EXPLAIN OSI MODEL.(Transport layer,session layer,presentation layer,Application layer)

Transport  Layer:

 

 

  • In order for the data to be sent across the network,the file must be broken up into usable small data segments(typically 512- 18K bytes).The transport layer breaks up the file into segments for transport to the network,and combines incoming segments into a contiguous file.The Transport layer does this logically , not physically, and it is done in software as opposed to hardware.
  • Services-point addressing: Computers often run several programs at the same time.For this reason,sourse-to-destination delivery means not only from one computer to the next but also from a specific process (running program)on one computer to a specific process(running program)on the other.Th transport layer header must therefore include a type of address called a service-point address(or port address).The network layer gets each packet to the correct computer;the transport layer gets the entire message to the crrect process on that computer.
  • Connection control: The transport layer can be either connectionless or connection oriented.A connectionless transport layer treats  each segment as an independent packet and delivers it to the transport layer at the destination machine .A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all the data are transferred,the connection is terminated.
  • Flow control: Like the data link layer,the transport layer is responsible for flow control.However,flow control at this layer is performed end to end rather than across a single link.

 

 

 

 

Session Layer:

 

  • The session layer manages the communications between the workstation and the network.The session layer directs the information to the correct destination,and identifies the source to the destination . The session layer identifies the type of information as data or control.The session layer manages the initial start-up of a session , and the orderly closing of a session.The session layer also manages log on procedures and password recognition.
  • Dialog control: The session layer allows two systems to enter into a dialog.it allows the communication between two processes to take place in either half duplex(one way at s time) or full-duplex(two ways at a time)mode.
  • Synchronization:The session layer allows a process to add checkpoints,or synchronization points,to a stream of data.for examples,if a system is sending a file of 2000pages,it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. in this case,if a crash happens during the transmission of page 523, the only pages that need to be resent after system recovery are pages 501 to 523. pages previous to 501 need not be resent. figure illustrates the relationship of the session layer to the transport and presentation layers.

 



 

Presentation Layer:

  • The network redirector sends CPU operating system native code to the netwotk  operating system : the coding and format of the data is not recognizable by the network  operating system . The data consists of file transfers and network calls by network aware programs.for example,when a dumb terminal is used as a workstation(in a mainframe or minicompuet network),the network data is translated into(and form)the format that the terminal can use  .The presentation layer presents data to and  form the terminal using special control characters to control the screen display(LF-line feed, CR-carriage return,cursor movement,etc..).The presentation of data on the screen would depend on the type of terminal that''s used:VT100,VT52,VT420,etc.
  • Similarly,the presentation layer strips the pertinent file from the workstation  operating system''s file envelope.The control characters,screen formating,and workstation  operating system envelope are all stripped or added to the file(if the workstation is receiving or transmitting data to the network).This could also include translating ASCII file characters from a PC world to EBCDIC in a IBM Mainframe world.
  • The presentation layer also controls security at the file level: this provides both file locking and user security.The DOS share program is often used for file locking ,When a file  is in use,it is locked from other users to prevent 2 copies of the same file from beling generated,if 2 users both modified the same file,and user A saved it,then user B saved it,then user A''s changeswould be erased!
  • Translation: the processes(running programs)in two systems are usually exchanging information in the from of character strings,numbers and so on.The information must be changed to bit streams before being transmitted.Because different computers use different encoding system,the presentation layer is responsible for interoperability between these different encoding methods.The presentation layer at the sender changes the information from its sender-dependent format into a common format.The presentation layer at the receiving machine changes the common format into its receiver-dependent format.
  • Compression: Data compression reduces the number of bits contained in the information .Data compression becomes particularly important in the transmission of multimedia such as text,audio and video.

Application layer:

  •   The application layer enablesthe user ,whether human or software ,to access the network.it provides user interfaces and support for services such as electronic mail,remote file accress and transfer,shared database management ,and other types of distibuted information services.
  • figure shows the relationship of the application layer to the user and the presentation layer. of the many application services available,the figure shows only three: XAOO(message-handling services),X.500(directory services),and file transfer,access,and management(FTAM).the user in this example employs XA00 to send an e-mail message.
  • Network virtual terminal:A network virtual terminal is a software version of a physical trminal,and it allows a user to log on to a remote host .To do so,the application creates a software emulation of a terminal at the remote host. The user''s computer talks to the software terminal which, in turn,talks to the host,and vice versa.The remote host belirves it is communicating with one of its own terminals and allows the user to log on.
  • File transfer,access,and management:This application allows a user to access files in a remote host (to make changes or read data),to retrieve files from a remote computer for use in the local computer,and to manage or control files in a remote computer locally.
  • Mail services: This application the basis for e-mail forwarding and storage.
  • Directory services:This application provides distributed database sources and access for global information about various objects and services.


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by maya    chaudhari  in Networking  on 9/17/2015 7:32:10 PM  


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