If a client monitored by link check leaves or enters the wireless cells it is assigned to, e.g. when a client is switched off or on, the AP triggers a simple network management protocol (SNMP) trap.

DCF (“Distributed Coordination Function”)
The abbreviations DCF and PCF describe two different methods of processing the data communication in radio networks. In DCF, all nodes are always “responsible for themselves”. A node only starts sending if no other signal is pending on its frequency and a receiver, which has received a message, intended for it, sends a confirmation message with which the sender detects the success of the transmission. DCF does not guarantee that a specific data volume is transferred within a maximum time interval. For this reason, it is primarily suitable for asynchronous data transmission (such as e-mail or web browsing).

PCF (“Point Coordination Function”)
The abbreviation PCF describes an access method defined in the 802.11 standard; however, the implementation of this method is not mandatory. The method is suitable to avoid some of the disadvantages of the DCF method. In PCF, not all network nodes have equal rights but one or several access points act as central administrators in the network. An access point then assigns time slots to the other nodes, the clients: Within these slots the frequency is reserved for these clients and they can send undisturbed.

PCF enables to ensure regular network access and the transmission of data within a specific period to the clients. For this reason, PCF is preferably suitable for applications which require continuous data flows. (Synchronous data transmission, e.g. video or audio streams) The achieved transmission periods, however, are in the range of several hundred milliseconds and also the speed of the change from one radio cell to the next does not meet real-time requirements.

iPCF (“Industrial Point Coordination Function”)
iPCF provides a proprietary alternative to PCF developed by SIEMENS, which solves a number of problems related to PCF. The access point poll the clients in their radio cell at regular, very short intervals. They can register their requirement to send longer data messages; however, they only start sending after having received the permission by the access point. These properties result in the following effects:

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 Access Points and Clients. iPCF reduces the handover times to less that 50ms and thus guarantees an almost uninterrupted communication.

Wireless is shared media; only one user is able to transmit data on selected communication channel. Other potential users cannot send data at the same time. The industrial QoS implemented by Siemens is a procedure that guarantees the required data and response time to selected essential clients.

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).