In the early 1970s, IBM discovered that large customers were reluctant to trust unreliable communications networks to properly automate important transactions. In response, IBM developed Systems Network Architecture (SNA).
A D V E R T I S E M E N T
"Anything that can go wrong will go wrong," and SNA may be unique in trying to identify literally everything that could possibly go wrong in order to specify the proper response. Certain types of expected errors (such as a phone line or modem failure) are handled automatically. Other errors (software problems, configuration tables, etc.) are isolated, logged, and reported to the central technical staff for analysis and response. This SNA design worked well as long as communications equipment was formally installed by a professional staff. It became less useful in environments when any PC simply plugs in and joins the LAN. Two forms of SNA developed: Subareas (SNA Classic) managed by mainframes, and APPN (New SNA) based on networks of minicomputers.
In the original design of SNA, a network is built out of expensive, dedicated switching minicomputers managed by a central mainframe. The dedicated minicomputers run a special system called NCP. No user programs run on these machines. Each NCP manages communications on behalf of all the terminals, workstations, and PCs connected to it. In a banking network, the NCP might manage all the terminals and machines in branch offices in a particular metropolitan area. Traffic is routed between the NCP machines and eventually into the central mainframe.
The mainframe runs an IBM product called VTAM, which controls the network. Although individual messages will flow from one NCP to another over a phone line, VTAM maintains a table of all the machines and phone links in the network. It selects the routes and the alternate paths that messages can take between different NCP nodes.
SNA Network Devices
host systems
terminals
Output devices
Communications controllers
Cluster controllers - Allow many devices to connect through
them. They connect ot a host or communications controller.
SNA Network Categories
Nodes
Type 2 - PCs, terminals and printers
Type 4 - Communications controllers
type 5 - Host computers used to manage the network
Data links - Connection between combinations of hosts, cluster
controllers, or nodes.
Possible SNA communications architectures
SDLS - Synchronous Data Link Control
BSC - Binary Synchronous Communication sends bits in frames which are
timed sequences of data.
Token-ring
X.25
Ethernet
FDDI
SNA units
NAU - Network Addressable Units
LU - Logical Units are ports that users use to access network resources
Type 1 - An interactive batch session
Type 2 - An IBM 3270 terminal
Type 3 - An IBM 3270 printer
Type 6.2 - A program to program session
Type 7 - An IBM 5250 family session
PU - Physical Units are a network device used to communicate with hosts.
Type 2 - Cluster controllers
Type 3 - Front end process
Type 5 - Host communications software
SNA software components
SSCP - Systems Services Control Point manages all resources in the
host's domain.
NCP - Network Control Program performs routing, session management
tasks. It runs in the communications controller.
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