OSI Model

Internet Model
OSI Model
In the late 1970’s ISO formulated a Reference (Open Systems Interconnection) Model:
To provide a common basis for the coordination of standards development
To allow existing and evolving standards to be placed into perspective with one another
has become the stand model for classifying communications functions
The Model is concerned with the structuring of Communication to provide a reliable, open communication service which is independent of any specific manufacturer’s equipment or conventions

ISO - OSI Model

Consists of seven layers:
Application
Presentation User
Session
Transport
Network
Data Link Network
Physical

Basic Concepts

Line Configuration
Topology
Transmission Mode
Categories of Networks
Internetworks
OSI Model (physiccal layer)

Note: The physical layer is responsible for transmitting individual bits from one node to the next

Define the characteristics of the interface between the devices and the transmission media
Encode bits into signals and decode signals to get bits
Define transmission rate, which must be the same for both sender and receiver
Synchronize clocks

The data link layer is responsible for transmitting frames from one node to the next

Framing: divide the data stream into manageable data units called “frames”
Physical addressing: insert the physical address of the next node into frame’s header
Flow control: prevent overflow at receiver
Error control: make sure that frames are correctly received
Access control: make sure that there is no link access conflict

The network layer is responsible for the delivery of packets from the original source to the final destination over multiple networks

Logical addressing: e.g., IP addresses
Routing: how to get to the destination

We want to send data from a node with network address A and physical address 10, located on one LAN, to a node with a network address P and physical address 95, located on another LAN. Because the two devices are located on different networks, we cannot use physical addresses only; the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.


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