v  Transport layer provides the logical connection to two applicationsprocesses running on different hosts within a network layers.v  Transport layer is responsible to send the entire message fromsender to the receiver also ensure that the message reached to the destinationin same sequence as it was sent from the sender.v  Transport layer makes the data transfer reliable as it divides the largedata into small chunks so data can be handling or transferred efficientlythrough the network.v  Transport layer provides different services Ø  Reliable transfer of data Ø  Control  flow of dataØ  Transmission of data in same order as it was sent from sender Ø  Multiplex ports      1.1.1.

      Functions of transport layer Ø  Addressing Transport layer include service point addresses which is portaddress.  This layergets the message to approve the process on the computer unrelated to theNetwork Layer, which gets each packet to correct the computer.Ø  Segmentation & end to end connection A single message is divided  into chunks and sent the segments one by oneand at destination point it again reassemble the segments and chunks of thatmessage to decrypt it into original from ,each segment will receive in samesequence as it was sent from sander.Ø  Quality of services                                                                                ·           ErrorRecovery                                                                                ·           Reliability                                                                                ·           Overcrowdingcontrol  Ø  Realization Software and hardware controls.Transportlayers involve different protocols TCP transfer control protocol is a connection oriented protocol TCP/IP  application which sends thedata receives acknowledgement or verification that the data was actuallyreceived and UDP is a connectionless protocol that does not provide thereliable transfer of protocol. Transport layer is responsible of transparenttransfer of data.

1.1.2.      Issues with transport Ø  Insecurity I transport layer when data is being transferred through wirelessnetworks or other mediums, we are not sure about the secure transmission ofdata because wireless devices are not always secure.Ø  Lack of central authorityIn wired networks it would be possible to monitor or check the datatraffic by applying some central security checks but in wireless networks thesesecurity checks are not available so lack of central check authority is theissue with transport layer. Ø  Physically vulnerable Nodes used for the transmission purpose are easily breakable whichcan cause vulnerable threat to the loss of data being transmitted.     Inabove diagram the data which is first divided into segments is sent fromapplication layer to network layer and these segments passes through networklayer and receives  to application layerand when data is received to the recipient it again merged into original formas it was before.

 1.2.   Session layerv  Session layer or port layer controls the connection between twocomputers or communication devices.v  It performs the full duplex, half duplex and simplex duplication operation.Ø  Full duplex Full duplex communication refers to the transmission of data in twodirections at same time means data can be sent and received at the same timethrough a single communication medium. Telephone is the example of full duplexmode.Ø  Half duplexHalf duplex mode was used in old networks it refers thetransmission of data only in one direction over a medium at same time, walkietalkie is the example of the half duplex modeØ  Simplex Simplex mode only allows the transfer of data in on direction itdoes not the switching of direction while transmission of data.

For example inradio broadcast transmissions information only transfer in one direction tomultiple receivers.  v  Synchronization of the information from different sources is also handlingby session layer, for example sessions are used in live television programswhere video and audio streams coming from two different sources merged together,sessions avoid overlapping and in case of connection loss it tries to recoveror manage the failed connection.v  Session layer deals with the sessions and coordination of theapplications and networks.v  Role of session layer is important in keeping the track of themultiple telnet connections, download of multiple files requested by the FTPapplications and retrieval of the multiple web pages from a server at a time. v  Dialogue control and synchronization are the functions of sessionlayer.

v  Check points in session layer are considered as recovery points incase of system failure these check points are considered to be recovery configurationsand action.v  Different protocols like NFS, NetBios names, RPC, SQL work at thislayer. 1.2.1.      Dialogue control Dialogue control allows two systems to get start communication inhalf duplex or full duplex mode.

1.2.2.      Synchronization Synchronization performs checkpoints into streams of data for exampleif a system is sending a file of 1600 pages it applies checkpoint after every100 pages to ensure the quality and acknowledgement of data transmittedsuccessfully or not . v  Web browsers and OSI protocol known as X.225 or ISo8327 are theexcellent examples of session layer protocols. 1.

3.    Presentation layerv  Presentation sometimes known as syntax layer is the 6thlayer of OSI model work to present data to application layer which is above topresentation layer.v  Presentation layer defines the syntax which is used by the hostthat communicates through a network.v  Functions of presentation layer involve characterization, datatranslation, encryption and decryption of data and data conversion as well.v  It works as a translator when different language is used betweensender and the recipient.v  Http (Hyper text transfer protocol), ASCII, EBCDIC, TIFF, GIF are the examples of thepresentation layer.

   1.4.   Application layerv  Application layer provide functions on for different applicationprograms. It provides and manages the connection of all lower layers.

v  It is also responsible to manage the both application and users endtasks and overall system management also dependent on the application layer. v  Application layer provides application services for the filetransfer this layer is closed to end users which mean both the OSI applicationlayer and the end udders can interact directly with the software application.v  It identifies the communication partners determine the resourceavailability through internet protocol network and provide synchronizedcommunication to end users. v  Examples of application layers are Telnet, FTP (File TransferProtocol), FTFP (Trivial File Transfer Protocol), NNTP (Network news transferprotocol) and Electronic or simple mail etc.  

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