Using Ethernet for Industrial Control

Introduction
Ethernet is widely used for information (i.e., office environment) communications today. The technology has availability, familiarity, and cost benefits. Until recently, however, creating an industrial control system using Ethernet was not feasible due to a number of factors, including Ethernet’s lack of determinism, the need for interoperability among devices, and security concerns.

Determinism is the ability to predict when information will be delivered. To guarantee this, an industrial control network must provide scheduled bandwidth (or time slots) that are reserved for time-critical data transfer. Communication over Ethernet, however, is based upon collision detection. If a device attempts to send a message, and that message collides with another message on the Ethernet media, the device backs off and waits to transmit. Thus Ethernet cannot guarantee determinism.

However, recent advances in switch technology have now enabled Ethernet to approach determinism. Switches, unlike traditional bridges and hubs, reduce traffic between the devices attached to their ports. Moreover, the IEEE 802.3 Standard provides for standardized full-duplex operation, which gives a single node - in a point-to-point connection to the switch - full wire concentration. As a result, full-duplex switched Ethernet networks are theoretically able to avoid collisions.

The requirement for device interoperability, the ability of products from different control vendors to communicate with each other, has been answered by the EtherNet/IP (industrial protocol) networking standard. This standard provides interoperability among products from a wide community of automation vendors. It is supported by three major networking organizations: ControlNet International (CI), the Industrial Ethernet Association (IEA) and the Open DeviceNet Vendor Association (ODVA)
Ethernet OSI Model
All installed Ethernet networks support one or more communication protocols that run on top of Ethernet and provide sophisticated data transfer and network management functionality. The communications protocol determines what level of functionality is supported by the network, what types of devices may be connected to the network, and how devices interoperate on the network.

TCP/IP (transmission control protocol/internet protocol) is the communications protocol of the Internet. TCP/IP is a layered protocol that can be mapped approximately to the OSI (Open System Interconnection) seven-layer network model shown in the following figure. The OSI model represents the components of a standard open network architecture.

In this model, Ethernet represents Layers 1 (Physical) and 2 (Data Link). The Internet Protocol (IP) maps to Layer 3 (Network). The TCP and UDP transports map to Layer 4 (Transport). The TCP/IP protocol suite has no specific mapping to Layers 5 and 6 of the model. The user services commonly associated with TCP/IP networks map to Layer 7 (Application).

Each layer of the OSI model uses the services provided by the layer immediately below it. For example, when a TCP connection needs to send a packet of data to another device over Ethernet, it passes the packet to IP for transmission. IP then handles the interface to Ethernet and ensures that the packet gets transmitted onto the Ethernet network to the destination device. On the receiving end, the IP layer receives the packet from the Ethernet interface, and passes it to the appropriate TCP connection within the receiver.