January/February/March 2020 I 27
Since the logical designs denote the base requirements
of the physical design, the intended logical topologies
and level of redundancy between each of the associated
network levels (i.e., core, distribution, access, and device)
must be identified. The core denotes the head end of the
network, typically incorporating a demilitarized zone
(DMZ) firewall connection to the IT/corporate network,
redundant layer 3 switches, server connections, and
redundant links down to all of the distribution switches.
The distribution switches then employ redundant connections
to their associated access switches, and finally
the access switches connect the end devices. The hierarchical
star topology is generally preferred and is the most
common for the network backbone (i.e., the interconnections
between the core, distribution, and access layer
switches). The interconnections between the access
switches and end devices could be in either star, ring
or linear topologies for industrial networks. Once the
preceding is understood, the designer is able to use
a methodical approach to address the physical infrastructure
requirements between each level of the network,
starting at the core and working down to the device level
as depicted in Figure 1.
BACKBONE LEVEL INFRASTRUCTURE:
CORE TO DISTRIBUTION AND
DISTRIBUTION TO ACCESS
Optical fiber is the preferred medium for all network
infrastructure backbone segments for several reasons. It
is immune to electromagnetic and radio frequency (RF)
interference, extends well beyond the 90 meter (approximately
300 feet) permanent link limit of balanced twisted-
pair without additional active hardware, and offers low
latency and fail-over times. Where multiple links are
required, a single multi-strand cable can support many
links. Therefore, the main selections become optical fiber
FIGURE 1: This depiction of an industrial network shows how the star, ring, and linear topologies can be utilized, as well as the
physical infrastructure requirements between each level of the network from the core to the device. Source: Rockwell Automation,
ON, Canada.