16.8 APPLICATION LAYER

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The application layer provides management functions to support distributed applications such as e-mail, file transfer, remote login, and the World Wide Web. With the widespread use of TCP/IP-based networks, applications using the ISO/ OSI protocol suite are very limited. Some of the prominent application layer protocols are these:

FTAM (file transfer, access and management) for file transfer applications.
X.400 MHS (message handling systems) for electronic mail.
X.500 for directory services.

The ISO/OSI reference model is fundamental to computer communication because it serves as a reference model for all protocol suites.

The application layer provides the functionality required for specific applications such as e-mail, file transfer and remote login. A separate protocol is defined for each application. Examples are X.400 for e-mail, FTAM for file transfer and X.500 for directory services.

Note

The application layer protocols of OSI are not widely used in practical networks. However, some application layer protocols used on the Internet are derived from the OSI application layer protocols. For example, Lightweight Directory Access protocol (LDAP) is derived from X.500.

Summary

The ISO/OSI protocol architecture is presented in this chapter. This layered model has seven layers: physical layer, datalink layer, network layer, transport layer, session layer, presentation layer, and application layer. Each layer has a specific functionality. The physical layer specifies the electrical, mechanical, functional, and procedural characteristics for transmission of the bit stream. The datalink layer's job is to establish and maintain the link and disconnect after the data transfer is complete. Error control and flow control are also done in this layer. The network layer takes care of routing of the packets in the network. The transport layer provides an end-to-end reliable data transfer. Even if the packets are received out of sequence or with error, the transport layer ensures that the packets are put in sequence and all the packets are received without error by giving the information about the erroneously received packets to the sender and obtaining the packets again. The job of the session layer is to establish and maintain sessions. The presentation layer takes care of the presentation formats, including compression and encryption. The application layer provides the interface to different application programs such as e-mail, file transfer, remote login, and so on.

Two important concepts in a layered model are protocol encapsulation and peer-to-peer communication. A layer sends its data in the form of a protocol data unit (PDU) to the layer below. The lower layer adds its own header to the PDU (without making any changes to the PDU) and then sends it to the layer below. This mechanism is known as protocol encapsulation. Finally when the data stream is received at the other end, the data passes through the layers again. Each layer strips off the corresponding header and takes appropriate action based on the information in the header. For instance, the header information of the transport layer is interpreted only by the transport layer at the other end. Even though the transport layers on two machines do not talk to each other directly, the header information is interpreted only by the peer layer (layer at the same level). This is known as peer-to-peer communication.

Though there are hardly any networks that run the ISO/OSI protocol, a good understanding of this protocol is important because it acts as a reference model for many protocol stacks.

References

Dreamtech Software Team. Programming for Embedded Systems. Wiley Dreamtech India Pvt. Ltd., 2002. This book contains chapters on serial communication programming and protocol conversion software development.
Joe Campbell. C Programmer's Guide to Serial Communications (Second Edition). Prentice Hall Inc., 1997. This is an excellent book for developing a variety of applications and systems software using serial communications. A must-read for getting good expertise in serial communication programming.
A.S. Tanenbaum. Computer Networks. Prentice Hall Inc., 1997.
W. Stallings. Data and Computer Communications (Fifth Edition). Prentice Hall Inc., 1999.

Questions

Explain the ISO/OSI protocol architecture.
Explain the serial communication protocol using the RS232 standard.
Explain the HDLC protocol.
List the application layer protocols of the OSI reference model.

Exercises

1.	Standardization of various protocols by the international standards bodies is done through consensus. Everyone has to accept the proposal, then only the proposal becomes a standard. For standardizing the ISO/OSI architecture, there were two proposals—one proposal based on six layers and the other proposal based on eight layers. Finally, seven layers were accepted (just the average of six and eight, but no other reason!). Develop six layer and eight layer architectures and study the pros and cons of each.
2.	Interconnect two PCs running the Windows 9x/2000/XP operating system through RS232 and experiment on the communication parameters.
3.	Interconnect two PCs running the Linux operating system through RS232 and experiment on the communication parameters.

Answers

1.	A six-layer architecture for computer communication can be just the elimination of the session layer in the ISO/OSI architecture. Session layer functionality is minimal and can be eliminated. An eight-layer architecture can have an additional layer to provide security features that runs above the transport layer.
2.	Windows 9x/2000/XP operating systems provide the Hyper Terminal with which you can establish communication between two systems through an RS232 link.
3.	In Linux operating system, you will have access to the complete source code for serial communication. You can interconnect a Linux system and a Windows system using RS232.

Projects

Interconnect two PCs using an RS232 cable. Write a program to transfer a file from one computer to the other through the RS232 link.
Interconnect two PCs using an RS232 cable and write a program for a chat application. Make the software modular to give a layered approach to software development.
If you have a processor-based board (such as 8051 or 8085/8086) in your microprocessor laboratory, interconnect this board with a PC using an RS232 link. You need to write the software on both the PC and the processor board to achieve communication.

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