Using Switches Instead of Ethernet Hubs in POWERLINK Networks

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There is often discussion about whether switches can be used instead of standard Ethernet hubs in POWERLINK networks. Since POWERLINK is based on standard Ethernet technology (IEEE 802.3), it is technically possible to use switches. It should be noted that any POWERLINK network built with devices other than Class 2 repeaters does not comply with the POWERLINK standard as defined in POWERLINK Specification DS_301. For these reasons, there is no specific recommendation for particular switches. In this post, I want to explain the technical possibilities, as well as the potential risks when using switches in a POWERLINK network.

Technical Possibilities of Using Switches:
Technically, a POWERLINK network can be set up using switches, as long as the specific requirements for jitter and latency are respected. Managed switches provide options for monitoring and controlling network behavior, but they require adjustments to the system configuration to minimize potential disturbances.

Negative Side Effects and Risks:

  • Non-compliance with Standards: According to the POWERLINK specification DS_301, using switches does not comply with the standards, as only hubs fully meet the requirements for low jitter and latency.
  • Increased Latency and Jitter: Switches can introduce extra delays and fluctuations in data transmission, which can reduce the real-time performance of the network.
  • User Responsibility: When switches are used, it is the responsibility of the user to ensure that the jitter and timing requirements are met. This may require advanced network analysis and special configurations to ensure reliable operation.

Recommendations:

  1. Prefer Hubs: To comply with the POWERLINK specification and ensure reliable real-time communication, hubs should be used.
  2. Use Managed Switches with Caution: If switches must be used, managed switches are recommended, as they offer more control over network behavior. However, the system configuration must be carefully adjusted to handle the additional latency.
  3. Cables and EMC Measures: Use shielded cables (S/FTP or SF/FTP, category 5 or higher) to ensure resistance to interference, and respect the maximum Ethernet cable length of 100 meters.

Further Information:
For detailed technical specifications, you can refer to the POWERLINK specification DS_301, especially section 3.1 “Topology”, which is available on the B&R website.

While it is technically possible to use switches in a POWERLINK network, it is not standard-compliant and may introduce risks in terms of latency and jitter. It is the user’s responsibility to ensure all requirements are met. For reliable real-time communication, the use of hubs remains the recommended solution.

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cool,thanks for that :slight_smile:

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If you would like to use a switch instead of a hub in a Powerlink network for some reason, then it might be a good idea to use a cut-through forwarding switch to reduce the latency instead of a store and forward one.

There is a short explanation about the differences:

You also might have to modify the response timeout setting for each Powerlink node to adapt it to the higher latency of a switch compared to a hub.

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