TCP/IP Model & Comparison with OSI Reference Model.

The Internet runs over TCP/IP, The Transmission Control Protocol/Internet Protocol, which is actually a suite of protocols, each performing a particular role to let computers speak the same language. TCP/IP was designed by the Defense Advanced Research Projects Agency (DARPA) in the 1970s—the design goal being to let dissimilar computers freely communicate, regardless of location. Most early TCP/IP work was done on Unix computers and now, TCP/IP is the de facto standard that unifies the Internet.

The OSI Reference Model and TCP/IP protocol suites were developed around the same time, and both of them had a model describing network communication. The OSI model is primarily used because of the amount of detail it provides. TCP/IP Model has four abstract layers, and focuses on layers 3 and 4 of the OSI reference model; the TCP/IP model groups the top three OSI layers into a single “Application” layer. This is because these functions typically occur before the data leaves the application itself. Also, because the Data Link and Physical layers of the OSI model are so closely related together, the TCP/IP model groups them into a single “Network Interface” layer. TCP/IP’s goal is to move messages through virtually any LAN product to set up a connection running virtually any network application. 

The graphic below compares both network models.

TCP/IP’s four abstract layers include:

Network interface: This allows TCP/IP to interact with all modern network technologies by complying with the OSI model.

• Internet: This defines how IP directs messages through routers over internetworks such as the Internet.
• Transport: This defines the mechanics of how messages are exchanged between computers.
• Application: This defines network applications to perform tasks such as file transfer, e-mail, and other useful functions.

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The Layered Architecture & Reference Models.

A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for an effective communication and divides them into logical groupings called layers. When a communication system is designed in this manner, it is known as a hierarchical or layered architecture. The most common Internetworking Models are OSI Reference Model, TCP/IP Model, and DoD Model.

Below are the advantages of Reference Models / layered architecture:

• Reduces complexity and accelerates evolution. A vendor may concentrate its research and development works on a single layer without worrying the details of other layers, because changes made in one layer will not affect other layers.
• Ensures interoperability among multiple vendors’ products, as vendors develop and manufacture their products based on open standards.

Explanation: It divides the network communication process into smaller and simpler components, facilitating component development, design, and troubleshooting.
It allows multiple-vendor development through the standardization of network components. It encourages industry standardization by clearly defining what functions occur at each layer of the model. It allows various types of network hardware and software to communicate. It prevents changes in one layer from affecting other layers to expedite development.

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The OSI Reference Model.

The International Organization for Standardization (ISO), an international engineering organization based in Paris, first published the Open Systems Interconnection (OSI) reference model in 1978. This seven-layer model has become the standard for designing communication methods among network devices since than.

The OSI Reference Model (Open System Interconnection Reference Model) is used as an excellent way to describe and fully understand network communication. The OSI model divides networks into seven functional layers; each of which describes a  specific function or set of functions (specific aspects of network communication) performed when data is transferred between applications across the network and therefore is often called the Seven-LayerSstack. (The OSI Model is a logical model, not a physical one).

In order to memorize the layers and their order, there are two handy memorization tips or acronyms that you can use to remember the layers:

• All People Seem To Need Data Processing, where each word contains the first letter of the layers from the top-down, or you can use 
• Please Do Not Throw Sausage Pizza Away, where each word contains the first letter of the layers from the bottom-up.

The OSI model has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other as well as with users. The bottom four layers define how data is transmitted end to end. 

Below describes the roles and functions of every layer in the OSI reference model:

• Application Layer: This layer interfaces directly with the network-aware applications, giving it access to network resources. Without this layer, no user application would be able to get access to the network.
• Presentation Layer: Encodes the data being sent or received into a generic format that will be understood by both devices. For example, a web browser might receive data in HTML format or a picture in JPG format, which are generic and well understood standards.
• Session Layer: Begins, ends, and manages the sessions between devices.
• Transport Layer: Handles the reliability of the connection and logical separation of applications. For example, if a computer is surfing the Internet with a web browser and at the same time listening to Internet-radio, this layer ensures the correct data arrives to the correct application. In addition, this layer handles flow-control (ensuring one side does not send information faster than the other can receive) and data integrity (ensuring the data is not corrupt). The most common Transport Layer protocol is TCP.
• Network Layer: Provides logical addressing services allowing a device to dictate the source and destination address used for end-to-end communication. This layer is also responsible for routing the packet from its source to its destination. The most common Network layer protocol is IP.
• Data Link Layer: Provides physical addressing services allowing a device to dictate the source and destination address used for local network communication. This layer permits communication between devices connected to the same network. This layer is also responsible for error detection (not correction).
• Physical Layer: Defines the physical standards used for network communication.

It is important to note here that, peer layers form protocol-independent virtual links; each layer in the stack relies on the layers above and below it to operate, yet each operates independently of the others, as if it were having an exclusive conversation with its counterpart layer on the other computer. Each layer on the device is said to have established a virtual link with the same layer on the other device. With all seven virtual links running, a network connection is established, and the two devices are talking as if they were wired directly together.

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What is a Network? & What are the Core Components of a Network?

"Collection of devices connected together to communicate and share resources is called a Network." or "The fabric that ties business applications (Web Browsers, FTP, Database Applications, Email, Instant Messengers, Online Gaming, Video conferencing/Streaming etc.,) together is called a Network".

The goal of a network is to establish communications and share resources throughout an organization or over the internet. Let’s take a look at some of the core building blocks (components) that make this communication possible:

• Personal Computers (PCs) and Servers: These devices serve as the endpoints in the network and are responsible for sending and receiving data to and from the network.
• Network Connections: You must have a way to attach a device to the network; this building block includes the network interface card (NIC), cabling and connectors.
• Hubs and Switches: These devices provide points on which all the end systems of a network can attach.
• Routers: Routers connect multiple networks together and find the best way to reach each network.
• WLAN devices: These devices connect wireless devices such as computers, printers, and tablets to the network. Since pretty much every device manufactured today has a wireless NIC, you just need to configure a basic access point (AP) to connect to a traditional wired network.
• Access Points or APs: These devices allow wireless devices to connect to a wired network and extend a collision domain from a switch. An AP can be a simple standalone device, but today they are usually managed by wireless controllers either in house or through the internet.
• WLAN Controllers: These are the devices that network administrators or network operations centers use to manage access points in medium to large to extremely large quantities. The WLAN controller automatically handles the configuration of wireless access points and was typically used only in larger enterprise systems.
• Firewalls & Intrusion Prevention Systems: These devices are network security systems that monitor and control the incoming and outgoing network traffic based on predetermined security rules, and is usually an Intrusion Detection & Protection System (IDS/IPS). Cisco PIX firewall and Cisco Adaptive Security Appliance (ASA) firewall typically establish a barrier between a trusted, secure internal network and the Internet, which is not secure or trusted.

These components can build a network within a local area (LAN) or across a wide area (WAN).

Interpretation of a Network Diagram or Placement of Core Network Components:

The following figures show placement of each of the core network components in a network topology:

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