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As well as including subarea networks, many networks today also use APPN. A purely APPN network is decentralized. It is made up of interconnected nodes that communicate as peers across local and wide-area networks.
A sample APPN network is shown in
Except for LEN nodes, each node in an APPN network contains a control point (CP) that is responsible for managing the node and its resources. A CP in one APPN node can establish a session with the CP in an adjacent node. These CP-CP sessions are used to exchange network information that enables resources to be located dynamically.
APPN networks include the following types of nodes that provide different levels of connectivity and networking support:
A low entry networking (LEN) node is a type 2.1 node. It contains the most basic functionality necessary for connection to an APPN network.
LEN nodes do not contain control points and so cannot establish CP-CP sessions with any other node. This means that they must be manually pre-configured with information about the remote LUs with which they can communicate.
A LEN node can communicate using a direct point-to-point connection to any other node in the APPN network. Additionally, it can communicate with a network node that provides access to other nodes in the network. To do this, the resources on the other nodes must be configured on the LEN node as though they were resources of the network node. If, in addition, the LEN node's resources are configured on the network node, other nodes in the network can locate the resources on the LEN node. In this case, the network node acts as a network node server (NNS) on behalf of the LEN node. To the LEN node, the entire APPN network appears to be located on the network node; to the rest of the APPN network, the LEN node appears to be part of its NNS.
An APPN end node provides limited APPN function support.
Each end node includes a control point; but it establishes CP-CP sessions with just one adjacent network node, which becomes its network node server. The end node can receive network information from its network node server so that remote LUs do not need to be defined. It can also register its own LUs with the network node server, so that they are visible to remote nodes without the need for them to be pre-configured at the remote nodes.
With the help of its network node server, an end node can establish LU-LU sessions with any other node in the APPN network. An end node can also communicate without the help of a network node server, but this is possible only if the remote node is directly connected and if the remote node and its LUs are configured manually.
Unlike network nodes, end nodes do not support intermediate sessions carrying traffic for other nodes, so they are restricted to the edges of the network.
An APPN network node provides the peer-oriented networking services that are fundamental to APPN.
Network nodes form the backbone of an APPN network. They can dynamically locate and connect to each other, exchanging topology and configuration information and supporting intermediate sessions that enable LEN nodes and end nodes to communicate across the network.
Each network node includes a control point that can establish a CP-CP session with any other adjacent network node, and with any adjacent end node for which the network node is the network node server.
Network nodes in an APPN network need to maintain topology information (about the location of other nodes in the network and the communications links between them), and to forward this information around the network when the topology changes. As the network grows in size, the amount of stored information and topology-related network traffic can become large and difficult to manage.
It is possible to avoid these problems by separating the network into subnetworks, so that each node only needs to maintain topology information about the nodes in its own subnetwork. However, this results in increased network traffic when trying to locate resources in other subnetworks.
The Branch Extender feature of APPN, illustrated in Branch Extender, provides a solution to these problems.
As the name implies, Branch Extender is designed for networks that can be divided into distinct areas such as separate branches of a large organization. It works by separating out branches from the main backbone APPN network (for example, the network in the organization's headquarters).
Each branch contains a node of a new type called Branch Network Node (BrNN), which is connected to a Network Node in the main APPN backbone network. The BrNN combines the functions of an APPN network node and an APPN end node.
To the backbone network, the BrNN appears as an End Node, connected to its Network Node Server (NNS) in the backbone network:
The nodes in the backbone network are not aware of the nodes within the branch, reducing the amount of topology information that must be stored.
Because the BrNN appears as an End Node, it does not receive topology information from the backbone network (topology information is transmitted only between Network Nodes).
The BrNN registers all resources in the branch with its NNS as though they were located on the BrNN itself. This means that the nodes in the backbone network can locate resources in the branch without having to be aware of the separate nodes in the branch.
To the branch network, the BrNN appears as a Network Node, acting as the network node server (NNS) for End Nodes in the branch. Each node in the branch sees the rest of the network as being connected through its NNS, in the same way as for a standard NNS.
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