OSI Model

Andres, Wen-Yuan Liao

Department of Computer Science and Engineering

De Lin Institute of Technology

andres@dlit.edu.tw

http://www.cse.dlit.edu.tw/~andres

Overview

Explains how standards ensure greater compatibility and interoperability between various types of network technologies

OSI reference model networking scheme

The basic functions of the OSI model

General Model of Communication

Using layers to analyze problems in a flow of materials Source, destination, and data packets Media Protocol The evolution of ISO networking standards

The concept of layers will help you understand the action that occurs during communication from one computer to another

Media

Telephone wires

Category 5 UTP (used for 10BASE-T Ethernet)

Coaxial cables (used for cable TV)

Optical fibers (thin glass fibers that carry light)

Protocol

A Protocol is a set of rules that make communication on a network more efficient. A set of rules, or an agreement, that

determines the format and transmission of data

The OSI Reference Model

The purpose of the OSI reference model The seven layers of the OSI reference model The functions of each layer Encapsulation Names for data at each layer of the OSI model

ISO OSI 7 Layers

In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. This separation of networking functions is called layering.

Application Layer

Is closest to the user

Provides network services to applications

Does not provide services to any other OSI layer

Think of browsers

Presentation Layer

The ensures that the information that the application layer of one system sends out is readable by the application layer of another system

Think of a common data format

Session Layer

Synchronizes dialogue between the two hosts' presentation layers and manages their data exchange

Offers provisions for efficient data transfer, class of service, and exception reporting

Think of dialogues and conversations

Transport Layer

Provide a data transport service that shields the upper layers from transport implementation details

Think of quality of service, and reliability

Network Layer

Think of path selection, routing, and addressing.

Data Link Layer

Is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control

Think of frames and media access control

Physical Layer

Defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems Voltage levels, timing of voltage changes,

physical data rates, maximum transmission distances, physical connectors, and other

Think of signals and media

Encapsulation

Encapsulation wraps data with the necessary protocol information before network transit

Peer-to-Peer Communications

Each layer of the OSI model at the source must communicate with its peer layer at the destination

Each layer's protocol exchanges information, called  protocol data units (PDUs), between peer layers

Comparison of the OSI Model and the TCP/IP Model

The TCP/IP reference model

The layers of the TCP/IP reference model

TCP/IP protocol graph

Comparison of the OSI model and the TCP/IP model

TCP/IP

TCP/IP reference model and the TCP/IP protocol stack make data communication possible between any two computers, anywhere in the world

U.S. Department of Defense (DoD)

Application Layer

Higher level protocols should include the session and presentation layer details

Handles high-level protocols, issues of representation, encoding, and dialog control

Transport Layer

Deals with the quality-of-service issues of reliability, flow control, and error correction Transmission Control Protocol (TCP)Connection-oriented SegmentPacket switching

Internet layer

Internet protocol (IP)

Best path determination and packet switching

Postal system

Network Access Layer

Host-to-network layer

Includes the LAN and WAN technology details, and all the details in the OSI physical and data link layers

Applications

FTP - File Transfer Protocol HTTP - Hypertext Transfer Protocol SMTP - Simple Mail Transfer protocol DNS - Domain Name System TFTP - Trivial File Transfer Protocol

The transport layer involves two protocols - transmission control protocol (TCP) and user datagram protocol (UDP)

There is only one network protocol - internet protocol, or IP

The network access layer, refers to the particular LAN or WAN technology that is being used

SimilaritiesBoth have layers Both have application layers, though they include very different services Both have comparable transport and network layers Packet-switched (not circuit-switched) technology is assumed Networking professionals need to know both

DifferencesTCP/IP combines the presentation and session layer issues into its application layer TCP/IP combines the OSI data link and physical layers into one layer TCP/IP appears simpler because it has fewer layers TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, typically networks aren't built on the OSI protocol, even though the OSI model is used as a guide

Summary

OSI reference model

Seven numbered layers

Encapsulation

Peer-to-Peer Communications