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of several fieldbus segments using a wireless backbone based on IEEE . with point coordination
function (PCF) is proposed.
In [], it is investigated how the MAP/MMS application layer protocol can be enhanced with
mobility. In the proposed system, the IEEE . WLAN with DCF is used; time-critical transmis-
sions and channel errors are not considered. In [], the same question was investigated with digital
European cordless telephone (DECT) as underlying technology.
24.7 Wireless Ethernet/IEEE 802.11
Instead of developing WLAN technology for the factory floor from scratch, existing technologies
might serve as a starting point. A good candidate is the IEEE . WLAN standard [,,],
since it is the most widely used WLAN technology. Some alternative systems are HIPERLAN [,],
Bluetooth [], and HomeRF [].
24.7.1 Brief Description of IEEE 802.11
IEEE . belongs to the IEEE .x family of LAN standards. he standard describes architecture,
services, and protocols for an Ethernet-like wireless LAN, using a CSMA/CA-based MAC protocol
with enhancements for time-bounded services. he protocols run on top of several PHY's: an FHSS
PHY, a DSSS PHY offering  and  Mbps [], a . and  Mbps extension of the DSSS PHY [], and
an OFDM PHY with  Mbps [].
The standard describes an ad hoc mode and an infrastructure-based mode. In the infrastructure
mode all communications is relayed through ixed APs. An AP constitutes a service set in, and mobile
stations have to associate with the closest AP. he APs are connected by a distribution system which
allows to forward data packets between mobile stations in different cells. In the ad hoc mode, there
are neither APs nor a distribution system, stations communicate in a peer-to-peer fashion. A detailed
description of IEEE . can be found in [].
The basic MAC protocol of . is called the DCF. It is a CSMA/CA protocol using the
RTS/CTS scheme described in Section .. and different inter-frame gaps to give control frames
(e.g., acknowledgments, CTS frames) priority over data frames. However, data frames cannot be
differentiated according to priorities. The IEEE . MAC provides a connectionless and semi-
reliable best-effort service to its users by performing a bounded number of retransmissions. The
user cannot specify any quality-of-service requirements for his packets, he can only choose between
contention-based and contention-less transmission (see below). Furthermore, it is not possible to
specify attributes like transmit power, modulation scheme, or the number of retransmissions on a
per-packet basis. This control would be desirable for treating different packet types differently. As
an example, one could transmit high-priority packets with high transmit power and low bitrate to
increase their reliability.
The enhancement for time-bounded services is called PCF and works only in infrastructure mode.
The PCF defines a superframe structure with variable—but maximum-length superframes. A super-
frameconsistsofasuperframeheaderfollowedbyacontention-freeperiod(CFP)andacontention
period (CP), both of variable length. During the CP all stations operate in DCF mode, including
the APs. To initiate the start of the CFP, the AP (also called point coordinator, PC) has to acquire
the wireless medium before it can transmit its beacon packet. herefore, beacon transmissions and
CFPs are not strictly periodic and isochronous services are not supported. he beacon indicates the
length of the CFP and all stations receiving the beacon are forbidden to initiate transmissions during
this time. Instead, they wait for being polled by the PC. If this happens, they can use the medium
exclusively for transmission of a single packet. After the CFP ends the stations return to their usual
DCF behavior and can initiate transmissions at their will.
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