Protocol Reference Model of OSI. Introduction to the OSI layer How OSI was created and why Comparison with TCP/IP Layers : Application layer.

Protocol Reference Model of OSI

Introduction to the OSI layer How OSI was created and why Comparison with TCP/IP Layers : Application layer Presentation layer Session layer Transport layer Network layer Data link layer Physical layer Conclusion Resources

Open Systems Interconnection (OSI) Each layer support the layers above it and

offers services to the layers below Each layer performs unique and specific

task A layer only has knowledge of its neighbour

layers only A layer service is independent of the

implementation

An attempt for a framework for developing networking technologies

OSI became a tool for explaining the Networking in general

Before OSI was created people created their Software/Hardware as they wanted it to be. There was not any compatibility. Now OSI is used as a rule set for all vendors to create their Software/Hardware by using the standards.


II OSI Overview

1. OSI - layered framework for the design of network systems that allows communication across all types of computer systems.

2. The OSI 7 Layers. ( Brief functional overview. )

3. Vertical and horizontal communication between the layers using interfaces. (defines what information and services should the layer provide to the layer above it. )

Each layer contains a logical grouping of functions

Each function receive an input(one or more) and produces an output

Protocol Reference Model of OSII History 1. The need for standardization

- many vendors, no interoperability

- no common framework

2. ISO and CCITT came up with

OSI (Open System Intercommunication) in 1984.

3. OSI Protocol Suite – unaccepted by vendors and users. (TCP won)

4. OSI – a standard, which allows communication between different systems without requiring changes to the logic of the underlying hardware and software.

Protocol Reference Model of OSILayer abstraction and the path of the message


OSI Overview

4. Data Encapsulation

a) PDU conception – each protocol on the diff. layer has its own format.

b) Headers are added while a packet is going down the stack at each layer.

c) Trailers are usually added on the second layer.

Pretty similar to OSI TCP/IP has less layers(four) Main difference in layers is after layer 4

Applications and Services run on it Enables human network to interface the underlying data

network Applications on that layer (E-mail clients, web browsers, Chats,

etc.) – top-stack applications (As people are on the top of the stack)

Applications provide people with a way to create message Application layer services establish an interface to the network Protocols provide the rules and formats that govern how data

is treated Protocols on the destination and the host must match


III The OSI Layers7. Application Layer

◦ Provides user interfaces and support for services

◦ Resource sharing and device redirection

◦ Remote file access◦ Remote printer access◦ Inter-process communication◦ Network management◦ Directory services◦ Electronic messaging (such

as mail)◦ Network virtual terminals

Coding and conversion of Application layer data to ensure that data from the source device can be interpreted by the appropriate application on the destination device.

Compression of the data in a manner that can be decompressed by the destination device.

Encryption of the data for transmission and the decryption of data upon receipt by the destination.

This is the layer at which application programmers consider data structure and presentation

Examples: GIF, JPEG, TIFF, etc. Sometimes n distinction is made between the presentation and

application layers. For example http/https. HTTP is generally regarded as an application layer protocol although it has Presentation layer aspects such as the ability t identify character encoding for roper conversion


The OSI Layers6. Presentation Layer

◦ Translation (connects different computer systems)

◦ Compression (transmission efficiency)

◦ Encryption (SSL security)

Functions at this layer create and maintain dialogs between source and destination applications

Authentication Permissions Session Restoration (Checkpoint or

recovery)


The OSI Layers5. Session Layer

◦ Session establishment, maintenance and termination (Deciding who sends, and when.)

◦ Session support (security, name recognition, logging )

Tracking the individual communication between applications on the source and destination hosts

Segmenting data and managing each piece Reassembling the segments into streams of application data Identifying the different applications Conversation Multiplexing Segments Connection-oriented conversations Reliable delivery Ordered data reconstruction Flow control TCP – Web Browser UDP – Video Streaming Applications


The OSI Layers4. Transport Layer

◦ Connectionless and connection-oriented services

◦ Process-Level Addressing◦ Multiplexing and

Demultiplexing ◦ Segmentation, Packaging

and Reassembly ◦ Connection Establishment,

Management and Termination

◦ Acknowledgments and Retransmissions

◦ Flow Control

Addressing (IPV4) Encapsulation (Inserts a header with source

and destination IPs) Routing (Move a packet over the Internet) Decapsulation (Open the packet and check

the destination host) IP is connectionless


The OSI Layers3. Network Layer

◦ Logical Addressing ◦ Routing (where the packet is

destinated to) ◦ Datagram Encapsulation ◦ Fragmentation and

Reassembly (handling too big packets )

◦ Error Handling and Diagnostics ( using status messages for example )


End to end packet delivery

It is the role of the OSI Data Link layer to prepare Network layer packets for transmission and to control access to the physical media.

Allows the upper layers to access the media using techniques such as framing

Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection

Frame - The Data Link layer PDU Node - The Layer 2 notation for network devices connected to a

common medium Media/medium - The physical means for the transfer of information

between two nodes Network - Two or more nodes connected to a common medium The Data Link layer is responsible for the exchange of frames between

nodes over the media of a physical network.


The OSI Layers2. Data Link Layer2.1. Logical Link Control (LLC )

◦ Establishment and control of logical links between local devices on a network.

2.2. Media Access Control (MAC) ◦ The procedures used by devices

to control access to the network medium.

◦ • Frame sequencing • Frame acknowledgment • Addressing• Frame delimiting• Frame error checking• PDU: frame

The role of the Physical layer is to encode the binary digits that represent Data Link layer frames into signals and to transmit and receive these signals across the physical media that connect network devices.

Copper cable Fiber Wireless


The OSI Layers1. Physical Layer

• Definition of Hardware Specifications (of cables, connectors, wireless radio transceivers, network interface cards )

• Encoding and Signaling (bit representation)

• Data Transmission and Reception (half duplex, full duplex )

• Topology and Physical Network Design (mesh, ring, bus)

• PDU: bit


OSI Summary

The way people learn Networking A standard for software A standard for hardware Seven layers architecture Each layer independent on the others Similar to TCP/IP(TCP/IP explained) OSI is used as a model for developing

network aware applications(Here I mean that people use its structure to model software)

Protocol Reference Model of OSI. Introduction to the OSI layer How OSI was created and why Comparison with TCP/IP Layers : Application layer.

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