"Powerlink goes wireless" – Evaluation of wireless Ethernet ...

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"Powerlink goes wireless" – Evaluation of wireless Ethernet ...

  1. 1. 1 “Powerlink goes wireless” – Evaluation of wireless Ethernet Powerlink transmission Dipl.-Ing. Johannes Trummer, Dipl.-Ing. Dr. Erich Leitgeb (Supervisor), Ing. Andreas Merdonig, Institute for Broadband Communication, University of Technology, Graz Abstract— Wireless applications are getting more interesting between Powerlink’s base technology Ethernet and Wireless in Automation today, also for Ethernet Powerlink networks. As LAN. Another advantage is the vast range of available there is no commercial solution available at the moment, a cost- hardware. effective way for wireless Powerlink transmission shall be found. Powerlink is based on IEEE 802.3 Ethernet standard, which For the implementation of wireless Powerlink connections means that there is a close relation to IEEE 802.11 Wireless not only radio transmission shall be evaluated. Optical LAN. Therefore the possibility of transferring Powerlink data wireless systems are also taken into account, also known as over WLAN networks is evaluated and tested. As Powerlink free space optics (FSO). As there are FSO systems available at belongs to the fastest Automation bus system, the critical point is the Institute for Broadband Communication, tests and to provide high data rates and low latency also for wireless measurements are done. transmission, as Powerlink´s characteristics shall be preserved in a wireless version. In addition to the radio type transmission optical wireless systems are also taken into account, which could II. POWERLINK be used for line-of-sight applications (FSO, free space optics). Index Terms—Wireless Powerlink, Industrial Ethernet, In order to have reference values for later measurements a Ethernet IEEE 802.3, Wireless LAN IEEE 802.11, optical wired Powerlink network consisting of a Managing Node wireless, free space optics (Master) and two Controlled Nodes (Slaves) connected by an Ethernet Hub was arranged in the laboratory (see figure 1). I. INTRODUCTION P owerlink is a very fast Industrial Ethernet bus system for Automation purposes. It offers low latency and guaranteed data transfer within fixed time slots using standard Ethernet hardware. Up to now transmission is done over CAT5 or CAT6 Ethernet cable and fibre optic links in combination with Ethernet media converters [5]. There is no commercial solution for wireless Powerlink transmission available at the moment. But like in many other technical areas, wireless connections are getting more important also in Automation. Fig. 1: Standard Powerlink network used for measurements There are numerous applications that could benefit from wireless Powerlink transmission. One example is the The latency of the network was measured using the free connection of battery powered mobile devices like machine network analyzer “Wireshark” [6]. As seen in figure 2, response times are very short on cable. After the Master has control units with the freedom of movement for the operator. sent his Polling Request (PReq) command to a specific slave, Among the radio systems there are lots of well known the slave responds with Polling Response (PRes) command commercial techniques like Bluetooth, Wireless LAN and within 6 to 8 microseconds. We should keep the values in Zigbee. If possible, for the radio type of the wireless mind for later comparison. The second important value for the Powerlink transmission one of those should be used in order correct function of a Powerlink network is the jitter, which to achieve a cheap implementation and save development indicates the repeated accuracy of the Start-of-Cyclic (SoC) costs. Due to the fact that Powerlink is a high data rate bus frame reaching the powerlink devices. The SoC is needed for system with low latency, Wireless LAN will probably be the synchronization issues. If jitter gets too high, the slave can not only technique with enough performance for Industrial take part in synchronous data traffic anymore. For later Ethernet use. In addition to that there is a close relationship comparison, jitter is measured directly on CN1’s device hub.
  2. 2. 2 [3][4]. Of course delay is also higher because of access point’s store-and-forward technology, signal processing and modulation/demodulation needed for wireless transmission Fig. 2: Delay measurement – cable connection Fig. 3: Jitter measurement – cable connection Fig. 4: ISO-OSI seven layer model Figure 3 shows that jitter is very low, the SoC accuracy differs only 1 to 3 microseconds from the 10000 microsecond cycle. B. Powerlink over Wireless LAN In this case the measurement inaccuracy of Wireshark and the used standard PC hardware is probably higher than the real As mentioned before, correct MAC addressing on ISO-OSI jitter. In a Powerlink network, all devices have a unique layer 2 is essential for Powerlink. In home or office use most hardware address (MAC). The Master addresses the slaves devices operate as clients associated to an access point using this MAC (Medium Access Control) address (see providing access to the network infrastructure. This destination addresses in figure 2). In order to get a synchronous Powerlink connection between the Master and its configuration works fine for IP data traffic but is not suitable Slaves also over a wireless link, it will be necessary to provide for Powerlink. If the WLAN devices in the Powerlink network correct MAC addressing and forwarding. shown in figure 5 are configured as Access Point (Managing Node) and Access Point client (Controlled Nodes), the Wireless LAN will change the MAC addresses of the data III. WIRELESS LAN packets transferred through it. On the wireless link the source and destination MAC addresses are substituted with those of As Wireless LAN (IEEE 802.11) is the wireless evolution of Ethernet (IEEE 802.3), the two technologies should be compared in order to find out if Wireless LAN could be used for Powerlink transmission. A. Fundamentals When comparing Ethernet and Wireless LAN in the ISO- OSI seven layer model (see figure 4), we can see that these two technologies only differ below the IEEE 802.2 Logical Link Control layer (LLC). This layer enables communication between different Ethernet based technologies. While Ethernet transmission uses cable connections, Wireless LAN has to cope with much more unreliable radio transmission. That is why a different access scheme (CSMA/CA vs. CSMA/CD) and packets with more overhead have to be used for WLAN Fig. 5: Wireless transmission in a Powerlink network
  3. 3. 3 the two involved WLAN devices, which means that from Because of the direct association and the absence of clients Managing Node’s view all Controlled Nodes use the same there is no need for Beacon signals and cyclic increase of MAC address. In home and office use there is no problem as latency is avoided. The only disadvantage of this IP routing on layer 3 directs the packet to the right recipient. configuration is that wireless point to multipoint transmission But when used with Powerlink the Managing Node can not is not possible, which means that one access point per bridge address the Controlled Nodes anymore because of the wrong is required on the Managing Node’s side. Experience showed MAC address. that an implemented WDS bridge mode is no guarantee for a Access Point configurations with clients implicate another working wireless Powerlink transmission. For example, disadvantage as a beacon signal is necessary for correct Linksys’ WAP54G [11] access point has a correctly association. It is also required for synchronizing the clients to implemented WDS bridge mode but was not able to handle the access point and has to be transmitted at least once per the vast amount of Powerlink data packets with the necessary second [10]. Every time the beacon is sent out by the access timing accuracy. This resulted in a shutdown of the point, the Powerlink data packets are delayed which results in synchronous data transfer. The conclusion is that the higher latency. If latency exceeds the allowed response time, computing power of the access point’s hardware has to be then the correct packet sequence can be disturbed. As shown adequate also. After having tested many Wireless LAN in figure 6, the response of CN1 exceeds its allowed time of devices only two could be used for wireless Powerlink. All 3000 microseconds and the master already sends out Poll measurements were done with deactivated wireless encryption Request for CN2 before getting CN1´s Poll Response. and good radio reception conditions. C. Compex WP54AG The Compex WP54AG (see figure 7) professional access point offers an Infineon ADM5120 175 MHz processor and 16 MByte SDRAM on board. It is equipped with a separate Compex WLM54AG dual band (2.4 GHz and 5 GHz) wireless module [7]. Fig. 6. Packet disturbance caused by Beacon transmission There is only one client operating mode enabling Powerlink communication with Access Point configurations. It is called Wireless Adapter mode and offers a direct WLAN connection for only one Ethernet client. Measurement shown in figure 6 was done with one Wireless Adapter per Controlled Node. MAC address is also replaced, but the Master Node is able to access the Controlled Node, as the Wireless Adapter is forwarding data transfer directly to him. Because of the beacon issue, this configuration is not recommendable for Fig. 7: Compex WP54AG 802.11a/g dual band access point practical use. The only operating mode that should be used for wireless Powerlink transmission is WDS bridge mode, which was originally intended as cable replacement for the interconnection of buildings. The tests showed that many of the available Access Points on the market do not offer this feature or the operating mode is not implemented correctly. Because of the fact that the WDS bridge is a cable replacement, the Ethernet packets from the cabled part of the network are entirely encapsulated in the WLAN packets including their MAC addresses. On the receiving side of the bridge the original Ethernet packet is unwrapped from the received WLAN packet and sent to the Ethernet port of the WLAN device, where the connected Controlled Nodes are addressed with their correct MAC address. Wireless bridges are point-to-point links where two Wireless LAN devices are directly associated to each other. Other clients in the surrounding are not allowed to participate. Fig. 8: Latency measurement Compex AP, 802.11g 2.4 GHz mode
  4. 4. 4 When configured as WDS bridge in 802.11g 2.4 GHz mode, the more computing power the wireless device must have. Controlled Node 1 responds within approximately 2800 Jitter values are approximately the same for 2.4 and 5 GHz microseconds, Controlled Node 2 after 2400 microseconds mode. (see figure 8). The same measurement was done for the 802.11a 5 GHz mode (see figure 9). Here CN1 responds within approximately D. PC Engines Alix.2C3 Board + Ubiqiti WLAN module 2000 microseconds, CN2 requires 1600 microseconds. Obviously the Compex AP’s performance is much better in In order to have a wireless device with more computing 802.11a than in 802.11g mode, therefore the Controlled Nodes power two miniature PC Engines Alix.2C3 mini PC boards [8] are able to respond more quickly. with separate 5 GHz 802.11a Ubiquiti XR5 [9] WLAN modules were tested. The boards are equipped with 500 MHz AMD Geode processors and 256 MB SDRAM on board. Generally they are miniature x86 PCs with linux operating system and drivers installed on a CF card. The Ubiqiti WLAN modules are installed in the mini PCI slot. Fig. 9: Latency measurement Compex AP, 802.11a 5 GHz mode Further we want to know about the jitter when using a wireless link. For this purpose, the jitter is measured directly at CN1’s built-in device hub (results see figure 10). Fig. 11: PC Engines Alix.2C3 system board Fig. 10: Jitter measurement Compex AP, 802.11g 2.4 GHz mode In comparison to cable connection Jitter is higher in principle because of the more complex wireless transmission. Taking a look at line 142 in figure 10 we can see that there are additional sporadic jitter increases of more than 30 microseconds. The reason for that are the two connected Controlled Nodes. The Compex AP on the Controlled Node’s side obviously doesn’t have enough computing power to Fig. 12: Ubiqity XR5 Wireless LAN module handle the data transfer for both CNs with the necessary timing accuracy. This means that the more CNs are connected
  5. 5. 5 The two devices were also configured as WDS wireless bridge IV. FREE SPACE OPTICS (FSO) in order to do the same measurements for comparison (see figure 13). Another interesting method for wireless Powerlink transmission in areas with free line of sight is the use of free- space-optics (FSO). This technology was derived from fibre optics and makes use of light transmission in free space. Due to its narrow beam, it is especially suited for point to point links. For maximum availability obstacles in the direct transmission path should be avoided. In addition to that FSO systems are sensitive to ambient conditions and can be influenced by direct sunlight, turbulent air and fog [2]. Two OptiKom wireless optical systems were available for measurements in the laboratory (see figure 15). Each of them is equipped with eight LEDs for sending and one PIN diode for receiving. Fig. 13: Latency measurement Alix.2C3 The higher computing power of the Alix boards allows much faster response times. Controlled Node 1 responds within approximately 420 microseconds, Controlled Node 2 after 380. That means that the response time of the Alix wireless bridge is nearly five times faster in comparison to the Compex bridge. Because of that also much shorter Powerlink cycles are possible. While the shortest possible cycle for two Controlled Nodes with the Compex bridge was 8000 microseconds in 2.4 GHz and 6000 microseconds in 5 GHz Fig. 15: OptiKom free-space-optics system mode, the Alix bridge was able to work at 1600 microseconds. The jitter measurement (figure 14) for a cyclic time of 1600 A point to point link consisting of two FSO systems was microseconds also shows very good behavior, the Start-of- inserted between Managing Node and Controlled Node of the Cyclic frames are delivered at Controlled Node 1 with Powerlink network (see figure 16). Because the FSO system accurate timing, just like it was connected over cable. A long was built for a data rate of 10 Mbps, a connection could not be term test for 60 hours also worked fine, both CNs stayed in established as Powerlink requires a physical 100 Mbps link. synchronous transfer mode. Therefore two 10/100 Mbps switches (1x Linksys SR216, 1x Surecom EP-816DX-A) converting between 10 and 100 Mbps were added in order to establish a connection between the Managing and the Controlled Node. Fig. 14: Jitter measurement PC Engines Alix.2C3 Fig. 16: FSO Powerlink transmission with Delight systems
  6. 6. 6 Adding two switches increases delay significantly because of most applications. Important requirements for failure free the store-and-forward technology switches are based on. transmission are good radio reception and avoiding Measuring the delay, CN1´s Poll Response was received after interference by other Wireless LAN devices that could use the 200 to 240 microseconds. On the one hand this is faster than same channel. the Wireless LAN transmission was, but on the other hand Optical wireless point to point links can easily be delay would be less without switches. As there is no FSO established with FSO in areas where movability is not an system with 100 Mbps media converters available, the delay issue. Line-of-sight areas are linked with even lower delay in of the two Ethernet switches has to be measured separately by comparison to WLAN. Transmission delay is mainly direct cable connection (see figure 17). In this configuration influenced by the media converters used for FSO systems. CN1`s Poll Response was received after approximately 60 A further point that has to be discussed for industrial use is microseconds (see figure 18). Powerlink’s fault-tolerance in case of transmission errors. As wireless connections are not as reliable as cabled ones, the Managing Node has to reactivate Controlled Nodes which have fallen out of synchronous data traffic after transmission errors or battery power loss. ACKNOWLEDGEMENT Work was carried out within the master thesis of Johannes Trummer at TU Graz [1] in cooperation with Bernecker and Rainer (B&R). Thanks to Pirmin Pezzei for supporting the FSO measurements. Fig. 17: Direct connection for measurement of switching delay REFERENCES [1] J. Trummer, “Technologievergleich und Zuverlässigkeitsabschätzung von Funkübertragungssystemen in der Automatisierung”, Master Thesis TU Graz, 2009 [2] E. Leitgeb, J. Bregenzer, M. Gebhart et al. „Free Space Optics – Broadband Wireless Supplement to Fiber-Networks”, Proceedings of SPIE Vol. 4975, 2003 [3] Ch. Lüders, Lokale Funknetze, first edition 2007, ISBN 978-3-8343- 3018-5 [4] J. Rech, Wireless LANs, third edition 2008, ISBN 978-3-9369931-51-8 [5] Ethernet Powerlink Website http://www.ethernet-powerlink.org [6] Wireshark Protocol Analyzer Website http://www.wireshark.org [7] Compex Products Website http://www.compex.com.sg [8] PC Engines embedded PC Hardware Website http://www.pcengines.ch Fig. 18: Result of delay measurement – Ethernet switches [9] Ubiqiti Products Website http://www.ubnt.com/products/xr5.php [10] Wifi-Planet Article 802.11 Beacons Revealed - Website http://www.wi-fiplanet.com/tutorials/article.php/1492071 Subtracting this delay from the previous FSO link [11] Linksys Products Website measurement, the conclusion is that the network including the http://www.linksysbycisco.com/US/en/products/WAP54G FSO link would have a delay of 140 to 180 microseconds when used without Ethernet switches. In comparison to the Wireless LAN systems the FSO link has much lower delay and there is room for further improvement by using faster media converters for the FSO systems. V. CONCLUSION Using Wireless LAN devices with enough computing power in WDS bridge mode already enables wireless Powerlink transmission with standard Wireless LAN hardware. Due to the complex WLAN transmission, delay is much higher in comparison to Ethernet but low enough for

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