ABSTRACT

The most prominent example of multidrop architecture is IBM’s SNA, which is a tree-structured hierarchical architecture, with a mainframe computer at the head. The architecture was first published in 1974 when processing was controlled by an expensive mainframe computer such as an IBM 360 that communicated with remote 3270 terminals over a network composed of voice-grade circuits. The physical components in an SNA network are hosts, communications controllers (also known as front-end processors), cluster controllers, and terminals. In the original network a mainframe computer runs a control program known as Advanced Communications Function/Virtual Telecommunications Access Method (ACF/VTAM), usually abbreviated VTAM. SNA establishes a logical path between network nodes and routes each message with addressing information contained in the protocol. SNA is defined in layers that are roughly analogous to the layers in ISO’s OSI model. SNA was the basis for much of the OSI model, but the layers do not line up exactly and it differs from OSI in several other respects. The original SNA protocol was built on the assumption that devices such as terminals and printers lacked processing capability so all traffic flowed through the host. The new SNA is called APPN. To communicate with another device, an end node sends a bind message to its network node, which in turn broadcasts a query that passes through the network. The node answering to that name responds to the query with a message that establishes the session and route. X.25, built on the first three layers of the OSI model, specifies the interface between DTE and a packet-switched network. . For example, a call setup packet would be used to establish the initial connection to the terminating device, which would return answer packets. VSAT is a Ku band satellite service that is an excellent medium for a widely dispersed operation. . Typical applications include LAN/WAN networking, Internet access, and videoconferencing. VSAT terminal is a small device with a 90 to 120 cm dish antenna. Transponder space is leased from the satellite provider or obtained from the hub provider. Data network applications can be separated into Inquiry/response, Conversational, Bulk data transfer, Bulk data transfer.