Why layered protocols

With connectionless protocols, no setup in advance is needed. The sender just transmits the first message when it is ready. Dropping a letter in a mailbox is an example of connectionless communication. With computers, both connection-oriented and connectionless communication are common.

In the OSI model, communication is divided up into seven levels or layers, as shown in Fig. Each layer deals with one specific aspect of the communication. In this way, the problem can be divided up into manageable pieces, each of which can be solved independent of the others. Each layer provides an interface to the one above it.

The interface consists of a set of operations that together define the service the layer is prepared to offer its users. In the OSI model, when process A on machine 1 wants to communicate with process B on machine 2, it builds a message and passes the message to the application layer on its machine.

This layer might be a library procedure, for example, but it could also be implemented in some other way e. The presentation layer in turn adds its own header and passes the result down to the session layer, and so on. Some layers add not only a header to the front, but also a trailer to the end. When it hits bottom, the physical layer actually transmits the message, which by now might look as shown in Fig.

When the message arrives at machine 2, it is passed upward, with each layer stripping off and examining its own header. Finally, the message arrives at the receiver, process B, which may reply to it using the reverse path. The information in the layer n header is used for the layer n protocol. As an example of why layered protocols are important, consider communication between two companies, Zippy Airlines and its caterer, Mushy Meals, Inc.

Every month, the head of passenger service at Zippy asks her secretary to contact the sales manager's secretary at Mushy to order , boxes of rubber chicken. Traditionally, the orders have gone via the post office. However, as the postal service deteriorates, at some point the two secretaries decide to abandon it and communicate by FAX. They can do this without bothering their bosses, since their protocol deals with the physical transmission of the orders, not their contents.

Similarly, the head of passenger service can decide to drop the rubber chicken and go for Mushy's new special, prime rib of goat, without that decision affecting the secretaries. These range from the specification of connectors, addresses of the communications nodes, identification of interfaces, options, flow control, reliability, error reporting, synchronisation, etc. In practice there are so many different functions, that a set also known as suite or stack of protocols are usually defined.

Each protocol in the suite handles one specific aspect of the communication. The protocols are usually structured together to form a layered design also known as a "protocol stack". All major telecommunication network architectures currently used or being developed use layered protocol architectures. The precise functions in each layer vary.

In each case, however, there is a distinction between the functions of the lower network layers, which are primarily designed to provide a connection or path between users to hide details of underlying communications facilities, and the upper or higher layers, which ensure data exchanged are in correct and understandable form. The highest layers are sometimes known as "middleware" because they provide software in the computer which convert data between what the applications programs expect, and what the network can transport.

Devices from different technology generations can co-exist thus the older units do not get discarded immediately newer technologies are adopted. Scalability — Experience has shown that a layered or hierarchal approach to networking protocol design and implementation scales better than the horizontal approach.

Mobility — Greater mobility is more readily delivered whenever we adopt the layered and segmented strategies into our architectural design.

Value Added Features — It is far easier to incorporate and implement value added features into products or services when the entire system has been built on the use of a layered philosophy.

Cost Effective Quality — The layered approach has proven time and time again to be the most economical way of developing and implementing any system s be they small, simple, large or complex makes no difference. This ease of development and implementation translates to greater efficiency and effectiveness which in turn translates into greater economic rationalization and cheaper products while not compromising quality. Modularity — I am sure that you have come across plug-ins and add-ons.

These are common and classical examples of the benefits to be derived from the use of a hierarchal layered approach to design. The Graduated, Blended Approach to Migration — Compatibility enables technologies to co-exist side-by-side which results in quicker uptake of newer technologies as the older asset investments can still continue to be productive.

Thus migration to newer technologies and standards can be undertaken in stages or phases over a period of time. This is what is known as the graduated blended approach; which is the opposite of the sudden adoption approach. This is due to the clearer and more distinct definition and demarcation of what functions occur at each layer when the layered approach is taken.

Task Segmentation — Breaking a large complex system into smaller more manageable subcomponents allows for easier development and implementation of new technologies; as well as facilitating human comprehension of what may be very diverse and complex systems.

Portability — Layered networking protocols are much easier to port from one system or architecture to another.