computer networks osi layer

7/29/2019 Computer Networks OSI LAYER

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Networking Standards

and the OSI Model

Free Notes For All Students By: ABURAFEY


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Network Communication

s Recognize data

s Divide data into manageable chunks

s

Add information to each chunk to:Determine where the data is coming from

Identify where the data is going to

s

Add timing and error checking informations Put the data on the network and send it on

its way


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Standard Protocols Needed

sNetwork operating systems follow strict

rules (protocols) to control how each of the

previous tasks are accomplished.

sNeed for standard protocols so that

software/hardware from different vendors

could communicate


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Standards Organizations

s ANSIAmerican National Standards InstituteDetermine standards for electronics industry

Represent US in setting international standards

s EIAElectronics Industry AllianceTrade organization representing electronic firms across

US

s IEEEInstitute of Electrical and Electronic

Engineers International society of engineering professionals

Promote development and education in engineering andcomputer science fields


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Standards Organizations

s ISOInternational Organization for

StandardizationCollection of organizations from 130 countries

Establish international technological standards to

facilitate global exchange of data

s ITUInternational Telecommunication Union

United Nations agency that regulates internationaltelecommunications including radio and TV

frequencies, satellite and telephone specifications, and

networking infrastructure


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OSI and 802 Models

s Two primary sets of standards

s Define rules for:How network devices communicate

Methods used to determine when to send data

Methods to ensure that data is received correctly

How the network is cabled

How the network maintains the flow of dataHow bits of data are represented


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Open System Interconnection (OSI)

s 1977-78 International Organization for

Standardization (ISO) began developing

specifications for network communicationss 1984 OSI model was released

s International standard

s Best known and most widely used guide tounderstanding network communications


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802 Model

s Model published by the Institute for Electrical and

Electronic Engineers, Inc. (IEEE)

s Project began in February 1980 (802)

s Predates OSI standard but developed in

cooperation with OSI

s Defines aspects of networking related to physical

cabling, connectivity, error checking, datatransmission, encryption, and emerging

technologies


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The seven-layer OSI model

s To help you remember

the order of the layersAll people seem to

need data processing

Please do not throw

sausage pizza away


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Relationship of OSI Layers

s Each layer of the OSI model must

communicate with the layer above and

below itFor example, the Presentation layer must

communicate with the Application layer (one

above) and the Session layer (one below)

s As data passes down through the OSI

layers, each layer (except Physical) adds

some information to the


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Relationship of OSI Layers

s When data reaches the receiving computer,

the information added by each layer of the

OSI model is read and processed by thecorresponding layer on the receiving

computer

s This is referred to as peer-layer

communications


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Relationships among OSI layersComputer A Computer B

Virtual

Communication

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical


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Packet Creation ProcessData Data


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Physical Layer

s Responsible for transmitting bits from onecomputer to another

s Physical topologys

Network connection typesNumber of pins on the network connector Passive and active hubs, terminators, cables, repeaters,

transceivers, etc.

s

Defines the electrical detailsBit encodingwhat represents 0 and 1How data will be synchronizedBaseband/broadband transmission


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Data Link Layer

s Accepts packets of data from Network layer andpackages the data into data units called frames

s Adds information such as frame type and

physical addresss Responsible for providing error-free transfer of

data framesFrame Check Sequenceerror checking algorithm is

added at the end of each frame so that receivingcomputer can detect damaged frames and request thatframes be resents Cyclic Redundancy Check (CRC)most commonly used

algorithm


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Data Link Layer

s Intelligent hubs, bridges, and switches are

addressed by the Data Link Layer

s

Defines how media will be accesseds Divided by 802 model into two sub-layers:

Logical Link Control (LLC)

Media Access Control (MAC)


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Project 802 Logical Link Control and

Media Access Control sub-layers


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Data Link Layer

s Logical Link ControlUses error check algorithm (FCS) to verify that

data is received correctly

Ensures that the rate of transmission is appropriatefor the receiving computer

Provides the link between the Data Link layer and

the Network layer


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Data Link Layer

s Media Access ControlControls the way multiple devices share the

same media channelsDefines the method the NAC will use to determine

when to send data

Communicates directly with the networkadapter card

MAC (physical) address for network adaptercards provide unique identifier for each NAC


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Network Layer

s Responsible for addressing messages acrossnetworks

s Translates network logical addresses into

physical machine addressess Breaks large segments into smaller packetss Determines routing across networkss Supports communications between logically

separate networksRouters and gateways (devices that pass data from

one network to another) operate in the Network layer


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Transport Layer

s Ensures data is delivered error-free in sequence

and without duplication or loss

s Breaks large packets from the Session layer into

segments to be sent to the receiving computer

s Adds sequencing information to segments

s Reassembles the segments into messages

s Sends acknowledgement to sending computer


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Session Layer

s Responsible for establishing and maintaining

communications between two nodes on the

network

s Allows applications on separate computers toshare a connection (called a session)

s Provides data synchronization and checkpointing

so that if there is a network failure only the data

sent after the checkpoint must be resent

s Provides name recognition and security needed to

allow two applications to communicate


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Presentation Layer

s Translates data between the format the

network requires and the format the

application expects

s Responsible for encrypting data,

changing or converting the character set

and interpreting graphic commands

s Manages data compression to reduce

number of bits to be transmitted


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Application Layer

s Provides services to support user

applications, such as file transfer, database

access, and e-mailNot a particular applicationbut services thatare provided to applications


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OSI Process

P h y s i c a l L aP l a c e s b i t s o n t o

D a t a L i n k L aP a c k a g e s d a t a

A d d s F C S ; a d d sp a s s e s t o P h y

N e t w o r k l a yA d d n e t w o r k a

p a s s e s d a t a t o

T r a n s p o r t l aS u b d i v i d e s d

a d d s s e q u e n cp a s s e s d a t a t o

S e s s i o n l a y e r A d d s a c o n t r o l f r a m

i n d i c a t e s t h a t y o u h a vp a s s e s d a t a t o T r a

P r e s e n t a t i o n l a ye n c y r p t s r e q u e s t ( i

a d d s a n y c o d e s r e q u i r p a s s e s r e q u e s t t o

A p p l i c a t i o n l e v e lf o r m u l a t e s r e q u e s tf o r d a t a a n d s e n d s

r e q u e s t t o P r e s e n t a t i o n l a y e

R e q u e s t f o r e - m a i l i s r e c e i v e d b y

o n r e c e i v i n g c o m p u t e r R e q u e s t i s p a s s e d u p t h e l a y e r s oE a c h l a y e r r e a d , p r o c e s s e s a n d r e m o v e s i n f

U s e r R e q u e s t s M a i l


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Network Packets (Frames)

s Small chunks of data and other information

s Packet StructureHeader

s Source address

sDestination address

s Instructions that tell the computer how to pass the

data along (routing)

sReassemble information

Data

Trailer containing error-check information


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A simple data packet

Destination ID Control CRC

Sender IDData

HeaderTrailer


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Two Major Frame Types

s EthernetNetworking technology developed in 1970

Most commonly used todayUsed in bus and star topologies

s Token Ring

Developed by IBM in 1980sUsed in ring topology


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Addressing

s MAC Addressphysical address burned

onto NIC card

Unique address for each NIC card produced inthe world

Consists of a Block ID and a Device ID

sEach manufacturer has one or more Block IDs

Added to frame by Data Link LayerMAC

sublayer


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Addressing

s Network addresslogical address assigned to anetwork device that identifies the network that adevice belongs

Can be assigned automatically when a computer isturned ons DHCP server provides IP (network) addresses to

computers

Can be assigned manually so that address remainsthe same each time computer is turned ons Addresses for servers and printers are assigned manually

so that other devices can always locate them


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By: Deepak Kumar

computer networks osi layer

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