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HDLC was developed by the International Standards Organization (ISO) as a superset of IBMs SDLC and the United States National Bureau of Standards (NBS) ADCCP protocols. A version of HDLC is the Link Access Protocol-Balanced (LAP-B), which is used in ITU-TS X.25 packet switched networks. While HDLC was built on SDLC and is very similar, the two generally are not compatible, depending on the framing conventions in the specific HDLC implementation.
A network architecture defines the communications products and services which ensure that the various components can work together. Early on, even the various systems of a given manufacturer did not interoperate, let alone afford connectivity with the products of other manufacturers. While the IBM Systems Network Architecture (SNA) and the Digital Equipment Corporation DECnet architectures have solved these internal problems, they still do not interoperate (nor are they ever likely to do so). Truly open systems architectures are still in the distant future, although great strides have been made in this regard through the Open Systems Interconnection (OSI) model fostered by the International Standards Organization (ISO).
A number of standard computer network architectures have been defined, many of which segregate various functions into discrete layers of responsibility, for ease of development and management. Network architecture examples include Xerox Networking System (XNS), DECnet, Advanced Research Projects Agency network (ARPAnet) (now the Internet), U.S. Department of Defense Network (DDN), IBMs Systems Network Architecture (SNA), and the Open Systems Interconnection (OSI) model.
Layered models serve to enhance the development and management of a network architecture. While they primarily address issues of data communications, they also include some data processing activities at the upper layers. These upper layers address application software processes, presentation format, and the establishment of user sessions. Each independent layer, or level, of a network architecture addresses different functions and responsibilities. The layers work together, as a whole, in order to maximize the performance of the process. The various functions address the functions of data transfer, flow control, sequencing, error detection, and notification.
Data Transfer allows the transfer of data from one node to another. Included are such issues as normal or expedited data flow; packet, block or frame sizing; and data assembly and segmentation. Flow Control controls the flow of packets through the network to reduce congestion and the resulting degradation of network performance. Sequencing of the data packets is required where packets may take different routes between nodes. Error Detection is required to ensure data integrity. Notification provides for the advice from receiver to transmitter of the receipt of packets and their condition.
Systems Network Architecture (SNA)
Developed in 1974 by IBM, SNA was a five-level design architecture that has grown into a seven-layer model. SNA comprises software and hardware interfaces to allow various IBM systems and software to communicate. SNA includes network nodes, physical units, and logical units.
While SNA is similar to, it is not compatible with, the OSI model.
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