IBM SNA Interoperability Concepts
IBM SNA Interoperability Concepts |
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As with the hierarchical network model, the type of software that is implemented in an APPN hardware component determines how it functions on an APPN network. Also, just as in an hierarchical network, the combination of hardware and software that is implemented in an APPN network device is called a PU. A PU represents a device, often called a node, to the SNA network.
In a pure APPN network, all nodes are PU type 2.1 nodes. PU type 2.1 is an upgrade of the PU type 2.0 standard used in hierarchical SNA networks. Like PU type 2.0 nodes in hierarchical SNA networks, peer-oriented type 2.1 nodes can communicate with type 5 host nodes. However, type 2.1 nodes provide the added capability to establish peer-to-peer communications with other type 2.1 nodes. As such, type 2.1 nodes do not require the use of mainframe system services control points (SSCPs) or communications controllers used in hierarchical SNA networks.
This capability allows SNA APPN networks to be constructed entirely of type 2.1 nodes, such as those implemented in IBM AS/400 systems and personal computers.
There are three types of PU 2.1 nodes in APPN networks, as shown in Figure A.7:
Figure A.7 PU 2.1 Node Types in an APPN Network
APPN network nodes contain the highest level of functionality of the three APPN node types. APPN network nodes can perform all the basic functions supported by the other APPN PU type 2.1 nodes. These basic functions include the ability to establish sessions with other APPN PU type 2.1 nodes. To maintain compatibility with hierarchical networks, APPN PU type 2.1 nodes can also establish sessions with type 5 (mainframe) host nodes. However, APPN network nodes (such as those implemented in IBM AS/400 computers) can provide the following additional services for the other PU type 2.1 node attached to it:
An APPN network node and the other PU type 2.1 nodes attached to it comprise an APPN domain. The APPN network node acts as a server for the other PU type 2.1 nodes in its domain. It can also act as an intermediate node, supporting routing functions in APPN networks that contain multiple APPN network nodes.
An APPN network node contains a control point (CP) that provides directory database updates and directory search services. The CP can establish sessions with CPs in adjacent APPN network nodes to keep the network structure and directory information up-to-date, without relying on the presence of a mainframe SSCP.
For example, when a new APPN end node is added to the APPN network, its APPN network node server automatically updates its directory and routing tables and propagates that information to the other APPN network nodes.
Note
APPN directory services differ from most other directory services that use non-SNA network protocols. Because no fixed addresses are assigned to APPN resources, APPN directories respond to LU directory requests by providing the location and route to the destination LU, rather than the address of the destination resource. Consequently, APPN network nodes must dynamically create route identifiers when LU to LU sessions are established.
Like all PU type 2.1 nodes, APPN end nodes can establish peer-to-peer sessions with other PU type 2.1 nodes (and with PU type 5 mainframe host nodes used in hierarchical SNA networks).
Unlike APPN network nodes, APPN end nodes cannot perform routing functions and cannot function as intermediate nodes in APPN networks. However, APPN end nodes can provide a subset of APPN session services and directory services for its own LUs.
APPN end nodes can also be attached to an APPN network node, which acts as the server for routing, session, and directory services. An end node can have links to more than one network node, but only one network node acts as the server for the end node at any given time.
Like APPN network nodes and APPN end nodes, LEN nodes can establish peer-to-peer sessions with other PU type 2.1 nodes (and with PU type 5 mainframe host nodes used in hierarchical SNA networks).
However, to communicate with APPN nodes that are controlled by other APPN network nodes, LEN nodes, must communicate through the APPN network node that is acting as its server. LEN nodes do not provide any APPN routing or session services for other nodes.
