The operating
system uses the LAT (local area transport) software to communicate
with terminal servers and other systems within
a local area network (LAN). Terminal servers are
communication devices dedicated for connecting terminals, modems,
or printers to a LAN. They offer the following features:
Provide a cost-effective method of
connecting many user terminals to a computer
Save on cable requirements
Maximize the number of devices that can access a
computer
With the LAT software, which implements the LAT protocol,
the operating system can offer resources, or services,
that the terminal servers can access. A system that offers LAT services
is called a service node. In addition, nodes
can access LAT services by enabling outgoing connections (using
LATCP) and using the DCL command SET HOST/LAT. (In
the remainder of this chapter, "servers" refers
both to dedicated terminal servers and to nodes that allow outgoing
access to other LAT services.)
A LAT service can consist
of all the resources of a computer system, or it can be a specific
resource on a computer system, such as an application program. You can set up
your system as a general timesharing service,
meaning that all of its resources are available to users in the
LAN, or you can restrict access to a specific service (application
program) on the system. This chapter and the HP OpenVMS I/O User's Reference Manual outline
the procedure you use to set up access to a dedicated application
program.
How the LAT Protocol Works The LAT protocol allows the terminal servers and computers
to communicate within a LAN, such as the Ethernet or the Fiber Distributed
Data Interconnect (FDDI). The LAT protocol matches terminals and
other devices to the computing resources (services) of the LAN.
Because LAT terminals are not connected directly to the computer
(service node) they are accessing, the local server must listen
for service requests from its terminals and be able to match the
terminals with computers that provide the desired services.
Using the LAT protocol, then, the operating system announces
its available services over the LAN. Servers listen to the LAN announcements
and build a database of service information so that they can locate
an appropriate system when a user terminal requests computing services.
For example, a user terminal might request general processing service
or a data entry program on the operating system. A server uses the
LAT protocol to establish and maintain a connection between the
requesting terminal and the operating system.
Sometimes the operating system can request services from a
terminal server. The LAT protocol allows systems to ask for connections
to printers or other devices attached to a terminal server.
Advantages of the LAT Protocol Using the
LAT protocol on your system has many advantages:
The LAT protocol lets you make the
resources of any computer on a local area network available to any user
in that network.
In addition to general processing resources, you
can set up terminals, printers, and modems so they are available
from multiple systems in the local area network. This lets you efficiently
use these resources and keep them available even if one of the systems
in the network must be shut down.
You can also set up application programs, such as
data entry programs or news services, as resources. When a user
requests a connection to the resource, the LAT protocol sets up
a connection directly to the application program. No login procedure
is necessary.
The LAT protocol provides load balancing features
and recovery mechanisms so users get the best, most consistent service
possible. In their broadcast messages, systems rate the availability
of their services so that servers can establish connections to computing
resources on the least busy node. If a node becomes unavailable
for any reason, the servers attempt to provide access to alternate
services.
Users can establish multiple computing sessions
on their terminals, connecting to several different computers and
switching easily from one computing session to another. After switching
from one session to another, users can return to the previous session
and pick up where they left off. This saves users the time normally
required to close out and reopen files or accounts and to return
to the same point in a session.
Finally, the LAT protocol can provide improved system
performance. Because the servers bundle messages onto a single LAN
interface, a server interface decreases the network traffic and
reduces the number of computer interrupts realized in systems where
terminals, modems, and printers each have a physical connection
to the computer.
HP OpenVMS System Manager's Manual, Volume 2:... 
Managing the LAT Software