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.