Osi Model, Sna & Tcp

8/9/2019 Osi Model, Sna & Tcp

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OSI MODEL, SNA & TCP/IPARCHITECTURE

PRESENTED BY:

SAURABH SINGH

260713


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INTRODUCTION

Open Systems Interconnection (OSI) model isa reference model developed by ISO(International Organization forStandardization) in 1984, as a conceptualframework of standards for communicationin the network across different equipmentand applications by different vendors. It is

now considered the primary architecturalmodel for inter-computing and inter-networking communications.


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The OSI model defines the communicationsprocess into 7 layers, dividing the tasks involvedwith moving information between networked

computers into seven smaller, more manageabletask groups. A task or group of tasks is thenassigned to each of the seven OSI layers. Eachlayer is reasonably self-contained, so that the

tasks assigned to each layer can be implementedindependently. This enables the solutions offeredby one layer to be updated without adverselyaffecting the other layers.


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CHARACTERISTICS OF LAYERS

Layers 7 to 4 deal with end to end

communications between data source and

destinations, while layers 3 to 1 deal withcommunications between network devices.

On the other hand, the seven layers of the

OSI model can be divided into two groups:

upper layers (layers 7 , 6 & 5 ) and lower layers(layers4,3,2,1).


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The upper layers of the OSI model deal withapplication issues and generally areimplemented only in software. The highest layer,

the application layer, is closest to the end user.The lower layers of the OSI model handle datatransport issues. The physical layer and the datalink layer are implemented in hardware and

software. The lowest layer, the physical layer, isclosest to the physical network medium (thewires, for example) and is responsible for placingdata on the medium.


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Layer 7: Application Layer

Defines interface-to-user processes for communication anddata transfer in network

Provides standardized services such as virtual terminal, fileand job transfer and operations

Layer 6: Presentation Layer

Masks the differences of data formats between dissimilarsystems

Specifies architecture-independent data transfer format

Encodes and decodes data; encrypts and decrypts data;compresses and decompresses data


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Layer 5: Session Layer

Manages user sessions and dialogues

Controls establishment and termination of logic links

between users Reports upper layer errors

Layer 4: Transport Layer

Manages end-to-end message delivery in network

Provides reliable and sequential packet deliverythrough error recovery and flow control mechanisms

Provides connectionless oriented packet delivery


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Layer 3: Network Layer

Determines how data are transferred between network devices

Routes packets according to unique network device addresses

Provides flow and congestion control to prevent network resource depletion

Layer 2: Data Link Layer

Defines procedures for operating the communication links

Frames packets

Detects and corrects packets transmit errors

Layer 1: Physical Layer Defines physical means ofsending data over network devices

Interfaces between network medium and devices

Defines optical, electrical and mechanical characteristics


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IBMs SYSTEM NETWORK ARCHITECTURE


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SNA is a computer networking architecture

developed by IBM to provide a network

structure for IBM mainframe, midrange, andpersonal computer systems. SNA defines a

set of proprietary communication protocols

and message formats for the exchange and

management of data on IBM host networks.


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SNA defines methods that accomplish the

following:

Terminal access to mainframe and midrangecomputer applications.

File transfer of data between computer systems.

Printing of mainframe and midrange data on

SNA printers. Program-to-program communications that allow

applications to exchange data over the network.


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IMPLEMENTATION

SNA can be implemented in the following two network models:

Hierarchical The hierarchical SNA networking model, also calledsubarea networking, provides geographically disparate terminal usersaccess to centralized mainframe processing systems. In the hierarchicalnetworking model, centralized host-based communication systemsmust provide the networking services for all users on the network.

Peer-to-Peer The more recently developed Advanced Peer-to-PeerNetworking (APPN) model makes use of modern local area network(LAN) and wide area network (WAN) resources and client/servercomputing. APPN networking enables a form of distributed processingby allowing any computer on the network to use SNA protocols to gainaccess to resources on any other computer on the network. Computers

on an APPN network do not have to depend on mainframe-basedcommunication services.

Because ofthe large installed base of legacy applications that run on IBMmainframe and midrange systems, both of these SNA networkingmodels continue to be widely used in enterprise networks.


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ADVANTAGES

Localizationofproblems in the telecommunications networkwas easierbecause a relatively small amount ofsoftware actually dealt withcommunicationlinks. Therewas a singleerrorreporting system.

Addingcommunicationcapability to an applicationprogramwas mucheasierbecause theformidable area oflinkcontrol software that typicallyrequires interrupt processors and software timers was relegated tosystem software and NCP.

With the advent ofAPPN, routingfunctionality was theresponsibility ofthecomputer as opposed to therouter(as with TCP/IP networks). Eachcomputermaintained a list ofNodes that defined theforwardingmechanisms. A centralizednode typeknown as a Network Nodemaintained Global tables ofallothernode types. APPN stopped the

need tomaintain APPC routing tables that explicitly definedendpoint toendpoint connectivity. APPN sessions wouldroute toendpoints throughother allowednode types until it found thedestination. This was similarto theway that TCP/IP routers function today.


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DISADVANTAGES

Connection tonon-SNA networks was difficult. Anapplicationwhich needed access to somecommunicationscheme, which was not supported in thecurrent versionofSNA, facedobstacles. Before IBM includedX.25 support

(NPSI) in SNA, connecting to anX.25 networkwould havebeen awkward. ConversionbetweenX.25 and SNAprotocols could havebeenprovidedeitherby NCP softwaremodifications orby anexternalprotocolconverter.

A sheafofalternatepathways betweenevery pairofnodesin a network had tobepredesigned and storedcentrally.

Choice among thesepathways by SNA was rigid anddidnot take advantageofcurrent linkloads foroptimumspeed.


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ThedesignofSNA was in theera when theconcept oflayeredcommunicationwas not fully adoptedby thecomputer industry.Applications, databases andcommunicationfunctions weremingledinto the sameprotocolorproduct, tomake it difficult tomaintainormanage. That was very commonfor theproducts created in that time.

Even after TCP/IP was fully developed, XWindow systemwas designedwith the samemodelwherecommunicationprotocols wereembeddedintographicdisplay application.

SNA's connectionbased architecture invoked huge statemachinelogicto "keep track" ofeverything. APPN added a newdimension to statelogicwith its concept ofdifferingnode types. While it was solidwheneverythingwas runningcorrectly, therewas still a needformanual

intervention. Simple things likewatching the Control Point sessions hadtobedonemanually. APPN wasn't without issues; in theearly daysmany shops abandoned it due to issues found in APPN support. Overtime, however, many ofthe issues wereworkedout but not before theadvent oftheWeb Browserwhich was thebeginningoftheendfor SNA.


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INTERNET PROTOCOL

ARCHITECTURE


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MEANING

The Internet Protocol Suite (commonly known

as TCP/IP) is the set of communication

protocols used for the Internet and other similar

networks.

The Internet Protocol Suite, like many protocol

suites, may be viewed as a set of layers. Each

layer solves a set of problems involving thetransmissio n o f data, and provides a well-defined

service to the upper layer protocols based on

using services from some lower layers.


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Upper layers are logically closer to the userand deal with more abstract data, relyingon lower layer protocols to translate data intoforms that can eventually be physicallytransmitted.

The TCP/IP consists of four layers (RFC-1122). From lowest to highest, these arethe Link Layer, the Internet Layer,the Transport Layer, and the ApplicationLayer.


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IMPLEMENTATION

Most operating systems in use today,

including all consumer-targeted systems,

include a TCP/IP implementation. Unique implementations include Lightweight

TCP/IP, an open source stack designed

for embedded systems and KA9Q NOS, a

stack and associated protocols foramateur packet radio systems and personal

computers connected via serial lines.

Osi Model, Sna & Tcp

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