SNA Terminology
Some special terms are used to refer to the hardware and components that implement the SNA architecture.
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This section presents several key terms and their definitions.
In general, entities in an SNA network represent functions rather than things. Therefore, most of the terms defined below identify combinations of hardware and software. In some cases, the software or hardware component of a specific network element has not been explicitly mentioned to make the definition more readable.
The Network Nodes, Links, and Subareas diagram illustrates a sample relationship among nodes, links, and subareas.
Nodes
An SNA network is made up of NODES which are connected to one another by one or more LINKS. An SNA node is that portion of a hardware component and its associated software components that implement the functions of the seven SNA layers. The architecture currently defines three types of nodes: host subarea nodes, communications controller subarea nodes, and peripheral nodes.
Host Subarea Nodes
A host subarea node consists of a processor containing a telecommunications access method such as VTAM. These nodes provide the control and management functions for the network.
Communications Controller Subarea Nodes
A communications controller running a network control program such as ACF/NCP makes up a communications controller subarea node. These nodes provide data routing and flow control for the network.
Peripheral Nodes
Numerous devices including cluster controllers (3274s, etc.), distributed processors (8100s, etc.), workstations (3178s, PCs, etc.), and printers (3287s, etc.) are peripheral nodes. They provide network access and services for the end-user.
Subareas
A subarea consists of one host or communication controller node and the peripheral nodes that are attached to it. Subareas are the major breakdown of the network for routing. Messages originating in a peripheral node in one subarea and bound for a peripheral node in another subarea are routed first to the destination subarea and from there to the appropriate peripheral node.
MICS Network Analyzer
identifies the subarea number associated with the session partners in files created from NPM data as follows:- Primary Logical Unit Subarea - PLUSA
- Secondary Logical Unit Subarea - SLUSA
Network Nodes, Links, and Subareas
H +-----------------------------------------+ O | HOST PROCESSOR | S | (3084) | T | (HOST SUBAREA NODE) | |-----------------------------------------| | APPLICATION | | (TSO) | S | (PRIMARY LOGICAL UNIT) | U |-----------------------------------------| B | SYSTEMS SERVICES CONTROL POINT | A | (ACF/VTAM) | R +-----------------------------------------+ E || A ============================||============================= || C +---------------------------+ C |COMMUNICATIONS CONTROL UNIT| U | (3725) | | ACF/NCP | S | (CCU SUBAREA NODE) | U +---------------------------+ B | A | R | /| E LINK |/ | A | | +------------------+ |CLUSTER CONTROLLER| | (3274) | |(PERIPHERAL NODE) | +------------------+ | | 3178 (SECONDARY LOGICAL UNIT)
Links
Links are the media through which adjacent nodes are connected. A link consists of a link connection and two or more link stations.
Link Connections
The physical medium of transmission between two nodes is called the link connection. These may be telephone wires, fiber optic cables, microwave beams, System 370 channel cables, etc.
Link Stations
The combination of hardware and software that allows a node to attach to, and provide control for, a link is a link station. Link stations transmit data over a link using data link control protocols.
Transmission Groups
Transmission groups (TGs) are groups of one or more links connecting adjacent nodes that are defined logically to the path control network as a single link. Use of transmission groups allows an installation to match the bandwidth (capacity) of a path to the requirements (i.e., as traffic increases, additional lines can be added to the transmission group). Transmission groups also tend to exhibit higher availability than individual links because if one link of a multi-link TG fails, the connection is not lost.
Peripheral Links
Peripheral links connect a subarea node and a peripheral node. Since peripheral nodes are placed at the logic extremities of the network, these links are sometimes called boundary links.
MICS Network Analyzer
retains the peripheral link name in the data element SLULINK in many files.Network Addressable Units (NAUs)
Network addressable units are sources, destinations, or elements in the path of network traffic. Network addressable units provide end-user access to the network and aid network operators in management and control functions. Each NAU has an associated address that the path control network uses to route data to it. There are three types of NAUs:
- Logical Units (LUs)
- Physical Units (PUs)
- System Services Control Points (SSCPs)
L
ogical Units (LUs)
Logical units are the access points to the network. End-user communication over the network is possible only when the respective end-user's LUs establish a mutual relationship called a session. Not all LUs support the same set of SNA functions; the architecture uses LU type to identify the supported functions. Currently seven LU types exist:
- LU Types 2, 3, 4 and 7 support communications between application programs and various types of workstations.
- LU Types 1, 4, 6.1, and 6.2 support communications between two programs.
MICS Network Analyzer
classifies logical units into one of two types:- Primary Logical Units (PLUs)
- Secondary Logical Units (SLUs)
The primary logical unit is responsible for activating the LU-LU session. The secondary logical unit receives the BIND Session Request. Generally, PLU names identify host applications (TSO, CICS, IMS, etc.) and SLU names identify terminals.
Physical Units (PUs)
Physical units are the hardware elements of the network. Every SNA node contains one physical unit to manage the links connecting the node to other nodes.
MICS Network Analyzer
identifies several types of physical units:o Secondary Logical Unit Physical Unit (SLUPU) o Primary Logical Unit Subarea Physical Unit (PLUSAPU) o Secondary Logical Unit Subarea Physical Unit (SLUSAPU)
Secondary logical unit physical unit (SLUPU) names typically identify terminal control units. Primary logical unit subarea physical unit (PLUSAPU) names usually specify the host processor in which the application resides. The secondary logical unit subarea physical unit (SLUSAPU) names the host processor or communications controller node in which the SLU resides.
Systems Services Control Points (SSCPs)
Systems services control points are the functions within host subarea nodes which activate, control, and deactivate network resources. The physical location of the SSCP is a host processor, but the SSCP itself is a logical entity.
Domains
The span of control of an SSCP is called its domain. A domain consists of one SSCP and the network resources (LUs, PUs, links, and link stations) that it controls. In some cases, network resources are shared by multiple SSCPs and thus reside in multiple domains. Resource sharing increases overall network availability by reducing the dependence on a single SSCP.
The following Network Addressable Units diagram illustrates the relationships among domains, subareas, and logical and physical units.
Network Addressable Units
+-----------------------------------------+ | | | HOST PROCESSOR A | | | | | S | | (HOST SUBAREA NODE) | H U | | | O B | | PRIMARY LOGICAL UNIT PHYSICAL UNIT | S A | |-----------------------------------------| T R | SYSTEMS SERVICES CONTROL POINT | E D | (ACF/VTAM) | A O +-----------------------------------------+ | M || | A --------------------||---------------------------------- I || C A +---------------------------+ C |COMMUNICATIONS CONTROL UNIT| U A | (PU) | | ACF/NCP | S | | (CCU SUBAREA NODE) | U | +---------------------------+ B | | A | | R | | E | | A | -------------------|-/|------------------------------- | ===================|/=|==================================== ----------------------|------------------------------- | | +----------+ | | | 3274 |__ 3279 | | | PU | LU C | | +----------+ C | | /\ / U | | / | +---------------------------+ S | |COMMUNICATIONS CONTROL UNIT| U | | (PU) | B | | ACF/NCP | A | (CCU SUBAREA NODE) | R D +---------------------------+ E O || A M ---------------------||--------------------------------- A || | I +-----------------------------------------+ | N | HOST PROCESSOR B | S | | H U B | (HOST SUBAREA NODE) | O B | | S A | | PRIMARY LOGICAL UNIT PHYSICAL UNIT | T R | |-----------------------------------------| E | | SYSTEMS SERVICES CONTROL POINT | A | | (ACF/VTAM) | | | +-----------------------------------------+ | |
The following sections discuss the addressing scheme used by SNA to refer to the various components discussed above, the protocols used to control the exchange of data at the seven SNA functional levels, and the message formats used to pass data between them.