wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

48
Wireless Local
Area Networks
Henning Trsek
Ostwestfalen-
Lippe University of
Applied Sciences
Juergen Jasperneite
Ostwestfalen-
Lippe University of
Applied Sciences
Lucia Lo Bello
University of Catania
Milos Manic
University of Idaho
Idaho Falls
48.1 Introduction.....................................................................................48-1
48.2 The 802.11 Family............................................................................48-2
48.3 Physical Layer...................................................................................48-2
Frequency Bands. •. Modulation Techniques
48.4 Medium Access Control.................................................................48-3
Distributed Coordination Function. •. Point Coordination Function
Enhanced Distributed Channel Access. •. HCF Controlled
Channel Access. •. Direct Link Protocol and Block ACK
48.5 Limitations of DCF and HCF for QoS Support
in Industrial Environments...........................................................48-8
48.6 Security Mechanisms......................................................................48-9
48.7 Fast Handover..................................................................................48-9
Mechanisms on the AP Side. •. Mechanisms on the Client Side
48.8 Future Enhancements...................................................................48-11
References..................................................................................................48-11
48.1 Introduction
Many industrial automation applications are designed to be very flexible in order to manufacture products
with increased efficiency. Thus, a flexible communication infrastructure is required, leading to a
growing demand for wireless networks. Wireless networks fulfill these requirements because they offer
more flexibility, cost reductions, and higher mobility to the automation system as compared to their
wired counterparts.
Since communication in future industrial automation systems will most likely be realized by real-time
Ethernet protocols like PROFINET [IEC06a,IEC06b], Ethernet/IP [Eth09], etc. (cf. Chapter 37 of this
book), the corresponding wireless technology at the field level has to be suitable and carefully chosen
with respect to the application requirements, in terms of both dependable and temporal behavior.
Therefore, IEEE 802.11 [IEE07] wireless local area networks (WLANs) are an interesting option because
they were initially designed to be the wireless extension of Ethernet [IEE05]. Some interesting application
scenarios are overhead monorail systems or automated guided vehicles (AGVs) that can be found in the
area of logistics. For instance, AGVs are mobile transport systems that can operate autonomously. They
carry loads and use either fork lifts or a conveyor system to move the objects to be transported. Basically,
they are used to deliver work pieces to specific manufacturing processes and have to be quite flexible by
definition. In a typical scenario, the mobile system consists of a local programmable logic controller (PLC)
to control the vehicle. The local PLC also communicates with the central controller that is responsible for
overall coordination and task assignment. The tasks are then independently executed by the autonomous
systems. The requirements are determined by the velocity and the number of vehicles in the entire system.
48-1
© 2011 by Taylor and Francis Group, LLC