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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.