Author: Neftaly Malatjie

Short for Wide Area Network, WAN is a collection of computers and network resources connected via a network over a geographic area. Wide-Area Networks are commonly connected either through the Internet or special arrangements made with phone companies or other service providers.

A metropolitan area network, or MAN, consists of a computer network across an entire city, college campus or small region. A MAN is larger than a LAN, which is typically limited to a single building or site. Depending on the configuration, this type of network can cover an area from several miles to tens of miles. A MAN is often used to connect several LANs together to form a bigger network. When this type of network is specifically designed for a college campus, it is sometimes referred to as a campus area network, or CAN.

1. VTAM (Virtual Telecommunications Access Method) resides in the host and contains the SSCP, the PU for the host, and establishes the LU sessions within the host.
   NCP (Network Control Program) resides in the communications controller (front end processor) and manages the routing and data link protocols.

   SNA Layers

   SNA is implemented in functional layers starting with the application that triggers the communications down to the bottom layers which transmit packets from station to station. This layering is called a “protocol stack.” The SNA stack is compared with the OSI model below. Although SNA had major influence on the OSI model, there are differences in implementation.
   

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1. The heart of a mainframe-based SNA network is the SSCP (System Services Control Point) software that resides in the host. It manages all resources in its domain.

   Within all nodes of an SNA network, except for Type 2.1, there is PU (Physical Unit) software that manages node resources, such as data links, and controls the transmission of network management information. In Node Type 2.1, Control Point software performs these functions.

   In order to communicate user data, a session path is created between two end points, or LUs (Logical Units). When a session takes place, an LU-LU session is established between an LU in the host (CICS, TSO, user application, etc.) and an LU in the terminal controller or PC.
   An LU 6.2 session provides peer-to-peer communication and lets either side initiate the session.

1. In SNA, nodes are end points or junctions, and data links are the pathways between them. Data links include high-speed local channels, the SDLC data link protocol, Token Ring and Ethernet.

   Nodes are defined as Type 5 (hosts), Type 4 (communications controllers) and Type 2 (peripheral; terminals, PCs and midrange computers). Type 2.0 nodes communicate only with the host, and Type 2.1 nodes communicate with other 2.1 nodes (peer-to-peer) without going to the host.