One of the most important concepts in networking is the open-systems interconnection (OSI) reference model. It serves as a framework within which communication protocol standards are developed.
In 1977, the International Organization for Standards (OSI) began an ambitious project to develop a single international standard set of communications protocols. By 1984, ISO had defined an overall model of computer communications calle the Reference Model for Open Systems Interconnection, or OSI Model. The OSI model, described in international standard ISO 7948, documents a generalized model of system interconnection.
ISO also developed a comprehensive set of standards for the various layers of the OSI model. The standards making up the OSI architecture were not widely implemented in commercial products for computer networking. However, the OSI model is still important. The concepts and terminology associated with the OSI model have become widely accepted as a basis for discussing and describing network architectures. The OSI model is often used in categorizing the various communications protocols that are in common use today and in comparing one network architecture to another.
The Seven Layers
The OSI model defines the seven functional layers listed in the table below. Each layer performs a different set of functions, and the intent was to make each layer as independent as possible from all others.
|Layer 7||Application Layer|
|Layer 6||Presentation Layer|
|Layer 5||Session Layer|
|Layer 4||Transport Layer|
|Layer 3||Network Layer|
|Layer 2||Data-Link Layer|
|Layer 1||Physical Layer|
The following sections briefly describe each of the seven layers of the OSI model, working from the bottom up.
|Layer 1 – Physical Layer
The physical layer defines the physical characteristics of the interface, such as mechanical components and connectors, electrical aspects such as voltalge levels representing binary values, and functional aspects such as setting up, maintaining, and taking down the physical link. Well-know physical layer interfaces for local area networks (LANs) include Ethernet, Token-Ring, and Fiber Distributed Data Interface (FDDI).
Layer 2 – Data-Link Layer
Layer 3 – Network Layer
Layer 4 – Transport Layer
Layer 5 – Session Layer
Layer 6 – Presentation Layer
Layer 7 – Application Layer
Protocol Model Comparison
The following table illustrates how some common protocol stacks map into the OSI model:
|Application||Network File System (NFS)||Netware Control Protocol||AppleShare||Server Message Blocks|
|Presentation||AppleTalk Filing Protocol (AFP)|
|Session||Telnet||FTP||SMTP||SNMP||Named Pipes||NetBIOS||ASP||ADSP||ZIP||PAP||NetBIOS||Named Pipes|
|Network||IP||IPX||Datagram Delivery Protocol (DDP)|
|Data-Link||LAN Drivers||LAN Drivers||LAN Drivers||LAN Drivers|
|Media Access Control||ODI||NDIS||LocalTalk||EtherTalk||NDIS|
The following table illustrates how some common services map into the OSI model: