2. The OSI Model and the TCP/IP Protocol Suite
The layered model that dominated data communication and networking literature before 1990 was the Open Systems Interconnection (OSI) model. Everyone believed that the OSI model would became the ultimate standard for data communications-- but this did not happen. The TCP/IP Protocol suite became the dominant commercial architecture, because it was used and tested extensively in the Internet; the OSI model was never fully implemented.
In this chapter, we first briefly discuss the OSI model and then we concentrate on TCP/IP as a protocol suite.
OBJECTIVES
The chapter has several objectives:
- To discuss the idea of multiple layering in data communication and networking and interrelationship between layers.
- To discuss and the OSI model and its layer architecture and to show the interface between the layers.
- To briefly discuss the functions of each layer in the OSI model.
- To introduce the TCP/IP protocol suite and compare its layers with the ones in the OSI model.
- To show the functionality of each layer in the TCP/IP protocol with some example.
- To discuss the addressing mechanism used in some layers of the TCP/IP protocol suite for the delivery of a message from the source to destination.
2.1 Protocol Layers
In Chapter 1, we discussed that a protocol is required when two entities need to communicate. When communication is not simple, we may divide the complex task of communication into several layers. In this case, we may several protocols, one for each layer.
Let us use a scenario in communication in which the role of protocol layering may be better understood. We use two examples. In the first example, communication is so simple that it can occur in only one layer. In the second example, we need three layers.
Example 2.1
Assume Maria and Ann are neighbors with a lot of common ideas. However, Maria speeks only Spanish, and Ann speeks only English. Since both have learned the sign language in their childhood, they enjoy meeting in a cafe a couple of days per week and exchange their ideas using signs. Occasionally, they also use a bilingual dictionary. Communication is face to face and happens in one layer as shown in Figure 2.1.
Example 2.2
Now assume that Ann has to move to another town because of her job. Before she moves, the two meet for the last time in the same cafe. Although both are sad, Maria surprises Ann when she open a packet that contains two samll machines. The first machine can scan and transform a letter in English to a secret code or vice versa. The other machine can scan and translate a letter in Spanish to the same code or vice versa. Ann takes the first machine; ,Maria keeps the second one. The two friends can still communicate using the secret code, as shown in Figure 2.2.
Communication between Maria and Ann happens as follows. At the third layer, Maria writes a letter in spanish, the language she is comfortable with. She then uses the translator machine that scans the letter and creates a letter in the secret code. Maria then puts the letter in an envelop and drops it to the post office box.
