Industrial Ethernet: The Hunt for CSMA/CD

By November 27, 2017Uncategorized

The unpredictable nature of Ethernet TCP/IP’s timing has long been a drawback for many industrial network applications, specifically those with stringent real-time capability requirements. Yet, as the overall cost vs. benefit performance has improved over time, industrial users have developed methods to overcome the apparent shortcomings.

In total, there are currently about thirty Industrial Ethernet systems in use around the world, but those most significant to the North American market are:

  • Ethernet/IP
  • Modbus/TCP
  • EtherCAT®
  • Ethernet Powerlink
  • Profinet

All these protocols have been developed with the challenges of real-time control in mind (constant cycle times, low latency, low jitter), specifically the design of preventive measures to avoid data collisions, and they approached the solution in different ways.

The one apparent difference to the standard Ethernet TCP/IP protocol is the implementation of additional Real Time Data Exchange modules that manage real-time communication either in place or on top of TCP/IP/UDP.

Still, all protocols continue support for the Ethernet (IEEE 802.3) physical and data layers and TCP/IP/UDP, hence providing an interface to standard information-level networks.

Protocols such as Ethernet/IP and Modbus/TCP appear to be the most vulnerable when it comes to hard real-time control, because they inherited all the features that render Ethernet TCP/IP non-deterministic. While the CSMA/CD issue (random delay times after bus arbitration) can be significantly suppressed through the use of Ethernet switches, these protocols still have to deal with the additional jitter caused by UDP and IP. Without careful design considerations, they will be questionable for hard real-time control, specifically for fast motion control applications.

Ethernet Powerlink circumvents the CSMA/CD issue very elegantly through bypassing the TCP/IP/UDP layers when it comes to real-time communication. Ethernet Powerlink utilizes dedicated and highly deterministic time slices assigned to each node in the network. The result is that only one node transmits data to the bus during its dedicated time slice, while all other nodes wait for their turn. This method effectively prevents any message collision and thus eliminates random dwell times. Obviously, the bypassing of TCP/IP/UDP also eliminates the IP/UDP jitter.

EtherCAT®, however, approached the CSMA/CD problem by eliminating it entirely at the physical level, which, in all consequence, requires the use of a dedicated hardware. While an EtherCAT® Master system requires only a regular Ethernet network interface, each EtherCAT® Slave device requires a dedicated controller. EtherCAT® still uses Ethernet as the physical layer, but the Ethernet messages are passed from node to node (daisy-chain), thus eliminating the need for bus arbitration per CSMA/CD.

Profinet, in comparison to all other protocols introduced here, has undergone a transition through all previously described scenarios: Profinet V1 (CBA) uses the standard Ethernet TCP/IP, Profinet V2 (RT) bypasses the TCP/IP/UDP layers, while Profinet V3 (IRT) utilizes a dedicated Ethernet controller.

Source by Wilfried F Voss

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