Improve Industrial Ethernet Network Uptime |

Improve Industrial Ethernet Network Uptime

November 092014
Improve Industrial Ethernet Network Uptime

By Robert Reid, Panduit, on behalf of Industrial IP Advantage

What we are seeing today in manufacturing is an update of plant network architectures with solutions that securely merge information and control data to improve performance, security and safety within the plant. Network uptime is becoming increasingly important as ethernet is starting to be deployed for critical and time-based processes, where keeping such discrete networks active through a fault without stopping critical processes is paramount.

Given that greater than 60% of ethernet link failures are related to physical infrastructure (Grenier, 2011), it is important from the outset to design and build a resilient network that is architected to recover (converge) quickly from a failure condition.

Switching and signaling delays are affected by media type (Fiber/Copper), media length, number of switch hops and transceiver type, whereas processing and reservation time are independent.

Convergence occurs as a result of a change in network topology, i.e., a physical link failure. When this occurs, a routing algorithm is run to build a new routing table based on the failure condition/location. Once all the routing tables have been updated, convergence is complete.

The convergence time to recover and restore from a failed path condition depends on several factors. In restoration, switching occurs after backup paths are computed following the receipt of failure notification. The convergence time to recover a single path is the sum of the following:
1.    Signal delay: time to signal a network failure between nodes (largest component)
2.    New path processing delay: time taken to compute an alternate path
3.    New path reservation delay: time required to reserve on newly computed path
4.    Switching delay: the time required to switch from affected path to new path

One of the main findings of this study is that fiber offers higher resilience through convergence times for uplinks and rings as compared to copper. In the network architectures covered, network availability and performance benefits described can be achieved by deploying robust fiber optic cabling channels as shown in the Converged Plantwide Ethernet (CPwE) Design and Implementation Guide (ENET-TD001).

One way to achieve high resiliency on uplinks is to deploy a redundant star topology utilizing a redundant pair of fiber links on a single switch, one active and one acting as a standby (Cisco FlexLinks and LACP for example). Convergence times here can be less than 50 ms.

High resiliency/low convergence time on rings can also be achieved through protocol (REP - Resilient Ethernet Protocol) with fiber where we have critical processes that require a few ms of convergence.

Obviously, not all industrial ethernet applications have strict requirements on convergence time. Required convergence times depend on the tolerance of the system to withstand a loss of communications and the risk posed. Convergence time requirements vary from information processing, such as Human Machine Interface (HMI) applications, where less than 1 second is acceptable, to critical motion control applications where a handful of msec is required. Choice of physical infrastructure (architecture, media types, transceiver sets) vs convergence requirements should be studied in the light of total installed cost.

The most common pushback to deploying fiber in such networks, is that solutions tend to be expensive and “craft sensitive” with high learning curves. Fiber solutions today have evolved to be much easier to deploy in factories and plants and there are new ways to terminate that are more “electrician friendly”. The “Deploying Panduit Polymer Coated Fiber (PCF) Cable” video shows deployment and termination of an Interconnect cable within a cabinet for an industrial application.

New installer friendly industrial automation fiber optic cables are designed with technicians in mind and are ideal for harsh, device-level industrial installations. Cost effective, large diameter, high strength GiPC fiber (Graded Index Plastic Clad Fiber), like hook-up wire, is easy to prepare and terminate with LC Crimp and Cleave connectors using hand-held tools and minimal training. A good resource for applying fiber with industrial applications is the Fiber Optic Infrastructure Application Guide.

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