XI International PhD Workshop OWD 2009, October Using Wireless Systems in Realtime Ethernet Fieldbuses Problems and Solutions

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1 XI International PhD Workshop OWD 2009, October 2009 Using Wireless Systems in Ethernet Fieldbuses Problems and Solutions Dipl.-Ing.(FH) Aurel Buda, Hochschule Bochum University of Applied Sciences Germany, Dipl.-Ing.(FH) Kristian Stieglitz, Hochschule Bochum University of Applied Sciences Germany, (Prof. Dr. Jörg F. Wollert, Hochschule Bochum University of Applied Sciences Germany) Abstract Within the last decade, the utilization of wireless solutions for industrial applications has become very popular. Especially in harsh environments, mobile and rotating scenarios, or at positions difficult to access, the advantages of radio technologies are obvious. In addition to that, there is a great potential on saving time and money during planning, installation, and commissioning of plant sections. However, due to the lower capacity and reliability of wireless links compared to wired ones, time critical domains like factory automation or motion control, can hardly be served by radio based solutions. In this context, the upcoming Ultra Wideband (UWB) technologies offer new opportunities, to meet the requirements of high speed industrial communication systems. This paper gives a short overview of radio technologies in industrial communication systems and shows that the new UWB standard ECMA-368 might give access to low latency wireless real time applications. 1. Introduction Besides the predominant utilization of wireless technologies in consumer products, their application in industrial communication systems is meanwhile well established. Several domains of industrial automation already benefit from the advantages of wireless applications. In harsh environments or rotating applications, radio links often offer a lower error probability compared to cable connections and sliding contacts. Certain mobile scenarios are only realizable, based on radio technologies. Furthermore wireless solutions may decrease time and costs concerning the planning, installation and maintenance of plant sections. It has been estimated that typical wiring costs in industrial installations are US $ 130,..,650 per meter and adopting wireless solutions could eliminate 20 %,..,80 % of these costs [1]. Nevertheless, due to the fluctuating nature of radio communication channels, it is hardly possible to address time critical low latency domains, like factory automation or motion control. Especially industrial environments, containing several metallic and moving obstacles, are highly challenging for a reliable wireless data communication. After a struggling process of competing interest groups, first standards for Ultra-Wideband (UWB) radio technologies have been published in 2006 and Although, UWB did not develop as fast as hoped for, it is very likely that products will enter the consumer market on a large scale within the next years. Besides, the technology has great potential to fulfill the requirements of high speed industrial communication systems. In general, UWB offers the following advantages: Low latency times, due to extreme short symbol durations, what additionally offers the possibilities for precise ranging. Robust against the effects caused by multipath scattering. Reflection and scattering are frequency selective. Using a high bandwidth reduces the probability of deep fadings over the whole frequency range. Energy efficiency, due to the low spectral density power. Particularly the first to statements underline the potential suitability of UWB for low latency realtime applications. Possible use cases might be the cable replacement in high speed realtime Ethernet connections or the application in wireless sensor actor networks (WSANs). The rest of the paper is organized as follows. Chapter two gives a short overview of the state of the art industrial communication systems. Chapter three comprises the state of the art wireless technologies for industrial communication systems. Chapter four introduces UWB, gives an overview of the ECMA-368 standard and points out its advantages and limits for industrial communication systems. Chapter five gives a summary of the paper and an outlook on future work. 296

2 2. Modern Industrial Communication Systems The traditional fieldbuses and the directly coupled signals are slowly being replaced by Ethernet based communication solutions [2]. The move towards Ethernet as the basic communication platform is mainly based on the efficient price/performance relationship of the technology. Corresponding to Figure 1, the performance of realtime Ethernet (RTE) protocols has historically evolved into three generations [3] ModBus/ IDA, Ethernet / Performance Prioritizing PROFINET RT Scheduling PROFINET IRT, Powerlink, EtherCAT Fig.1. Classification of industrial-ethernet Protocols[3]. The protocols using the whole Ethernet TCP/ stack and on top a realtime-specific application layer belong to the first generation. Good examples are Ethernet/ [4] and Mod-Bus/IDA [5]. Since the whole TCP/ protocol stack is used, the realtime performances are limited. Guaranteed update times of about 100 ms can typically be reached. Protocols of the second generation are a tradeoff between native Ethernet standard versus achievable realtime performance. The transport and network layers are bypassed in order to achieve a more efficient realtime communication. By means of this optimization, update times in the range of about 10 ms can be reached. A good example is PROFI- NET RT [6]. Protocols which are changing/replacing the original MAC scheme are part of the third generation. For those protocols, specific hardware or software is necessary. Good examples are Ethernet Powerlink [7], EtherCAT [8] and PROFINET IRT[6]. Depending on the protocol, summation or individual frames and cut-through operations are used, in order to exchange process date at extreme low latency times. The update times of third generation RTE protocols reach down to below 250 µs, according to the number of nodes joining a network. 3. Wireless Technologies in Industrial Communication Systems The utilization of wireless technologies in industrial communication systems has exponentially evolved within the last ten years. First applications, aiming at non-critical data logging or downloading, monitoring, and configuration, based on simple cable replacements. Nowadays, wireless solutions reach from i/o-communication of decentralized peripherals, to fieldbus bridges in point-to-point and pointto-multipoint topology, and up to wireless sensor and actor networks in star or full meshed multihop topologies. Target applications reach right up to closed-loop control systems with moderate latency requirements. In order to reduce costs and guarantee a world wide harmonized operation, unlicensed frequency bands are typically used. This tendency is very pronounced for the 2.45 GHz ISM (Industrial, Scientific, and Medical) frequency band. Because of the high availability, transceiver chips of commercial standards are often applied on the PHY and MAC layers. Good examples are the technologies of IEEE /WLAN [9], IEEE [10] /Bluetooth [11] or IEEE [12]. Previous analyses [13], [14], [15] attested the capabilities of these technologies for industrial applications. In order to comply with the requirements in automation, respecting determinism, reliability, and availability, above the PHY and/or MAC layers, adapted protocols are usually implemented. Further performance improvements are achieved by applying diversity techniques. In doing so, current wireless solutions have reached a state of serving applications with update times of 10 ms 20 ms. Below these timelines, the reserve for packet retransmissions and error corrections is insufficient to guarantee a reliable communication. Additional improvements may be attained, using upcoming technologies like IEEE n or Bluetooth Low Energy, both mainly operating within the 2.45 GHz frequency band. But due to the increasing number of wireless solutions, sharing this frequency spectrum, interferences caused by coexisting technologies have become a major problem, especially for time critcal applications. By operating at different frequency ranges, UWB might ease these issues, as well. 4. Ultra-Wideband The first regulation for UWB devices within a frequency range between 3.1 GHz and 10.6 GHz was published by the FCC in 2002 [16]. The maximum e.i.r.p. power density is limited to dbm/mhz. More restrictive regulations concerning the frequency ranges, channel occupation, and maximum power spectral densities followed for Europe, Japan, Korea, and China since The regulation s definition of UWB is very simple: The relative bandwidth has to be larger than 20 % and the absolute bandwidth has to be at least 500 MHz at a 10 db cut-off frequency. 297

3 Fig.2. ECMA-368 band group allocation (Source: Standard ECMA-368 3rd Edition). UWB follows the approach of a parallel utilization of the frequency spectrum with a large bandwidth and a low spectral density power, hence being immune and appearing as noise to coexisting narrow band technologies. Classically, UWB is based on Impulse Radio [17], which transmits information via impulses in the baseband without modulation. The UWB spectrum is generated due to extreme short durations (< 1ns) of these impulses. Since the regulations give no restrictions on signal forming and modulation, modern UWB technologies use well known modulations, like OFDM, as well. Based on the specifications of the WiMedia Alliance [18], the first UWB standard ECMA-368 [19] was published in late 2006 and is available in version 3.0 since It uses a Multiband OFDM (MB-OFDM) scheme and supports high datarates of up to 480 Mb/s on the PHY layer. The amendment IEEE a [20] is the second standard for UWB PHY and MAC layers and was published in The standard uses Direct Sequence UWB, bursts of impulses, to generate signals and aims at a ultra low power and low datarate communication (100 kb/s to 27 Mb/s) with precise ranging capabilities. Since IEEE a transceiver chips are not available yet, the rest of the paper focuses on ECMA ECMA-368 ECMA-368 defines high datarate UWB PHY and MAC layers and is based on the specifications of the WiMedia Alliance. The standard builds the foundation for a set of commercial protocols, like Certified Wireless USB (CW-USB) [21], and WiNET for TCP/ support. The Bluetooth Special Interest Group is evaluating the standard for a next generation Alternate MAC/PHY (AMP) for Bluetooth. The standard defines fourteen 528 MHz frequency bands, which are split up into six band groups according to figure 2. Each band group consists of three bands with the exception of band group five, which consists of only two bands. Band group 6 was introduced in version 2.0 of the standard and supports a worldwide harmonized operation. Transmission channels are realized, using time frequency codes (TFI). TFIs define the band switch pattern per symbol within a band group. For each band group, consisting of three bands, ten channels are defined (three channels for band group five). An example of TFC-1 for band group one (specified as channel 9) is given in figure 3. Fig.3. Example for a transmission in band group 1 with TFC-1 (Source: Standard ECMA-368 3rd Edition). Data is encoded using MB-OFDM with 122 subcarriers (100 data, 10 guard, 12 pilot). All OFDM symbols are of the same length (312,5 ns) and have a raw datarate of 640 Mb/s. Thereby data is always coded across all data carriers in the frequency domain and six consecutive symbols in the time domain, having a duration of µs. Datarate Coding rate FDS TDS /3 yes yes 80 1/2 yes yes 106,7 1/3 no yes 160 1/2 no yes 200 5/8 no yes 320 1/2 no no 400 5/8 no no 480 3/4 no no Tab.1. Data coding versus datarate. The different datarates are realized, using a convolutional code with coding rates of 1/3 to 3/4, time domain spreading (TDS), and frequency domain spreading (FDS), as illustrated in table 1. TDS makes use of time diversity, by redundantly transmitting data over two consecutive symbols. FDS makes use of frequency diversity, by redundantly transmitting data over two OFDM carriers. Depending on the datarate, distances between three to ten meters are achieved at an output power of dbm/mhz. This is an sufficient range for applications in typical production cells. 298

4 Fig.4. The ECMA-368 Superframe (Source: Standard ECMA-368 3rd Edition). Figure 5 shows a theoretical comparison of the physical transmission duration of ECMA-368 for different datarates, IEEE g at 56 Mb/s, and IEEE Draft n at 300 Mb/s (greenfield) for different amounts of payloads. Because of the shorter protocol overhead, even at 53.3 Mb/s ECMA-368 outperforms IEEE Draft n for payload length of > 100 bytes. Thereby, the energy consumption of ECMA-368 is approximately one magnitude lower compared to IEEE On the PHY and MAC layer, the standard defines two types of interframe spaces. Short interframe spaces (SIFS) have a duration of 10 µs and minimum interframe spaces (MIFS) with µs duration. When transmitting frames from one device to another in a burst, the interframe spacing after a frame shall at least be MIFS. In all other cases, the interframe spacing shall at least be SIFS. The equivalent SIFS and RIFS (Reduced IFS) of IEEE Draft n 2.0 have longer durations of 16 µs and 2 µs, respectively. Fig.5. Comparison of the physical transmit durations between ECMA-368, IEEE g and IEEE Draft n. The standard specifies a fully distributed MAC layer with no explicit coordinating device. According to figure 4, a transmission channel is divided into superframes, each consisting of 256 medium access slots (MAS). Each MAS has a duration of 256 µs. A superframe is composed of a beacon period (BP) at the beginning and a following data period. The BP constists of a variable number of beacon slots with a duration of 85 µs. Withtin the beacon slots, devices exclusively send beacon frames, containing information on the device. With each device joining a network, called beacon group, the beacon period gets extended by one beacon slot. Two beacon slots are always reserved for devices to join in the future. Beacons are also used to negotiate and coordinate the channel access among the device within the data periods of superframes. The standard defines the two access methods distributed reservation protocol (DRP) and prioritized contention access (PCA). The DRP gives exclusive access to reserved blocks of MASs for a reservation target and a reservation owner, where the target is a single device and the owner may also belong to a multicast group. Data transmission is always initiated by the target device. Within blocks of non reserved MAS, devices may access the medium using PCA. PCA is a CSMA/CA scheme, using a backoff procedure with alternating interframe spacings (AIFS), to give access to the medium with four priority levels. Besides these fundamentals, the standard supports a strong encryption, using AES-128 CCM and a four-way handshake, and ranging capabilities, optionally. The basic characteristics of ECMA-368 make it well suited for the utilization in industrial communication systems. However, in order to get full access to the PHY capabilities, the following weak points of the MAC have to be overcome: Depending on the number of devices, participating in a beacon group, the length of the BP varies between 512 µs and 8.16 ms. At these times, no data transfer is allowed. In principle, DRP offers a time synchronised access to the medium, suitable for deterministic realtime protocols. But the duration of 256 µs of a MAS is alligned with respect to large payloads. When using frames with small payloads, like it is common for industrial protocols, about 80 % of this time would be left unused. PCA gives more flexibility for devices to access the channel. But contention aware protocols do not guarantee a deterministic channel access. 299

5 The last statement is often accepted for modified IEEE solutions. 5. Conclusion As a whole, one can say that the ECMA-368 PHY has the capabilities, to achieve a reliable wireless data transfer beyond the timelines given by state of the art industrial wireless solutions. However, without modifying the MAC, it will be hardly possible to get full access to this capabilities. With its limited range, it is well suited for the local operation within dedicated production cells. With concern to coexisting technologies, the short range actually is of advantage for this use case. Within the scope of the project Ultra Wideband Interface for Factory Autmation (UWIfac), funded by the german ministry for education and research (BMBF), further theoretical and practical performance evaluations regarding latency, error, and coexistence properties, will be accomplished. Besides ECMA-368, IEEE a will be a subject of these analyses, as well. The results will be published in the future. References [1] Chehri, A.; Fortier, P.; Tardif, P.: An Investigation of UWB-Based Wireless Networks in Industrial Automation, proceedings of IJCSNS col. 8, no. 2, February [2] Ethernet-Based Device Networks Worldwide Outlook. Market analysis and forecast through 2012, ARC Advisory Group [3] Jasperneite, J.; Schumacher, M.; Weber, K.: Limits of increasing the performance of industrial Ethernet protocols, proceedings of ETFA 2007, pp , [4] Ethernet/, Ethernet Industrial Protocol (Ethernet/), [5] ModBus, [6] PROFINET, PROFIBUS International, [7] Powerlink, Ethernet Powerlink Standardization Group, [8] EtherCAT, Ethernet Technology Group (ETG), [9] LAN/MAN Standards Committee of the IEEE Computer Society. Information technology Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Revision of [10] LAN/MAN Standards Committee of the IEEE area net-works Specific requirements Part 15.1: Wireless Me-dium Access Control (MAC) and Physical Layer (PHY) Specifications for Wireless Per-sonal Area Networks (WPANs). [11] Bluetooth, [12] LAN/MAN Standards Committee of the IEEE area net-works Specific requirements Part 15.4: Wireless Me-dium Access Control (MAC) and Physical Layer (PHY) Specifications for Low Rate Wire-less Personal Area Networks (LR-WPANs), Revision of [13] Vedral, A.; Wollert, J. F.; Buda, A.; Altrock, R.: The Capability of Bluetooth for Real-Time Transmission in Automation, proceedings in IASTED Network and Communication Systems (NCS 2006), March 2006 [14] Vedral, A.; Wollert, J. F.: Analysis of Error and Time Behavior of the IEEE PHY-Layer in an Industrial Environment, proceedings of the IEEE Workshop on Factory Communication Systems (WFCS 2006), Jun. 2006, pp [15] Willig, A.; Matheus, K.; Wolisz, A.: Wireless Technology in Industrial Networks, Proceedings of the IEEE, vol. 93, no. 6, pp , [16] Federal Communications Commission FCC: 02 48A1 Revision of Part 15 of the Commission s Rules Regarding Ultra-Wideband Transmission Systems, February 2002, revision of [17] Nekoogar, F.: Ultra-Wideband Communications- Fundamentals and Applications, Prentice Hall Communications Engineering and Emerging Technologies Series, ISBN: , [18] WiMedia Alliance, [19] Ecma International: Standard ECMA-368: High Rate Ultra Wideband PHY and MAC Standard, 3rd Edition, 2008 [20] LAN/MAN Standards Committee of the IEEE area networks Specific requirements Part 15.4a: Wireless Me-dium Access Control (MAC) and Physical Layer (PHY) Specification for Low-Rate Wire-less Personal Area Networks (LR-WPANs), Amendment 1: Add Alternate PHYs. [21] USB Implementors Forum, Wireless USB 1.0 specification, Authors: Dipl.-Ing.(FH) Aurel Buda Hochschule Bochum University of Applied Sciences Lennershofstr Bochum - Germany tel. +49(0) fax. +49(0) aurel.buda@hs-bochum.de Dipl.-Ing.(FH) Kristian Stieglitz Hochschule Bochum University of Applied Sciences Lennershofstr Bochum - Germany tel. +49(0) fax. +49(0) kristian.stieglitz@hs-bochum.de 300