2. USAGE CONSIDERATIONS › 2.2 General Analysis Considerations › 2.2.2 SNA Overview › 2.2.2.1 SNA Terminology
2.2.2.1 SNA Terminology
Some special terms are used to refer to the hardware and
components that implement the SNA architecture. Therefore, a
basic understanding of these terms is germane to any
discussion of SNA. 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.
Figure 2-4 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.
CA MICS Network Analyzer identifies the subarea number
associated with the session partners in files created from
NPM data as follows:
o Primary Logical Unit Subarea - PLUSA
o Secondary Logical Unit Subarea - SLUSA
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)
Figure 2-4. Network Nodes, Links, and Subareas
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. CA 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:
o Logical Units (LUs)
o Physical Units (PUs)
o System Services Control Points (SSCPs)
LOGICAL 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:
o LU Types 2, 3, 4 and 7 support communications
between application programs and various types of
workstations.
o LU Types 1, 4, 6.1, and 6.2 support communications
between two programs.
CA MICS Network Analyzer classifies logical units into one
of two types:
o Primary Logical Units (PLUs)
o 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. CA 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.
Figure 2-5 illustrates the relationships among domains,
subareas, and logical and physical 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) | | |
+-----------------------------------------+ | |
Figure 2-5. Network Addressable Units
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.
1 - SNA Network Addressing
2 - SNA Data Formats
3 - SNA Protocols
