why osi layer is needed n networking????

Answer 1

Pretty simple really. It allows programmers and hardware manufacturers to focus on a single layer or two at most.

For example. Lets say that you manufacture network cards. The OSI layer model allows you to focus on layers 1 and maybe 2. You're not concerned with the protocols that will be used or the applications that the user will run. Those don't matter to you. Without the OSI model you'd theoretically need to write drivers for every possible combaination of your hardware, network protocol, and application that a user might need. Obviously that would be extremely difficult if not impossible.

Answer 2

OSI is an ABSTRACT description of all the functions needed to support communication between devices. ANY connection between hosts will have software or hardware that will perform all functions described for layers 1-7.
In the real world, we normally use twisted pair for layer 1, Ethernet for layer 2, then TCP/IP for everything else. Because the TCP/IP protocol stack was developed before OSI, it has protocols that perform all layer 4-7 functions. IF you must, you may say IP is layer 3, TCP is layer 4+5 and all TCP applications are layer 6-7.
BUT, the point is that all OSI functions will have to be performed somewhere, even if you are using a protocol stack that disregards the OSI layer system completely.

Answer 3

Layer 3 is the Network layer.