The stack’s highest three levels comprise the applications-layers set, whose applications generally execute through software. From the top down, that set includes applications, or Layer 7; presentation, or Layer 6; and session, or Layer 5. Interfacing with end-users through software, Layer 7 contains familiar Transmission Control Protocol/Internet Protocol (TCP/IP) suites such as hypertext transmission protocol (HTTP); simple network management protocol (SNMP); simple mail transfer protocol (SMTP), for e-mail; file transfer protocol (FTP); and others. The bottom four levels define the transport-layers set. It contains transport, or Layer 4, which includes TCP; network, or Layer 3, which includes IP; datalink, or Layer 2; and physical, or Layer 1. Those two lowest layers, which employ both software and hardware, encompass Ethernet. Specified in the Institute of Electrical and Electronics Engineers’ (IEEE, www.ieee.org) standard 802.3, Ethernet is the standard local area network (LAN). Interconnecting seamlessly like hardware and software on a personal computer, Layers 1 and 2 support the OSI stack’s upper layers by handling transmission of message frames onto network media, states Krishna Mendu, consulting engineer with automation vendor Invensys Process Systems (www.invensys.com), Foxboro, Mass. Layer 1, which defines the electrical and mechanical interfaces to the network, connects to network media such as cabling, and places information on it. Typical Ethernet speeds range from 10 million bits per second (Mbps) to 10 gigabits per second (Gbps). Depending on transmission speed, data flow through coaxial cables, twisted-pair wires or fiber-optic cables. Layer 2’s main task is connecting the network layers of two machines or systems trying to communicate. The datalink’s upper sublayer, Logical Link Control (LLC), manages interdevice communications via a single network link. Layer 2’s lower sublayer, Media Access Control (MAC), describes addresses for each device connected to the datalink layer. To transmit messages from an interface on a node, the OSI model’s higher layers send data packets to Layer 2, Mendu says. “It deals with transfer of data frames between nodes of a network through one or more Ethernet interfaces on each node.” On the network, each station or node can have one or more interfaces, he continues, noting that each interface is assigned a MAC address. Find an addressTo aid one-to-one communication between nodes, Layer 2 can be configured to accept only message frames with the destination MAC address matching the MAC address of the receiving Ethernet interface, he adds. LLC tells the datalink layer what to do with a received packet. “When data frames are received from other nodes, error checking is performed and good frames are sent to the upper layers,” Mendu explains. Ranging in length from 72 to 1,518 bytes, each frame, which is a data-packet format, contains a header with source and destination addresses, as well as a trailer having error-correction data. Mendu notes that when multiple nodes simultaneously transmit frames, collisions can occur in half-duplex configurations—those patterns in which data transmission is bidirectional, but in only one direction at a time. However, the Ethernet layers handle those collisions and message retries, he states. Unquestionably, because Ethernet makes possible system-to-system communications, its use broadens. For example, several open industrial protocols are now using TCP/IP on Ethernet and are finding widespread utilization. These include EtherNet/IP, Modbus TCP, and Profinet. C. Kenna Amos, [email protected], is an Automation World Contributing Editor.
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