Extending CAN protocol with ISA100.11a wireless network - IEEE Xplore

Extending CAN protocol with ISA100.11a wireless network Soo Young Shin

Fadillah Purnama Rezha

School of Electronic Engineering Kumoh National Institute of Technology Email: [email protected]

School of Electronic Engineering Kumoh National Institute of Technology Email: [email protected]

Abstract—Hybrid network of wired and wireless technology has become hot topic research as a result of increasing demands in applying wireless network for industrial application. Hybrid network introduces different delay characteristic, depending on what technology that compose the hybrid network. In this paper, we present a framework for building a hybrid network composed of CAN protocol and ISA100.11a industrial wireless network. Simulation result have demonstrated that bottleneck of the hybrid network lies on ISA100.11a network. Keywords – Hybrid network, CAN, ISA100.11a, delay characteristic.

I. I NTRODUCTION The traditional wired control networks have been successfully implemented in industrial applications for decades. The ability of traditional wired control networks to provide high speed, high reliability and bounded delay makes them a popular option to be implemented in industrial plants. However, the wired control networks require infrastructure and space for installation. Adding new machine or system to the existing plant using wired control networks often create new problem of cable routing difficulties and sometimes require system engineer to re-design the whole cable routing plan. Hence, the traditional wired networks would be no longer suitable to meet the requirement of these scenarios. Having a wireless networks as an alternative such environments would be exceptionally valuable. Bayilmis et al in [1] proposed an interworking design to enable CAN nodes to communicate over IEEE 802.11b WLAN using encapsulation method. Similarly, in [2] Ren et al proposed a design of to integrate the CAN protocol wired technology with Bluetooth wireless technology. Another wired control network, Profibus-DP, is studied in [3] to extend Profibus-DP network with IEEE 802.11 wireless LAN for mobile devices. Those related works point out the necessity to extend wired control networks with the wireless one to meet the satisfy industrial needs. The work presented in this paper deals with extending (Controller Area Network) CAN protocol using ISA100.11a to meet real-time control applications in both automotive and industrial control. CAN protocol is a serial data communication protocol that provides high reliability and good real-time performance with very low cost. The CAN protocol is chosen to be studied in this paper due to its widely usage in wide range of applications [4]. On the other hand, ISA100.11a is an emerging open standard for process control dedicated 978-1-4673-4828-7/12/$31.00 ©20122 IEEE

for industrial automation and related applications [5], which attracts the attention of vendors and researchers. This paper, to best of our knowledge, is one of the first efforts in considering hybrid network of 2 popular communication protocol in industrial applications, CAN protocol and ISA100.11a. Both protocol’s ability to support message priority and provide deterministic delay makes them a suitable choice to be implemented in industry. The remainder of this paper is organized as follows. The following section presents an overview of CAN protocol and ISA100.11a network. Section III highlights the network architecture that is used to describe the hybrid network. In Section IV, we present the proposed framework for interconnecting CAN protocol and ISA100.11a followed by presentation of results obtained from simulations in Section V. Finally, the conclusions and future work are presented in Section VI. II. T ECHNICAL OVERVIEW OF CAN PROTOCOL AND ISA100.11 A A. CAN protocol CAN is a serial bus communications protocol developed by Bosch in the early 1980s [6]. CAN is essentially a broadcast network, where a message transmitted will be received by all nodes that are attached to the bus. In general, a CAN-compliant node consists of three parts: the host processor, the CAN controller, and the transceiver. The host processor sees all the messages on the bus, interprets received messages and decides which messages it wants to transmit. The CAN controller is basically a buffer for both incoming and outgoing messages. CAN controller stores received bits serially from the bus until an entire message is available and can be retrieved by the host processer. CAN controller also stores outgoing messages, which it receives from the host processor, and transmit the message serially via the bus. Finally CAN transceiver is the PHY layer of CAN, which converts the transmit-bit received from the CAN controller into a signal that is sent onto the bus, and vice versa. CAN protocol uses CSMA/AMP (Carrier Sense Multiple Access with Collision Detection and Arbitration on Message Priority) mechanism. In case of simultaneous transmission, arbitration is performed based on the priority level of the

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Figure 2. Figure 1.

ISA100.11a mesh network.

CAN protocol arbitration.

message ID, in which a logic zero is dominant over a logic one, and enables the message whose ID has the highest priority to be delivered immediately. If two nodes start transmitting at the same time and one of them receives a dominant bit (0) when a recessive bit (1) has been transmitted, the node loses arbitration and stops transmitting. Fig. 1 illustrates the bitwise arbitration in CAN protocol. The CAN protocol specification has 2 versions, 2.0A and 2.0B. The difference between CAN 2.0A and CAN 2.0B is located in the format of the message header. The original CAN protocol specification (2.0A) allocates 11-bit identifier, while the extended version (2.0B) contains 29-bit identifier. CAN protocol is optimized for short messages, with size of data field is between 0 and 8 bytes. The data rate achievable by CAN protocol depends on the length of the bus. The maximum data rate supported by CAN protocol is 1 Mbps, with bus length no more than 40 meters. A data rate of 125 kbps would allow a network length of up to 500 meters. However, it is allowed to use bridge-devices or repeaters to increase the allowed distance to more than 1 km.

modulation [9]. ISA100.11a only operates in 2.4-GHz band, using IEEE 802.15.4 channels 11-25, with maximum data rate of 250 kbps. Channel 26 is defined as optional in ISA100.11a. Each channels are spaced 5-MHz apart and has bandwidth of 2 MHz. The MAC of ISA100.11a implements synchronized time base and Time division multiple access (TDMA) for channel access with assigned bandwidth by use of ”contracts” [10]. CSMA/CA mode is optional in ISA100.11a because the focus of the standard is to provide a deterministic protocol for industrial applications. The timing axis of each device is divided into configurable fixed timeslot duration, varying from 10 ms to 12 ms. A collection of timeslots repeating on a cyclic schedule forms a superframe. The superframe’s length is configurable and can vary from one device to another. In general, longer-period superframes result in higher data latency and lower bandwidth, but with reduced energy consumption and less concentrated allocation of digital bandwidth [5].

B. ISA100.11a The ISA100.11a protocol is intended to provide reliable and secure wireless operation for non-critical monitoring, alerting, supervisory control, open loop control, and closed loop control applications [5], [7], [8]. The components of an ISA100.11a network consist of Security Manager, System Manager, gateway, backbone routers, and field devices as depicted in Fig. 2. The system manager performs policybased control of the network runtime configuration, monitors and reports on communication configuration, performance, and operational status. The security manager provides security services, while gateway provides an interface between ISA100.11a field network and plant network. Backbone router enables external networks to carry ISA100.11acompliant packet by encapsulating the PDUs for transport, allowing ISA100.11a network to use other networks, including CAN protocol as discussed in this paper. ISA100.11a protocol supports star, mesh, and combination of both topology. The physical layer of ISA100.11a devices is constructed based on IEEE 802.15.4 PHY standard that uses Direct-Sequence Spectrum Spreading and O-QPSK

Figure 3. Three channel hopping schemes of ISA100.11a: slotted hopping, slow hopping, and hybrid hopping.

ISA100.11a standard defines three types of channel hopping operations, which are slotted hopping, slow hopping, and hybrid hopping, as illustrated in Fig. 3. Each timeslot in slotted hopping uses a different channel in a hopping pattern to accommodate one transaction, while in slow hopping, one channel occupies a collection of successive timeslots with duration of 100-400 ms designated by system manager. The disadvantage of the slow hopping

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is that the receivers must continuously listen the channel for incoming traffic, which increases power consumption compared with slotted hopping. However, devices with imprecise timing settings or devices that temporarily lost contact with the network may have benefits from longer duration of timeslot. The hybrid hopping uses combinations of the slotted hopping and slow hopping superframes, where slotted hopping accommodates periodic data and slow hopping is available on a contention basis for sporadic data such as alarms and retries.

III. S YSTEM M ODEL A network topology model shown in Fig. 4 is used in this paper to describe the hybrid network technology of CAN protocol and ISA100.11a. The network model is intended to represent a common network architecture in industry where the main focus is applications such as monitoring and process control as stated in ISA100.11a standard. Process control is performed in distributed control system (DCS), which consists of several distributed control unit (DCU) as controller element. The DCUs are distributed throughout the system and connected by CAN protocol network for communication. DCU transmits control message, which usually has small size, to field devices, such as sensors and actuators. The field devices are connected to a gateway and communicates with the gateway through ISA100.11a wireless network. The field devices report monitoring message to DCU with varying size, depending on application type.

IV. I NTERCONNECTING CAN PROTOCOL OVER ISA100.11 A WIRELESS NETWORKS A framework is proposed to interconnect CAN2.0A networks that communicate through ISA100.11a wireless networks. An important function of the gateway is to translate any incoming traffic from CAN node into ISA100.11acompliant message format and forward the traffic to designated sensor node, and vice versa. The maximum message size that can be accommodated by CAN2.0A messages is 8 bytes, while ISA100.11a supports maximum message size (MaxDSDUSize) of 96 bytes. This specification enables message arriving from CAN nodes to be encapsulated directly into ISA100.11a DSDU frame after CAN header is stripped. After the message from gateway is received at designated ISA100.11a node, header parts of ISA100.11a are decapsulated and CAN 2.0A messages are extracted from ISA100.11a DPDU to be processed by ISA100.11a nodes. However, the amount of data transmitted from ISA100.11a node may be more than 8 bytes [11], hence fragmentation is necessary for carrying an ISA100.11a message in CAN2.0A protocol format. Incoming message from ISA100.11a nodes to gateway with size of more than 8 bytes must be fragmented into small fragment of 8 bytes before encapsulated into CAN message format and forwarded to CAN node. Upon receiving message from gateway, MAC layer of CAN node buffers incoming messages to construct the original data from ISA100.11a node before sending it to higher layer.

(a) Encapsulation

Figure 4.

CAN protocol-ISA100.11a network architecture.

Three types of node are defined in this model, CAN node, gateway, and ISA100.11a node. The CAN node only has CAN protocol technology and represents a distributed control unit (DCU). In this paper we use the CAN 2.0A version to model the CAN node. The gateway node acts as interface for both CAN protocol and ISA100.11a and understands the MAC layer of both network technologies. Gateway also represents the system manager and the security manager in the ISA100.11a system. Finally, ISA100.11a nodes is ISA100.11a-compliant devices that represent sensor, actuator, or end devices in industry.

(b) Fragmentation Figure 5. Message translation: (a) from CAN node to ISA node (b) from ISA node to CAN node

Both CAN protocol and ISA100.11a support message prioritization, where the message with higher priority has better probability to gain access to channel. In ISA100.11a, up to 16 message priority classes are supported, with only 5 class usage suggested by standard. In CAN2.0A protocol, with 11-bit identifier, 2048 message classes are possible. A standard point of view for message prioritization is proposed in the gateway model so that a message priority can be

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Figure 6.

Simulation scenario with 10 ISA nodes, 6 CAN nodes, and 1 gateway.

by the standard. All CAN nodes and ISA100.11a nodes generate packet with the same generation rate Npps . The simulation duration is 100 s and packets are generated with different start time which follows uniform distribution from 0 to 5. We ran each setting three times and took the average results. To model priority on ISA100.11a, each ISA100.11a node produces only one kind of priority packets, with constant size of 100 bits. Ten ISA100.11a nodes are divided into five categories to represent five priority classes used in ISA100.11a. Each priority is separated by a delay time before performing CCA τ , where τ is set to be 0.8 ms [12]. Therefore, simple FIFO queuing is used instead of priority queuing. In CAN node, data is generated with CAN ID that is uniformly distributed from 0 to 2047, with constant size of 64 bits. The reason behind such small data size is because in industrial application, especially in control ISA100.11a or monitoring system, the data size is generally small but frequently generated. Gateway node is set not to generate 1 any packet except beacon for ISA node and only do packet 2 fragmentation/encapsulation and packet forwarding.

translated properly, both from ISA100.11a nodes to CAN nodes, and conversely. In our proposed model, packet priority is classified into the same class in both CAN protocol and ISA100.11a. Maximum (2048) message classes are still supported by CAN node in order to perform contention to access CAN protocol. The message priority translation is illustrated as follows. Using table 1, gateway converts message priority into 5 classes. A message which arrives at gateway from ISA100.11a node with class 3 will be forwarded into CAN node with CAN ID that is uniformly distributed from 816 to 1224. Similarly, a message from CAN node with CAN ID of 1500 will be translated into an ISA100.11a message with packet class of 4. Table I PACKET CLASSES Category

Control

Monitoring

Type Closed loop regulatory control Closed loop supervisory control Open loop control Alerting Logging

Gateway 1

CAN ID 0-408

2

409-815

3 4 5

816-1224 1225-1632 1633-2047

3 4 5

B. Performance Evaluation

V. S IMULATION A. Simulation model Simulation is performed using OPNET modeler in a network scenario shown in Fig. 6. The simulation scenario consists of 6 CAN nodes and 10 ISA100.11a nodes, where each ISA100.11a nodes are connected to a gateway in star topology. Maximum data rate of both CAN nodes and ISA100.11a nodes is set to 250kbps. The superframe length of ISA100.11a node is always kept constant of 25 slots, with timeslot duration of 10 ms. The dedicated slot is configurable from 1 to 24, depending on the request of ISA100.11a end node. The rest slots among 25 slots that are not used as dedicated slot are allocated as contention slots. The reason behind this setting is because sporadic data rarely happens in industrial application. Packet lifetime of ISA100.11a packets is set to be 30 s, as suggested

In this section, the delay characteristic of the proposed hybrid network CAN-ISA100.11a is evaluated. A simulation is carried out with different network load Npps which follows poisson distribution, from 1 packet/second to 9 packet/second generated by each node. A higher value of Npps results in heavier traffic rate on the network. Fig. 7(a) and 7(b) show the delay characteristic of packet transmission from ISA100.11a node to CAN node and from CAN node to ISA100.11a node, respectively. In packet transmission from CAN node to ISA100.11a node, the obtained results show that more than 95% of the total end-to-end delay is contributed by path from ISA100.11a node to gateway, while delay from gateway to CAN node is relatively constant and insignificant compared to the total end-to-end delay. The reason behind this high delay contribution from ISA100.11a nodes is because of the slotted CSMA/CA mechanism employed in ISA100.11a standard. The standard requires exponential backoff to be applied to resolve collisions, with timeslot duration as a backoff period.

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packet generation as gateway cannot compensate the number of packet received from CAN nodes with the number of packet sent to ISA100.11a nodes. VI. C ONCLUSION In this paper, we presented a framework for building a hybrid network composed of CAN protocol and ISA100.11a industrial wireless standard. Packet encapsulation and fragmentation is employed to forward packet to different network technology. Through simulation we study the delay characteristic of CAN-ISA100.11a hybrid network. Simulation result shows that the ISA100.11a network becomes the bottleneck of the system and contributes more than 95% of the total end-to-end delay. In our future work, we will investigate the packet delay for each packet priority in such hybrid network. An algorithm or scheme to enhance the performance of the hybrid network is also left as future work.

(a) From ISA node to CAN node

ACKNOWLEDGMENT The authors gratefully acknowledge the anonymous reviewers for their constructive comments on the paper. This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(2012R1A1A1009442). R EFERENCES (b) From CAN node to ISA node. Figure 7.

Delay characteristics of packet transmission in hybrid network

This means when a node has a packet to transmit in shared slot and detects that the channel is busy, the shortest waiting period before next retransmission attempt is the duration of timeslot, which we choose to be 10 ms in our simulation. While in CAN protocol, a node can start to transmit a message as soon as it detects that the bus is free. The longest waiting time after a collision detection is the transmission time of CAN message with maximum data size, which is 111 bits in CAN2.0A standard. Therefore with data rate of 250kbps, the longest waiting time for a CAN node is 0.44 ms in our simulation. Different delay characteristic is obtained in packet transmission from CAN node to ISA100.11a node. A sudden increase of delay occurs when the value of Npps exceeds 6, which is also contributed mostly by delay in wireless transmission (from gateway to ISA100.11a nodes). This result implies the limit of gateway capacity to transmit ISA100.11a packet. Since all transmitted packet from CAN nodes must pass through gateway, gateway load capacity becomes bottleneck of the hybrid network. When the number of packet generated by all CAN nodes is high enough, at one point gateway cannot forward the packet immediately to meet deadline anymore. Once this capacity limit is surpassed, the delay becomes worse with higher number of

[1] C. Bayilmis, I. Erturk, C. Ceken, and I. Ozcelik, “A can/ieee 802.11b wireless lan local bridge design.” Computer Standards and Interfaces, vol. 30, no. 3, pp. 200–212, 2008. [2] X. Ren, C. Fu, T. Wang, and S. Jia, “Can bus network design based on bluetooth technology,” in Electrical and Control Engineering (ICECE), 2010 International Conference on, june 2010, pp. 560 –564. [3] K. C. Lee and S. Lee, “Integrated network of profibus-dp and ieee 802.11 wireless lan with hard real-time requirement,” in Industrial Electronics, 2001. Proceedings. ISIE 2001. IEEE International Symposium on, vol. 3, 2001, pp. 1484 –1489 vol.3. [4] F. Li, L. Wang, and C. Liao, “Can(controller area network) bus communication system based on matlab/simulink,” in Wireless Communications, Networking and Mobile Computing, 2008. WiCOM ’08. 4th International Conference on, oct. 2008, pp. 1 –4. [5] Wireless systems for industrial automation: Process control and related applications, ISA100.11a Working Group Std., 2009. [6] Can specification version 2.0. [Online]. Available: http://www.bosch.com [7] S. Petersen and S. Carlsen, “Wirelesshart versus isa100.11a: The format war hits the factory floor,” Industrial Electronics Magazine, IEEE, vol. 5, no. 4, pp. 23 –34, dec. 2011. [8] G. Wang, Comparison and Evaluation of Industrial Wireless Sensor Network Standards ISA100.11a and WirelessHART. Gothenburg, Sweden: Chalmers University of Technology, 2011. [9] Wireless medium access control (MAC) and physical layer (PHY) specifications for low rate wireless personal area networks (WPANS), IEEE STD 802.15.4 Std., 2006. [10] B. Akyol, H. Kirkham, S. Clements, and M. Hadley, A survey of Wireless Communications for the Electric Power System. United States of America: U.S. Department of Energy, 2010. [11] What do we expect from wireless in the factory. [Online]. Available: http://docbox.etsi.org/Workshop/2008/200812 WIRELESSFACTORY/ CAMBRIDGE WHITTAKER.pdf [12] N. Q. Dinh, S.-W. Kim, and D.-S. Kim, “Performance evaluation of priority csma-ca mechanism on isa100.11a wireless network,” in Computer Sciences and Convergence Information Technology (ICCIT), 2010 5th International Conference on, 30 2010-dec. 2 2010, pp. 991 –996.

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