The Institute of Electrical and Electronics Engineers (IEEE) has provided several standards for local area networks (LANs) collectively known as IEEE 802, that include the IEEE 802.3 standard for a Carrier Sense Multiple Access/Collision Detect (CSMA/CD) protocol commonly known as the Ethernet. The Ethernet has three basic elements being a physical medium connecting stations (or client devices), an interface at each station for transceiving data across this “Ethernet channel” according to specific rules, and an Ethernet frame that comprises a standardized data format.
An Ethernet system has no central system controller. Each station (or node) shares the same physical medium (multiple access) by contending with other stations for frame transmission opportunities, which it does by ‘listening’ for openings on the physical medium (carrier sense) and, after starting each transmission, determining whether it is transmitting simultaneously with another station (collision detect). These functions are performed by a medium access control (MAC) sublayer, which operates as the interface between each Ethernet station and the physical medium.
This Ethernet protocol offers a number of advantages in LAN. The physical medium is simple, usually being a cable comprising a twisted pair of copper wires. The rules of CSMA/CD are simple and well known, and the hardware interface is generally inexpensive. An Ethernet network can support high data rates, with typical Ethernet networks operating at 10 base-T (10 million bits per second (Mbps)), 100 base-T (100 Mbps), or some networks at a data rate above one billion bps. Ethernet interfaces are widely available and many types of Ethernet stations can share the same physical medium, regardless of the hardware, operating system, and application software being used by each station. For these reasons, Ethernet protocol has been widely adopted for LAN implementations.
The Ethernet does have several limitations that generally renders it ineffective for real-time networking applications. For example, Ethernet does not guarantee delivery of messages, nor does it guarantee the correct message sequence for multi-frame data sequences. Where this type of functionality is desired, a transport protocol such as the transport control protocol (TCP) may be added. Even with such an additional protocol, the Ethernet is non-deterministic in the sense that delivery of any data frame cannot be guaranteed within a specific amount of time. This limitation greatly restricts the utility of Ethernet in real-time applications.
When one skilled in the art refers to industrial or real-time Ethernet, they often refer to industrial Ethernet standards including EtherCAT, PROFINET, Ethernet/IP, Sercos III or Ethernet POWERLINK. These industrial Ethernet standards support a minimum cycle time in the tens of microseconds range. The term “cycle time” for Ethernet is defined as a constant time period during which network devices (e.g., field devices such as sensors or actuators and process controllers) exchange process data, and “real-time” for Ethernet refers to an event, such as the cycle time to exchange process data occurring at a deterministic time. The particular industrial Ethernet standard dictates the minimum cycle time, where for example PROFINET IRT v2.3 which cannot have a cycle time faster than 31.25 μs due to its 64 byte frame size and this protocol requiring multiple frames for each data exchange.
Many industrial networks in factory and process automation use a cycle time between 500 μs and 10 ms to exchange process data within the network. The cycle time is chosen once during the engineering phase of the network and does not change during run time. Although PROFINET IRT v2.3 supports a cycle time as low as 31.25 μs, the majority of industrial networks in factory automation and control use cycle times greater than 500 μs, often a cycle time in the millisecond range.