Overview of TCP/IP Model
In 1970, Defense Advanced Research Projects Agency (DARPA) of U.S. provides TCP/IP model for internet communication prior to the OSI model (1978).
United States Department of Defense (USDOD) needed a standard model by which its computers can exchange data by understanding each other. So, DARPA, an agency of USDOD, created TCP/IP model for defense computer communication. Later on, uses of this model do not stick to only defense computer networks as wide-spreading use of internet starts all over the world.
TCP/IP model describes that data should be transmitted over internet by 4 layers compared to 7 layers in OSI model (hyperlink). These Layers are:
- Application Layer
- Transport Layer
- Internet Layer (Network Layer)
- Link Layer
TCP/IP combines the presentation and session layer issues into its application layer. It also combines OSI data link and Physical layer into Link Layer.
Let’s understand these four layers with a real-life example. You are living in UK. You write a letter to your friend living in US. None of these two friends care about how the letter will be sent and received. You and your friend are working in Application Layer……communicating with each other who don’t care about delivery mechanism. In telephony world, VoIP software installed in a computer or server works in Application Layer.
If you write the wrong address of your friend on envelop. The postal employee in US will stamp it as ‘address not found’ and send it back to you if it was a registered or priority letter (TCP). In VoIP communication transport layer programmed in software (like softswitch) handles this in transport layer.
This layer uses two protocols TCP and UDP. With TCP protocol the softswitch will monitor the data transmission between it and Media Gateway. It will repeatedly try to send the portion of data if it does not move from MGC to MG; if it fails then it will give a failure notice. Email delivery uses TCP in transport layer as it needs reliable delivery. But, the scenario is not practical in voice data delivery.
If TCP sends lost data packages repeatedly, then your earlier conversions may received by your friend after later conversions which is not desirable in a typical telephone conversation. That’s why VoIP communication uses UDP protocol in Transport layer because UDP never deals with lost data, once it is lost it is lost for ever (like a postage of normal letter)
An Airplane has to fly your letter to US. The pilot does not care about Gordon Brown or Prince Charles; who writes those latter. He only cares about all those packets are addressed to US and his responsibility is to carry those letters to US central postal delivery office. This airplane is working in Internet Layer.
In VoIP communication, this address is written as IP address (Give hyperlink, discuss about IP). Internet layer send the voice packets to the network node closer to the final destination.
Now, your letter has reached US. It needs to be moved within US by trucks or vehicles to reach your friends home. These delivery trucks are working in link layer.
Link layer works in the local network with which the host is attached. TCP/IP is designed to be hardware independent; it does not have physical layer like OSI model. That means the postman do not care who received the letter after he drops it in the postbox of your friend’s home. That’s why Link layer is the lowest layer in TCP/IP model.