Issue link: https://www.e-digitaleditions.com/i/1192126
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 con- nections to their associated access switches, and finally the access switches connect the end devices. The hierar- chical star topology is generally preferred and is the most common for the network backbone (i.e., the interconnec- tions 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 infrastruc- ture 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 (approxi- mately 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.