ISO-OSI.pdf

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THE OSI REFERENCE MODEL

The International Organization for

Standardization (

Standardization (ISO

ISO

) developed the

Open System Interconnection (

Open System Interconnection (OSI

OSI

))

Reference Model to promote

communication between dissimilar

computers as a guide for defining a set

of open protocols.

This model has served as one of the

most basic and essential, elements of

computer networking since 1984.

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The OSI model provides a framework

(functional guideline) for defining

standards to connect heterogeneous

computers.

The OSI model solves the complicated

problem of communication between

computer systems by dividing the

communication into sub-tasks (layers).

Each task executes a specific function by

using the services of the task below it

and providing services to above task.

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OSI

LAYER MODEL

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Most communication environments

separate the communication functions

and application processing. This

separation of networking functions is

called

called LAYERING

LAYERING..

OSI divides the big task of host-to-host

networking called

networking called INTERNETWORKING

INTERNETWORKING

into a vertical stack. This OSI stack

contains these seven numbered layers,

these indicate distinct functions.

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The layered approach to OSI offers

several advantages to system

implementers. By separating the job of

networking into logical smaller pieces,

vendors can more easily solve network

"problems" through

"problems" through divide-and-conquer

divide-and-conquer

..

The OSI layers afford extensibility. New

protocols and other network services will

generally be easier to add to a layered

architecture.

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Upper layers Upper layers 7. application 7. application 6. presentation 6. presentation 5. session 5. session Lower layers Lower layers 4. transport 4. transport 3. network 3. network 2. data link 2. data link 1. physical 1. physical Higher-level facilities Higher-level facilities Application Application Presentation Presentation Session Session Network communication Network communication Transport Transport Network Network Physical medium Physical medium Data Link Data Link Physical Physical

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ADVANTAGES

•Divide the interrelated aspects of

network operation into less complex

elements.

•Define standard interfaces for "Plug and

Plug" Compatibility and multi-vendor

integration.

•Divide the complexity of

internetworking into Discrete, more

easily learned operations.

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APPLICATION LAYER APPLICATION LAYER

The application layer provides different The application layer provides different

services to user applications. It interfaces services to user applications. It interfaces directly to these services and provide

directly to these services and provide

conversion between associated application conversion between associated application

processes, after that it issues requests to the processes, after that it issues requests to the presentation layer. It contains a variety of presentation layer. It contains a variety of protocols that are commonly needed.

protocols that are commonly needed. Another application layer function is file Another application layer function is file

transfer. Different file systems have different transfer. Different file systems have different file naming conventions. Transferring a file file naming conventions. Transferring a file between two different systems requires

between two different systems requires handling all incompatibilities.

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Application Layer 7 protocols Application Layer 7 protocols F

FTTP P FFiille e TTrraannssffeer r PPrroottooccooll TF

TFTP TP TrTriviviaial l FiFile le TrTranansfsfer er PrPrototococolol, , a a sisimpmple le fifile le trtranansfsfer er prprototococolol HT

HTTP TP HyHypeperTrTexext t TTraransnspoport rt PrPrototococolol, , usused ed in in ththe e WoWorlrld d WiWide de WWebeb P

POOPP3 3 PPoosst t OOffffiicce e PPrrototococool l VVeerrssiioon n 33 SN

SNMMP P SiSimpmplle e NeNetwtwoork rk MMaannagageemmeennt t PProrottococolol S

SMMTTP P SSiimmpplle e MMaaiil l TTrraannssffeer r PProrottooccooll Te

Telnlnet et A A reremomote te tetermrmininal al acaccecess ss prprototococolol Gn

Gnututelella la A A pepeerer-t-to-o-pepeer er fifilele-s-swawappppining g prprototococolol D

DNNS S DDoommaaiin n NNaamme e SSeerrvviiccee B

BOOOOTTP P BBooototssttrraap p PPrroottooccooll R

Rlloogigin n A A UUNNIIX X rreemmootte e llooggiin n pprroottooccolol MI

MIME ME MuMultltipipururpopose se InInteternrnet et MaMail il ExExtetensnsioionsns A

AFFP P AApppplle e FFiilliinng g PPrroottooccooll S

SSSH H SSeeccuurre e SSHHeellll IM

IMAP AP IntterInernnet et MMeessssagage e AAcccceesss s PProrototococoll N

NTTP P NNeettwwoorrk k TTiimme e PPrroottooccooll Go

Gophpher er A A prprececuursrsor or of of weweb b sseaearcrch h enengigineness N

NCCP P NNeettwwaarre e CCoorre e PPrroottooccooll Fi

Finngeger r GiGivves es ususeer r pprorofifile le ininfoformrmaatitionon N

NNTNTP P NeNewws s NNetetwworork k TTrranansfsfeer r PrPrototococolol LD

LDAP AP LiLighghtwtweieighght t DiDirerectctorory y AcAccecess ss PrPrototococolol D

DHCHCP P DynDynaammiic c HHosost t CoConnfifigugurarattioion n PrPrototococolol IIRRC C IInntteerrnneet t RReellaay y CChhaatt

Ja

Jabbbbeer r An An ininststanant-t-memessssagagining g prprototococolol We

WebDbDAV AV WeWeb b DiDiststriribubuteted d AuAuththorioring ng anand d VeVersrsioioniningng D

DIICCT T DDiiccttiioonnaarry y pprroottooccooll BA

BACnCnet et BuBuilildiding ng AuAutotomamatition on anand d CoContntrol rol NeNetwtworork k prprototococolol RAD

RADIUS IUS An An autauthenhenticticatiation, on, autauthorhorizaizatiotion n and and accaccounountinting g protprotocoocoll DIA

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PRESENTATION LAYER PRESENTATION LAYER

This layer provides data representation and code This layer provides data representation and code

formatting. It provides a uniform means for a remote formatting. It provides a uniform means for a remote host to inform the local client how to present the data host to inform the local client how to present the data to the application or client.

to the application or client.

It ensures that the data that arrives from the network It ensures that the data that arrives from the network can be used by the application, and it ensures that

can be used by the application, and it ensures that

information send by the application can be transmitted information send by the application can be transmitted on the network.

on the network.

In some cases, the presentation layer directly In some cases, the presentation layer directly

translates data from one format to another. Other translates data from one format to another. Other functions that may correspond to the presentation functions that may correspond to the presentation layer are data encryption/decryption and

layer are data encryption/decryption and compression/decompression.

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The presentation layer is responsible for the delivery The presentation layer is responsible for the delivery and formatting of information to the application layer and formatting of information to the application layer for further processing or display. It relieves the

for further processing or display. It relieves the

syntactical differences in data representation within syntactical differences in data representation within the end-user systems.

the end-user systems.

Note: An example of a presentation service would be Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an

the conversion of an EBCDIC-coded text file to an ASCII-coded file.

ASCII-coded file.

In many widely used applications and protocols, no In many widely used applications and protocols, no distinction is made between the presentation and distinction is made between the presentation and

application layers. For example, HTTP is an application application layers. For example, HTTP is an application layer protocol, also identify character encoding for

layer protocol, also identify character encoding for proper conversion in the presentation layer.

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SESSION LAYER SESSION LAYER

The session layer establishes, manages and The session layer establishes, manages and terminates session between application. It terminates session between application. It responds to service requests from the

responds to service requests from the

presentation layer and issues service requests presentation layer and issues service requests to the transport layer.

to the transport layer.

It provides the mechanism for managing It provides the mechanism for managing end-user, host, application process operations and user, host, application process operations and establishes check pointing, termination, and establishes check pointing, termination, and restart procedures.

restart procedures.

This layer is responsible for dialog control This layer is responsible for dialog control

between nodes. Dialog is a formal conversation between nodes. Dialog is a formal conversation in which two nodes agree to exchange data.

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The communications between applications The communications between applications

across a network can take place and controlled across a network can take place and controlled in one of three dialog modes, are Simplex,

in one of three dialog modes, are Simplex, Half-duplex, and Full-duplex.Sessions can Half-duplex, and Full-duplex.Sessions can allow traffic to go in both directions at the allow traffic to go in both directions at the

same time, or in only one direction at a time. same time, or in only one direction at a time. Testing for out-of-sequence packets is handled Testing for out-of-sequence packets is handled also.

also.

Sessions, enable nodes to communicate in an Sessions, enable nodes to communicate in an organized manner. Each session has three

organized manner. Each session has three phases as Connection establishment, Data phases as Connection establishment, Data transfer, Connection release.

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Session Layer 5 protocols Session Layer 5 protocols N

NCCP P NNeettwwoorrk k CCoonnttrrool l PPrroottooccooll S

SMMB B SSeerrvveer r MMeessssaagge e BBlloocckk N

NFFS S NNeettwwoorrk k FFiille e SSyysstteemm A

ASSP P AApppplleeTTaallk k SSeessssiioon n PPrroottooccooll A

ADDSSP P AApppplleeTTaallk k DDaatta a SSttrreeaam m PPrroottooccooll D

DLLC C DDaatta a LLiinnk k CCoonnttrrooll N NP P NNaammeed d PPiippeess NetBIOS NetBIOS NWLink NWLink P

PAAP P PPrriinntteer r AAcccceesss s PPrroottooccooll Z

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TRANSPORT LAYER TRANSPORT LAYER

It translates system names into addresses and It translates system names into addresses and divides messages into fragments that fit within divides messages into fragments that fit within the size limitations established by the network. the size limitations established by the network. This layer is responsible for addressing,

This layer is responsible for addressing, determining routes for sending, managing determining routes for sending, managing network traffic problems, packet switching, network traffic problems, packet switching, routing, data congestion, and reassembling routing, data congestion, and reassembling data at the receiving end to recover the

data at the receiving end to recover the original message.

original message.

Ensure that segments delivered will be Ensure that segments delivered will be acknowledge back to sender.

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It provides retransmission of segments those It provides retransmission of segments those are not acknowledged. Put segments back into are not acknowledged. Put segments back into their correct sequence at the destination.

their correct sequence at the destination.

To enable packets reassembly in their original To enable packets reassembly in their original order, this layer includes a

order, this layer includes a messages sequencemessages sequence

number

number in its header.in its header.

It allows users to segment/reassemble several It allows users to segment/reassemble several upper layer application on to the same

upper layer application on to the same

transport data stream. It insures host-to-host transport data stream. It insures host-to-host session connection.

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In some protocol suites this layer also provides In some protocol suites this layer also provides multiple conversations (multiplex) between the multiple conversations (multiplex) between the same two end-points. Delivery may be

same two end-points. Delivery may be

'guaranteed' (streams, like TCP) or 'best effort' 'guaranteed' (streams, like TCP) or 'best effort' (datagram, like UDP).

(datagram, like UDP).

The transport layer also is responsible for The transport layer also is responsible for

delivering messages from a specific process on delivering messages from a specific process on one computer to the corresponding process on one computer to the corresponding process on the destination computer.

the destination computer.

In OSI terms the transport layer assigns a In OSI terms the transport layer assigns a

service access point (SAP ID) to each packet. service access point (SAP ID) to each packet. In TCP/IP terms that is called a

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One more responsibility of the transport layer One more responsibility of the transport layer is detecting errors in transmitting data.

is detecting errors in transmitting data.

Two general categories of error detection can Two general categories of error detection can be performed by the transport layer:

be performed by the transport layer: Reliable delivery

Reliable delivery : Reliable delivery does not: Reliable delivery does not

mean that errors cannot occur, only that errors mean that errors cannot occur, only that errors are detected if they do occur.

are detected if they do occur. Unreliable delivery

Unreliable delivery : Unreliable delivery does: Unreliable delivery does

not mean that errors are likely to occur, but not mean that errors are likely to occur, but rather indicates that the transport layer does rather indicates that the transport layer does not check for errors.

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Transport Layer 4 protocols Transport Layer 4 protocols S

SPPX X SSeeqquueenncceed d PPaacckkeet t EExxcchhaannggee T

TCCP P TTrraannssmmiissssiioon n CCoonnttrrool l PPrroottooccooll U

UDDP P UUsseer r DDaattaaggrraam m PPrroottooccooll S

SCCTTP P SSttrreeaam m CCoonnttrrool l TTrraannssmmiissssiioon n PPrroottooccooll R

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NETWORK LAYER NETWORK LAYER

The network layer operates independently of The network layer operates independently of the physical medium, which is a concern of the the physical medium, which is a concern of the physical layer.Since routers are network layer physical layer.Since routers are network layer devices, they can be used to forward packets devices, they can be used to forward packets between physically different networks.

between physically different networks.

When the network layer receives a message When the network layer receives a message from upper layers, the network layer adds a from upper layers, the network layer adds a header to the messages that includes the

header to the messages that includes the

source and destination network address. This source and destination network address. This combination of data plus the network layer is combination of data plus the network layer is called a

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It translates logical addresses and names into It translates logical addresses and names into physical addresses for Packets delivery.

physical addresses for Packets delivery. The addressing which makes this delivery The addressing which makes this delivery possible can be thought of as a universal possible can be thought of as a universal

address as compared to the local addressing of address as compared to the local addressing of the MAC.

the MAC.

It determines routes for sending, managing It determines routes for sending, managing network traffic problems, packet switching, network traffic problems, packet switching, routing, data congestion, and reassembling routing, data congestion, and reassembling data.

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This layer may also be concerned with This layer may also be concerned with end-to-end flow control and the segmentation and

end flow control and the segmentation and reassembly (SAR) functions for the

reassembly (SAR) functions for the

transmission of large packets of data which transmission of large packets of data which

exceed the maximum transmission unit (MTU) exceed the maximum transmission unit (MTU) of the data link layer.

of the data link layer.

The Internet Protocol (IP) is the most common The Internet Protocol (IP) is the most common example, and Novell's Internetwork Packet

example, and Novell's Internetwork Packet eXchange (IPX) is another popular

eXchange (IPX) is another popular implementation.

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Network Layer 3 protocols Network Layer 3 protocols

IIP P IInntteerrnneet t PPrroottooccool l vveerrssiioon n 44 IIPPvv6 6 IInntteerrnneet t PPrroottooccool l vveerrssiioon n 66 A

ARRP P AAddddrreesss s RReessoolluuttiioon n PPrroottooccooll R

RAARRP P RReevveerrsse e AAddddrreesss s RReessoolluuttiioon n PPrroottooccooll IICCMMP P IInntteerrnneet t CCoonnttrrool l MMeessssaagge e PPrroottooccooll IIGGMMP P IInntteerrnneet t GGrroouup p MMaannaaggeemmeennt t PPrroottooccooll M

MPPLLS S MMuullttii--PPrroottooccool l LLaabbeel l SSwwiittcchhiinngg R

RIIP P RRoouuttiinng g IInnffoorrmmaattiioon n PPrroottooccooll O

OSSPPF F OOppeen n SShhoorrtteesst t PPaatth h FFiirrsstt E

EGGP P EExxtteerriioor r GGaatteewwaay y PPrroottooccooll B

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DATA LINK LAYER DATA LINK LAYER

The data link layer sends data from network layer to The data link layer sends data from network layer to physical layer. Manages physical layer communications physical layer. Manages physical layer communications between connecting systems. It is responsible for

between connecting systems. It is responsible for providing node-to-node communication on a single, providing node-to-node communication on a single, local network.

local network.

To provide this service, the data link layer must To provide this service, the data link layer must perform two functions.

perform two functions.

1.It must provide an address mechanism that 1.It must provide an address mechanism that

enables messages to be delivered to the correct enables messages to be delivered to the correct

nodes, Also, it must translate messages from upper nodes, Also, it must translate messages from upper layer into bits that the physical layer can transmit. layer into bits that the physical layer can transmit. 2.Data units at the Data Link layer are most

2.Data units at the Data Link layer are most

commonly called frames, although the term packet commonly called frames, although the term packet is used with some protocols.

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The Data Link layer is comprised of a single layer in the The Data Link layer is comprised of a single layer in the OSI model but actually incorporates two sub-layers

OSI model but actually incorporates two sub-layers when viewed from the perspective of the IEEE Logical when viewed from the perspective of the IEEE Logical Link Control (LLC) model. The lower of the two Link Control (LLC) model. The lower of the two

sub-layers is called the media access control (MAC) and the layers is called the media access control (MAC) and the upper sub-layer is the LLC layer.

upper sub-layer is the LLC layer.

The MAC layer provides local addressing, error The MAC layer provides local addressing, error correction, and in some cases includes a field to correction, and in some cases includes a field to distinguish the frame type from other frame types. distinguish the frame type from other frame types. This component determines who is allowed to access This component determines who is allowed to access the media at any one time.

the media at any one time.

The LLC provides Service Access points (SAP) for The LLC provides Service Access points (SAP) for

passing off the frame to higher level. This component passing off the frame to higher level. This component determines where one frame of data ends and the next determines where one frame of data ends and the next one starts. Examples of data link standards include

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Data link Layer 2 protocols Data link Layer 2 protocols A

ARRP P AAddddrreesss s RReessoolluuttiioon n PPrroottooccooll C

CDDP P CCiisscco o DDiissccoovveerry y PPrroottooccooll D

DCCAAP P DDaatta a LLiinnk k SSwwiittcchhiinng g CClliieennt t AAcccceesss s PPrroottooccooll Ethernet

Ethernet F

FDDDDI I FFiibbeer r DDiissttrriibbuutteed d DDaatta a IInntteerrffaaccee H

HDDLLC C HHiiggh h LLeevveel l DDaatta a LLiinnk k CCoonnttrrooll L

L22F F LLaayyeer r 2 2 FFoorrwwaarrddiinng g PPrroottooccooll L

L22TTP P LLaayyeer r 2 2 TTuunnnneelliinng g PPrroottooccooll P

PPPP P PPooiinntt--ttoo--PPooiinnt t PPrroottooccooll P

PPPTTP P PPooiinntt--ttoo--PPooiinnt t TTuunnnneelliinng g PPrroottooccooll S

SLLIIP P SSeerriiaal l LLiinne e IInntteerrnneet t PPrroottooccooll Token ring

Token ring X.25 X.25 F

Frraamme e rreellaay y A A ssiimmpplliiffiieed d vveerrssiioon n oof f XX..2255 A

ATTM M AAssyynncchhrroonnoouus s TTrraannssffeer r MMooddee M

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PHYSICAL LAYER PHYSICAL LAYER

This transmits data over a physical medium or on This transmits data over a physical medium or on network hardware, like cables, cards, etc. This layer network hardware, like cables, cards, etc. This layer relates the electrical, optical, mechanical and

relates the electrical, optical, mechanical and

functional interface to the cable. It is responsible for functional interface to the cable. It is responsible for transmitting bits (zero & ones) from one computer to transmitting bits (zero & ones) from one computer to another.

another.

This layer is concerned with the signaling of the This layer is concerned with the signaling of the message and the interface between the sender or message and the interface between the sender or receiver and the medium. It is defined by one of the receiver and the medium. It is defined by one of the standards bodies and carries a designation that

standards bodies and carries a designation that

indicates the characteristics of the connection. Among indicates the characteristics of the connection. Among frequently used physical layers standards are frequently used physical layers standards are EIA-232-D, ITU V.35, and some of the X series (X.21/X.21bis). D, ITU V.35, and some of the X series (X.21/X.21bis).

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Physical Layer 1 protocols Physical Layer 1 protocols

IISSDDN N IInntteeggrraatteed d SSeerrvviiccees s DDiiggiittaal l NNeettwwoorrkk P

PDDH H PPlleessiioocchhrroonnoouus s DDiiggititaal l HHiieerraarrcchhy, y, T-T-ccaarrririeer r ((TT11, , TT3 3 eettcc)) RS

RS--23232 2 A A ssereriial al liline ne iintntererfaface ce ooririggininalally ly dedevevellopoped ed tto o ccoonnnnecectt modems and computer terminals