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

WirelessHART, ISA100.11a, and OCARI 53-3
53.2.1 the WirelessHART Physical Layer
The lowest layer on the OSI model is the PHY; it is responsible for sending the bits across the network
media. It enables the transmission and reception of the protocol data across the physical radio channel.
In particular, the WirelessHART PHY is based on a tailored adoption of the IEEE STD 802.15.4-2006
[KHP08], in order to reduce the overall device cost. This standard is intended to conform to regulations
in Europe, Canada, Japan, China, and the United States. Only a simplified set of services are defined
and only the 2.4.GHz band, which uses OQPSK-HSS with DSSS modulation, is supported. In order to
ensure worldwide availability, channel 26 (the one center around 2480.MHz) is not supported since it
is not legal in many locales. In this way, transceivers readily available “off the shelf” or custom radio
transceivers that are compliant to IEEE 802.15.4 can be adopted. Transceivers must only support some
additional restrictions on channel switching time and power up sequence to satisfy the medium access
strategy requirements and lower the power consumption. The radio must be capable to set the transmitting
power (from −10 up to 10.dBm) and must provide some mechanisms to perform the so-called clear
channel assessment (in particular, only the CCA mode 2, carrier sense, of IEEE 802.15.4 is supported).
Finally, the radio can operate in two different states: awake, during normal operation; and sleep, when
it is inactive.
53.2.2 the WirelessHART Data Link Layer
According to the OSI model, the data link layer (DLL) is the lowest information-centric layer, which exploits
services offered by the PHY entities and provides basic low-level messaging among different peer entities.
It provides only local addressing; it can forward messages to neighbors, i.e., devices within the local
addressing domain, but cannot modify message addresses. Addresses at the DLL have only local scope
and can be duplicated in other local links. The MAC layer is part of the DLL and sits directly on top of the
PHY; since it controls the radio, it has a large impact on the overall energy consumption, and hence, the
lifetime of a node. MAC protocols decide when and how nodes (devices) may access the shared medium,
i.e., the radio channel, and try to ensure that no two nodes are interfering with each other’s transmissions.
WirelessHART uses TDMA and channel hopping to control accesses to the network. It is well known
that TDMA is a technique that provides collision-free, deterministic communications and it is the preferable
solution when time deadlines must be satisfied. When the TDMA approach is used, communications
between devices can occur in time slots. Time slots are grouped together in structures called superframes,
which are repeated continuously. Each device must be capable to manage several superframes. Both the
slot size and the superframe length (in number of slots) are fixed and form a network cycle with a fixed
repetition rate. A schematization of this mechanism is proposed in Figure 53.2.
Destination listens for incoming message
Source now listening
Data
ACK
Cycle N
Slot Cycle N + 1
Superframe
FIGURE 53.2
Basics of WirelessHART TDMA mechanism.
© 2011 by Taylor and Francis Group, LLC