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

WirelessHART, ISA100.11a, and OCARI 53-7
packet’s final destination is called a hop. WirelessHART networks can be implemented in a wide variety
of topologies. In a high-performance scenario, a star topology is probably preferable (i.e., all devices are
one hop to the gateway), while a multi-hop mesh topology is useful for a less demanding (e.g., monitoring)
application. Obviously, any topology in between can also be realized.
There can be also devices that lack the connection with the process but have only communication
facilities; they are routers, handheld devices (used for commissioning and/or maintenance purposes),
and adapters (used to connect legacy hardware with the wireless network).
Routing of data packets is based on graph routing rather than on the (optional) source address routing.
Each pair of nodes is interconnected by several graphs, i.e., a directed list of paths that connect
them. Both upstream (toward the gateway) and downstream graphs are used. Only the network manager,
who is responsible for correctly configuring each graph, knows the entire route; the graph information
within a node only indicates the next hop destinations. Consequently, devices in a graph route
must be configured prior to its use; i.e., it must contain a list of all the links that can be used to forward
a packet along the graph. Graph routing is redundant, highly reliable, and should be used for normal
communications.
On the contrary, a source route is a single, directed route between a source and a destination device.
The source route is statically specified in the packet itself; for this reason, it is vulnerable and it is used
only during the commissioning phase.
53.2.4.1 the Network Layer Datagram
The network layer datagram consists of the following fields (see Figure 53.7):
• A 1 byte control field
• The 1 byte time-to-live (TTL) hop counter
• The least-significant 2 byte of the absolute slot number (latency count)
• A 2 byte graph ID
• The (final) destination and (original) source addresses
• Optional routing fields
• A security layer is encapsulated within this datagram together with the enciphered payload
• Enciphered network protocol data unit (NPDU) payload
Communication between a pair of network addresses is organized in sessions for security purposes.
Four sessions are generally set up as soon as any device joins the network, even if additional sessions
may be added:
• Unicast session between the device and the network manager—it is used by the network manager
to manage the device.
• Broadcast session between the device and the network manager—it is used to globally manage
devices. For example, this can be used to roll a new network key out to all network devices. All
devices in the network have the same key for this session.
Enciphered payload
Control TTL
Network layer
ASN
snippet
Graph
ID
Destination
address
Security layer
Source
address
Security
control
Counter
(Expanded routing
information)
MIC
FIGURE 53.7
The network layer datagram.
© 2011 by Taylor and Francis Group, LLC