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

53-2 Industrial Communication Systems
recently. In particular, in this chapter are described the proposal of the Hart Communication Foundation
(HCF), also known as WirelessHART [SHM07,KHP08], the proposal of the International Society of
Automation (ISA), also known as ISA100 [ISA09], and the Optimization of Ad hoc Communications
in Industrial networks (OCARI [D08]) project, funded by the French national research council—ANR.
53.2 WirelessHART
The HART• (Highway Addressable Remote Transducer) Communication Protocol is one of the earliest
fieldbus to appear on the market. It was originally designed to preserve compatibility and improve communication
over legacy 4–20.mA analog instrumentation wiring, sharing the pair of wires used by the older
system. According to some, considering the huge number of 4–20.mA devices installed throughout the
world, the HART protocol is probably the most diffused protocols for industrial communications today.
The protocol was proposed by Rosemount Inc. in the mid-1980s for their smart field instruments.
It was based on the Bell 202 communications standard that AT&T developed for a modem operating
in the audio frequency range. Starting from 1986, it was made an open protocol, thus speeding up its
enhancement, as proved by several successive revisions to the specification.
As regards performances, it has been developed having in mind process automation as the target
scenario, and when used in multichannel I/O system, it allows for about one transaction per second
(if frequency shift keying (FSK) modulation is adopted), with latencies on the same order of magnitude
(depending on the number of nodes). In addition, it is able to work on distances up to 1500.m.
WirelessHART [H09] is an extension of wired HART; it has been developed with the aim of preserving
the same performance of the wired counterpart (or even improve them), allowing the implementation
of a truly autonomous sensor. In fact, as suggested by its name, it uses radio frequencies (RF) as the
communication medium with a nominal coverage area of about 100.m (line of sight) and devices may
be powered either by wire, battery, solar, or a combination of these sources. Communicating devices
are described in terms according to the well-known Open Systems Interconnection (OSI) Seven-layer
communication model [H88]; a comparison with the traditional OSI model communications layers is
given in Figure 53.1.
OSI layer
Application
WirelessHART features
Command oriented. Predefined data types and application
procedures. Auto-segmented transfer of large data sets.
Presentation
Presentation
Session
Secured session between network devices
Transport
Reliable stream transport, negotiated segment
Network
Power-optimized redundant path, mesh to the edge network
Data link
Secure and reliable, Time synced, TDMA/CSMA,
frequency agile with ARQ
Physical
2.4 GHz wireless, 802.15.4 based radios, 10 dBm Tx power
FIGURE 53.1
OSI Seven-layer model and WirelessHART comparison.
© 2011 by Taylor and Francis Group, LLC