2 basics of networking

8/10/2019 2 Basics of Networking

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The Open Systems Interconnect Model (OSI)

Introduction To The Open SystemsInterconnect Model (OSI) IntroductionOSI is a standard description or "reference

model" for how messages should betransmitted between any two points in atelecommunication network. Its purpose isto guide product implementers so that their

products will consistently work with other products.

The Model The OSI model was created bythe IEEE committee so different vendors products would work with each other. You

see the problem was that when HPdecidedto create a network


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The Open Systems Interconnect Model (OSI)Contd..

product, it would be incompatible with similar products of a different vendor e.g IBM. Sowhen you bought 40 network cards for your company, you would make sure that the rest ofthe equipment would be from the same vendor, to ensure compatibility. As you wouldunderstand things were quite messy, until the OSI model came into the picture.

As most would know, the OSI model consists of 7 layers.

Each layer has been designed to do a specific task. Starting from the top layer (7) we will see how the data which you type gets converted into segments, the segments intodatagrams and the datagrams into packets, the packets into frames and then the framesare sent down the wire, usually twisted pair, to the receiving computer.


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The Seven OSI Layers


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The OSI Layer (Quick Summary)


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OSI Layer 1 - Physical Layer

The first four layers define how data istransmitted end-to-end.

There are no protocols which work at thePhysical layer. As mentioned, Ethernet, TokenRing and other topologies are specified here.


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OSI Layer 1 - Physical Layer Contd

The Physical layer has two responsibilities:

It sends bits and receives bits. Bits come only in values of 1 or 0.

This layer helps us in identifying whether we have a link up situation with end systems.

The Physical layer's connectors (RJ-45, BNC etc.) and different physical topologies (Bus,Star, Hybrid networks) are defined by the OSI as standards, allowing different systems tocommunicate.


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OSI Layer 2 Data-link Layer

The Data-link ensures that messages are delivered to the proper device andtranslates messages from the Network layer into bits for the Physical layer totransmit.

It formats the message into data frames and adds a customized headercontaining the hardware destination and source address.

The Datalink layer is subdivided into two other sub layers, the Media AccessControl (MAC) and the Logical Link Control (LLC).


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OSI Layer 2 Data-link Layer Contd..

Media Access Control (MAC) 802.3

This defines how packets are placed on the media (cable). Contention media (Ethernet)access is first come first served access where everyone shares the same bandwidth. Physical

addressing is defined here. What's Physical addressing? It's simple.

You will come across 2 addressing terms, Logical addressing & Physical addressing.

Logical addressing is basically the address which is given by software e.g IP address.

Physical addressing is an address which is given not by the software, but the hardware.Every network card has a "MAC" address which is burnt into the card's EPROM and thisspecial address is used to uniquely identify your computer's network card from all the otherson the network.


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OSI Layer 2 Data-link Layer Contd..

Logical Link Control (LLC) 802.2

This sub layer is responsible for identifying Network layer protocols and then encapsulatingthem when they are about to be transmitted onto the network or decapsulate them when itreceives a packet from the network and pass it onto the layer above it, which is the Networklayer.

Some common protocols which work at the Datalink layer are: ARP, RARP, DCAP.


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OSI Layer 3 - Network Layer

The Network layer is responsible for routing through an internetwork

The Network layer is responsible for transporting traffic between devices that are notlocally attached. Routers, or other layer-3 devices, are specified at the Network layer and

provide routing services in an internetwork.

Some common protocols which work at the Network layer are: IP, DHCP, ICMP, IGRP,EIGRP, RIP, RIP2.


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OSI Layer 4 - Transport Layer

The Transport layer is responsible for providing mechanisms for multiplexingupper-layer application, sessionestablishment, data transfer and tear down ofvirtual circuits. It also hides details of anynetwork-dependent information from thehigher layers by providing transparent datatransfer.

Services located in the Transport layer bothsegment and reassemble data from upper-layer applications and unite it onto the samedata stream. TCP and UDP: TCP is areliable service and UDP is not.
http://www.firewall.cx/udp.phphttp://www.firewall.cx/tcp-intro.php


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OSI Layer 5 - Session Layer

The last 3 layers of the OSI model are referredto the "Upper" layers. These layers areresponsible for applications communicating

between hosts. None of the upper layers knowanything about networking or networkaddresses.

The Session layer is responsible for setting up,managing and then tearing down sessions

between Presentation layer entities. TheSession layer also provides dialog control

between devices, or nodes. It coordinatescommunication between systems and serves toorganize their communication by offeringthree different modes: simplex, half-duplex

and full-duplex. The session layer basicallykeeps one application's data separate fromother application's data.


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OSI Layer 5 - Session Layer Contd

Some examples of Session-layer protocols are:

Network File System (NFS)Structured Query Language (SQL)

Some common protocols which work at the Session layer are: DNS, LDAP, NetBIOS.


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OSI Layer 6 - Presentation Layer

The Presentation Layer gets its name from its purpose: It presents data to the Applicationlayer. It's basically a translator and provides coding and conversion functions. A successfuldata transfer technique is to adapt the data into a standard format before transmission.Computers are configured to receive this generically formatted data and then convert thedata back into its native format for reading. By providing translation services, thePresentation layer ensures that data transferred from the Application layer of one systemcan be read by the Application layer of another host.

The OSI has protocol standards that define how standard data should be formatted. Taskslike data compression, decompression, encryption and decryption are associated with thislayer. Some Presentation layer standards are involved in multimedia operations. Thefollowing serve to direct graphic and visual image presentation :

JPEG

MIDI

MPEG


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OSI Layer 6 - Presentation Layer Contd

There are no protocols which work specifically at the Presentation layer, but the protocolswhich work at the Application layer are said to work on all 3 upper layers.


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OSI Layer 7 - Application Layer

The Application layer of the OSI model is where users communicate with the computer. TheApplication layer is responsible for identifying and establishing the availability of the intendedcommunication partner and determining if sufficient resources for the intended communicationexist. The user interfaces with the computer at the application layer.

There are various protocols which are used at this layer. FTP, TFTP, Telnet, SMTP and other protocols work on the first three layers of the OSI model, which obviously includes theApplication layer.

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