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LAT Concepts

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2.5 LAT

Digital Equipment Corporation’s LAT (Local Area Transport) networking software is designed to ease the

process of accessing and managing local area networks.

LAT is significantly different from other protocols in two important ways. First, LAT is not routable. There

is no way to divide LAT networks into smaller subnetworks and use routers to reduce traffic between nodes.

Second, LAT is a timer-driven protocol. Packets are expected at certain intervals, and the protocol cannot

adapt to slow network links dynamically. For these reasons, LAT traffic over wide areas is typically carried

inside (or encapsulated in) TCP/IP or IPX/SPX packets. The latter two protocols are fully routable, and

can handle wide-area, slow network links.

Note: If LAT is bridged across slow links, session time-outs and errors are likely.

LAT software is built around the concept of services. A service may be provided by a dedicated device,

such as a printer, or by a network host. A device that offers one or more services, such as your Server, is

called a node.

In general, all services offered by the Server are associated with one or more ports; exceptions to this rule

will be noted later. Figure 2-2 shows an example of services offered on a network.

Figure 2-2: Example of Network Services

Nodes advertise their services to the network by broadcasting occasional messages about them. These

messages, referred to as multicasts, contain the node’s name and its list of services. By monitoring

multicast messages, all hosts on the network know what nodes and services are available and can provide

this information to their interactive users. The Show Services and Show Nodes commands display this

information.

Node Vax2, with

service Vax2

(logins to other

users)

Node Phred, a computer

offering services

"LaserPrinter" (for

printing) and "Phred"

(for logins)

Node Server_1, offering

service "modem"

PWR OK NET RCV XMT

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