open system models - osi reference model

8/9/2019 Open System Models - OSI Reference Model

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Open System Models


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The OSI Reference Model

Developed by the International Standards Organization(ISO) as a first step towards connecting open systems. The principles behind OSI(O S I ) Model are as

follows:

Separate layer for a different level of abstraction . Well-defined functionality. Defining internationally standardized protocols.

The layer boundaries should be chosen to minimizethe information flow across the interfaces.

Each layer performs independently but incoordination.


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The OSI Reference Model


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Application Layer

Is the software applications that you use on your screen

It is concerned with file access and file transfer It serves as a window for application to access the

network resources

Examples could be FTP, Telnet


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Presentation LayerTranslates data received from session layer into a format so that

each application can understand. It is also called as the networks translator .Translates and encrypts data.

Implementation of these tasks: Data representation

Encodes and decodes data making it suitable for transmission. ASCII-to-binary conversion in one direction and binary-to-ASCII conversion

the other direction Data compression

Compressing the data before sending reduces the cost and space in memory. Network security and privacy

Encryption technique is followed in order to enable reliable communication oveuntrusted channels

For example, layer 6 defines what happens when it tries to display UNIX-style data on an MS-DOS screen


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Session Layer Handles the actual connections between systems I.e

Concerned with opening dialog between sender and receiver Handles two applications to communicate over the

network(bi-directional) Makes sure communication continues in a controlled fashion.

Implementation through following three steps: Connection establishment Data transfer Connection release

Defines the order of data packets to be transmitted Breaking of large document into smaller packets manages records of transmission sent


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Transport Layer Is like the registered mail system Layer 4 is concerned with ensuring that data gets

to its destination The transport layer of the reference model is

responsible for final communication of lower layers to upper layers.

Handles errors Solves problems related to transmission

Solves problems related to reception of datapackets

Ensures the error free message delivery This is where the TCP in TCP/IP does its work


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Network Layer

Provides an addressing scheme Works with layer 2 to translate data packets logical

network addresses into hardware based MACaddress and moves the packets toward their destination

It is also the lowest layer that isnt concerned withthe hardware

At this layer the term protocol really comes intoplay the IP in TCP/IP is what this layer of thesoftware does.


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Data link Layer

Sends data frames from the Network layer to thePhysical layer.

It package the bits into logical units for reliable delivery.

Ensures error free data frame transfer from onecomputer to another

Here network card addresses become important


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Physical Layer

Transmits bits from one computer to another Defines how the cable connects with the network

card adapter

Connects distinct networks physically. Types of connection

Point to pointe.g pc directly to printer

Multipointe.g connecting central CPU to dumb terminals


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Relationships among OSI

layersVirtual Communication Is the communication between peer layers of the

two computers communicating over

the network Exchanges information placed between the

adjacent layers of each machine Allows data flow from top to the bottom layers in

the sending computer Allows data flow from bottom to the top layers in

the receiving computer


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Relationships among OSIlayersComputer A Computer B

VirtualCommunication

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical


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Data transmission in OSI

model

PhysicalBits and bytes

Data link Frames

Network Packets

TransportTPDU

SessionSPDU

PresentationPPDU

ApplicationUser data


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DOD Model and TCP/IP


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Introduction For establishing a network a set of common

protocols is needed, collectively called a protocolsuite.

TCP/IP is the most commonly used protocolsuite and the very basis of todays internet.

The model for TCP/IP suite is the DOD(Department Of Defense) reference model.


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DOD Model In the late 1960s, United States DOD

research wing - Advanced ResearchProjects Agency devised a reference model- the DOD reference model

The DOD model is the very first reference

model and hence the very basis of networking.


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DOD Model

Process / Application

Internet

Network Access

Host-to-Host


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DOD vs. OSI model

Process / Application

Network Access

DOD Model

Network

Transport

Application

Presentation

Session

Data Link

Physical

OSI Model

Host-to-Host

Internet


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What is TCP/IP Suite? It is a set of protocols used for

interconnecting networks. It is named after two of its major constituent

protocols Transmission Control Protocol(TCP) and Internet protocol (IP).

It is based on the DOD reference model. Most of theO/Ss support TCP/IP


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TCP/IP Protocols Process/Application layer protocols:

Telnet

File Transfer Protocol (FTP) Network File System (NFS) Simple mail transport protocol (SMTP)

Simple Network Management Protocol(SNMP)


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TCP/IP Protocols Host to Host layer protocols

Transmission control protocol is responsiblefor verifying the correct delivery of data fromone machine to another. It is a connection-oriented protocol.

User Data gram Protocol is used to do the jobof TCP in wireless based networks


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TCP/IP Protocols Internet layer protocols

Internet protocol is used to maintain the networkaddresses and help the packets to route properly in anetwork.

Address Resolution protocol is used to identify thehardware address of the machine having a particular IPaddress.

Reverse Address Resolution protocol maps the

hardware address to the IP address. ICMP is management protocol and messaging service

provider for IP


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TCP/IP Protocols

Network access layer protocols Ethernet (IEEE 802.3) Token ring (IEEE 802.5) X.25


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IP Addressing

An IP address is a numerical identifier assignedto each machine on an IP network. It designates the location of the device in a

network. An IP address is a 32 bit binary number These 32 bits are divided into four sections each

containing one byte, referred to asoctets.

Eg.10000100.10000010.10100011.01111010same as 132.130.163.122 |octet

IP addresses are assigned according to the five

classes of networks


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IP address classes Class A addresses :

These are used for the systems with a small number of networks and a large number of hosts.

These addresses use the first one byte for specifyingthe network and the last three for the host.

The Class A format could be depicted as follows:

Network.Node.Node.Node


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IP address classes

Class B addresses : These provide an equal number of networks

and hosts by assigning the first two bytes tothe network and the last two bytes to the host.

The format for Class B network could bedepicted as follows: Network.Network.Node.Node


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IP address classes

Class C addresses: These classes use the first three bytes of the

address to specify the network and the lastbyte to specify the host.

The format for Class C network could be

depicted as follows: Network.Network.Network.Node


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Sub-network addressing

The Internet is divided into sub-networks It helps to

reduce the traffic,

optimize network performance, simplify management and facilitates spanning large geographical areas.

Internet

Subnets


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Sub-network addressing

The network administrator creates a 32-bitsubnet mask comprising of ones and zeros.

The ones in the subnet mask represent thepositions that refer to the subnet addresses. The zeros in the subnet mask represent the

positions that refer to the host part of theaddress.


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Domain Name System

There are two type of network addresses IP addresses and

Character type NetBIOS addresses NetBIOS addresses of a machine consist

of the domain name, the sub-domain andthe machine name.


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Domain Name System Examplesales01.reliance.com

Internet

Com Gov Org Edu Net

Reliance

Sales01 Sales02 Sales03 Sales04 Sales05

intel Microsoft Wipro


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Address resolution

The process of mapping the IP address of a machine to its hardware or MAC address

This job of address resolution is done bythe ARP The ARP maintains a table which is used to

store the IP address as well as the MACaddress of a machine.


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Address resolution

If the destination machine is in the localnetwork then a message is broadcasted in

the network. The destination answers the broadcast bysending a reply packet that contains thehardware address of the destination.

This is then entered into the ARP cache.


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Internet Protocol (IP) The IP is an integral part of the Internet layer. IP can be viewed as a pathfinder for packets,

telling the packets to go where they want to go.

It defines the basic unit of data transfer throughouta network based on TCP/IP, i.e. the IP packet or IPdata gram..

IP performs the routing function for the network. IP defines a set of rules that embody the idea of connectionless packet delivery.


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The TCP in detail

TCP is one of the most important part of theTCP/IP suite. It is a connection-oriented protocol. TCP acts as a message-validation protocol Handles the retransmission of lost or corrupted

packets Processes the receipt of data from the

destination in the form of acknowledgement

messages. makes a reliable connection between two

processes or the two peer upper layers, throughsockets.


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TCP functions

To establish a reliable connection.

To convert data into a suitable format/ datastream.

To maintain the connection.

To ensure reliable data delivery.


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World Of Internet


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Scope

Internet Intranet

Extranet How the net work? Client/Server basics

Middleware Security


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What is the Internet? A set of computers talking over fibre optics,

phone lines, satellite links, and other media. An ocean of resources waiting to be

explored. A place, which helps in ones research for

thesis or presentation.

Gives access to thousands of libraries andarchives that will open at ones fingertips. Provides unlimited commercial opportunity.


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What is Internet?

In a nutshell: It is a collection of small and big Networks.

All networks are interconnected All networks use TCP/IP protocols for

communicating with each other.


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Intranet

Network within the organization. Collection of Two or more LANs. Links more than one kind of networking

technology using TCP/IP. Uses firewall to keep the larger Internet

out of this smaller network. Owned by a particular organization.


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Intranet

Hub

Bridge

LAN usingStar Topology

Router (Delhi )

FFFDDI

Network

10-Mbps Ethernet

WAN

Router (Bombay )

Bombay OfficeDelhi Office

ISDN lines


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Intranet

Intranet benefits: Reduces the expenditure incurred by anorganization

Helps in improving the productivity of staff. Helps in improving companys relations with

customers Helps in improving the speed and accuracy of

the communication Helps in managing and accessing data

warehousing information Helps in Knowledge Management


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Intranet


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How does the Net work?

All data are moved on the net in the unitscalled packets. Size of packets depend upon the application

those packed them. Every network is attached to the net with the

help of routers. Packets are forwarded in the network with

the help of routers to their destination. Different packets may follow different path.


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Client/ Server basics

Server Server is a special purpose privileged programor application dedicated to providing oneservice.

Server can handle multiple clients at the sametime.

Connected to the network so that remote clientcan reach for the service.

Usually resides on a powerful computer.


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Client/Server basic

Example of server applications are: Web server Mail server FTP server

News server


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Client/Server basics

Client Client is the application or program, which

request for particular service from server It is designed to directly interact with server. Acts as a Interface between user and server.

Client/server model is known as two tier architecture.


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Client/server basics

Examples of client applications are: Browsers E-mail software's FTP client

Telnet


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Middleware The problems with the traditional client/server

(now often called "two-tier" client/server) wereaddressed by the multitier client/server architecture.

Conceptually, an application can have anynumber of tiers Most popular multitier architecture is three-tier

Model It partitions the system into three logical tiers: the

user interface layer, the business rules layer,and the database access layer


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Middleware

The user interface layer communicates only withthe business rules layer or Middleware, never directly with the database access layer.

The business rules layer, in turn, communicateswith the user interface layer on one side and thedatabase access layer on the other.

Thus, changes in the database access layer willnot affect the user interface layer because theyare insulated from each other.


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Middleware

Databaseserver

GUIlayer

Business object or

Middleware


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Middleware

This architecture enables changes to be made inthe application with less likelihood of affectingthe client component

Because the multitier client/server architecturepartitions the application into more componentsthan traditional two-tier client/server, it alsoallows more flexibility in deployment of theapplication.


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Advantages Of Networking

d f k


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Advantages of Networking

Sharing of Information and Resources Optimised use of CPU Power Reduction in cost of peripherals

Ease of operation - no manual data transfer Better organised file system Improved Security system On-line Information Update


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