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The OSI Reference Model

In 1984 the International Standards Organization (ISO) released a network reference called the Open System Interconnect (OSI) model. This model defines a network operating system as having seven layers, each layer performing a specific task.

No real world network operating systems conform exactly to the OSI model, but it is useful as a reference when describing existing systems. It is difficult to study network devices such as routers, switches, and gateways without using the model. It is also difficult to describe and compare networking protocols such as TCP/IP and IPX/SPX without using the OSI model as a reference.

Layer 7

Application

Layer 6

Presentation

Layer 5

Session

Layer 4

Transport

Layer 3

Network

Layer 2

Data Link

Layer 1

Physical

Frequently you will see a table of the layers inverted from that shown above, with Layer 1, the Physical layer on top. This causes confusion in the learning process when data is described as moving up the OSI model or when a higher layer is referred to. Be alert to this possibility.

OSI Layers

Layer

Description

Application

This layer provides the interface to the network for the Network Operating System (NOS). It provides network services and applications such as HTTP, FTP, TELNET and SMP.

Presentation

This layer provides character set conversion and formats the data, It performs encryption and decryption, compression and decompression.

Session

This layer authenticates security and establishes a connection ID. It establishes, synchronizes, maintains and ends sessions.

Transport

This layer repackages messages that are too long into smaller segments. It adds segment sequencing numbers, provides message multiplexing and manages flow control. At the receiving end it provides error detection and recovery, and reassembles the segments in the proper order.

Network

This layer breaks data into smaller units and assigns logical addresses. It determines the route from source to destination. On the receiving end it translates logical addresses into physical addresses and reassembles the units. This layer manages network traffic and routing.

Data Link

This layer organizes data into frames and assigns the physical address. It provides flow control and packages bits from the physical layer into frames and provides error checking and correction.

Physical

This layer describes the physical components of the network, which includes network interface cards, cables and connectors. It converts digital bits into electronic signals for sending on the network, and converts received signals into digital bits.

Donít worry if you donít understand the descriptions of each layer at this time. Further explanations will follow. If you will need to be able to recall the order of the layers. You can do that by memorizing the phrase: All People Seem To Need Data Processing.

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