Q: What is Wireless Distribution System, WDS?

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.

Q: What is Rapid Roaming?

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.

Q: How does one power up the Access Point?

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.