Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)
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16.1 OSI REFERENCE MODEL
The International Organization for Standardization (ISO) has developed the Open Systems Interconnection (OSI) protocol architecture, which is a seven-layer architecture for computer communications. This standard is specified in ISO 7498 and ITU-T X.200 recommendations.
The Open Systems Interconnection (OSI) protocol architecture developed by the International Organization for Standardization (ISO) is a seven-layer architecture. This architecture is defined in ITU-T Recommendation X.200.
During the late 1980s, a number of vendors started marketing software based on ISO/OSI protocol suite. However, by that time, there already was a large number of TCP/IP-based networks, and in the race between ISO/OSI and TCP/IP, TCP/IP won without much effort.
The ISO/OSI model (also referred to as the ISO/OSI protocol suite) still is an important model to study because it is considered as the reference model for computer communications. We can map any protocol suite on to the ISO/OSI model for studying the functionality of the layers.
| Note | In the battle between ISO/OSI protocol architecture and TCP/IP protocol architecture, TCP/IP won mainly because of the large installation base of the TCP/IP protocol software. Still, studying the ISO/OSI protocol architecture is important because it is acknowledged as the reference model for computer communication protocols. |
The ISO/OSI protocol suite is shown in Figure 16.1. The seven layers are (from the bottom)
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Physical layer
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Datalink layer
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Network layer
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Transport layer
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Session layer
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Presentation layer
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Application layer
| Application Layer |
| Presentation Layer |
| Session Layer |
| Transport Layer |
| Network Layer |
| Datalink Layer |
| Physical Layer |
| Note | What is the rationale for seven layers? Why not six layers or eight layers? During the standardization process, there were two proposals—one proposal with six layers, and the other with eight layers. To achieve consensus, finally the seven-layer architecture was standardized—just the average of six and eight. That is how standardization is done! |
Each layer performs a specific set of functions. If we consider two systems, the protocol stack has to run on each system to exchange useful information for a particular application. As shown in Figure 16.2, the two application programs on the two end systems communicate with each other via the protocol suite. The application program on one end system sends the data to the layer below (application layer), which in turn adds its header information and sends it to the layer below (presentation layer). Each layer adds the header and forwards to the layer below. Finally, the physical layer sends the data over the communication medium in the form of bits. This bit stream is received by the other system, and each layer strips off the header and passes the remaining data to the layer above it. The header information of one layer is interpreted by the corresponding layer in the other system. This is known as peer-to-peer communication. For instance, the header added by the transport layer is interpreted by the transport layer of the other system. So, though the two peer layers do not communicate with each other directly, the header can be interpreted only by the peer layer. We will study the functionality of each layer in the following sections.
Each protocol layer adds a header and passes the packet to the layer below. Because the header is interpreted only by the corresponding layer in the receiving system, the communication is called peer-to-peer communication. Peer means a layer at the same level.
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