1-71.2 Overview of ET-LAN Unit Functions1.2.1 Function ModelThe functions of the ET-LAN unit are shown in the diagram below.I/O and shared memory are used for the interface to the user program (CPU unit).A maximum of eight simultaneous connections are possible for each of the computer link, data transfer,and transparent communication functions.The layers beneath the transport layer of the ET-LAN unit provide the following communication services.TCP (Transmission Control Protocol):TCP is a connection-based communication method which provides the virtual circuit. In the TCPcommunication method, since communication services including re-transmission, sequence and flowcontrol for the communication data are provided, high communication reliability is guaranteed at theprotocol level.UDP (User Datagram Protocol):UDP is a connectionless communication method which provides only data communication in IP units. Inthe UDP communication method, since no re-transmission, sequence, or flow control for thecommunication data is provided, support at the application level is required to guarantee communicationreliability.IP (Internet Protocol):IP is used to transmit data in units of datagrams to partner node specified by an IP address. It providesfunction such as the dividing and reassembling of communication data and communication servicesbetween networks via a router.ICMP (Internet Control Message Protocol):ICMP is used to transmit the error message in the IP. The ET-LAN unit supports the echo replay optiononly to the ping command.ARP (Address Resolution Protocol):ARP is used to transmit the Ethernet (physical) address, which is essential to Ethernet communications,by specifying the IP address. When an ET-LAN unit accesses a station with unknown Ethernet address,you only need to specify its IP address using the broadcast method.AUI (Attachment Unit InterfaceAUI is a transceiver cable that connects the ET-LAN unit with a transceiver.
