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Safeguarding PTP Protocol with Parallel Redundancy Protocol SOURCE: CISCO
This article shares a high-level view into PRP, PTP, and how one safeguards the other. The complex nature of today's industrial networks relies on a deep understanding of protocols' specifications and network design best practices.
Figure 1. PTP Network INDUSTRY IS EMBRACING INDUSTRIAL automation to bring new and better services and to protect their critical infrastructures. In the process, they are quickly realizing the need for a secure and resilient industrial grade network. One of the key requirements from this network is to achieve time synchronization with high accuracy across crucial functions. The network needs to deliver the robust protocols that can handle both unexpected and planned events while shielding the complex systems and services that it supports. Two key protocols answer the call for high precision time synchronization and enhanced redundancy – Precision Time Protocol (PTP) and Parallel Redundancy Protocol (PRP). PTP establishes a dynamic packet-based time synchronization mechanism that achieves sub-microsecond accuracy. PRP increases infrastructure reliability by providing lossless failure redundancy using two parallel networks and packet duplication. Each of these protocols brings enormous value to industrial use-cases. Both integrate 09.202 1
their operation over Local Area Networks (LANs). Both protocols utilize Local Area Networks, lower the cost of deployments and reduce overall network complexity. Industry verticals that can most benefit include telecommunications, financial, manufacturing, and energy sectors, among others. All of these require high availability, reliability, and highly accurate time synchronization to ensure services consistency. Implementing PTP over a PRP environment isn’t as simple as combining both protocols. PRP’s redundancy works on packet duplication over parallel ethernet networks. PTP time synchronization accuracy relies on a clear understanding of path and transit delays. Duplicating PTP packets would complicate the delivery of accurate timing information, potentially confusing end devices. Standards bodies introduced protocols’ modifications that eliminate these challenges, ensure their smooth coexistence, and help to highlight the advantages of their combined strengths. This article shares an overview of how PRP
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interoperates with PTP and vice versa. Let’s start with a high-level view of PTP and PRP to set a baseline.
Precision Time Protocol (PTP)
Precision Time Protocol (PTP) is a time precision and synchronization protocol standardized by the IEEE in IEEE Std. 1588. PTP, a message-based protocol, runs on packet networks, such as Ethernet networks. It enhances the time synchronization mechanisms (phase, frequency, absolute time) from older, less accurate implementations to one that takes advantage of a Local Area Network where data and synchronization would use the same network infrastructure while providing sub-microsecond accuracy. PTP automatically establishes a timing hierarchy. This allows network devices to protect timing accuracy by participating in its measurement and adjustment. Other older, less accurate time synchronization methods relied on their own dedicated infrastructures for analog and serial mechanisms, e.g., PPS
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