THE OPEN SYSTEMS INTERCONNECTION (OSI) MODEL Open Systems Interconnected Model – was created in the 70’s by the ISO (although the CCITT came up with their own model)10 different people got together and considered all functions of communications was created because people realized that our computers needed to talk to each other(and there was no one dominant computer system) ISDN was a big factor 7 layers = 4 upper and 3 lower
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The OSI reference model is used in many ways: – – – – To provide assistance when troubleshooting network problems To provide a common terminology and framework for networking technology developers To facilitate the development of connectivity standards needed for flexible open architectures To enable the development of protocol stacks that allow network nodes to communicate with one another
The OSI model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers. An instance of a layer provides services to its upper layer instances while receiving services from the layer below. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer. According to recommendation X.200, there are seven layers, each generically known as an N layer. An N+1 entity requests services from the layer N entity. At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU).
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A Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower...