This chapter acquaints readers with some of the concepts and terms that apply to Local Area Transport (LAT) networks and explains the role of 4DLT in the LAT environment. It also contains introductory information on using 4DLT from a LAT terminal. This chapter includes these sections:
LAT is network software that specializes in communications between user terminals and host computers on an Ethernet local area network (LAN). The purpose of a LAT network is to increase the number of resources available to terminal users. LAT enables these users, usually restricted to the resources offered by their local host, to access resources on various nodes throughout the LAT network. Local Area Transport.
LAT protocolsLAT software consists of a set of protocols originally developed by Digital Equipment Corporation and used primarily for DEC networks. Subsequent implementations of LAT have been developed to allow non-DEC equipment to participate in a LAT network.
4DLT is Silicon Graphics' implementation of LAT protocols for IRIS systems. A Silicon Graphics workstation running 4DLT can supply resources to the LAT network, and the workstation user can access LAT resources by means of 4DLT utilities.
The principal elements of a LAT network are user terminals, terminal servers, and service nodes and the Ethernet LAN and LAT software that enable communication among them. The printers, modems, and other devices available to users are also considered part of the LAT network, but only components running LAT software (that is, terminal servers and service nodes) are considered LAT nodes. Figure 1-1 shows the elements of a LAT network.
LAT terminals are keyboard devices that enable users to interface with the LAT network. Frequently these are dumb terminals, such as VT100s; however, some LAT terminals are intelligent stations running software that enables them to make terminal-type connections. An Silicon Graphics workstation running 4DLT can be used as a LAT terminal.
ALAT terminal server is an intelligent device that accepts commands from users to establish connections to LAT services. Once the connection is established, the terminal server manages the communications between the user and the service. A terminal server is frequently a special-purpose hardware device, such as a DECserver 90L, but it can also be a network host running terminal server software. An IRIS workstation running 4DLT can function as a LAT terminal server.
In addition to managing terminal-host connections, terminal servers maintain a data base of available LAT services. Users can display the list to determine the LAT services available to them.
Figure 1-2 illustrates a terminal server in a LAT network.
printers (LAT)Notice from Figure 1-2 that LAT terminal servers can also support devices such as printers and modems. When a terminal server is providing a resource (such as a printer), the terminal server has an inbound connection from the network. When a terminal server is receiving a service on behalf of one of its users, the terminal server has an outbound connection to the network
A LAT service node is a host, such as a VAX™ or Silicon Graphics workstation, that offers services to terminal users. Service nodes advertise their services to terminal servers, then wait for terminal servers to send them connection requests. A user must have an active connection to a service node to use its resources.
A single service node can offer many services, such as user logins, printers, file systems, and applications. Frequently, several service nodes may offer a service by the same name. When this is the case, the terminal server connects to the service node that is likely to give the best response in providing the service (see “LAT Service Ratings” in Chapter 2).
Figure 1-3 illustrates service nodes in a LAT network.You can see from Figure 1-3 that a service node can also function as a terminal server. In addition to functioning as terminal servers and service nodes, LAT nodes can function simultaneously as DECnet or TCP/IP nodes.DECnet nodes and LAT TCP/IP nodes and LAT
4DLT software provides two primary functions in a LAT network. One component of 4DLT operates as a terminal server for the Silicon Graphics workstation user, supporting outbound connections between the user and services on the network. Another 4DLT component operates as a service node, supporting inbound connections between remote LAT users and the resources available on the workstation. Inbound connections to the workstation are not noticeable to the user.
Figure 1-4 illustrates the LAT functions that 4DLT performs in a Silicon Graphics workstation.
