Modern
computer networks are designed in a highly structured way.
To reduce their design complexity, most
networks are organized as a series of layers, each one built upon
its predecessor. The OSI Reference Model is based on a proposal developed
by the International Standards Organization (ISO).
The model is called
ISO OSI (Open systems Interconnection) Reference Model because it deals
with connecting open systems - that is, systems that are open for
communication with other systems.
The OSI model has seven
layers.
|
Physical
Layer
|
Relates to the physical medium, i.e. cables, cards, mechanical interfaces
to cables. |
|
Data Link
Layer
|
Packages raw bits from the Physical layer into data frames.
-
LLC(Logical Link Control) Upper Sublayer
of the Data Link layer that manages data link communication and defines
the use of Service Access Points (SAP).
-
MAC (Media Access Control) Sublayer of
the Data Link layer that communicates with the network adapter card.
|
|
Network
Layer
|
Responsible for addressing messages and translating logical addresses
and names into physical addresses. Also determines route to the destination
computer and manages switching, routing, and data congestion. |
|
Transport
Layer
|
Ensures that the messages are delivered error free, in sequence,
and with no losses or duplications. Repackages messages for efficient transmission. |
|
Session Layer
|
Allows to computers to establish and end a connection. Controls
Security and name recognition to allow computers to communicate over a
network. Provides syncronization between user tasks, and regulates
which side transmites, when, and for how long. |
|
Presentation
Layer
|
Determines which format will be used to exchange data. Manages security
with data encryption. Provides rules for data transfer and provides data
compression to reduce the number of bits needed for transfer. |
|
Application
Layer
|
Serves as the window that application processes use to access network
services. |
|
