Today a lot of discussion is done about industrial and deterministic networks. Evolution of technology in combination with heavy industry requirements make industrial networks a necessity. Networks in this scientific field have the oddity that they often need to support real time applications. For this to be done often mathematical models are used in combination artificial intelligence. To be able to model real-life decision problems, there is a requirement to specify and reason with probabilistic and deterministic information.
It is critical at this point to define what an industrial network is. An industrial network is used to connect various devices for factory automation and process control applications. These networks which are typically referred to as "buses", have as basic components sensors, actuators and controllers. The functions that supervise the whole system are usually automatic and require no human interference. An exception is done when the system encounters errors which cannot correct by itself. An industrial network provides data link services at layers 1 and 2 of the OSI model and various commands at layer 7. Some functions in layer 3 through 6 may be partially or completely provided Fig1. Industrial networks can be formed by many industrial protocols like PROFIBUS, DeviceNetâ„¢, CAN, InterBus and Foundation Field Bus, or newer industrial Ethernet protocols, which include PROFINET, EtherNet/IPâ„¢, Powerlink, EtherCAT®, Modbus® TCP and SERCOS III.
The subject of this survey is to analyze every possible aspect of these networks, to describe algorithms and protocols that are used here. Various problems, limitations and drawbacks that are met will be described here also. The structure of is survey is organized as described below.
The first section contains brief descriptions of protocols that are used today. In the second part problems and various issues that are met are discussed. In the third part Models and algorithms that are used are described and finally on the fourth part the various aftermaths are analyzed.
Network models have a wide usage in operations research. They are used for a large number of diverse applications that include transport of goods, design of communications and pipeline systems, assignment of people to jobs, routing of vehicles, bid evaluation and production planning. Network models are a building block in many other formulations such as plant location, manufacturing models and cash flow analysis.
Osi-model.png
Fig 1. The OSI reference model [1]
By the term determinism it is meant that each node (actuator, controller etc) has a predefined access time t on the network. That way equal and fair access to the network by all the components is ensured. Some essential features in industrial networks are [2]:
Response times that can be predicted.
Affordable nodes.
Ensured compatibility with existing systems.
Control architecture.
Data transfer that is connectionless.
Environments that can be presented.
Ability to tolerate faults.
Network Synchronization
Latency that can be predicted
Many media participating in the network.
Peer to Peer (P2P) architecture
Affordable development cost
Open Architecture
Existence of API (Application Programming Interface)
Topology which is connection based
Efficiency
Data channels which are multicast
Compliance with the OSI model
Open availability and support
Object orientation
There is a lot of discussion about deterministic protocols should or should not be used in Industrial networks as time t is known. Many researchers claim that only deterministic protocols should be used. The truth is that it is not a matter of protocol selection, but it is mostly a matter of the layers above the MAC sublayer whether a network will function.[2]
Protocols that are used generally in industrial environments are divided into
Token passing protocols
Polling based protocols
Priority arbitration protocols
Protocols
A brief description of most common protocols used in industrial networks follows.
The first protocol is the ProfiBus protocol one of the most popular FieldBus systems. Its origin comes from German Government in cooperation with automation manufacturers which in 1989 implemented on ASIC chips produced by multiple vendors. It is based on RS485 and on the European EN50170 Electrical specification.
The formats it supports are: ProfiBus DP (Master/Slave), ProfiBus FMS (Multi-master/Peer to Peer which is a peer to peer messaging format, which allows masters to communicate with one another. In this case scenario just as in ProfiBus DP there is a possibility that up to 126 nodes are available and all can be masters if desired. It is found that FMS messages consume more overhead than DP messages), and ProfiBus PA (intrinsically safe). It uses the following connectors: 9-Pin D-Shell connector or 12mm quick-disconnect. The maximum number of nodes it can support is 127 whereas the maximum distance is from 100m to 24km including repeaters. The Baud rate varies from 9600 to 12M Bit/sec. The message size is 244 bytes of data per node per message. The messaging format that is used are Polling, Peer-to-Peer. The main support organization is the ProfiBus Trade Organization (www.profibus.com)
The Typical Applications that this protocol is used are 1) Process Control and large assembly, and material handling machines. 2) In Single-cable wiring of multi-input sensor blocks, 3)In pneumatic valves, 4) In complex intelligent devices, 5) In smaller sub-networks (such as ASI), and operator interfaces. The Advantages that ProfiBus is the most widely accepted international networking standard. It is widely used in Europe and also popular the American continent and to some parts of Africa and Asia. The main characteristic of ProfiBus is that it can handle large amounts of data at high speed and serve the needs of large installations. The various versions it has cover the majority of automation applications and needs. The main Disadvantages here are high overhead to message ratio for small amounts of data, the fact that there is no power on the bus and finally the slightly higher cost than some other buses.[3]
Another important protocol is DeviceNet: It is the FieldBus that fits best for Low and Mid-Level Factory Networking. Its Origin is from Allen-Bradley, which constructed it in 1994. It is based on Controller Area Network technology, which is borrowed from the automotive industry, and the RS485 electrical specification. The Maximum Number of Nodes it can support is 64. It is based on popular 'Mini' 18mm and 'Micro' 12mm waterproof quick-disconnect plugs and receptacles, and 5 pin phoenix terminal block connectors. The Distance of operation varies from 100m to 500m. The Baud rate is 125, 250 and 500 Kbits/sec. The Maximum Message size is 8 bytes of data per node and per message, while the Messaging formats are polling, strobing, change-of-State and cyclic. It is mostly based on the model of producer/consumer. The Supporting Trade Organization of this protocol is open DeviceNet Vendor Association (www.odva.org). The typical applications that are found are in assembling, welding and material handling machines. Also it is very common in single-cable wiring of multi-input sensor blocks, in smart sensors, in pneumatic valves, in barcode readers, in drives and operator interfaces. The main advantages here are the affordability, the popularity the high reliability, and capability of efficient use of network bandwidth, and the fact that power is available on the network. The main drawbacks are the limited bandwidth, the limited message size and maximum length.[3]
The third protocol used in industrial networks is ControlNet. Its main purpose is for the high level, mission critical FieldBus. It was designed and produced by Allen-Bradley in 1995. It is based on RG6/U cabling and Rockwell ASIC chip. The maximum number of nodes that can be supported are 99. The connectors that are used here are twin redundant BNC, whereas the maximum distance is 250 to 5000M with repeaters installed. The baud rate is 5M bit/Sec and the message size varies from 0 to 510 bytes. The messaging formats that are met here are based on producer/consumer model; multi-master, peer to peer, fragmented, prioritized and deterministically scheduled repeatable messages; dual transmission paths for built-in redundancy. The organization that supports it is ControlNet International (http://www.controlnet.org). It is used mostly in critical mission areas such as networking among multiple PC's, PLC's and sub-networks. It is also found on process control and in situations that require high-speed transportation of both time-critical Input/Output and messaging data. Such examples are uploading /downloading of data configuration programming and peer-to-peer messaging.The advantages that it has are 1)Determinism, 2)repeatability, 3)efficient use of network bandwidth, 4) It is able of providing redundancy at lower cost compared to the most other existing networks including Ethernet. It can be transmitted on any IP transport protocol through Ethernet, Firewire or USB. The disadvantages that it has are limited support from multiple -vendors support.
After that, InterBus is the next protocol. Some characteristics of this are : the High Speed, Maximum Diagnostics FieldBus. It was discovered by Phoenix Contact, 1984. The maximum number of nodes that can be supported are 256. It is using 9 Pin D-Shell and 23mm circular DIN connectors. It offers many cabling options which allow twisted pairing connections, fiber optic connections, slip ring, infrared or SMG connections. The maximum distance it can support varies from 400m to 12.8 km Total. The baud rate is 500 Kbits/sec, while the message size is 512 bytes of data per node and can support unlimited block transfers. The messaging formats include Input output scanning and PCP channel for data transfer. The organization that Supports this bus is the InterBus Club which can be found at (www.interbusclub.com). The typical applications that this bus is used are in assembling lines, in welding machines and general in production lines. Other use includes multi-input sensor blocks where singe cabling solution is applied. Also in pneumatic valves, on barcode readers, on drives and operator interfaces. Additional usage is with Sensor Loop and AS-I sub-networks.
The advantages of this bus include auto-addressing capability which makes startups very simple. Extensive diagnostic capability is another feature while it has gained widespread acceptance (especially in Europe), in addition it has low overhead, it responds rapidly and is efficient in using bandwidth. It also supports powering options for input devices that available on the network. On the drawbacks one could include entire network breakdown by a failed connection and limited ability of transferring great amounts of data.
Furthermore, EtherNet is the following protocol. Ethernet: The Worldwide Defacto Standard for Business and PC Networking. It was discovered by Digital Equipment Corporation, Intel and Xerox, 1976 and it was implemented on Multitudes of chips produced by many vendors. It based on IEEE 802.3
Formats: 10 Base 2, 10 Base T and 100 Base T, 100 Base FX, 1 Gigabit; Copper
(Twisted Pair / Thin Coax) and Fiber
Connectors: RJ45 or Coaxial
Maximum Number of Nodes: 1024, Expandable with Routers
Distance: 100M (10 Base T) to 50 KM (Mono mode, Fiber with Switches)
Baudrate: 10M to 100M Bit/sec
Message size: 46 to 1500 bytes
Messaging format: Peer-to-Peer
Supporting Trade Organization: Industrial Ethernet Association
(www.IndustrialEthernet.com) and IAONA
(www.iaona.com).
Typical Applications: Nearly universal in office / business Local Area Networks. Widely used
also in PC to PC, PLC to PLC and supervisory control applications. Gradually working its way toward the "sensor level" in plant floor applications.
Advantages: Ethernet is the most widely accepted international networking standard. Nearly universal worldwide. Ethernet can handle large amounts of data at high speed and serve the needs of large installations.
Disadvantages: High overhead to message ratio for small amounts of data; No power on
the bus; Physically vulnerable connectors and greater susceptibility to EMI/RFI than most fieldbuses; Confusion based on multiple open and proprietary standards for process data.
Characteristics of Industrial Networks/ Requirements
3.1 The environment
The industrial environments present some specific issues and requirements. For
example, the I&C domain concerns instrumentation, supervisory and control of the
processes. I&C focuses mainly on three levels that can be represented as below:
This functional architecture has several level-to-level interfaces requirements that need to be achieved by the communications network. These requirements are:
• Level 0 to level 1: real-time control (persistent stable duplex communications links), deterministic data transfer (robustness of the LLC stack), short ranges communications, always-connected, lower data rate communications.
• Level 1 to level 2: wide range communications, hybrid-type physical medium, not-always connected, higher data rate conmiunications, possible data aggregation.
3.2 The benefits from going wireless
The benefits one can expect from going wireless for a sensor network in the I&C
domain can be listed:
• cables cost reduction
• mobile points of acquisition
• self-healing communications architecture
" low-power consumption
• adaptable topology (star, tree, mesh)
• ad-hoc communications
• harsh wiring conditions and difficult environment [Wireless Networks in industrial
environments: State of the art and Issues]
3.3 Reliability: High reliability is a key requirement in process automation. Reliability is a measure of the percentage of transmitted data packets received at the receiver node .
3.4 Interference: Wireless is an open access medium and therefore, operation in a license free band network will need to be immune to interference. Adaptability to both short-term
and long-term interference is vital.
3.5 Delivery: Wireless networks can be affected by noise and interference which can degrade channel performance. Appropriate and timely measures should be adopted to provide data retransmission in case of packet loss.
3.6 Mobility: The network should offer connectivity to both static (e.g. fixed equipment) and mobile (e.g. user terminal) nodes in the field.
[ Wireless Communication in Process Automation: A Survey of Opportunities,
Requirements, Concerns and Challenges
Waqas Ikram & Nina F. Thornhill]
Problems and Challenges
Energy supply and low power operation: In some fieldbus systems the same cable can be used for communication purposes as well as to supply a station with energy. If the cabling were to be dropped completely, alternative ways to supply stations with energy would have to be found. Some options are wireless energy transmission, energy-scavenging methods or using batteries. For battery-driven stations, energy is a scarce resource and should be used economically. Replacing batteries may be infeasible or can lead to machine downtimes. Several mechanisms to conserve energy in protocols and applications have been developed in the context of wireless (sensor) networks . In the design of fieldbus protocols, however, the main concern was real-time communications, not energy-efficiency. There are efforts to combine both targets .[ Wireless Technology in Industrial Networks ]
Real-time behaviour:
Real-time operation is necessary for control functions. In this connection, "real-time" means
that the system must be able to response to control requests timely, so that corrections still
have their desired effect on process operation. This presumes both the real-time operation
system and data transfer; deterministic operation is the most important requirement.
Real-time requirements depend on the application and they can be divided in four
categories (Neumann, 2007):
non-real-time applications in diagnosis, maintenance, commissioning, slow mobile
applications
soft real-time applications: in process and factory automation, mainly in data
acquisition and monitoring
hard real-time applications in process and factory control, fast mobile applications,
machine tools
isochronous hard real-time applications especially motion control.
[Wireless Sensor Networks
in Industrial Automation
Marko Paavola and Kauko Leivisk]
Wireless Networks (not wired): Although wired sensor networks can also be deployed for this kind of applications, WSNs present additional advantages. In fact, their wireless capability allows deployments in hostile environments, where vibrations or moving parts may prevent the use of cables that would be damaged or even broken. In addition to reduce cabling costs, the WSNs provide network flexibility, as the sensor nodes may be relocated quickly without necessitating time-consuming cable installation and maintenance. [Survey on Wireless Sensor Network Technologies for Industrial
Automation: The Security and Quality of Service Perspectives
Delphine Christin 1;?, Parag S. Mogre 2 and Matthias Hollick]
Problems with wired technology:
Pre-planning requirements, higher installation and maintenance costs of the wired network .
Difficulty in troubleshooting connectors .
Less flexible infrastructure due to fixed connections.
Wired networks have to be designed with spare capacity on cards, marshalling cabinets, junction boxes and so forth, to cater for future expansion .
Rotating equipment cause constant twisting of cables which results in fatigue and communication failure.
The use of wireless technology can assist the industry to overcome the limitations of wired networks, and benefit from the mobility and design freedom it offers.
Harsh environment and rotating equipment: The use of wired communication is limited in certain applications due to either technical or economical reasons. Access to harsh environments and rotating equipment are some examples. Wireless communication offers an opportunity to
access these locations and replace slip-rings and festoon cable .
[Wireless Communication in Process Automation: A Survey of Opportunities,
Requirements, Concerns and Challenges
Waqas Ikram & Nina F. Thornhill]
The main concern of an industry management staff is to find a solution that is sufficiently cheap
to justify its purchase and at the same time is robust and guarantees that it can handle all plant equipment.
Models and Algorithms
Security Issues
Security: The wireless medium is an open medium and without countermeasures, it is easy for an attacker to eavesdrop, to insert malicious packets, or to simply jam the medium, this way challenging reliable and timely transmission. [Wireless Technology in Industrial Networks
Andreas Willig, Member, IEEE, Kirsten Matheus, Member, IEEE, Adam Wolisz, Senior
Member, IEEE]
Eavesdropping, insertion of malicious packets and producing destructive interference (jamming) would require to physically tap a cable, which can be prevented by simple administrative measures.
Fieldbus systems were supposed to be used exclusively in the manufacturing plant with no direct connection to other networks like the Internet. Therefore, the nowadays common threats like hackers, denial-of-service attacks, viruses, etc. were not anticipated. Both reasons are not valid anymore. Todays automation networks tend to be more and more integrated with other
networks, for example to allow cost-effective remote monitoring and maintenance of machine plants. There are many techniques to protect a network against attackers from outside, for example firewalls. For this source of threats the (wireless) fieldbus system needs no own security mechanisms, since firewalls are typically placed at the fringe of factory Intranets, some hops away from the fieldbus. But when using wireless media an attacker which is close enough to the network (say, on a company's parking lot) can do the following things:
eavesdropping: an attacker might record process data and commands. Encryption could be used to prevent this but since the set of messages transmitted from a sensor / towards
an actuator tends to have low entropy cryptanalysis is comparably easy and may render the extra overhead for encryption useless if no additional measures are taken.
jamming: an attacker might generate noise and prevent any useful transmission, this way harming reliable and timely data transfers (denial of service). One way to prevent jamming from outside the manufacturing plant is to weave metal threads into its walls to create a Faraday cage, or to use narrowband-jamming-resistant transmission schemes like spread-spectrum communications. Further mechanisms have to be developed.
injecting packets: an attacker might generate false sensor data or malicious command packets for actors, send management packets to include or exclude stations from a network etc. To prevent this, mechanisms for ensuring authentication ("who sent this message?") and message integrity ("is this the message originally sent?") are needed to create mutual trust relationships between mobile stations and access-points / wired-towireless gateways. Such mechanisms are often implemented using shared secrets and public key cryptography, calling in turn for proper key distribution protocols. To avoid replay attacks proper sequence numbers / session keys have to be introduced into these protocols. In the case of hybrid systems, these protocols have to take into account that the wired stations do not run any security-related protocols and cannot participate.[ Wireless LAN Technology in Factory and
Industrial Automation]
Applications
Conclusion
