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http://www2.sea.siemens.com | FAQ - Wireless | 2007-07-26
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A: If a Wireless LAN needs to be upgraded, it is sometimes not possible to connect the access points over Ethernet to make communication beyond cells possible. Possible reasons:
Temporary equipment required for commissioning
Cable channels cannot be expanded or do not exist
Installation difficult (for example in a sandpit)
In such situations, the access points can be used in the Wireless Distribution System mode without being wired.
In WDS mode, it is important that an access point can "see" its neighbors (otherwise the chain would be interrupted!). This can make it necessary to use special, distant antennas instead of the supplied antennas. As shown in Figure stations are perfectly capable of taking part in wireless operations in this mode. One important property emerges from its; namely, that stations and access points must share the entire data rate available. This situation can be made more critical because in WDS mode, ALL access points and stations work on the same wireless channel (for example 802.11g and channel 1, 2.4 GHz at 54 Mbps).
The performance of the wireless infrastructure in the above figure can be improved if dual access points (SCALANCE W788-2PRO) are used (see Figure 6). One wireless card can then be used to set up the wireless infrastructure (backbone) and the other wireless card provides the stations with a cell at the location of the dual access point with which they can access this wireless infrastructure. Once again, it is possible that special distant antennas may be required.
A: Rapid Roaming is fast reestablishment of a connection when a mobile node changes from one radio cell to the other. Standard commercially (office grade) available products require several hundred milliseconds to reestablish connection in this case. To get quick deterministic data communication an industrial version of the Point Coordination Function (iPCF) has to be implemented in the Scalance Access Points and Clients. iPCF reduces the handover times to less that 50ms and thus guarantee an almost uninterrupted communication.
A: There are three different ways of supplying power to the Access Points.
Supply 24VDC directly on the M12 connector as shown below.
Supply Power and Ethernet thru the hybrid cable (order no. 6XV1870-2J) and using the hybrid connector that is supplied with the AP.
Connect a POE injector to the Hybrid connector (IP67 rated) on the AP. The Scalance APs are IEEE 802.11af compliant.
A: With this technique, it is possible to achieve a significantly more reliable wireless link in a "difficult" environment in which reflections and multipath reception interfere with wave propagation. The use of antenna diversity is easy to recognize because two antennas are available for one wireless card. The recipient can then evaluate the information from two different antennas and select the better antenna dynamically during reception. When transmitting, the diversity function automatically selects the other antenna after a set number of failed attempts.
A: When the link check is used, if there is no communication taking place, frames are sent to the node at selectable cyclic intervals to check its presence in the wireless network. This method provides a high degree of operating reliability.
A: IP-alive monitors the cyclic communication connections that are extremely common in automation engineering environments. On selected IP connections, the monitoring function checks whether packets are actually exchanged at the times prescribed by the cycle time. If packets are missing, errors can be reported in various ways just as with the link check mechanism (error LED, log file, E-mail, SNMP trap). The user can configure the method of reporting to suit the situation.
A: Wireless LAN implemented according to IEEE 802.11 provides a powerful wireless connection that can be used as it stands for many applications in the office and home environment. Unfortunately, the standard does not support applications with real-time and deterministic requirements. Wireless LAN according to the IEEE 802.11 standard is a "shared medium" in which all stations must share access to the common medium. While this access is controlled, it is not fully predictable.
A: With this method, Industrial Wireless LAN provides the option of guaranteeing both a minimum data rate and a "worst-case" transmission time for selected clients (Quality of Service, QoS). These parameters are negotiated prior to communication.
To illustrate the mechanism described above, let us assume an access point at which 6 stations are logged on. If an application requires that clients 1, 2, and 3 (for example mobile controllers) interface with the factory network over the Industrial Wireless LAN, it must be guaranteed that these controllers can send a status message at fixed, cyclic points in time. This is possible only if an additional mechanism is available to assign the right to transmit. By reserving the data rate in SIMATIC NET, in the example above, clients 1 and 3 have the opportunity to access the access point in the first phase although clients 4 and 5 obviously have large files to transmit. This is followed by a period in which all other stations have their turn according to the normal rules. In Figure 2, this is first client 5 and then client 6. This is once again followed by the phase in which the stations with a reserved data rate can access the access point. In the schematic in Figure 2, it is also clear that client 4 is a "victim" of the IEEE 802.11 access method. Since it is not a client with previously configured assured performance (QoS).
A: The IEEE 802.11 standard does not specifies that an access point must be connected to the wired network over Ethernet. Moreover, it is not even specified that a wired network must be available at all. This means that a station in the cell does not immediately recognize whether or not the wired interface is impaired or interrupted. Such a fault has far-reaching consequences if the station is a mobile controller that sends important process data to the control room. If the wired interface to access point 1 is interrupted in the access point detects the fault and automatically turns off its cell (prerequisite: the user has selected this option in the Web interface of access point). If the cell is designed redundantly (overlapping cells), the station roams to the next available access point. Without this mechanism, there is no way that the mobile controller can keep up the connection to the controlling computer.
A: Scalance W can work as a FAT access point or a THIN access Point. A FAT access point is self-contained unit, supporting transmission, security and management functions.
A: THIN access point handles transmission functions locally but depend on central security and management system or a Wireless Controller. Scalance W can work as a THIN access point with several Wireless Centralized Controller manufacturers.
A: Port Mirroring allows one to monitor traffic on a network by copying traffic from monitored port to the mirror port. This can improve the performance of a network because it allows one to monitor network for problems. Traffic analyzers can capture all traffic on the network when used with port mirroring.
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