, maximum cell transfer delay, bounded jitter and a specified cell loss ratio for the connection. For the cabling in ATM networks shielded or unshielded twisted pair, optical fiber, plastic optical fiber or hard polymer clad fiber may be used. The communication distance varies with the media deployed and the bandwidth required. For the use on the sensor/actuator level at a bit rate of 155 MBit/s and distances of up to 100 meter unshielded twisted pair or multimode fiber is sufficient. If cable lengths are kept shorter (up to 50 meters) plastic optical fiber may be used. With hard polymer clad fiber the communication distance depends on the minimum bend radius. Due to the communication range of up to 1000 meters on the control layer, only multimode fiber (155 MBit/s) or single mode fiber (622 MBit/s) can be installed. On the factory level, with distances of several kilometers only single mode fiber may be used. With respect to network topology today’s ATM systems support a star topology, where individual end systems are connected to an ATM network node (switch). This structure might not be desirable on the sensor/actuator level where up to 100 devices are attached to a single controller. However there are first approaches to combine small ATM switching elements with network interface chips into a single device [13]. This will allow alternate network topologies, like an ATM ring or bus, which are examined in the next chapter. 5. SIMULATIONS For the performance evaluation of an ATM-based sensor/actuator network for automation systems simulations have been conducted [14]. As a simulation tool an event-driven simulation kernel developed at the Department of Computer Science IV has been used
[15]. The simulation program operates cell-based, which means that for each transmission of an ATM cell a single event is generated and processed. In order to evaluate the suitability of ATM for the transmission of real-time control data, different network topologies, different cell scheduling strategies inside the ATM switches and the influence of concurrent video transmissions have been investigated.
Sensor / Actuator ATM-Switch Controller 155 MBit/s 25 MBit/s
5.1 Network Topologies The network topology determines the transmission path of the control data through the ATM network. For all topologies it has been assumed that the data is generated at a sensor and is then transmitted to a controller for processing. There is a single controller in the ATM network that collects data from all sensors and sends out control commands to the attached actuators. Furthermore a certain degree of locality has been assumed, in the sense that the sensors generating the data are attached to the same ATM switches as the influenced actuator. The following network topologies have been chosen as typical candidates.
Fig. 3 Ring Topology
Circle. This is a special variation of the ring topology, where all sensors, actuators and the controller have a build-in ATM switching device. The network components are directly coupled by 155 MBit/s lines with their adjacent neighbors, see Fig. 4. This setup is realized by some of today’s fieldbus systems on the sensor/actuator level.
Star. In this network all sensors and actuators are attached via 25 MBit/s connections to a single ATM switch. The controller is connected to the central ATM switch by a 155 MBit/s line, see Fig. 2.
Sensor / Actuator + ATM Switch Controller + ATM Switch 155 MBit/s
Sensor / Actuator ATM-Switch Controller 155 MBit/s 25 MBit/s
Fig. 4 Circle Topology
Tree. The sensors and actuators are evenly distributed over 10 ATM switches, which are attached to a single root switch. All connections have a bandwidth of 25 MBit/s. The controller is linked to the root switch by a 155 MBit/s line, see Fig. 5. Sensor / Actuator
Fig.2 Star Topology
ATM-Switch
Ring. In this topology 11 ATM Switches are linked by a uni-directional 155 MBit/s ring. The sensors and actuators are evenly distributed over 10 ATM switches and attached via 25 MBit/s transmission paths. The controller is connected to the 11th ATM switch by a 155 MBit/s link, see Fig. 3. A bi-directional link of the 11 ATM switches would result in a bus topology, which is investigated in the following.
Controller 155 MBit/s 25 MBit/s
Fig. 5 Tree Topology
Bus (mid). This topology shows a bi-directional ring, which is interrupted at one point. 11 ATM switches are connected by bi-directional, 155 MBit/s links. The controller is attached to the ATM switch in the middle. The sensors and actuators are evenly distributed over the other 10 ATM switches and connected by 25 MBit/s lines, see Fig. 6.
Sensor / Actuator ATM-Switch Controller 155 MBit/s 25 MBit/s
Fig. 6 Bus (mid)
Bus (side). This topology is similar to bus (mid), with the only difference, that the controller is attached to an ATM switch on one side of the bus, see Fig. 7.
preferred for ATM cell scheduling [17]. They can be subdivided into two groups: simple and rate-based algorithms. Rate-based cell scheduling algorithms, like Weighted Fair Queueing (WFQ) or Virtual Clock (VC), provide a coupling between the allocated bandwidth and the delay requested. On the sensor/actuator level we face a combination of small data sizes (mostly only a few bits) and short response times. For this reason rate-based cell scheduling algorithms lead to over allocation of bandwidth and to poor link utilization.
Sensor / Actuator ATM-Switch Controller 155 MBit/s 25 MBit/s
In our simulations we compared three different simple cell scheduling algorithms: •
FIFO (first in, first out), where all cells are treated in the same way and no priorities are assigned.
•
Static Priority (PRIO), where the cells are inserted into 4 separate priority queues (alarm, fast control, slow control, other) according to Tab.2. Each priority class is served in a FIFO manner.
•
Earliest Deadline First (EDF), where the cells are scheduled with respect to their remaining tolerable delay. Cells with a delivery deadline in the near future are sent out first. Therefore the control data in the simulation is sent together with its requested arrival time.
Fig. 7 Bus (side)
5.2 Cell Scheduling Algorithms The data that is sent from the sensors to the controller and further to the actuators, consists of different data types, that have specific response times. Tab. 2 shows the response times and the occurrence of the different control data types used in the simulations. The response time is defined as the duration between the transmission of the sensor data and the reception of the control command by the actuator.
Data Type
Response Time
Occurrence
Alarm
100 µs
10 %
Fast Control
1 ms
20 %
Fast Control
10 ms
40 %
Slow Control
100 ms
20 %
Slow Control
1s
10 %
Tab. 2 Control Data Types
In order to ensure the cell transfer delay needed by the different real-time data streams, ATM switches implement a prioritization of cell streams. This is realized by executing an algorithm for the selection of the next cell inside the output queue to be sent and is called cell scheduling. There are two types of algorithms for cell scheduling [16].
5.2 Multimedia Transmissions One advantage of the use of ATM is the possibility to send video and audio streams in parallel to the realtime control data over the same network. In order to analyse the influence of video streams on the network performance one camera for video input and one display for output are connected to each ATM switch on the sensor/actuator level. At the switch where the controller is attached, a 155 MBit/s link to the control level is assumed, where the input video is processed and the video output is provided. As an example Fig. 8 illustrates the multimedia setup for the topology bus (mid). Control Level
Sensor/Actuator ATM Switch Controller 155 MBit/s 25 MBit/s
Work conserving strategies always schedule a cell for transmission if the output queue is not empty. Non work-conserving allow the link to go idle, even if there are cells waiting for transmission. Therefore these strategies lead to a higher average delay and average buffer occupancy than work-conserving algorithms. Due to their simplicity work-conserving strategies are
Display Camera
Fig. 8 Multimedia Setup
In order to accommodate several video connections
6. RESULTS
7,599 7,643 7,695
For the simulations it is assumed that all control data can be transmitted inside one ATM cell within the payload of 48 bytes and that there is no processing time needed inside the controller. The simulations covered a period of 10 seconds of system operation and the sensors generated control data according to the distribution shown in Tab. 2. If not otherwise stated, 100 sensor/actuator pairs are attached to the network, 10 to each ATM switch. The switching delay of the ATM switches is 30 µs. In most cases the 99% confidence intervals are below 1 microsecond and are therefore omitted. 0,938
1,0 0,9
# ATM switches crossed
max. buffer
min.
avr.
max.
occ. [cells]
Star
2
2
2
100
Tree
4
4
4
91
Bus (mid) 4
8
12
44
Ring
12
12
11
Bus (side) 4
13
22
11
Circle
201
201
3
In order to discuss the effects of the different scheduling strategies, the star topology was used. All other topologies showed in principle the same behavior. 0,6
Star
Tree
Minimum
0,3 0,2 0,1
Ring
Average
0,560 Bus (side)
1
PRIO
0,372 Bus (mid)
0,4
10
100
1000
Required Response Time [ms]
EDF
FIFO
Fig. 10 Average Response Time for different Scheduling Algorithms Circle
Maximum
Fig. 9 Response Times for different network topologies
Fig. 9 shows the minimum, average and maximum response times for the different network topologies using FIFO as a scheduling strategy. The star topology yields the shortest transmission times, but has a high cabling overhead, since all 200 devices have to be attached at a single ATM switch. Star and tree have large worst case response times compared to the average time, since the maximum buffer occupancy at the output port to the controller is very high, see Tab. 3. Both bus topologies show a large difference between the minimum and maximum response time, because the number of ATM switches crossed varies strongly from the position of the sensor/actuator pair, see Tab. 3. The ring provides a small variation of transfer times, because of its symmetrical layout. Due to the switch delay of 30 µs the circle provides unsatisfying slow response times and is therefore not considered any more in the forthcoming analysis.
Fig. 10 proofs that non of the scheduling strategies was able to meet the stringent transmission time of 0.1 ms. With the use of FIFO there are no differences for the 5 classes of control data. Earliest Deadline First (EDF) shows a linear increase as a response to relaxed timing requirements. Static Priority assigns 3 priority queues to the 5 classes of control data, which results in a stepping curve. FIFO performs very well in this scenario, because 10 % of the sensors belonging to the alarm message class generate 80% of the network traffic. These messages are handled by EDF and PRIO also in a FIFO manner. 0,6 Max. Response Time [ms]
0,0
0,5
0,1
0,209
0,1
0,124 0,162
0,2
0,209 0,238
0,3
0,209
0,4
0,496
0,5
201
0,0 0,548 0,559 0,599
0,677
0,6
12
Tab. 3 ATM switches crossed and buffer occupancy
0,7
0,411
Response Time [ms]
0,8
Topology
Aver. Response Time [ms]
with a high frame rate on a 155 MBit/s link, the video streams need to be compressed. Because the video pictures taken by the cameras on-site need to be compressed in real-time, M-JPEG is used. The average network load of one M-JPEG stream in the simulation was 3 MBit/s, with a peak rate of 4.2 MBit/s. For the video playback a MPEG-1 coded video was transmitted from the control level to all attached displays with an average bandwidth of 1.26 MBit/s.
0,5 0,4 0,3 0,2 0,1 0,0 0,1
1
10
100
1000
Required Response Time [ms]
PRIO
EDF
FIFO
Fig. 11 Maximum Response Time for different Scheduling Algorithms
14
With respect to the number of attached devices, Fig. 12 presents the maximum response time for the Static Priority (PRIO) class „fast control“. This class consists of messages with a required transmission time of 1 or 10 ms. It can be seen, that for the topology bus (side) this deadline is missed when 150 or more sensor/actuator pairs are attached. With 200 pairs connected to the ATM network also the bus topology with the controller on one side misses the requirement. All other topologies barely meet the requirements.
Max. Response Time [ms]
1,4 1,2 1 0,8 0,6 0,4 0,2 0 50
Tree
100 150 Number of attached Sensor/Actuator Pairs
Bus (side)
Star
Bus (mid)
200
Ring
Fig. 12 Maximum Response Times for Priority Class „Fast Control“
Regarding the integration of video transmissions, Fig. 13 illustrates the average transmission times for MJPEG and MPEG-1 frames for the scheduling strategies EDF and PRIO. All delays are below the required bound of 200 ms, but the maximum buffer occupancy at the link to the control level reached 1200 cells. With static priority the response times for the real-time control data were not affected. EDF assigns a higher priority to ATM cells carrying video data, than to control data having a tolerable delay of 1 s. Therefore these data cells are delayed in the presence of video transmissions. Nevertheless the required response time of 1 s could be met.
Aver. Transfer Times [ms]
12
Fig. 11 illustrates the more important maximum transmission times for the different scheduling strategies. Static Priority (PRIO) provides the best results for the message class with a required response time of 100µs, although the achieved transmission time of 175µs does not meet the deadline. EDF shows only little advantage over the FIFO strategy without any prioritization.
10 8 6 4 2 0
Star
Tree
PRIO MJPEG
Bus (mid)
EDF MJPEG
Bus (side)
PRIO MPEG1
Ring
EDF MPEG1
Fig. 13 Average Transfer Times for Video Frames
7. CONCLUSIONS From the requirements for factory communication systems introduced in chapter 2, it can be concluded that strict QoS guaranties for network connections like offered by ATM are the key issue for the reliable and safe operation of a plant control system. It is absolutely crucial that in a machine operated environment alarm and emergency messages as well as control information are transmitted on time and correct. Based on these assumptions ATM offers a most promising approach to meet these goals. ATM is the only data transfer technology that is connection oriented and provides traffic control functions, which can effectively assure the network performance for a single, logical connection. It offers a single, seamless network technology for all three layers in the automation environment, with a simple integration of wide are networks, like the Internet. New developments, like the integration of switching elements in end system devices, allow the deployment of ATM even on the sensor/actuator level in a bus or ring topology, using cheap copper wiring. Problems that still need to be solved include the configuration of ATM networks, the support of fault tolerant ring topologies and the high cost of ATM equipment. As discussed in chapter 3 applications in the automation environment can benefit from improved network services and performance only, if appropriate interfaces are provided. Therefore the adaptation of application layer protocols and the development of suitable middleware is an important task. The simulation results presented in chapter 6 proof that ATM is a suited network technology even for the sensor/actuator level, where stringent real-time requirements have to be met. Scheduling strategies help to isolate different priority classes, even though response times of 0.1 ms could not be reached. One advantage of the use of ATM is the transmission of multimedia streams over the same network, without affecting the control data. 8. REFERENCES
[1] M. Solvie, Zeitbehandlung und MultimediaUnterstützung in Feldkommunikationssystemen. Hanser, 1996 [2] C. Partridge, Gigabit Networking. AddisonWesley, 1994 [3] J. R. Pimentel, „Implications of Emerging Technologies on Factory Communication Systems“, Proceedings of the IEEE International Workshop on Factory Communication Systems (WFCS’97), Barcelona, 1997 [4] J.-F. Guillaud, M. R. Pokam, G. Michel, „Information Superhighway enters the manufacturing world“, Proceedings of the 3rd IEEE Workshop on the Architecture and Implementation of High Performance Communication Subsystems (HPCS’95), Mystic, Connecticut, 1995 [5] P. Drews, P. Fromm, „Advanced Robot Sensor Communication“, Proceedings of the 1995 IIW/IIS Conference, Stockholm, 1995 [6] C. Cseh, J. Jasperneite, „Emerging Data Transfer Technologies for Factory Communication“, Proceedings of the 24th Annual Conference of the IEEE Industrial Electronics Society, Aachen, 1998 [7] C. Shen, „On ATM Support for Distributed Real-Time Applications“, Proceedings of the IEEE Real-Time Technology and Applications Symposium, Boston, 1996 [8] ITU-TSS (CCITT), „Recommendation I.321, BISDN Protocol Reference Model and its Application“, Geneva, 1991 [9]
The ATM Forum, http://www.atmforum.com
[10] M. DePrycker, ATM - Solution for Broadband ISDN. Prentice-Hall, 1995 [11] D. E. McDysan, D. L. Spohn, ATM Theory and Application. McGraw-Hill, 1994 [12] ATM Forum Technical Committee, „Traffic Management Specification, Version 4.0“, ATM Forum 1996 [13] B. Maaref, S. Nasri, P. Sicard, „Performance Evaluation of an MMS/ATM Implementation“, Proceedings of the IEEE International Workshop on Factory Communication Systems (WFCS’97), Barcelona, 1997 [14] M. Janßen, „Evaluation of ATM Deployment in Automation Systems“, Diploma Thesis, University of Technology Aachen, Dep. of Computer Science IV, 1998 [15] P. Davids, „ATLAS Version 6.0 Reference Manual“, University of Technology Aachen, Dep. of Computer Science IV, 1993
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