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Sad project


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Sad project

  1. 1. WHAT IS SYSTEMS ANALYSIS AND DESIGN? Systems analysis and design is about developing software, but it is more about developing a complete automated information system which includes hardware, software, people, procedures, and data These five components exist in virtually all automated information systems although the amount of each will vary with respect to the specific system being developed. All of these components must be considered and addressed during systems analysis and design. If any one of them is slighted or overlooked, the system will more than likely not be successful. When you buy software at a retail store or the campus bookstore you are buying more than the software. For example, you expect the software to work with certain hardware, have its own set of procedures for using it, work with the data you want to give it, and interact with you; hence the five components have been considered. The culmination of the systems analysis and design process is to produce an acceptable automated information system for use in one or more of the following ways: 1) software to be used within the business that it was developed for (e.g., a payroll system developed by XYZ Corporation for its own internal use to produce paychecks for its employees), 2) software for sale via retail stores, mail order catalogs, or direct from the company that created it, or 3) software to be used within products produced by a business (e.g., an automated teller machine sold by IBM). In all three of these scenarios the other four components of an automated information system—hardware, data, procedures and people—have been analyzed and designed to work with the software. HDLC (High-level Data Link Control) is a group of protocols or rules for transmitting data between network points (sometimes called nodes). In HDLC, data is organized into a unit (called a frame) and sent across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent. HDLC is one of the most commonly-used protocols in what is layer 2 of the industry communication reference model called Open Systems Interconnection (OSI). (Layer 1 is the detailed physical level that involves actually generating and receiving the electronic signals. Layer 3 is the higher level that has knowledge about the network, including access to router tables that indicate where to forward or send data. On sending, programming in layer 3 creates a frame that usually contains source and destination network addresses. HDLC (layer 2) encapsulates the layer 3 frame, adding data link control information to a new, larger frame. Now an ISO standard, HDLC is based on IBM's SDLC protocol, which is widely used by IBM's large customer base in mainframe computer environments. In HDLC, the protocol that is essentially SDLC is known as Normal Response Mode (NRM). In Normal Response Mode, a primary station (usually at the mainframe computer) sends data to secondary stations that may be local or may be at remote locations on dedicated leased lines in what is called a multidrop or multipoint network. (This is not the network we usually think of; it's a nonpublic closed network. In this arrangement, although communication is usually half-duplex.) Variations of HDLC are also used for the public networks that use the X.25 communications protocol and for frame relay, a protocol used in both and wide area network, public and private.
  2. 2. In the X.25 version of HDLC, the data frame contains a packet. (An X.25 network is one in which packets of data are moved to their destination along routes determined by network conditions as perceived by routers and reassembled in the right order at the ultimate destination.) The X.25 version of HDLC uses peer-to-peer communication with both ends able to initiate communication on duplex links. This mode of HDLC is known as Link Access Procedure Balanced (LAPB).