OSI LAYER - CCNA

OSI ( open source interconnection )

why do we go for OSI ?

* standardization

* interoperability

--> here two different vendor system communicates with each other

--> ISO developed OSI

for example: there are two systems and each have its own application on it.when both the system wants to communicate, there are 7 layers which it should go across it..

they are:

1.Application Layer

2.Presentation Layer

3.Session Layer

4.Transport Layer

5.Network Layer

6.Datalink Layer

7.Physical Layer

where,

* 1 - 3 are called as upper layers or software layers

* 4 is the heart of the OSI models

* 5 - 7 are called as lower layers or hardware layers

PHYSICAL LAYER:

for two different system to communicate, we need a physical layer connection.

* physical layer is responsible for transmitting bit stream over transmission media.

TOPOLOGY:

there are four types:

* Bus

* Mesh

* Star

* Ring

BUS:

here all the systems are connected to one common backbone cable bandwidth.

MESH:

here each system maintains a dedicated link to reach other system in a network.

STAR:

here all the systems are connected to one central device.

RING:

here all the systems are connected in the form of ring.

TRANSMISSION MODES:

there are 3 types:

* simple duplex

* half duplex

* dull duplex

SIMPLE DUPLEX:

in this mode, only one way communication is possible.

(ie) one can send the information and other can only receive it.

HALF DUPLEX:

in this mode, two way communication is possible.but only one can send at a time.

FULL DUPLEX:

in this mode, two way simultaneous communication is possible which means both can send & receive it at a time.

Different types of cables:

* coaxial cable

* twisted pair

COAXIAL CABLE:

it is used for only half duplex communication because it is having only one copper conductor.

TWISTED PAIR CABLE:

there are two pairs in it:

--> one pair is used to send the signal and the other pair is to receive the signal.

--> twisted pair supports only full duplex.

* physical layer also define sthe rate of speed.

DATALINK LAYER:

* responsible for delivering frames from one system to another.

* it defines about the netwrok.

MAC ( MEDIA ACCESS CONTROL ) :

* it is represented in hexa decimal format.

NETWORK LAYER:

* delivering packets from one network to another network across multiple links.

* logical addressing ( IP address ).

* best path determination.

* routing

* repeaters

ROUTERS: it maintains routing table for forwarding decision.

TRANSPORT LAYER:

* it is responsible for end to end communication.

* its function is segmentation & re-assembling.

* to identify segments in network, sequence numbers are used.

* before exchanging any data, sequence number synchronization should take place.

SESSION LAYER:

* it is used to establish, maintain & also to termionate the sessions.

PRESENTATION LAYER:

* it says how to present the data in the network.

APPLICATION LAYER:

* it acts as an interfacebetween user & application.

* it creates an environment for communication.

various application layers are:

HTTP - web browsing

FTP - file transfer

SMTP - mail transfer

SNMP - network management

TELNET - remote access

TFTP - configurating file transfer