Image Processing Reference
In-Depth Information
The AP has a poll list of station addresses. The polling scheme itself is not fully specified. A sta-
tion which desires to be polled has to signal this to the AP during the association process. he poll
list membership ends upon disassociation or when the station reassociates itself without requesting
contention-free service.
24.7.2 Real-Time Transmission over IEEE 802.11
The PCF is designed to provide time-bounded services. Many studies [,,] confirm that indeed
packets transmitted during the CFP receive substantially smaller delays than those transmitted dur-
ing the CP, however, at the cost of substantial overhead. In [] the authors show a scenario where
eight voice calls of each having a data rate of kbps require approximately % bandwidth of a Mbps
transmission medium (without channel errors).
When transmission has to be both timely and reliable despite channel errors, retransmissions are
needed. When a time-critical packet transmitted during the CFP fails, the station can try the retrans-
mission during the following CP or during the next CFP one superframe later (except the case where
multipleentriesinthepollinglistareallocatedtothemobileandthusitreceivesmultiplepollsduring
the same CFP). Hence, retransmissions of important packets receive no priority in ..
24.8 Summary
This chapter presented some problems and solution approaches to bring WLAN technology to the
factory plant and to benefit from reduced cabling and mobility. The basic problem is the tension
between the hard timing and reliability requirements of industrial applications on the one hand and
the serious error rates and time-varying error behavior of wireless channels on the other hand. Many
techniques have been developed to improve the reliability and timeliness behavior of lower layer
wireless protocols, but up to now wireless fieldbus systems have not been deployed on a large scale
as the problem of reliable transmission despite channel errors is not solved satisfactorily. It is not
clear which combination of mechanisms and technologies has the potential to bound the number of
deadline misses under realistic channel conditions.
It seems to be an open question whether just some more engineering is needed to make wireless
transmission suitable for fulfilling hard real-time and reliability requirements, or whether there is
really a limit of what can be achieved. Fortunately, wireless communications and WLAN technol-
ogy is a very active field of research and development. New technologies are created and existing
technologies are enhanced. As an example, the IEEE .g and IEEE .e working groups are
working on delivering higher bitrates and better quality of service to users. It will be exciting to see
how industrial applications can benefit from this.
References
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