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

53-10 Industrial Communication Systems
53.3.3 time Keeping
An ISA100.11a wireless network uses International Atomic Time (TAI) for its internal operation. It
has been already stated that synchronization is fundamental to ensure a correct events reporting and
communication protocols behavior. For this reason, devices must share a common sense of time; when
a device sends a message to a clock source, it may receive a clock offset update in the acknowledge and,
using this information, can correct its clock skew. Other devices could overhear and update their clocks
based on the same message exchange, even if the ISA proposal does not address this issue. Obviously,
each device that joins the network must specify the quality of its clock.
53.3.4 the ISA100.11a Network Layer and Topologies
The network layer header fields have been influenced by 6LoWPAN proposal [MG07,MKH07], and an
attempt is made to facilitate future compatibility. In the ISA proposal, the concept of a network made
up of ISA100 devices only (called DL subnet in the following) with respect to a backbone network has
been separated; a backbone device is defined as a device interfaced to a non-ISA100 network, e.g., an
industrial Ethernet or any other network within the facility interfacing to the plant’s network. The
aim is to take into account power and bandwidth requirements on one side and allow for a huge variety
of topologies on the other one. For this reason, several datagram formats have been considered
for the network layer (e.g., using short addresses, 16 bit, for DL subnet devices and long addresses,
64 bit identifier for the joining process and 128 bit addresses for backbones). In particular, three formats
have been considered: the basic header is only used in a DL subnet (it is expected to be the most
common format and minimizes the overhead); also the contract-enabled must be used only over a DL
subnet but allows to include more information; the full header is the IPv6 header and is intended for
use over the backbone. It is a network layer task to translate addresses and to manage packet fragmentation.
It must be remembered that at the network layer, any device within a DL subnet is a single
network hop away.
53.3.5 the ISA100.11a Upper Layers
A transport layer that provides connectionless services with optional security has been defined in the
standard; it extends the UDP over IP version 6 and 6LoWPAN. The APL defines software objects that
mimic real-world objects and also defines the communication services necessary to enable object-toobject
communication.
53.4 OCARI
OCARI is an industrial wireless protocol specification [D08] that proposes a deterministic MAC layer
for time-constrained communication and a network layer that has an energy-aware and proactive routing
strategy for the non-time-constrained communication period. Its APL is based on the specification
defined by the ZigBee Alliance. The following figure illustrates OCARI protocol stack architecture
(Figure 53.10).
53.4.1 Physical Layer
The PHY of OCARI is based on IEEE 802.15.4-2006 PHY over 2.4.GHz frequency band. This RF is
chosen because it is an ISM band that is usable worldwide. Moreover, there is a large choice of RF
transceivers that implement IEEE 802.15.4 PHY 2.4.GHz. Major chipset makers such as TI, Freescale,
Atmel, etc. propose optimized RF transceivers over this frequency band.
© 2011 by Taylor and Francis Group, LLC