Network Communication Models (5.3)–Cisco Communication Principles
Network communication models help you understand the various components and protocols used in network communications. These models help you see the function of each protocol and their relationship to other protocols.
Video—Network Protocols (5.3.1)
Refer to the online course to view this video.
Video—The Protocol Stack (5.3.2)
Refer to the online course to view this video.
The Protocol Stack (5.3.3)
Successful communication between hosts requires interaction between many protocols. These protocols are implemented in software and hardware that are installed on each host and networking device.
The interaction between the different protocols on a device can be illustrated as a protocol stack, as shown in Figure 5-6. A stack illustrates the protocols as a layered hierarchy, with each higher-level protocol depending on the services of the protocols shown in the lower levels.
Figure 5-6 A Protocol Stack for Internet Communications
The separation of functions enables each layer in the stack to operate independently of others. For example, you can use your laptop computer connected to a cable modem at home to access your favorite website, or you can view the same website on your laptop using a wireless connection at the library. The function of the web browser is not affected by the change in the physical location or the method of connectivity.
The protocols in Figure 5-6 are described as follows:
- Hypertext Transfer Protocol (HTTP)—This protocol governs the way a web server and a web client interact. HTTP defines the content and formatting of the requests and responses that are exchanged between the client and server. Both the client and the web server software implement HTTP as part of the application. HTTP relies on other protocols to govern how the messages are transported between the client and server.
- Transmission Control Protocol (TCP)—This protocol manages the individual conversations. TCP is responsible for guaranteeing the reliable delivery of the information and managing flow control between the end devices.
- Internet Protocol (IP)—This protocol is responsible for delivering messages from the sender to the receiver. IP is used by routers to forward the messages across multiple networks.
- Ethernet—This protocol is responsible for the delivery of messages from one NIC to another NIC on the same Ethernet local-area network (LAN).
The TCP/IP Model (5.3.4)
Layered models help you visualize how the various protocols work together to enable network communications. A layered model depicts the operation of the protocols occurring within each layer, as well as the interaction with the layers above and below it. The layered model has many benefits:
- Assists in protocol design, because protocols that operate at a specific layer have defined information that they act upon and a defined interface to the layers above and below
- Fosters competition because products from different vendors can work together
- Enables technology changes to occur at one level without affecting the other levels
- Provides a common language to describe networking functions and capabilities
The first layered model for internetwork communications was created in the early 1970s and is referred to as the Internet model. It defines four categories of functions that must occur in order for communications to be successful. The suite of TCP/IP protocols that are used for Internet communications follows the structure of this model, as shown in Table 5-2. Because of this, the Internet model is commonly referred to as the TCP/IP model.
Table 5-2 The Layers of the TCP/IP Model
| TCP/IP Model Layer | Description |
| Application | Represents data to the user, plus encoding and dialogue control |
| Transport | Supports communication between various devices across diverse networks |
| Internet | Determines the best path through the network |
| Network Access | Controls the hardware devices and media that make up the network |
The OSI Reference Model (5.3.5)
Two basic types of models are used to describe the functions that must occur in order for network communications to be successful: protocol models and reference models.
- Protocol model—This model closely matches the structure of a particular protocol suite. A protocol suite includes the set of related protocols that typically provide all the functionality required for people to communicate with the data network. The TCP/IP model is a protocol model because it describes the functions that occur at each layer of protocols within the TCP/IP suite.
- Reference model—This type of model describes the functions that must be completed at a particular layer but does not specify exactly how a function should be accomplished. A reference model is not intended to provide a sufficient level of detail to define precisely how each protocol should work at each layer. The primary purpose of a reference model is to aid in clearer understanding of the functions and processes necessary for network communications.
The most widely known internetwork reference model was created by the Open Systems Interconnection (OSI) project at the International Organization for Standardization (ISO). It is used for data network design, operation specifications, and troubleshooting. This model is commonly referred to as the OSI model. The OSI layers are described in Table 5-3.