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

Wireless Local Area Networks 48-9
performance with a varying CW size. Another relevant work is [Kon04], which uses three-dimensional
Markov chains to analyze the behavior of the different ACs when both the CWs and the AIFS are varied.
Among the approaches proposed in the literature to tune the CW, there are the Adaptive EDCF (AEDCF)
proposed in [Nao05], the Adaptive EDCA (AEDCA) proposed in [Rom03], and the adaptive technique
presented in [Vit08]. While the AEDCA and AEDCF approaches do not provide for changing the values
of CWmin and CWmax, but simply choose the best one in a suitable calculated range, the work [Vit08]
proposes an adaptive technique to increase the channel-access probability of the highest priority AC
in a general industrial scenario, using a fuzzy controller to dynamically find the most appropriate CW
range, on the basis of the observed network conditions.
In [TJK06], the performance of the 802.11e channel access mechanisms HCCA, using the proposed
simple scheduler, and EDCA in industrial automation systems were compared and evaluated. Even
though it was found that HCCA is superior to EDCA in scenarios with a large number of stations,
improving and adapting the HCCA scheduling algorithm would lead to much better results, which has
also been proven in several works, [CLM07], [GMN03].
48.6 Security Mechanisms
Although advantageous to local area networks (LANs), WLANs in industrial environments introduce
additional security challenges. Compromising of industrial WLANs can vary from costly downtimes
and decreased system performance, to more serious consequences, such as industrial espionage, physical
infrastructure damage, and even loss of human lives.
Some of the typical WLAN security issues entail authentication, access control, encryption, jamming,
interception, and hijacking. Security mechanisms include various encryption protocols, such are
Wired Equivalent Privacy (WEP), WiFi Protected Access (WPA and WPA2), Temporal Key Integrity
Protocol (TKIP), Counter Model with CBC-MAC Protocol (CCMP), and WLAN Authentication and
Privacy Infrastructure (WAPI).
Although deprecated, WEP (based on Rivest Cipher 4 [RC4] encryption algorithm) is still in wide
use. WPA and WPA2 protocols introduced improved security relative to the TKIP-based WEP approach
by introducing a larger key length and a new Advanced Encryption Standard (AES) based algorithm
known as the Counter Mode with Cipher Block Chaining Message Authentication Code Protocol
(CCMP). WEP2 also introduced TKIP. TKIP was designed with a goal of replacing the WEP protocol
allowing legacy hardware to remain in use. TKIP introduced multiple master keys, a unique RC4 for
each frame generated by a maser key, a new integrity check hashing algorithm, etc. CCMP was designed
with the goal of replacing both TKIP and WEP.
WLAN Authentication and Privacy Infrastructure (WAPI) is the Chinese National Standard for the
Wireless LAN based on the Authentication Service Unit (ASU).
Following the guidelines for a secure WLAN deployment, common security measures (e.g., signal strength
limitation, MAC address filtering, SSID broadcast disabling, and using the most recent encryption protocols)
are highly recommended and should not be ignored.
Similarly to WLAN, the ubiquitous wireless personal area networks (WPAN) formed of Bluetooth,
Zigbee, and mesh networks suffer from certain security issues. While these issues include those of
encryption and authentication, they also relate to concerns relative to the “discovery mode,” “pairing,”
and elements of eavesdropping and location tracking.
48.7 Fast Handover
Especially when it comes to widely distributed industrial systems, a WLAN has to consist of more than
one AP to cover the whole area. Hence, special attention has to be paid to the handover procedure
between APs, because it causes a connection disruption that might not be tolerable for specific industrial
real-time applications. Usually, the handover consists of four different phases: the search phase, the
© 2011 by Taylor and Francis Group, LLC