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

A Survey of Wireless Sensor Networks for Industrial Applications 12-7
entire lifetime. This makes it susceptible both to interference from other networks operating on the
same frequency and to noise from other sources in the environment. As a result, ZigBee has thus not
been regarded as robust enough for harsh industrial environments [PDA08]. To combat this challenge,
the ZigBee Alliance released the ZigBee PRO specification [ZigBeeP] in 2007. ZigBee PRO is specifically
aimed at the industrial market, having enhanced security features and a frequency agility concept
where the entire network may change its operating channel when faced with large amounts of noise
and/or interference.
The ZigBee Alliance announced in April 2009 that it will incorporate standards from the Internet
Engineering Task Force (IETF) into future ZigBee releases, thereby opening up for IP-based communication
in ZigBee networks. Of special interest for the ZigBee Alliance in the IETF standard portfolio
is the 6loWPAN working group that has created a Request for Comments (RFC4944) investigating the
transmission of IPv6 packets over IEEE 802.15.4 networks [6loWPAN].
12.5.3 WirelessHART
WirelessHART is a part of the HART Field Communication Specification, Revision 7.0 [HART],
which was ratified in September 2007 as the first standard specifically targeting industrial applications.
WirelessHART is based on the PHY and MAC layers of the IEEE Std 802.15.4, although the MAC
layer has been modified to allow for frequency hopping. Another modification is that WirelessHART
operates exclusively in the 2.4.GHz band and to allow for full global availability, channel 26 as defined
by the IEEE 802.15.4 is not utilized, since it is, due to national regulations, not legal to use in some
countries.
WirelessHART employs a multi-hop full mesh network topology, using time-division multiple access
(TDMA) as the channel access method [KHP08]. With TDMA, the network communication is divided
into guaranteed time slots (GTS), where each GTS is reserved for a specific communication link. This
ensures contention-free utilization of the radio channel. Each time slot is 10.ms long, providing enough
time for the transmission of one data packet from the source device and the acknowledgment from
the recipient of the data packet. A collection of time slots is called a superframe, and the superframe
is repeated periodically throughout the network lifetime. At the beginning of a new superframe, the
gateway transmits a beacon which is used for time synchronization of the network.
With capabilities such as self-configuration and self-healing, deploying a WirelessHART network should
require minimal detailed understanding of low-level communication and radio propagation aspects.
12.5.4 ISA-100
The goal of the ISA-100 standards committee of the International Society of Automation (ISA) is to
deliver a family of standards defining wireless systems for industrial automation and control applications
[ISA-100]. The ISA-100.11a, ratified in September 2009, was the first standard to emerge. ISA-100.11a
aims to provide secure and reliable wireless communication for noncritical monitoring and control
applications, while critical applications are planned to be addressed in later releases of the standard.
The ISA-100.11a is based on the IEEE Std 802.15.4 PHY and MAC, operating in the 2.4.GHz band,
defining a frequency hopping, multi-hop mesh network. Like WirelessHART, TDMA is used as the channel
access method, along with network self-configuring and self-healing algorithms. The ISA-100.11a also
enables a network to carry existing wired fieldbus protocols such as FOUNDATION Fieldbus [IEC61158],
PROFIBUS [IEC61158], and HART [HART]. Existing wired installations can thus be conveniently
converted to a wireless infrastructure, with a transparent data transfer between systems. ISA-100.11a also
supports the IPv6 protocol, having adopted 6loWPAN [60loWPAN] as their network layer.
The ISA-100 has established the ISA-100.12 subcommittee to investigate options for the convergence
of WirelessHART and ISA-100.11a. The goal of the committee is to merge the two standards into a single
standard, which will then be published as a future release of the ISA-100.11a.
© 2011 by Taylor and Francis Group, LLC