THE OSI MODEL

• Open System Interconnection (OSI).
• Developed by the International Organization for Standardization (ISO)
• Model for understanding and developing computer-to-computer communication architecture that is flexible, robust and interperable.
• it is not a protocol.
• Developed in the 1980s.
• Divides network architecture into seven layers. 
• Each layer performs a subset of the required communication functions
• Each layer relies on the next lower layer to perform more primitive functions
• Each layer provides services to the next higher layer
• Changes in one layer should not require changes in other layer
• Layer 1,2,3 are the network support layer, deals with the physical aspects of moving data from one device to another.
• Layer 5,6,7 are the user support layer, allow the interoperability among unrelated software.
• Layer 4 ensures that what the lower layer have transmitted is in a form that the upper layers can use. 

                                                    7. APPLICATION LAYER

                                                    6. PRESENTATION LAYER
                                                    5. SESSION LAYER

                                                    4.TRASPORT LAYER

                                                    3. NETWORK LAYER

                                                    2. DATA LINK LAYER

                                                    1. PHYSICAL LAEYER

 PROTOCOL DATA USING (PDU)

• At each layer, protocols are used to communicate.
• Control information is added to user data at each layer
• For example, the transport layer may fragment user data
• Each fragment has a transport header added
• Destination Address
• Sequence Number
• Error detection code       
• This creates a transport protocol data unit (TPDU)

LAYER 1: PHYSICAL LAYER

RESPONSIBLE OF:

• Transmitting individual bits from one to the next.

• Synchronize of bits

• Line configuration

• Physical topology

• Transmission mode

• Data rate.

• Representation of bits: a stream of bit(0s,1s),

• Physical characteristics of interface and media.

LAYER 2: DATA LINK LAYER

—  Moving frames from one hop (node) to the next.

—  Framing: divided the stream of bits received from the network layer manageable data units called frames.

—  Physical address (MAC address).

—  Flow control.

—  Error control: added trailer to the end of frame.

—  Access control.

—  Hop to hop delivery

LAYER 3: NETWORK LAYER

¢ The network layer is responsible:

—   The delivery of individual packets from the original source to the final destination.

—  Logical addressing: if the packet passes the network boundary, we need another addressing system to help (source to destination) connection.

—  Routing: route or switch the packet to final destination.

—  Source-to-destination delivery (End-to-End).

SOURCE-TO-DESTINATION DELIVERY

 

LAYER 4: TRANSPORT LAYER

¢ The transport layer is responsible for:

—  Service point or Port addressing

—  Segmentation and reassembly:  a message is divided into transmittable segments each segment containing a sequence no.

—  Connection Control:  connection oriented or connectionless.

—  Flow control

—  Error control

LAYER 5: SESSION LAYER

• Dialog control: design to establish, maintain, and synchronize the interaction between communicating systems.

• Synchronization: it allows a process to add checkpoints or synchronization points to a data stream.

LAYER 6: PRESENTATION LAYER

• Design to the handle the syntax and semantic of the information exchanged between 2 systems.
• And design for data translation, encryption, decryption, and compression.

LAYER 7: APPLICATION LAYER

• The application layer is responsible for providing services to the user.

• Mail services

• File transfer, access and management

• Remote log-in or network virtual terminal

• Accessing the World Wide Web

• Directory service