TCP/IP is the most important model in the system structure. 
If you want to know more about howthe computer deal with the
 order which you input.You must study TCP/IP well.For example,
now we want to surf online. input the website “www.cisco.com”.
then the computer will change thewords to ASCII(just a example..) 
and send it to Application layer. Application layer receive the 
data, will add message to show where and how the data will send.
 Now the data have arrivedat transport layer, here the data will 
be made into segment and sent. When the segment reach Network layer,
it will be made into packet and  then sent to Data link layer. In the 
Data link layer,the packet become framing. Now the framing  arrive 
Physical layer. “Hi,welcome. but I will send you to the host which 
named 'www.cisco.com'“ the Physical layer said. It is right?   In fact , 
The physical layer don't  know what the framing is. He justcan send bit 
message .






  


The Model of  TCP/IP.


Application layer   the highest layer in the structure and for offerring 
                    the service between the processes which user ordered.
                    There are some protocol such as HTTP protocol ,FTP 
                   protocol, SMTP protocol ,etc..









Transport layer     for the communication between two processes in the host.
                    There are two types  protocol. one is connectionless call
                    TCP (Transmission Control Protocol)
                    and other is connection-oriented call UDP(User Datagram Protocol)
Network layer      for transmission between the different hosts in the packet network. 
                   There are two action: make into packet and  choose the suited routing.
                   There is a protocol call Internet Protocol (IP) on the layer.
Data link layer    for making into framing .
Physical layer      for sending the bit..

Why to carve up the model like this?
Maybe it can become more simple and easier to vindicate???