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.
5. SESSION LAYER
- 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
