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OSI Model

Honolulu Community College

Cisco Academy Training Center

Semester 1

Version 2.1.1

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Overview Past 2 decades has seen a tremendous growth

in networking. Manufacturers developed their own equipment,

leading to problem of compatibility between equipment of different vendors.

Proprietary systems like DECnet, SNA and TCP/IP would not communicate with each other.

To address this problem, ISO (International Organization for Standardization) conducted a study, and in 1984 released the OSI model.

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What is the OSI Model? The OSI reference model is the primary model for

network communications. OSI model represents the communication process,

how data is moved from one workstation to another on the network.

A layered model: Breaks the communication process down into 7

tasks (represented by the 7 layers). Network functions are specified for each layer.

Provides a standard to insure compatibility and interoperability.

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Why a Layered Model? Reduces complexity. Standardizes interfaces. Facilitates modular

engineering. Ensures interoperable

technology. Accelerates evolution. Simplifies learning and

teaching.

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Network Communications Originates at a source,

and travels to a destination. Information that travels on

the network is the data, packet, or data packet.

Data packet includes the source information along with other elements needed to make communication possible and reliable. Source & destination addresses. Information for error detection and error correction. Sequence numbers. Information identifying the application.

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Media Media is the material

through which data packets travels. Telephone wires Category 5 UTP (unshielded twisted pair) STP (shielded twisted pair) Coaxial cables (used for cable TV)

Thinnet - 10base2 Thicknet - 10base5

Optical fibers (thin glass fibers that carry light) Multi-mode (62.5/125 microns) Single-mode

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Protocol A set of rules, or an agreement, that determines

the format and transmission of data. Protocol is like a language.

If devices do not use the same protocol, they will not understand each other.

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OSI Model Provides standards. Represents the

communication process in a network.

Breaks communications process into 7 separate, independent tasks.

Each layer represents one task.

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Application Layer (7) Provides network services to applications.

Email, FTP - file transfer protocol, Terminal emulation, Telnet

Presentation Layer (6)Data Representation

Ensures data is readable.Data format and structure.

TIFF, GIF, JPEG, Quicktime, MIDI,etc.Negotiated data transfer syntax.Coding.Compression.

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Session Layer (5) Interhost communications.

Establishes, manages and terminates sessions between applications.

Provides support for layers above it.

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Transport Layer (4)

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Network Layer (3)

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Data Link Layer (2)

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Physical Layer (1) Foundation layer of the OSI model. Defines the electrical, mechanical, procedural, and

functional specifications for activating, maintaining, and deactivating the physical link between end systems. Voltage levels and timing of voltage changes. Physical data rates. Maximum transmission distances. Physical connectors.

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OSI Layers - key words Application - browsers. Presentation - common data format. Session - dialogues and conversations. Transport - quality of service, and reliability. Network - path selection, routing, and addressing. Data link -frames and media access control. Physical - signals and media.

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Encapsulation If one computer wants to send data to another

computer, the data must first be packaged in a process called encapsulation.

Encapsulation is packaging the data, as you would do to a letter when sending it through the mail.

Encapsulation wraps data with the additional protocol information before transmission. Additional information is in the form of headers,

and/or trailers.

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Data Encapsulation Each layer ‘encapsulates’ the data from the upper

layer. Data is wrapped with

the appropriate headers and trailers.

This happens at each layer as data flows down the OSI model.

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Data Encapsulation Example Consider sending email. In the OSI model, the email program

user interface is the application layer. The host layers (upper 4 layers) provide

the data, which is then physically delivered by the media layers (lower 3 layers) over the network.

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Details: Email Example (host layers) Application Layer: the email program user

interface, used to create the email message. Presentation Layer: the alphanumeric

characters are coded in ASCII or some other code.

Session Layer: the characters are formed into an email message for transmission.

Transport Layer: segments the data for transport through the network (TCP header).

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Email Example (media layers) Network Layer: segmented data is put into

packets (datagrams) with a network header. header has source and destination logical

addresses (IP). Data Link Layer: packet is put into a frame.

Frame header has source and destination physical addresses (MAC), frame trailer is also added.

Frame is encoded for transmission. Physical Layer: signals transmitted over the

networking media.

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Peer to Peer Communication Each layer uses its own layer’s protocol to

communicate with peer layer in the other system. Uses PDU - protocol data unit

Peer-layer communications uses services of the layers below it. Each layer takes the upper layer’s data and

puts it in its own PDU. This is data encapsulation.

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TCP/IP Reference Model The historical and technical open standard of the

Internet is TCP/IP. TCP/IP reference model & TCP/IP protocol

stack make data communication possible. Made up of only 4 layers. Application layer - handles

high-level protocols, issues of representation, coding, and dialog control. Combines application,

presentation & session OSI layer functions.

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TCP/IP Reference Model (cont.)

Transport layer - deals with the quality-of-service issues of reliability, flow control, and error correction. Similar to OSI transport layer.

Internet layer - sends data packets through internetwork & have them arrive at the destination independent of the path and networks they took. Similar to OSI network layer.

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TCP/IP Reference Model (cont.)

Network layer - also called the host-to-network layer. Concerned with all of the issues

that an IP packet requires to actually make a physical link.

Includes the LAN and WAN technology details.

Combines Data link and Physical OSI functions.

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TCP/IP Protocol Graph Illustrates some of the common protocols that

are specified by the TCP/IP reference model.

Application

Transport

Internet

Network

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TCP/IP Protocol Graph (cont.) Transport layer uses two protocols - TCP & UDP.

TCP - connection oriented, reliable. UDP - connection-less, unreliable.

Only one network protocol - IP (internet protocol). IP serves as a universal protocol,

Application

Transport

Internet

Network

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Summary The OSI reference model provides

a standard which ensures greater compatibility and interoperability between various types of network technologies.

Encapsulation - process in which data is wrapped with a particular protocol header/trailer before transmission.

Peer-to-Peer Communications - each layer uses it own protocol data units (PDUs) to communicate with its peer layer.

The End