Computer Networks

A Computer Network is a set of two or more connected computers for sharing their resources e.g. CD-ROM,Files etc.These computers are connected through wires,without wires,cable lines etc. Computer network is complex system, so we divide it in layers and explain it. OSI(Open System Interconnection) is one of these layering definitions and it is developed by International Standard Organization(ISO).OSI network model has seven layers,these are-
Application Layer,Presentation Layer,Session Layer,Transport Layer,Network Layer,Data link Layer and Physical Layer.
Physical Layer:
It is responsible for transmission of data from node to node in the real physical medium e.g wire,cable,OFC cable etc.It is bottom layer of the OSI model and takes frames from data link layer as input and transmits it one bit at a time. OFC,Twisted pair cable,NIC(Network Interface Card),repeater etc. are the devices used by this layer.This layer is also responsible for feeding data to data link layer,the layer above it.
Data link layer:
It is the layer 2 of OSI model.This layer provides reliable transmission of packets over unreliable physical layer by the help of techniques like framing,acknowledgment,sequence numbering,error detection and flow control.Error detection and correction for a particular transmission is determined by the data link layer.Data link layer contains two sub layers- LLC(Logical Link Control) and MAC(Media Access Control).LLC is responsible for identifying network layer protocols and then identifying them,it also provides flow control and sequencing of control bits.MAC defines how packets are placed on media.MAC also give unique physical address for devices,as well as logical topologies. Devices like switches are data link layer device.
Network Layer:
This no.3 layer of OSI model is responsible for source to destination delivery of packets.Routing of the data packets to the destination is the main concern here.IP addressing is done in this layer.Router is a device responsible for this action.It has the information about the whole network of it,and knows how to transfer data packets to a certain IP.IP,IPX,RIP,EIGRP,OSPF are the protocols mainly used here.
Transport Layer:
This layer is responsible for host to host transfer of data.The transport layer accepts data from the session layer and segments the data for transport across the network.This layer is also responsible for error-free and sequential transfer of data.TCP and UDP are main protocols used here.
Session Layer:
This is the layer used for creation of sessions e.g logical connection between the devices.It is also responsible for creating and maintain session.During the transfer of data between two devices if the session breaks down,this layer is responsible for creating the session  again.
Presentation Layer:
This layer is concerned with the syntax and semantics of the data transferred.It's basically a translator and provides coding and conversion functions.JPEG,MIDI,MPEG,Quick Time etc serve to direct graphic and visual image presentation.
Application Layer:
This is the layer closest to the users.This is the layer user interacts with.FTP,SMTP is the main protocols used here.

OSI Model