Kocaeli University, Technical Education Faculty,. Electronics and Computer ... Internetworking Unit (WIU) that is capable of connecting remote CAN 2.0A nodes ...
Extending CAN Segments with IEEE 802.11 WLAN Cuneyt Bayilmis, Ismail Erturk and Celal Ceken Kocaeli University, Technical Education Faculty, Electronics and Computer Education Department, 41300 Kocaeli, Turkey {bayilmis, erturk, cceken}@kou.edu.tr
Abstract The Controller Area Network (CAN) is currently employed in many distributed real-time control applications in the industrial environments. CAN-based distributed control systems have two main problems. These are the size of distributed area and the need for communication with other LANs and with remote CAN segments. A straigh(forward solution is to use interworking devices with wireless support to extend CAN segments, exploiting an IEEE 802.11 WLAN that is nowadays a low cost technology with high data rates. This research study aims at designing and implementing such an interworking device called Wireless Internetworking Unit (WIU) that is capable of connecting remote CAN 2.0A nodes over IEEE 802.11b WLAN using encapsulation method. Computer modeling and simulation of the proposed approach realized using OPNET Modeler and analysis of the simulation results obtained are also presented.
1. Introduction
CAN protocol is one of the most advanced and prominent autobus protocols in the communications industry. Although initially intended for use in only automotive applications, currently it is also deployed in many other industrial applications due to its high performance and superior characteristics. CAN protocol based on a CSMAlCD+CR access mechanism with the use of priorities is a serial communication protocol and it is used to support distributed real-time control and multiplexing. Its common applications include intelligent motor control, robot control, intelligent sensors/counters, laboratory automation and mechanical tools [1]. Extensive use of several CAN networks (e.g., each dedicated to a portion of an industrial plant, connected by suitable devices) in automotive and other control applications in modem industrial plants results in also need for intemetworking between CAN networks as well as between CAN and other major public/private networks. The main hold back of this topological solution is that several CAN networks have to be crossed for information to be transmitted between two specific CAN
0-7803-8735-X/05/$20.00©2005 IEEE
networks not directly connected. Employing a backbone (e.g., an FDDI) with large amount of bandwidth to interconnect CAN networks is one of the solutions to overcome this problem. However, there may be certain difficulties in some industrial scenarios where such a traditional wired backbone is deployed. For example, when an existing plant, in which cable laying would be quite difficult, requires installation of new communication systems. In addition, there could be some other cases where the use of a wired backbone is not feasible, for example, in an integrated factory where several robots work under the control of one or more supervision systems. In such a scenario, each robot may have more than one CAN FieldBus interconnecting the local sensors. Consequently these CAN networks can apparently not be interconnected using conventional wired systems. Having a wireless backbone in such scenarios to interconnect CAN networks would be exceptionally valuable [1]. As indicated in [2] some FieldBus systems (e.g. the IEC/ISA) already benefit from wireless communication systems at the physical level. Nevertheless, the use of a wireless FieldBus system as a backbone to interconnect other wired FieldBus systems is not a suitable solution because they do not provide possibility of interconnecting different FieldBus communication systems. One wireless network which currently possesses the features needed in an industrial control environment, that is, easy integration with several communication systems and capability to ensure critical time constraints, is the IEEE 802.11 standard. This paper focuses on extending CAN2.0A segments using the IEEE 802.11b WLAN. IEEE 802.11 standard possesses a centralized access mechanism capable of minimizing the time required to transfer the time--critical information exchanged between different interconnected FieldBus systems. The paper describes CAN and IEEE 802.11 briefly in section 2. Section 2 also introduces the proposed approach for interconnection of the CAN segments using IEEE 802.11 WLAN over radio links. Finally computer modeling and simulation results of the proposed scheme are presented in section 3.
2. Interconnecting CAN Segments over IEEE 802.11 WLAN 2.1 Controller Area Network (CAN)
Allowing the implementation of peer-to-peer and broadcast or multicast communication functions with lean bus bandwidth use, CAN applications in vehicles are gradually extended to machine and automation markets. As CAN semiconductors produced by many different manufacturers are so inexpensive, their widespread use has found a way into such diverse areas as agricultural machinery, medical instrumentation, elevator controls, public transportation systems and industrial automation control components [3-8]. [3] and [4] supply a detailed overview of the CAN features that can be summarized as high speed serial interface, low cost physical medium, short data lengths, fast reaction times and high level of error detection and correction. CAN utilizes the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as the arbitration mechanism to enable its attached nodes to have access to the bus. Therefore, the maximum data rate that can be achieved depends on the bus length. For example, the maximum data rates for 30 meter and 500 meter buses are 1 Mbitls and 100 Kbit/s respectively. As CAN employs a priority based bus arbitration process, the node with the highest priority will continue to transmit without any interruption. Thus CAN has a very predictable behavior and in fact CAN networks can operate at near 100% bus bandwidth. CAN messages include (}-8 bytes of variable data and 47 bits of protocol control information (i.e., the identifier, CRC data, acknowledgement and synchronization bits, Figure 1) [1], [3], [5-8]. Note that two versions of CAN exist, and they only differ in the size of the identifier (i.e., 11 and 29 bit identifiers with CAN 2.0A and with CAN 2.0B, correspondingly). The identifier field serves two purposes: assigning a priority for the transmission and allowing message filtering upon reception. The CRC data field holds a 16 bit CRC code calculated by both the transmitter and the receiver independently in order to find out any errors occurred during transmission of the frame. 2.2 Wireless LAN IEEE 802.11
IEEE 802.11 WLAN is a local area network implemented without wires. The main advantages of WLAN are mobility and cost-saving installation. Any WLAN aims to offer all the features and benefits of traditional LAN technologies (e.g., Ethernet and Token Ring) but without the limitations of being tethered to a cable.
IEEE 802.11 supports two different topologies, namely independent Basic Service Set (BSS or ad-hoc network) and infrastructure BSS. The stations can communicate directly with each other in independent BSS which is the simplest type of IEEE 802.11 LAN. In access point (AP) based networks (or infrastructure BSS) all mobile stations communicate through an AP that is connected to a wired network. IEEE 802.11 employs Carrier Sense Multiple Accessl Collision Avoidance (CSMAlCA) as the channel access method and operates in the 2.4 GHz unlicensed ISM (Industrial, Scientific and Medical) band. IEEE 802.11 WLAN standards are usually based on three physical layer (PHY) specifications: two for radio frequency (RF Direct Sequence Spread Spectrum, DSSS and Frequency Hopping Spread Spectrum, FHSS) and one for infrared (IR). Each technique provides a different data transfer rate. Generally radio frequency technology is preferred in WLANs due to the limitations of the infrared medium. WLAN packet types vary according to the PHY employed [9-l3]. The PHY layer is the interface between Medium Access Control (MAC) layer and wireless media. It consists of two sublayers, namely Physical Layer Convergence Procedure (PLCP) sublayer and Physical Medium Dependent (PMD) sublayer. The PLCP Protocol Data Unit (PPDU) is unique to the DSSS PHY layer. The PPDU frame consists of a PLCP preamble, PLCP header, and MAC Protocol Data Unit (MPDU) (Figure 2) [11]. IEEE 802.11 family consists of several substandards such as IEEE 802.11a operating in 5 GHz unlicensed U NIl (Unlicensed National Information Infrastructure) band with 54 Mbps data rate, and IEEE 802.11b operating in 2.4 GHz ISM band with 11 Mbps data rate and IEEE 802.11g (built on IEEE 802. l 1b) operating in 2.4 GHz ISM band with 54 Mbps data rate. 2.3. CANIIEEE 802.11 Wireless Internetworking Unit
Internetworking Units (IUs) are high-performance devices that are used to interconnect similar or dissimilar LANs. While a pass-through forwarding process is sufficient for the interconnection of similar LANs, both a translation process and a forwarding process are required for the interconnection of dissimilar LANs [14]. In this study, the proposed Wireless Internetworking Unit (WIU) interconnects two CAN2.0A segments communicating through an IEEE 802.11b; therefore, both the translation and forwarding processes are required. The main function of the Wireless Internetworking Unit is that the Protocol Data Units (PDU) of the CAN messages are encapsulated within those of the IEEE 802.11 b DSSS frames to be carried over wireless channels. Since a CAN 2.0A message is 108 bits, it can
easily be fitted into one IEEE 802.11b frame MPDU (Figure 3). Thus, neither segmentation/reassembly of CAN messages nor data compression is necessary for carrying a CAN message in one IEEE 802.11 frame. At
the destination WIU, preamble and header parts of the IEEE 802. l l b frames are stripped off, and the CAN messages extracted from the IEEE 802. l l b MPDUs can be processed. Data Field
Arb Itra tio n Fie Id s o F
Figure 1.
11 -bit
0-8
den tlfler
eRe Field
bytes
16-bJtCRC
CAN 2.0 A message format
I+--
1
PLCP-Header
PLC P-Pream ble ------" ....---
S yn chro nization (128 bits)
Figure 2.
IEEE 802.11b DSSS PLCP packet format CAN Frame
M PD U
Figure
(108 bits)
(108 bits)
3. Encapsulation of a CAN frame into an IEEE 802.11b DSSS frame
The proposed bridge has two ports which are capable of interconnecting two CAN segments using an IEEE ,
�
I
- - - - - - - - - - - -- ,
N
1
N2
802. l l b WLAN. Its network model and layered system architecture are shown in FigA, in Fig.5, respectively. .,.. ---------- - ....
, \
N5
N6
N7
N8
� Wireless WIU N3
1
medium \
N4
CAN segment
,
... _------------'"
CAN segment 2
1
WIU: Wireless Internetworking Unit P : Priority N
Figure 4.
: Node
The proposed network model CAN Node
CAN/IEEE 802.11 WIU
IEEE 802.11/CAN WIU
CAN Node
Application
Bridging
Bridging
Application
Data Link Layer (DLL)
Data Link Layer (DLL)
Physical Layer
I Figure 5.
Logical Link Control (LLC)
Logical Link Control (LLC)
----------
I
Layered system architecture of WIU
Data Link Layer (DLL)
PLCP r u b a1'�___ __ :_ PMD sub layer
Physical Layer
Physical Layer
I
I
MAC Layer
MAC Layer
PLCP sublayer Physical ----------Layer PMD sublayer
CAN Bu s
----------
Data Link Layer (DLL)
�
Wireless Channel
CAN Bus
, \
The functional model of the WID shown in Fig.6 contains following entities: • The CAN Interface Entity (CIE) provides the means for communication with CAN bus and has a bus receiver and a bus transmitter. • The WLAN Interface Entity (WIE) provides the necessary functions for communication over wireless medium and has a wireless receiver and a wireless transmitter. • The CAN Bus Receiver is a buffer that stores CAN messages delivered from the CIE. • The WLAN Receiver is a buffer that stores WLAN IEEE 802.11 frames delivered from the WIE. • The Look-up Table (LT) is the most important part of a WIU. It is built up during a Learning Process (LP) in which every WIU finds out its own attached CAN nodes' local messages and remote messages destined from/to any other CAN segment. After that the messages with certain priorities are associated with the relevant WID connected to the destination CAN segment. • The CAN Learning, Filtering, and Translating Entity (CLFTE) contains the LP. It also compares
CAN Bus
Figure
•
•
identifiers of the messages received from the CAN bus with the ones in the LT to realize whether the messages are local or remote. If the CAN message identifier has a match in the LT (i.e., it is destined to another CAN segment) then it is converted to WLAN frame format and sent to CAN Translating Buffer. Otherwise the CAN message is filtered as it is a local message. The WLAN Filtering and Translating Entity (WFTE) extracts CAN messages from the WLAN frames delivered from the WIE. It then checks out whether there is a CAN message identifier matching with the one extracted in the LT. If this is true (i.e., the CAN message destined to this CAN segment) the CAN message is sent to the WLAN Translating Buffer. Otherwise it is filtered as not destined to this CAN segment. FIF02 and FIF03 are the CAN Translating and the WLAN Translating buffers, respectively.
WLAN IEEE 802.11
6. Functional block diagram of WIU
In the proposed WID model, each port has a different protocol, frame/message format, and frame/message reception/transmission mechanism. Thus, the processes to be performed at each port of the WID are different. The flowcharts of these processes are shown in Fig. 7. It includes both CAN to WLAN data transfer processes and WLAN to CAN data transfer processes. Learning process is used to determine remote messages in every CAN segment and it works in conjunction with the LT. The LT is updated when either a new CAN node or a new CAN message is introduced to
the network as this is a very common case in any industrial process control environment. All CAN messages received from the CAN bus through the CIE is checked for a match in the LT. If there is a match with this CAN message identifier then the CAN message is converted into the WLAN frame format using the encapsulation method, and it is sent to the FIF02. Otherwise the CAN message is discarded as it is destined to a local CAN node. Meanwhile, when the FIF03 is full, meaning that it contains a CAN message delivered from the WFTE, the CAN message is passed to the CAN bus (Fig. 7-a).
Processes in the WLAN part of the WIU (Fig.7-b) contain receiving/sending WLAN frames from/to the wireless medium. If there is an incoming WLAN frame then the CAN message is extracted from it. After that the CAN message identifier is checked out in the LT. If there is a match then the CAN message is sent to the FIF03; otherwise, it is discarded. Meanwhile, when the FIF02 is full, meaning that it contains a WLAN frame encapsulating a remote CAN message delivered from the CLFTE, and the wireless medium idle then the WLAN frame is sent to wireless medium as broadcast.
3. Computer Modeling and Simulation of the Proposed Scheme
N
message from
FIF03 and send
it to CAN Bu s
(a)
The node model of the proposed CAN2.0A/IEEE 802.lIb WIU shown in Fig. 8 is realized using OPNET 9.0 Modeler Radio Module. The CAN�roc executes functions of the CAN Interface Entity (CIE), CAN Bus Receiver, and CAN Learning, Filtering and Translating Entity (CLFTE) that are given in the functional block diagram (Fig.6). Similarly, the WLAN�roc executes functions of the WLAN Interface Entity (WIE), WLAN Receiver, and WLAN Filtering and Translating Entity (WFTE). CAN_buffer and WLAN_buffer are used as CAN Translating buffer and WLAN Translating buffer, respectively.
Figure
(b) Flowcharts of the designed WIU processes a) CAN to WLAN data transfer process b) WLAN to CAN data transfer process Figure 7.
8. Node model of the proposed WIU
The ranges of CAN implementations are so versatile that there is no general model for a CAN application, not even a benchmark. Despite this, a CAN example can be used to illustrate the communication behavior and to discuss the system perfonnance where the proposed WIU is employed [14]. The simulation model shown in Fig. 4 consists of two CAN segments each with eight CAN nodes and a WIU. Every node introduces to the network a single CAN message with a certain priority. Four nodes in each segment produce local messages destined to another CAN node in the same CAN segment while the others produce remote messages destined to a CAN node in the other CAN segment. Product CAN messages in CAN segmentl and CAN segment2 are given Table 1.
Table 1.
SCN 1 2 3 4 5 6 7 8 SCN DL P LM Ns-n
Local (L) and remote (R) messages of the CAN Segments
CAN Segment I CAN Seg ment 2 MT ML MT ML P DL P DL DCN SCN 8 8 RM5 LMI 14 14 14 9 13 NI-3 14 14 RMI RM6 1 10 8 8 N21 6 6 14 11 LM5 10 6 14 LM2 3 N1-1 5 8 14 12 2 14 RM2 LM6 N24 7 8 8 14 12 14 LM3 13 NI-7 RM7 7 11 6 14 14 RM8 6 6 14 N2-- 4«!:>
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-+- 50150%
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Figure 12. The IEEE 802.l 1b WLAN to CAN Will process time under varying loads with different locaVremote (L/R%) messages ratios.
4. Conclusion The aim of this work has been to design a Will that provides a service to achieve the wireless interconnection of two CAN2.0A segments using an IEEE 802.l1b WLAN. Considering their easy usage in many industrial areas, CAN nodes emerge inevitably to need this type of wireless intemetworking for greater flexibility for their applications to be controlled and/or programmed remotely. In summary, the functional model of the designed CANIIEEE 802.11b WLAN Will includes four phases of operation. First, it receives a CAN message or a WLAN frame from one of its ports, i.e. the CAN side or the IEEE 802.11b side, respectively. Second, it decides whether or not to forward the message/frame. Third, the message/frame is reformatted into the required type to be, lastly, transmitted to the other system. Considering the message delivery deadline of the Society of Automotive Engineers (SAE) Benchmark (e.g., 100 ms for a CAN system with 12 nodes) [3], it can be concluded from the simulation results that the maximum process time of the messages from CAN to WLAN in the designed WID and end-to-end message delays are acceptable.
Load (MessageISecond)
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