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

WirelessHART, ISA100.11a, and OCARI 53-9
Data link payload
(possibly encrypted)
Data link layer
Authenticated by MMIC
IEEE 802.15.4
MAC header
ISA100.11a
DL header
MMIC
FCS
Physical layer
Preamble Delimiter Length
FIGURE 53.8
ISA100.11a datagram structure (DL Header: additional ISA100 Data Link layer header).
In addition, the compatibility with the IEEE 802.15-4-2006 has been preserved; in other words, the packet
at the DLL preserves the header and footer specified in the IEEE standard; see Figure 53.8. Nevertheless, it
must be highlighted that protocol itself is different; there are no IEEE MAC full function devices (FFDs),
the superframe is defined in a different way; even if CSMA/CA is used in both solutions, details are not the
same and the IEEE MAC backoff and retry mechanism is not used. Instead, the ISA standard implements
its own retries, involving spatial diversity (retries to multiple devices) and frequency diversity (retries on
multiple radio channels). Neighbor discovery and joining are also implemented differently.
Three general modes of operation are supported: the slotted channel hopping; the slow channel hopping;
and hybrid combinations of slotted and slow hopping. In slotted hopping, each timeslot uses a
different radio channel in a hopping pattern and it is intended to accommodate one message and its
acknowledgment (see Figure 53.9). On the contrary, in slow hopping, a collection of contiguous timeslots
is grouped on a single radio channel. Timeslot (bandwidth) allocation to devices may be centralized,
i.e., handled by the system manager, or delegated. For instance, a simple form of delegation is based on
channel hopping offsets; a frequency offset is assigned to each infrastructure device and it is free to create
superframes using that offset.
According to the specs, packets forwarding within the wireless network is performed at the DLL,
using the same graph routing adopted also by the WirelessHART solution. Routes are configured by
the system manager, based on reports from devices that indicate instantaneous and historical quality
of wireless connectivity to their immediate neighbors. The system manager accumulates these reports
of link quality to make routing decisions. These reports are standardized, but not the routing decision
process within the system manager.
D
Time
Time slot
A
RF
Chan.
A
B
B
C
C
D
B
A
C
D
A
B
Link
FIGURE 53.9
ISA100.11a link and channel hopping concept.
© 2011 by Taylor and Francis Group, LLC