The OSI (Open System Interconnection) Reference Model is a 7 layer communication model for computer networks. Originally created in the early 1980s by the ISO (International Standards Organization), it defines a layered protocol for host to client communications. This article briefly describes the layers of the OSI model.

Introduction

The standard is published in the ISO document OSI 7498-1. The 7 layers consist of:

Layer 7: Application Layer (HTTP, SMTP, FTP)
Layer 6: Presentation Layer (MIME, SSL)
Layer 5: Session Layer (Port Addressing)
Layer 4: Transport Layer (TCP, UDP)
Layer 3: Network Layer (IP)
Layer 2: Data Link Layer (Ethernet, PPP)
Layer 1: Physical Layer (DSL, T1, V.35, 100BASE-T)

Description of the Layers

The Application Layer allows a user to interact with an application that is connected to the network. Example application layer protocols are FTP (File Transfer Protocol), SMTP (Simple Mail Transfer Protocol) and HTTP (Hypertext Transfer Protocol).

The Presentation Layer transforms data into a format suitable for the application layer. Examples of presentation layer protocols include data compression schemes, and MIME types.

The Session Layer is responsible for controlling the network connection through the dialog between a host and client. Full and half duplex communications are examples of session layer protocols. The duplex of a connection determines if the host and client require message acknowledgement (handshaking), or not (asynchronous).

The Transport Layer is designed to provide a reliable and transparent data link, freeing the layers above it from managing data transportation issues. Packet protocols like TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are examples of transport layer protocols.

The Network Layer is responsible for handling the routing of data through the network and reporting routing errors. The IP (Internet Protocol) addressing scheme is the most widely used of the network layer protocols.

The Data Link Layer handles the physical connection of data links in a network. The most common data link layer protocol is Ethernet.

The Physical Layer consists of the physical devices that connect the hardware together. Common physical layer specifications are DSL, T1, V.35, and 100BASE-T.

Summary

The OSI Model is a 7 Layer protocol standard for computer network communications was created to supercede the earlier 5 layer TCP/IP model. When the TCP/IP model was created computer networks were primarily mainframes with terminals. Since then, the two-tier network architecture has been largely replaced by the three-tier and four-tier distirbuted network architecture. The additional presentation layer and middle-tier increases the complexity of the upper levels, and the Application Layer was split to accomodate this complexity. The lower layers remain essentially the same.

For a more detailed description of TCP/IP and UDP/IP transport layer protocols see the separate article available on this site.

References

Wikipedia OSI Model
http://en.wikipedia.org/wiki/OSI_Model

ISO Standard 7487-1
http://standards.iso.org/ittf/PubliclyAvailableStandards/s020269_ISO_IEC_7498-1_1994(E).zip

Tags: Networking, OSI