different layers of osi model.doc

1. Explain Explain Difer Diferent ent layers layers o OSI model? model?  The OSI model is a layered layered ramework or the design o network systems that allows allows commn commnica icatio tion n !etwe !etween en all types types o compt compter er system systems. s. It consists o se"en separate !t related layers# each o which de$nes a part o the proce process ss o mo"ing mo"ing inor inormat mation ion across across a networ network. k. The difer diferent ent layers in OSI model are represented represented in the ollowing $gre. 7

 

6 5 4 3 2 1

 

%pplication &resentation Session Transport 'etwork Data link &hysical

1. Physical Layer:  The physical layer coordinates the nctions re(ired to car carry a !it !it str stream eam o"er o"er a phys physic ical al medi medim m.. It deal deals s with with the the mecha echani nica call and and ele electr ctrical cal sp spec eci$ i$c catio ations ns o the int intera erace ce and and transmission medim. It also de$nes the procedres and nctions that physical de"ices and interaces ha"e to perorm or transmission to Occr. )igre shows the position o the physical layer with respect to the transmission medim and the data link layer. layer.

 The physical layer is also concerned concerned with the ollowing* ollowing* Physical character characteristi istics cs of interface interfaces s and medium: medium: The a. Physical physical layer de$nes the characteristics o the interace !etween the de"ices and the transmission medim. It also de$nes the type o transmission medim. !. Representation of bits* The physical layer data consists o a stream o !its +se(ence o Os or 1s, with no interpretation. To !e transmitted# !its mst !e encoded into signals--electrical or

optical. The physical layer de$nes the type o encoding +how Os and I s are changed to signals,.

Data rate:  The transmission rate-the nm!er o !its sent c. each second-is also de$ned !y the physical layer. d. ynchroni!ation of bits:  The sender and recei"er not only mst se the same !it rate !t also mst !e synchronied at the !it le"el. In other words# the sender and the recei"er clocks mst !e synchronied. e.Line con"#uration: The physical layer is concerned with the connection o de"ices to the media. In a point-to-point con$gration# two de"ices are connected throgh a dedicated link. In a mltipoint con$gration# a link is shared among se"eral de"ices. . Physical topolo#y:  The physical topology de$nes how de"ices are connected to make a network. De"ices can !e connected !y sing a mesh topology +e"ery de"ice is connected to e"ery other de"ice,# a star topology +de"ices are connected throgh a central de"ice,# a ring topology +each de"ice is connected to the next# orming a ring,# a !s topology +e"ery de"ice is on a common link,# or a hy!rid topology +this is a com!ination o two or more topologies,. g. $ransmission mode: The physical layer also de$nes the direction o transmission !etween two de"ices* simplex# haldplex# or ll-dplex. /. Data Lin% Layer:  The data link layer is responsi!le or mo"ing rames rom one hop +node, to the next. It makes the physical layer appear error-ree to the pper layer +network layer,. The ollowing )igre shows the relationship o the data link layer to the network and physical layers.

Other responsi!ilities o the data link layer inclde the ollowing* a. &ramin#:  The data link layer di"ides the stream o !its recei"ed rom the network layer into managea!le data nits called rames. !. Physical addressin#: I rames are to !e distri!ted to diferent systems on the network# the data link layer adds a header to the rame to de$ne the sender and0or recei"er o the rame. I the rame is intended or a system otside the senders network# the recei"er address is the address o the de"ice that connects the network to the next one. c. &lo' control: I the rate at which the data are a!sor!ed !y the recei"er is less than the rate at which data are prodced in the

sender# the data link layer imposes a 2ow control mechanism to a"oid o"erwhelming the recei"er. d. (rror control:  The data link layer adds relia!ility to the physical layer !y adding mechanisms to detect and retransmit damaged or lost rames. It also ses a

mechanism to recognie dplicate rames. Error control is normally achie"ed throgh a trailer added to the end o the rame. e. )ccess control* 3hen two or more de"ices are connected to the same link# data link layer protocols are necessary to determine which de"ice has control o"er the link at any gi"en time. 4. *et'or% Layer:  The network layer is responsi!le or the deli"ery o  indi"idal packets rom the sorce host to the destination host possi!ly across mltiple networks +links,. 3hereas the data link layer o"ersees the deli"ery o the packet !etween two systems on the same network +links,# the network layer ensres that each packet gets rom its point o origin to its $nal destination. I two systems are connected to the same link# there is sally no need or a network layer. The ollowing shows the relationship o the network layer to the data link and transport layers.

Other responsi!ilities o the network layer inclde the ollowing* a. Lo#ical addressin#:  The physical addressing implemented !y the data link layer handles the addressing pro!lem locally. I a packet passes the network !ondary# we need another addressing system to help distingish the sorce and destination systems. The network layer adds a header to the packet coming rom the pper layer that# among other things# incldes the logical addresses o the sender and recei"er. !. Routin#: 3hen independent networks or links are connected to create internetworks +network o networks, or a large network# the connecting de"ices +called routers or switches) rote or switch the packets to their $nal destination. One o the nctions o the network layer is to pro"ide this mechanism. 4. $ransport Layer:  The transport layer is responsi!le or process-toprocess deli"ery o the entire message. % process is an application program rnning on a host. 3hereas the network layer o"ersees sorce-to-destination deli"ery o indi"idal packets# it does not recognie any relationship !etween those packets. It treats each one independently# as thogh each piece !elonged to a separate message# whether or not it does. The transport layer# on the other hand# ensres that the whole message arri"es intact and in order# o"erseeing !oth

error control and 2ow control at the sorce-to-destination le"el. The following Figure shows the relationship of the transport layer to the network and session layers.

Other responsi!ilities o the transport layer inclde the ollowing* a. er+ice,point addressin#: 5ompters oten rn se"eral programs at the same time. )or this reason# sorce-to-destination deli"ery means deli"ery not only rom one compter to the next !t also rom a speci$c process +rnning program, on one compter to a speci$c process +rnning program, on the other. The transport layer header mst thereore inclde a type o address called a service point address +or port address,. The network layer gets each packet to the correct compter6 the transport layer gets the entire message to the correct process on that compter. !. e#mentation and reassembly: % message is di"ided into transmitta!le segments# with each segment containing a se(ence nm!er. These nm!ers ena!le the transport layer to reassem!le the message correctly pon arri"ing at the destination and to identiy and replace packets that were lost in transmission. c. -onnection control:  The transport layer can !e either connectionless or connection oriented. % connectionless transport layer treats each segment as an independent packet and deli"ers it to the transport layer at the destination machine. % connection oriented transport layer makes a connection with the transport layer at the destination machine $rst !eore deli"ering the packets. %ter all the data are transerred# the connection is terminated. d. &lo' control: 7ike the data link layer# the transport layer is responsi!le or 2ow control. 8owe"er# 2ow control at this layer is perormed end to end rather than across a single link. e. (rror control: 7ike the data link layer# the transport layer is responsi!le or error control. 8owe"er# error control at this layer is perormed process-to-process rather than across a single lin%: The sending transport layer makes sre that the entire message arri"es at the recei"ing transport layer withot error +damage# loss# or dplication,. Error correction is sally achie"ed throgh

retransmission

9. ession Layer:  The ser"ices pro"ided !y the $rst three layers +physical# data link# and network, are not s:cient or some processes.  The session layer is the network dialog controller. It esta!lishes# maintains# and synchronies the interaction among commnicating systems. Speci$c responsi!ilities o the session layer inclde the ollowing* a. Dialo# control:  The session layer allows two systems to enter into a dialog. It allows the commnication !etween two processes to take place in either hal dplex +one way at a time, or ll-dplex +two ways at a time, mode. !. ynchroni!ation:  The session layer allows a process to add checkpoints# or synchroniation points# to a stream o data. )or example# i a system is sending a $le o /;;; pages# it is ad"isa!le to insert checkpoints ater e"ery 1;; pages to ensre that each 1;;-page nit is recei"ed and acknowledged independently. In this case# i a crash happens dring the transmission o page 9/4# the only pages that need to !e resent ater system reco"ery are pages 9;1 to 9/4. &ages pre"ios to 9;1 need not !e resent. The ollowing )igre illstrates the relationship o the session layer to the transport and presentation layers.