Are Wireless Networks Good Enough to Support Real-Time Traffic for Industrial Control Applications - Case Study Example

Are Wireless Networks Good enough to Support Real-time Traffic for Industrial Control Applications? Name Course Name and Code Instructor’s Name Date Abstract This paper has identified IEEE 802.11, IEEE 802.15.1 and IEEE 802.15.4 based technologies as the major wireless networks with potential for being applied in real time industrial control. The paper argues that the reliability and timeliness of these networks are not up standard to allow them to be used in industrial control application. The main concern that has been found to limit the adoption of these technologies in industrial control applications is the interference within and between networks. From the discussion it has been established though that some of these issues have been addressed to some extend in newer versions of wireless networks. However, there is still more that is needed to enable them to replace the wired networks. Thus it has been concluded that, wireless networks are not good enough to support real-time traffic for industrial control applications in their current state. Introduction Consumer goods industry has received unprecedented utilization of wireless technologies. The success in this sector has led many organizations to start adopting these technologies in industrial floors. Their uses are deemed to have manifold benefits. One of these benefits is that it reduces time and costs required to install and maintain large number of cables (Zhao, 2010). The wireless systems also enable plants to be flexible in that stationary systems can be wirelessly coupled to any mobile subsystems or mobile robots to attain connectivity that could otherwise be impossible (Balasubramanian, 2007). Wireless networks also can simplify temporary accessibility of the plant machinery for programming or diagnostic purposes (Guosong and Yu‐Chu, 2010). Wireless networks can also be employed in industrial applications such as localization and tracking of unfinished parts, coordination of mobile robots and autonomous transport vehicles in addition to applications involving distributed control (Guosong and Yu‐Chu, 2010). The main networks used in industrial control applications include Bluetooth, IEEE 802.15.4 and IEEE 802.11 (Walke, Mangold and Berlemann, 2006). Bluetooth and IEEE 802.15.4 are examples of wireless personal area networks (WPAN) that are designed for connecting devices wirelessly while taking energy efficiency into consideration (Guosong and Yu‐Chu, 2010). They support medium data rates in the order of hundreds of Kbit/s to few Mbit/s and have on the order of a few meters. Most equipment offered by vendors complies with these standards (Xiao-ying and Zhen-chao, 2010). On the other hand IEEE 802.11 family of standards is an example of wireless local area networks (WLAN) which are designed to provide users with high data rates of up to tens of Mbit/s over ranges of tens to hundreds of meters (Zhao, 2010). This allows users to have untethered access to Ethernet (Balasubramanian, 2007). For industrial application wireless networks ought to be reliable, adaptable and scalable. The reliability of wireless networks is determined by the signal reliability between a radio transmitter and receiver that is dependent on path loss, radio frequency (RF) interference and transmission power (Jonsson and Kunert, 2009). Adaptability requires that the wireless network be able to integrate seamlessly with the environment (Niazi and Hussain, 2011). Scalability requires that wireless network be able to scale gracefully as the number of endpoints increase. Critical evaluation of these approaches Bluetooth Bluetooth is an example of WPAN that has a possibility of being used in industry for real time control. It is also known as the IEEE 802.15.1 standard. It is based on a wireless radio system which is designed for short range and cheap devices to replace cables in connecting a computer to its peripherals such as keyboards, mice, printers and joysticks. Bluetooth has two connectivity topologies (Balasubramanian, 2007). The first one is the piconet which is a WPAN that is formed by a Bluetooth device which serves as a master in the piconet (Guosong and Yu‐Chu, 2010). The second topology is made up of one or more Bluetooth devices which serve as slaves. Each device is defined by a frequency hopping channel based on the address of the master (Zhao, 2010). The master’s clock is used to synchronize all devices that participate in communications within any given piconet. The slaves only communicate with the master in a point to point fashion and this communication is controlled by the master (Balasubramanian, 2007). The master can either transmit via point to point or point to multipoint (Xiao-ying and Zhen-chao, 2010). When two piconets connect they form a scatternet and a Bluetooth may participate in several piconets at ago (Sankar, 2005). A devise can only be a master in a single piconet but can be a slave to several piconets in a scatternet. IEEE 802.15.4 This is a standard which defines two physical layers (PHYs) which represent three license free frequency bands that include 16 channels at 2.4 GHz, 10 channels at 902 to 928 MHz and one channel at 868 to 870 MHz. The maximum data rates for each of these bands are 250 kbps, 40 kbps and 20 kbps respectively. All the PHYs use Direct Sequence Spread Spectrum (DSSS). Examples of wireless devise which runs on this protocol are ZigBee and WirelessHART (Guosong and Yu‐Chu, 2010). ZigBee networks are designed for low duty cycle sensor networks. ZigBee provides self organized multi-hop and reliable mesh networking with long battery lifetime (Sankar, 2005). HART is a digital protocol for two way communication between a host application and smart field instruments (Zhao, 2010). It provides access to diagnostics, configurations and process data. WirelessHART is based on IEEE 802.15.4 like ZigBee. Both ZigBee and WirelessHART operate on low rate WPAN (LR-WPAN) (Jonsson and Kunert, 2009). LR-WPAN network allows two different device types to participate in communication (Xiao-ying and Zhen-chao, 2010). One of the devices is a full function device (FFD) while the other is a reduced function device (RFD). The FFD can serve as a PAN coordinator, a device or a coordinator in three modes. The RFD can only communicate to FFD (Balasubramanian, 2007). RFD is mainly used inn simple applications such as a passive infrared sensor or a light switch. They are mostly associated with a single FFD (Kunert, Uhlemann and Jonsson, 2010). FFD can become a PAN coordinator by establishing its own network once it is activated. It does so by choosing a PAN identifier that is not being used by another network within the radio sphere of influence. This is followed by the PAN identifier allowing other devices to join its network. Any of FFD in a network may become a coordinator and provide synchronization services to other devices or other coordinators. IEEE 802.11 This is wireless local area networks (WLAN) standard which are designed to provide users with high data rates of up to tens of Mbit/s over ranges of tens to hundreds of meters. This allows users to have untethered access to Ethernet. The IEEE 802.11 architecture has several components which interact to provide a wireless LAN that helps in supporting station mobility in a transparent manner to upper layers. An IEEE 802.11 has a basic service set (BSS) which act as its basic cell (Zhao, 2010). This is a set of fixed or mobile stations. Once a station moves out of its BSS it can no longer communicate directly with other members of the BSS (Guosong and Yu‐Chu, 2010). IEEE 802.11 employs the independent basic service set IBSS and extended service set (ESS) network configuration which allows a BSS to form a component of an extended form of network that is built with multiple BSSs to form a distribution system (DS) (Balasubramanian, 2007). The DS and APs allow IEEE 802.11 to form an infrastructure network that has an ESS network of arbitrary size and complexity (Xiao-ying and Zhen-chao, 2010). An example of IEEE 802.11 based device is Wi-Fi that can run on IEEE 802.11a/b/g. Wi-Fi allows users to access the internet at broadband speeds when connected to an access point (AP) or in ad hoc mode (Jonsson and Kunert, 2009). The table below provides the main differences between the three networks Standard Bluetooth ZigBee Wi-Fi IEEE spectrum IEEE 802.15.1 IEEE 802.15.4 IEEE 802.11a/b/g Frequency 2.4 GHz 868/915 MHz; 2.4 GHz 2.4 GHz; 5GHz Maximum signal rate 1mb/s 250 kb/s 54 mb/s Nominal range 10 m 10-100 m 100 m Nominal TX power 0-10 dBm (-25)-0 dBm 15-20 dBm Number of RF channels 79 1/10; 16 14 (2.4 GHz) Channel bandwidth 1 MHz 0.3/0.6 MHz; 2MHz 22 MHz Modulation type GFSK BPSK (+ ASK), O-QIPSK BPSK, QPSK COFDM, CCK, OFDM Spreading FHSS DSSS DSSS, CCK, OFDM Co-existence mechanism Adaptive frequency hopping Dynamic frequency selection Dynamic frequency selection, transmit power control: (802.11h) Basic cell Piconet Star BSS Extension of the basic cell Scatternet Cluster tree, mesh ESS Maximum number of cell nodes 8 ˃65,000 2007 Encryption EO stream cipher AES block cipher (CTR, counter mode) RC4 stream cipher (WEP), AES block cipher Authentication Shared secret CBC-MAC (ext of CCM) WPA2 (802.11I) Data protection 16-bit CRC 16-Bit CRC 32-bit CRC Acronyms: ASK (amplitude shift keying), GFSK (Gaussian frequency SK), BPSK/QPSK (binary/quardrature phase SK), O-QPSK (offset-QPSK), OFDM (orthogonal frequency division multiplexing), COFDM (coded OFDM), MB-OFDM (multiband OFDM), M-QAM (M-ary quadrature amplitude modulation), CCK (complementary code keying), FHSS/DSSS (frequency hopping/direct sequence spread spectrum), BSS/ESS (basic/extended service set), AES (advanced encryption standard), WEP (wired equivalent privacy), WPA (Wi-Fi protected access), CBC-MAC (cipher block chaining message authentication code), CCM (CTR with CBC-MAC), CRC (cyclic redundancy check). Comparison of Bluetooth, ZigBee and Wi-Fi For most industrial users, any problem with the equipment translates to economical loss and hence reliability is a primary concern for these users (Zhao, 2010). Thus parameters such as network robustness, reliable message delivery, authentication and integrity are all important for industrial use (Wozniak, Konorski and Katulski, 2009). In addition, the threat of industrial espionage requires that the networks to be deployed in industrial applications ought to have encryption for data security purposes (Xiao-ying and Zhen-chao, 2010). ZigBee is argued to lack industrial grade robustness. This is because it lacks frequency diversity since its entire network shares the same static channel which makes it susceptible to both intended and unintended jamming (Guosong and Yu‐Chu, 2010). This implies that the sever frequency selective fading due to the metal rich propagation environments in plants can potentially stop communication based on ZigBee platform. In addition, the static nature of its channels imply that interference is increased for other systems such as WPAN based platforms such as Wi-Fi and consequently increase delay as the network size grows and collisions forces retransmissions (Zhao, 2010). Another reason for lack of robustness in ZigBee is that the network lacks path diversity implying that any link breakages will require setting up of a new path from source to destination (Balasubramanian, 2007). As a consequence delay and overhead are increased and route discovery eventually consumes all bandwidth available in environments with unstable routes (Sankar, 2005). In addition, the lack of robustness implies that ZigBee is not well suited for control application (Xiao-ying and Zhen-chao, 2010). However, ZigBee has an encryption which can protect it from industrial espionage although it requires that equipment used be from vendors which support the necessary mechanisms (Walke, Mangold and Berlemann, 2006). WirelessHART is said to address the concerns raised about ZigBee by the industry. This network is designed to be robust and to allow secure communication. It has hopping and retransmission features which limits the effects of temporal and frequency interference (Zhao, 2010). This also limits interference with other networks. WirelessHART also has a mesh networking feature which has graph routing that provides path redundancy and self-healing properties. This limits the impact of broken links. Thus the robustness of this network provides a possibility of it being employed for wireless control especially in slow non critical processes (Sankar, 2005). The network also has time division multiple access (TDMA) and prescheduled timeslot features which help in preventing message collisions and allows devices to increase their power saving since the device only requires to keep the radio on during the needed timeslots (Walke, Mangold and Berlemann, 2006). WirelessHART is a secure network and has several layers of protection (Balasubramanian, 2007). All of its traffic is secured and payload is encrypted and all messages are authenticated in both single hop basis and end to end (Guosong and Yu‐Chu, 2010). WirelessHART however, requires that all devices be provided with a secret Join Key and Network id in order to join the network hence raises issues in its scalability. Bluetooth lacks coordination between different piconets and hence two piconets located close to each other may result in packet collision and interference with each other. This effect is however minimized by fast frequency hopping over the bandwidth of the 2.4 GHs (Balasubramanian, 2007). An improvement has made in newer Bluetooth versions by inclusion of an adaptive frequency hopping scheme (AFH) that allows the exclusion of certain carriers where corruption of packets occur at their frequencies. It is worthy noting that AFH is intended to improve the performance of Bluetooth piconet in presence of non hoping systems. Bluetooth provide an adaptive frequency hoping to avoid channel collision while ZigBee and Wi-Fi use dynamic frequency selection and transmission power control (Walke, Mangold and Berlemann, 2006). Studies have indicated that Bluetooth and Wi-Fi frequencies can be interfered with by 802.15.2. Potential interferences have also been documented between ZigBee and IEEE 802.11g (Balasubramanian, 2007). In addition coexistence issues have been raised concerning Bluetooth, ZigBee, Wi-Fi and microwave. All networks have encryption and authentication mechanism to protect them (Zhao, 2010). Transmission time for data depends on the data rate, the message size and the distance between the two nodes. Bluetooth has synchronous connection oriented (SCO) packets which support real time traffic by reserving time slots at periodic intervals. The extended SCO links in newer versions of Bluetooth allows a limited number of retransmissions (Guosong and Yu‐Chu, 2010). Given the short range of Bluetooth and the small number of slaves that are active at any given time several independent Bluetooth piconets can co-exist on a factory floor. Studies have indicated that when two piconets overlap when operating on different frequencies the signal quality is good (Xiao-ying and Zhen-chao, 2010). However, when they hop to the same frequency packets may be destroyed beyond recognition. In order reduce interference and obtain a good throughput it is recommended that long types of packets be used (Walke, Mangold and Berlemann, 2006). However, for industrial purposes such long packets are untenable (Balasubramanian, 2007). The Bluetooth network provides reliable security through authentication and encryption (Lucan, Simacek, Seppala and Hannu, 2006). It also has enhanced data rates that allow faster transmission of data. Unlike WPAN based networks, IEEE 802.11 networks such as Wi-Fi have been optimized to transmit large data files. Thus it has suboptimal performance when majority of data is made up of short control packets. IEEE 802.11 uses MAC layer acknowledgements and retransmissions. It has been argued that it is possible to have IEEE 802.11 ad hoc networks which consist of solely mobile stations. The network has limited timeliness due to super frame stretching which causes some jitters in the start times of super frames and foreshortened CFP’s (Zhao, 2010). In order to overcome this problem there is need to extend the basic MAC protocol. The network is also associated with complexity. IEEE 802.11 supports several authentication processes for security purposes (Guosong and Yu‐Chu, 2010). However, IEEE 802.11 lacks predictability owing to its stochastic access mechanism with random back off times and station contention. Issues/problems/further improvements identified The main issue that seems to arise in all the networks is the co-existence issue. As noted earlier most of these networks interfere with each other (Guosong and Yu‐Chu, 2010). This problem can however by carrying out carrier sensing operation in order to avoid interference with ongoing transmission (Loo, 2007). However, this depends on the ability of the carrier sensing operation to detect other types of wireless networks. The IEEE 802.15.4 standard for instance allows users to choose between various carrier sensing modes (Xiao-ying and Zhen-chao, 2010). This help in detection of a wide variety of networks hence help in avoiding interference (Zhao, 2010). Other networks can also implement a variety of carrier sensing modes to help resolve this problem (Walke, Mangold and Berlemann, 2006). The interference has however been relaxed with the widespread use of IEEE 802.11 in industrial environment and use of AFH for newer version of Bluetooth (Bluetooth V.1.2) (Balasubramanian, 2007). However, Bluetooth creates more interference to one another due to limited number of channels available. In addition when IEEE 802.11 is too close to Bluetooth, the Bluetooth receivers can be blocked from receiving (Zhao, 2010). This can be overcome by use of a joint scheduler to help enable a fair coexistence. As mentioned above IEEE 802.11 lacks predictability. Polling or token passing on top of the IEEE 802.11 can be used to eliminate contention Conclusion The major wireless networks with potential for being applied in real time industrial control include IEEE 802.11, IEEE 802.15.1 and IEEE 802.15.4 based technologies. The reliability and timeliness of these networks are not up standard to allow them to be used in industrial control application. The interference within and between networks is the main issue that raises issues with timeliness. Even though these issues have been addressed to some extend in newer versions of wireless networks, there is still more that is needed to enable them to replace the wired networks. Thus, wireless networks are not good enough to support real-time traffic for industrial control applications. References Balasubramanian, K. 2007. Channel adaptive real-time medium access control protocols for industrial wireless networks. Thesis for Masters Degree. 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