LAYER 4 – The TRANSPORT Layer, LAYER 5 – The SESSION Layer And LAYER 6 – The PRESENTATION Layer




LAYER 4 – The TRANSPORT  Layer

  Manages the transmission (transport) of data across a network.
  Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium)
  Reassembles chunks into their original sequence at the receiving end.
  Provides acknowledgements of successful transmissions and requests resends for packets which arrive with errors.








The transport layer is responsible for the delivery
of a message from one process to another
.





LAYER 5 – The SESSION Layer

  Enables two networked resources to hold ongoing communications (called a session)  across a network
  Applications on either end of the session are able to exchange data for the duration of the session.
  This layer is:
  Responsible for initiating, maintaining and terminating sessions.
  Responsible for security and access control to session information (via session participant identification)
  Responsible for synchronization services, and for checkpoint services.







The session layer is responsible for dialog 
control and synchronization.




LAYER 6 – The PRESENTATION Layer

  converts data from network format to computer expected format and vise versa (the network’s translator)
  For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand.
  This layer is also responsible for
protocol conversions,
 data encryption/decryption,
 data compression/decompression
  A special software facility called a “redirector” operates at this layer to determine if a request is network related or not.
For example connect remote network printer to the computer.







The presentation layer is responsible for translation, compression, and encryption.



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