IBM®’s System Network Architecture (SNA) defines a set of rules that
systems use to communicate. These rules define the layout of the data that
flows between the systems and the action the systems take when they receive
the data. The layout of the data is known as the format,
and the action the systems take when they receive that data is known as the protocol. Together, formats and protocols constitute the
architecture.
SNA does not specify how a system should implement the architecture. Indeed,
a fundamental objective of SNA is to allow systems that have very different
internal hardware and software design to communicate. The only requirement
is that the network flows meet the rules of the architecture.
One consequence of this independent implementation is that each system
brings its own terminology into its SNA implementation, and this can lead
to a confusion of terminology when you start to establish a connection between
two different systems. This section introduces those particular SNA concepts
and terms that you will encounter when configuring CICS® systems.
- network
- A network is a collection of interconnected computers and devices, together
with the physical and logical connections that connect them.
- network address
- A network address is a unique code that is assigned to every device
in a network. With a personal computer, for example, it is likely to be the
medium access control (MAC) address in its network adapter card.
- link and node
- A link connects two nodes, where a node is any device in a network that
transmits and receives data.
- logical unit (LU)
- A logical unit represents the logical destination of a communication
data flow. The formal definition of an LU is that it is the means by which
an end user gains entry into a network, and an end user is defined as the
ultimate source, or destination, of data flow in a network. SNA supports several
different types of LUs. These are grouped together in numbered LU types, such as LU type 2 for 3270 display terminals, and LU type
4 for printers. The LU type for CICS-to-CICS communication is LU type 6.2,
and is frequently referred to as advanced program-to-program
communication (APPC). Each LU is given a unique name that identifies
it in the network, and this is referred to as the LU name.
Sometimes the name of the network that the LU is in is appended to
the name of the LU. It is then known as the network-qualified
LU name, or the fully qualified LU name, and it
takes the form network-name.LU-name.
- physical unit (PU)
- A physical unit is the hardware and software components in a device
that manage its network resources. LUs reside within a PU, and one PU may
hold many LUs. There are several different types of PU. VTAM® running in
a mainframe host is a PU type 5, and NCP running in a 37x5 network controller
is a PU type 4. When workstations connect together in a peer-to-peer manner
they act as PU type 2.1. When a workstation connects to a mainframe host in
a hierarchical manner, it acts as a PU type 2.0. The PU type 2.1 is described
as an independent node (because it is independent of a mainframe host), and
the PU type 2.0 is a dependent node.
- control point (CP)
- The SNA concept that relates LUs to PUs is a control point. A CP can
be thought of as that part of a PU that manages the LU.
- exchange identification (XID)
- Associated with the PU and CP is the exchange identification. XID is
actually the name of a data flow that PUs exchange during the early stages
of their attempt to establish a connection, but, in the context of SNA configuration,
XID refers to one of the fields within that XID flow. It is a hexadecimal
field which the PUs use to confirm the identity of each other.
- session
- SNA uses the term session to refer to various types of data flow in
a network. To avoid ambiguity, it should always be qualified by a description
of the type of data flow, for example CP-CP session. However, when used by
CICS for APPC, it can be assumed to refer to data flow between LUs, and so
is an LU-LU session. There are usually several sessions between any two LUs,
and these are known as parallel sessions.
- connection
- CICS uses the term connection to refer to a group of sessions that connect
two CICS systems.
- transaction program (TP)
- In SNA, the term transaction program refers to the application program
in an APPC environment. The TP uses the LU to gain access to the network.
When CICS is using APPC, the TP is a CICS transaction.
- conversation
- In SNA, the term conversation describes the communication between two
TPs. That is, when two APPC TPs are in communication, they are said to be
holding a conversation. Conversations flow on LU-LU sessions. Each conversation
is allocated a session for its own private use. When the conversation ends,
the session is free to be used by another conversation. There can only be
one conversation between any two TPs, but one TP could have multiple conversations
with different TPs.
- mode
- There may be a choice of many routes and paths in the network that an
LU-LU session could use. One route might be suitable for large volumes of
batch data, another might be reserved for smaller, high speed exchanges. SNA
allows for these different route types to be grouped into modes. A TP can select an appropriate mode when it first establishes
a conversation. The conversation will flow on an LU-LU session that follows
the route defined by the node.
- local, remote, partner
- These terms are used in many contexts. For example, you could refer
to the local system, the remote system, and the partner system. When you are
at a workstation, you regard the workstation as the local system, the machine
your workstation is communicating with is the remote system, and that remote
system is your partner. However, these terms are all relative to your point
of reference, so the remote system regards itself as being a local system
with your workstation being its remote system, and each system is a partner of
the other.
In summary, it may help you to understand these terms if you visualize
a session as being like a pipe that links LUs. When the LU inserts data into
the pipe it is inevitable that the data will be passed to the LU at the other
end of the pipe. Pipes with similar characteristics are grouped together in
a mode. These pipes are created when the systems are first initialized and
SNA is started, or when a TP first requests the use of one. Several pipes
usually run in parallel between two CICS systems. When a CICS transaction wants
to hold a conversation with a transaction on another system, it requests the
use of a pipe. It is given the sole use of a pipe. When the transaction ends,
the pipe is returned to the pool and can be allocated to the next transaction.
CICS does not provide its own SNA support. That is, CICS does not structure
the network data flows into the SNA format, nor is it responsible for initiating
the SNA protocol when a data flow is received. These SNA functions are provided
by a separate SNA product, and CICS uses the services of that product. On
a System/390® host the SNA product is VTAM, and on an AIX® machine it is AIX SNA Server/6000 (AIX
SNA).
When you configure your systems, you have to configure both the CICS product
and the associated SNA product.
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