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

48-8 Industrial Communication Systems
The block acknowledgment is also an optional feature that increases the throughput efficiency. With
this option enabled, a station is allowed to transmit several frames within one TXOP. All transmitted
frames form a single block that is acknowledged by only one block acknowledgment frame in the end,
leading to a reduction of the necessary control message exchanges.
48.5 Limitations of DCF and HCF for QoS Support
in Industrial Environments
The DCF mechanism suffers from various limitations from the point of view of supporting the traffic flows
typically found in industrial environments, for several reasons. First, as collisions may occur, the available
bandwidth is lowered and the medium access time is nonpredictable. The lack of predictability represents
a major problem when time-constrained traffic flows have to be handled, as there is no way to provide
guarantees on deadline meeting, even in statistical terms. Moreover, there is a potential for capture effect,
as once a station gains access to the channel, it may keep the channel busy for as long as it chooses. A way
to prevent a station with heavy non-real-time traffic to monopolize the channel, thus significantly reducing
the probability that real-time traffic will suffer from heavy contention and be significantly delayed
due to non-real-time traffic, is wireless traffic smoothing. The traffic smoother is at the middleware level,
located between the network layer (IP) and the wireless MAC that handles RT and NRT traffic differently,
injecting them in the MAC layer at a different rate. An adaptive traffic smoother that implements
the harmonic increase multiplicative decrease (HIMD) algorithm to regulate non-real-time traffic bursts
before they are sent over the network, thus privileging real-time flows, is proposed in [Jai03]. In [LoB06], a
Wireless Traffic Smoother (WTS) for industrial WLANs is presented and evaluated. The WTS, based on a
fuzzy controller, is able to provide end-to-end soft real-time communications with low round-trip times.
Both approaches, [Jai03] and [LoB06], do not entail any modification in the 802.11 protocol.
Many works have addressed the limitations of the DCF for real-time traffic in industrial and robotic
applications. For example, in [San05], the performance of the 802.11b protocol using broadcast and
unicast transmissions in different uncontrolled load scenarios was analyzed in the context of real-time
communication between mobile robots. The paper concludes that as long as the non-real-time traffic
sharing the channel with the real-time traffic is not excessively bursty, broadcast transmissions outperform
unicast ones in terms of packet losses. However, when the non-real-time traffic features very bursty
patterns, the broadcast reliability significantly degrades.
The lack of mechanisms to provide QoS guarantees is another limitation of the IEEE 802.11 standard.
To solve this issue, IEEE (Task Group E) published the IEEE 802.11e standard [IEE05e] as an
amendment to the original 802.11 standard intended to enhance the MAC support for applications
with QoS requirements. The proposed mechanisms of this standard were discussed in Section 48.4.3
and in Section 48.4.4.
However, recent literature outlined some limitations of the 802.11e protocol when different kinds of traffic
are supported on the same channel, and the total offered workload is high. Some works dealt with the
issue through simulation-based assessments, others through analytical considerations. The work [Mor06]
showed through simulations that the default parameter values of the EDCA mode are not able to guarantee
industrial communication timing requirements, when the highest priority class (AC_VO) is used to support
real-time traffic in shared medium environments where other types of traffic are present. The paper
concludes stating that new communication approaches must be devised in order to adopt IEEE 802.11e
networks on the factory floor. In [Vit07], it was shown by simulation that it is beneficial to adapt CWmin and
CWmax for the AC_VO class to allow for a larger spectrum of backoff values, thus reducing the number of
collisions inside that class. The reason for these results is that the CWmin and CWmax settings provided by
the standard for the AC_VO class determine a narrow range of backoff values for the packets in the class.
Among the works addressing the sensitivity of IEEE 802.11e performance to changes in the CWs
depending on the network load, there is the one in [Xia04] which extending the approach in [Bia00],
models the EDCA protocol by means of three-dimensional Markov chains and analyzes the network
© 2011 by Taylor and Francis Group, LLC