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

53-8 Industrial Communication Systems
• Unicast between the device and the gateway—it carries normal communications (e.g., process
data) between the gateway and the device.
• Broadcast session between the device and the gateway—it is used by the gateway to send the
identical application data to all devices.
In addition, each device always has a join session that cannot be deleted. The join key is unique; it is the
only key that can be written by directly connecting to maintenance port physically implemented on the
device itself. It can also be written by network manager. In fact, only the network manager may create
or modify sessions and their corresponding keys.
53.2.5 the WirelessHART Upper Layers
A simple transport layer is provided in WirelessHART, which ensures data exchange reliability. It implements
a master–slave transaction, where a “master” issues a request packet and one or more “slaves”
reply with a response packet; or a slave cyclically publishes a response packet. The application layer
(APL), instead, is the same of the wired counterpart. HART application layer is command based with
standard data types and procedures. Universal, Common Practice, Device Family, and Wireless commands
are specified. Extensive standard and device-specific status are available, including quality
assessment and status for all process variables. A detailed discussion of these commands is outside the
scope of this chapter.
53.3 ISA100.11a
The ISA100 [ISA09] committee was formed by International Society of Automation (ISA) to satisfy the
need of a wireless manufacturing and control systems. In particular, the ISA10.11a is devoted to the
development of a reliable and secure communication system for both noncritical monitoring and process
control applications, which can tolerate latencies on the order of 100.ms. ISA100.11a networks can
have a very rich structure. Several multi-hop networks having a mesh topology, each one built among
devices with routing capabilities to which simple non-routing devices are associated, can be interconnected
via a backbone. Routing support for inter-mesh communications is provided and each mesh
can have one or more gateways to other types of plant networks. However, it must be underlined that
the ISA100.11a is a very recent standard (it has been officially approved by the ISA association during
September 2009), and only preliminary implementations are available.
53.3.1 the ISA100.11a Physical Layer
Similarly to WirelessHART, ISA100.11a also utilizes radios operating in the 2.4.GHz ISM band and
based on the IEEE 802.15.4-2006 standard. Also in this case, the last channel is not used to be compliant
with regulations in some countries and some requirements on switching time of transceivers are
provided in order to implement a frequency agility mechanism.
53.3.2 the ISA100.11a Data Link Layer
Also, the DLL has many similarities with the one implemented in WirelessHART. ISA100.11a defines
devices according to their role in the network; basically, there is a system manager that manages the network,
a security manager that manages keys, routers, and field devices (other roles are defined, for instance,
for clock sourcing or multitier networking but are not reported for sake of simplicity). Time slots, superframes,
and links are the building blocks that permit communications to occur. A timeslot is a single, nonrepeating
period of time; a superframe is a collection of timeslots repeating on a cyclic schedule; links are
connections between devices (each link refers to one timeslot or a group of timeslots within a superframe).
However, in the ISA specs, time slots do not have a fixed length but must be aligned to a 250.ms boundary.
© 2011 by Taylor and Francis Group, LLC