All communication, whether face-to-face or over a network, is governed by predetermined rules called protocols. These protocols are specific to the characteristics of the conversation. In our day-to-day personal communication, the rules we use to communicate over one medium, like a telephone call, are not necessarily the same as the protocols for using another medium, such as sending a letter.
Think of how many different rules or protocols govern all the different methods of communication that exist in the world today.
Successful communication between hosts on a network requires the interaction of many different protocols. A group of inter-related protocols that are necessary to perform a communication function is called a protocol suite. These protocols are implemented in software and hardware that is loaded on each host and network device.
One of the best ways to visualize how all of the protocols interact on a particular host is to view it as a stack. A protocol stack shows how the individual protocols within the suite are implemented on the host. The protocols are viewed as a layered hierarchy, with each higher level service depending on the functionality defined by the protocols shown in the lower levels. The lower layers of the stack are concerned with moving data over the network and providing services to the upper layers, which are focused on the content of the message being sent and the user interface.
Using layers to describe face-to-face communication
For example, consider two people communicating face-to-face. As the figure shows, we can use three layers to describe this activity. At the bottom layer, the Physical layer, we have two people, each with a voice that can utter words aloud. At the second layer, the Rules layer, we have an agreement to speak in a common language. At the top layer, the Content layer, we have the words actually spoken-the content of the communication.
Were we to witness this conversation, we would not actually see "layers" floating in space. It is important to understand that the use of layers is a model and, as such, it provides a way to conveniently break a complex task into parts and describe how they work.
Network Protocols
At the human level, some communication rules are formal and others are simply understood, or implicit, based on custom and practice. For devices to successfully communicate, a network protocol suite must describe precise requirements and interactions.
Networking protocol suites describe processes such as:
- The format or structure of the message
- The method by which networking devices share information about pathways with other networks
- How and when error and system messages are passed between devices
- The setup and termination of data transfer sessions
Individual protocols in a protocol suite may be vendor-specific and proprietary. Proprietary, in this context, means that one company or vendor controls the definition of the protocol and how it functions. Some proprietary protocols can be used by different organizations with permission from the owner. Others can only be implemented on equipment manufactured by the proprietary vendor.
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