Q: What do you mean by Antenna Diversity?

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.

Q: What is Link Check?

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.

Q: What is IP Live?

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.

Q: What is Deterministic, Real-Time?

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.

Q: What is Reservation of the Data Rate?

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).

Q: What is Forced Roaming?

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.

Q: What is a ‘FAT’ Access Point?

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.

Q: What is ‘THIN’ Access Point?

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.