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HP OpenVMS System Manager's Manual, Volume 2:... |
Managing the LAT Software |
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The LAT network consists of the following components:
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Service nodes
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Service Nodes
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Terminal server
nodes
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Terminal Server Nodes
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Nodes allowing
outgoing connections
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Nodes Allowing Outgoing Connections
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LAN cable
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Components of a LAT Network
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Service nodes supply computing resources for the local network, while terminal server nodes (or nodes allowing outgoing connections) port their terminals, modems, or printers to those resources upon request from a user terminal or an application program.
Note that in a LAT network, nodes that access services are often referred to as master nodes, which distinguishes them from nodes that only provide services.
You can use the LAT Control Program (LATCP) to configure the LAT characteristics for your system. LATCP allows you to set up your system to support:
The systems that support incoming LAT connections are service nodes. (Using LATCP, you can also set up your system so that it supports neither incoming nor outgoing access.)
Service Nodes
A service node is one type of node in a LAT network. (Nodes
that are not running an OpenVMS operating system can also be used
along with the OpenVMS nodes in a LAT network.) A service node is
an individual computer in a LAN
that offers its resources to users and devices. Because the OpenVMS
operating systems contain the LAT protocol, any OpenVMS system can
be configured as a service node within a LAT network.
Types of Services
Each node offers its resources as a service.
Often, a node offers a general processing service, but it can offer limited
services or special application services as well. Any or all of
the services can be specialized applications.
For example, your service node might offer services for the following items:
The general processing service would allow the use of the general computing environment. The data entry and stock services, on the other hand, would be restricted environments, with connections to the application service but to no other part of the service node.
Each service is distinguished by the name the system manager assigns to it. In an OpenVMS Cluster, HP recommends that the service name be the same as the cluster name. In an independent node, HP recommends that the service name be the same as the node name. With special service applications, the service holds the name of the application.
Service Announcements
A service node announces its services over the LAN at regular
intervals so that terminal servers (and OpenVMS systems that allow
outgoing connections) know about the availability of these network
resources. The service announcement provides the physical node name,
the service names, a description of services, and a rating of service
availability. Servers listen to the LAN announcements and record
information in a database. On nodes allowing outgoing connections,
this database is maintained by the LAT Ancillary Control Process
(LATACP). (See
Managing the LATACP Database Size for more information about managing the LATACP database.)
Whenever a user terminal or application program requests a service, the server node connects to the appropriate service node.
Note that
you can disable a local node from multicasting service announcements
by using the /NOANNOUNCEMENTS qualifier with the LATCP command SET NODE. However, because remote nodes must rely on the LAT service
responder feature in the LAT protocol Version 5.2 (or higher) to
connect to the local node, HP recommends that you use this
qualifier only in a networking environment where newer model terminal
servers and hosts are present (all LAT hosts, terminal servers,
and PCs are running at least Version 5.2 of the LAT protocol). Otherwise,
systems running versions of the LAT protocol prior to Version 5.2
(for example, DECserver 100, 200, and 500 systems) will be unable
to connect to any of the systems that have LAT service announcements
disabled.)
Print Requests
In some cases, service nodes can request services from terminal
servers. The most common situation is when the system wants to use
a printer that is connected to a terminal server port. The system
submits the print request to the terminal server print queue that
is set up and initialized in the OpenVMS startup procedure. Then
the LAT symbiont (the process that transfers data to or from mass
storage devices) requests the LAT port driver to create and terminate
connections to the remote printer.
For information about setting up queues for printers connected to LAT ports, see the HP OpenVMS System Manager's Manual, Volume 1: Essentials.
Terminal Server Nodes
A terminal server node is the second
type of node in a LAT network. A terminal server node is usually located
near the terminals and printers it supports. The terminals and printers
are physically cabled to the terminal server; the terminal server
is physically connected to the LAN cable.
Locating Service Nodes
Terminal servers build and maintain a directory of services
from announcements advertised over the network. Then, when terminal
servers receive requests from terminal users, they can scan their
service databases and locate the computer that offers the requested
service.
Terminal servers not only look for the node that provides the requested service, but they can also evaluate the service rating of that node. If a requested service is offered by more than one node, then the service rating is used to select the node that is least busy. A server establishes a logical connection between the user terminal and the service node.
Setting Up Connections
One logical connection carries all the data directed from
one terminal server node to a service node. That is, the server
combines data from all terminals communicating with the same node
onto one connection. A terminal server establishes a logical connection
with a service node only if a logical connection does not already
exist.
If a connection fails for any reason, a terminal server attempts to find another node offering the same service and "rolls over" the connection so users can continue their computing sessions.
Even though terminal connections are bundled together, each terminal can be uniquely identified by its name. A terminal name consists of two parts: the first part is the name of the port on the terminal server that the terminal line plugs into; the second part is the name of the terminal server node.
Servicing Nodes
Although terminal servers are usually the requesting nodes
in a LAT network, sometimes service nodes request service from terminal
servers. Most commonly, a service node queues print requests to
remote printers connected to terminal servers.
Nodes
Allowing Outgoing Connections
Nodes can be set up to allow incoming connections, outgoing
connections, or both. Nodes (excluding those that offer incoming
connections only) such as terminal server nodes can locate service
nodes and set up connections. The database of information about
available nodes and services is maintained by the LAT Ancillary
Control Process (LATACP). (See
Managing the LATACP Database Size for more information about managing the
LATACP database.)
On a node that is set up to allow outgoing LAT connections,
a user can connect to another node in the LAT network by entering
the SET HOST/LAT command. For more information, refer to the SET HOST/LAT command in the HP OpenVMS DCL Dictionary.
Components of a LAT Network
Components of a LAT Network is an
example of a LAT network. The network consists of an Ethernet cable
connecting service nodes and terminal server nodes.
The three service nodes in Components of a LAT Network, named MOE, LARRY, and ALEXIS, each offer services to terminal server nodes on the network.
Two of the service nodes, MOE and LARRY, belong to the OFFICE cluster. (The cluster is distinguished by its computer interconnect [CI] and star coupler.) Because MOE and LARRY are clustered, their service names are the same as their cluster name. Because both service nodes offer an OFFICE service, terminal server nodes can assess the work load on both OFFICE nodes and establish a connection to a node that offers the service that is less busy.
The third service node, ALEXIS, is an independent node in the LAT network so its service name is the same as its node name.
In addition to its primary OFFICE service, node MOE offers an application service called NEWS. With this specialized service, user terminals can connect directly to the online news program, without any login procedure but also without general access to the general computer resources of the node.
The node FINANCE, shown in Components of a LAT Network, is a terminal server node; it supports a number of interactive terminals, a modem, and printers. The node PROCESSING allows outgoing connections; it supports several interactive terminals. The node FINANCE can accept print requests from any of the three service nodes, provided each service node has set up print queues to support remote printers on the terminal server.
Node PROCESSING is also a service node. It offers the service COMPUTE.
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Figure 1 Components of a LAT Network |
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