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

7-8 Industrial Communication Systems
2.4 and 5.GHz unlicensed band. The peak data rates of 11.Mbps are supported by 802.11b and 54.Mbps
for 802.11a and 802.11g. Different transmission technologies such as infrared, orthogonal frequency
domain multiplexing (OFDM), FHSS, and direct sequence spread spectrum (DSSS) can be employed.
For any given bandwidth, these standards support different data rates by selecting different modulation
schemes, i.e., DBPSK, DQPSK, 16-QAM, etc. Besides, half rate convolution codes are used with
puncturing to ensure reliable data transfer for a given QoS. Synchronization is achieved by sending a
predefined sequence which alerts the receiver that a signal is present. This is followed by start frame
delimiter which defines the beginning of a frame. The next generation for 802.11 is 802.11n, which is
designed to effectively replace all the previous 802.11 standards and enables speed up to 540.Mbps.
The Bluetooth technology is the commonly adopted standard for low-cost, short-range radio links
between different portable devices [MB00]. Also, based on the portions of the Bluetooth specification,
the IEEE 802.15 has been put forward as a WPAN standard. Like 802.11, the Bluetooth system is operating
in the 2.4.GHz industrial, scientific, and medicine band (ISM). The modulation uses Gaussian
frequency shift keying (GFSK) with a maximum frequency deviation of 140–175.kHz. The transmission
technology adopted is FHSS, where hopping at up to 1600 hops/s among 79 channels, which are spaced
at 1.MHz separation. The maximum transmission power depends upon which of the three power classes
(as defined by the standard) are supported and ranges from 1 to 100.mW. A maximum base-band data
rate of 723.2.kbps is supported for each link, with options for 1/3 rate repetition and 2/3 rate Hamming
forward error correction codes.
7.3 Performance Evaluation
Traditionally, the performance evaluation of ad hoc networks has been done on a layer-by-layer basis.
New protocols are proposed at a certain layer, and performance studies compare them with one another
for a number of scenarios and varying conditions. The key issues in the design and performance of
MANETs are comprehensively presented in the authoritative reference [CCL03]. Scores of studies compare
MAC protocols for both random access and reservation-based schemes for use in ad hoc wireless
networks presenting novel ideas. Similarly at the network layer, the performance evaluation of a number
of routing protocols for a variety of workload scenarios has been simulated [B04]. More recent and
comprehensive simulation results using OPNET Modeler 10.5 [IR1] have been carried out for the most
commonly used routing protocols, for instance AODV, DSR, and TORA [AA06]. Similarly, the performance
of transport layer protocols over MANETs has been considered in a number of studies and
schemes such as delaying ACKs [CGLS08] or using a fractional window increment (FeW) have been
recently presented [WHX09]. In the same vein, the impact of P2P traffic on various routing protocols
in MANETS has been evaluated in [OSL05]. A consideration that cuts across all studies is the choice
of the mobility model. Most studies assume a random way point mobility model which may not be the
most appropriate model for ad hoc networks. A comprehensive framework for evaluating the impact of
mobility in ad hoc networks is presented in [BSH03], and more recent studies have evaluated the impact
of the mobility model on routing protocols [TBD07]. A further consideration especially for sensor networks
is energy efficiency, and a number of studies present energy-efficient schemes both at the MAC
and at the network layer in order to increase node life.
The performance modeling of ad hoc networks is a complex problem, primarily due to the fact that
the research community does not have a unified framework for understanding the interaction of MAC
layer, congestion, interference, network coding, and reliability [KHP08]. The performance evaluation
of ad hoc networks is further compounded by the choice of the mobility model as well as by the traffic
patterns [PDH07], wherein it has been demonstrated that the choice of more representative traffic
patterns might produce results that are different than traditional choices of traffic sources. Cross-layer
designs have been a major theme, and novel stacks have been proposed to improve the performance of
ad hoc and sensor networks. Understanding the role and interactions between each subcomponent will
improve our understanding of these complex distributed systems.
© 2011 by Taylor and Francis Group, LLC