Page 43

Industrial Ethernet Book 103

Technology SOURCE: BELDEN commands. Thus any longer interruption on the communication network might cause the stop of an AGV which could lead to disruptions in the manufacturing process. The most important quality indicators of how a wireless network can meet the requirements of both applications are: • Packet loss rate: the percentage of sent messages (or packets/frames) that are not successfully received by the intended recipient • Latency: the delay in transmission for the delivery of a message via a wireless connection • Data throughput of the wireless connection: the ability to transmit a certain amount of data within a specified time • Interruption: a break in transmission that takes place when a client roams from one access point to another • Communication range: the area covered by an access point or the seamlessness in the coverage of a facility that determines whether the Wi-Fi connections are strong enough to reach all necessary locations Generally speaking, the importance of each parameter varies according to the application. When it comes to train-toground communication and AGVs, reliable communication has top priority. The wireless network must deliver a certain data throughput with minimal packet loss at every point of the area. A standard requirement of a train-to-ground installation is 20 to 80 Mbit/s data throughput with less than 1% packet loss. Especially the requirement on high reliability is similar for AGV scenarios, since any interruption in communication might cause the AGV to stop its operation. Quality of wireless networks To ensure this reliability can be achieved, the installation must have sufficient network coverage; in addition, the interruptions of a mobile client during the switch from one access point to another should be as short as possible (typically < 50 ms). Insufficient coverage results in a stark reduction of the data throughput, and frequent interruptions that are too long lead to extreme packet loss. For these reasons, an optimal mechanism for changing the connection from the client to the access points factors into these both aspects. Roaming needs to occur as quickly as possible and must be initiated precisely when the client leaves the range of the current access point and the next access point offers a stronger signal transmission which leads to a more reliable data throughput. State-of-the-art technologies Presently, there are various technological wireless network capabilities to enable client A mobile client on a train or AGV moves through the wireless networks of different access points. devices to rapidly change between access points. Since the security of the wireless network should be ensured at all times, including in scenarios with high mobility, there should be no compromises of the implemented security technology in favor of faster roaming times. Therefore, technologies for faster roaming should always be viewed in the context of the underlying security mechanisms. These roaming enhancements are often specific to special hardware or software features and therefore are only available on certain wireless network products. For example, the current BAT devices of the Hirschmann access point series support the following technologies: Fast roaming Although a mobile client moves through the transmission range of several different access points, the reliability of the communication and the available bandwidth must be guaranteed at all times. Ideally, to optimize bandwidth, neighboring access points with overlapping radio coverage should operate on different channels to minimize interference. A mobile client can connect automatically to the access point with the best signal. Fast roaming between wireless network access points has been possible for a long time. Interruptions of less than 50 ms can be achieved; however, even faster roaming requires further technical tricks and achieving such fast roaming times with proper security is even more challenging. Reducing scan times When roaming between two access points, an on-train client must first identify the next target access point. This is not as simple as it may sound, because in order to avoid interference between adjacent access points, these access points typically operate on different channels, meaning different frequencies. However, a client can only communicate with access points on one channel at a time. Therefore, when searching for candidate target access points, the client must deactivate its current communication connection in order to search other channels/ frequencies for suitable access points. A mobile client must therefore periodically interrupt its established connection to scan all eligible channels/frequencies to obtain an overview of signal strengths of the other access points in its environment. Only with this information can a client decide whether there is a possible connection with a better quality than the present quality, and then initiate the roaming process. Depending on the train’s speed and the associated changes in the environment of the WLAN client, the scanning processes must be performed repeatedly. Since the active connection cannot be used during these scans, it is not possible for the client to transfer the packets for the application during the scan – the network is not available whenever the client scans. For this reason, scan processes should be as short as possible. Secure fast roaming Whenever a client decides to switch its connection to a different access point, it will initiate the procedure for the fast BSS (Basic Service Set) transition defined in the IEEE 802.11 standard, meaning the actual roaming to the better access point. In consideration of the highest WiFi security, fast roaming is usually labelled as Fast BSS Transition. The security of a WiFi connection can only be guaranteed if a client properly authenticates at the target access point when connecting and if a valid key for this connection is provided for encryption of the data packets. This takes time and must be repeated with every roaming process, unless special techniques are used. Fast roaming is therefore only possible using a faster authentication mechanism. Over time, more and more (necessary and important) security mechanisms have been added to wireless networks, so that wireless networks today are very secure. But this security comes at a price: the connection setup and connection switching between access points is slower because the necessary security parameters must first be negotiated and exchanged. Here too, a certain level of technical trickery is needed to create both 43 11.2017 industrial ethernet book


Industrial Ethernet Book 103
To see the actual publication please follow the link above