When you move into the realm of Cisco networking, you have entered a world of building the roads that makes business possible. Most of the time, users and other network administrators take these roads for granted, just like you take them for granted when you drive a car. You simply assume that the roads will be there and that they’ll carry you through to your destination. However, a poorly timed construction project (network maintenance) or unscheduled road closure (network outage) will bring the entire infrastructure crumbling down. The goal of a network is to establish communications throughout an organization. Let’s take a look at the core building blocks 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.

These components can build a network within a local area (LAN) or across a wide area (WAN). In recent
years, the lines between a LAN and WAN have begun to blur, since Wireless and Fiber Optic technology
can extend the reach of a LAN much further than older technology ever could. Regardless, the following
definitions still stand strong:

• Local Area Network (LAN) –  a computer network covering a small geographic area such as a
  home, office or group of buildings.
• Wide Area Network (WAN) – a computer network covering a large geographic area such as a city, state, nation or globe.

Interpreting a Network Diagram

The following figure shows the placement of each of the core network components:

Interpreting a Network Diagram

Types of Network Communication
Not that long ago, network communication was solely restricted to data: internal corporate data or external
Internet data. Nowadays, the network has evolved to support all types of communication. Organizations
have begun to merge their telephone system with the network, creating Voice over IP (VoIP)
network traffic. Users have begun to mount video cameras on their computers and in conference rooms to
create streaming video traffic. What’s more, the network has begun to become so entirely saturated with
different application types, network administrators now require a way to divide the different traffic types
into application classes, some of which are far more important than others. Because of this, all of Cisco’s
newer equipment supports Quality of Service (QoS) features allowing you to manage the priorities of data
crossing the network. For example, at a major bottleneck in the network (such as the transition from the
high-speed LAN to the low-speed WAN), you could set up a system stating that the VoIP traffic is sent first,
followed by the streaming video, followed by the business-critical applications and so on.

Using the OSI Model

With all of this network communication occurring, trying to understand the network can become very complex,
very quickly. In order to try to make sense of it all, you can use the handy OSI Model. It’s a little known
fact that the OSI Model was never meant to be just a model that describes network communication. There
is also an OSI protocol (technically called the “OSI Networking Suite”) that was designed to compete with
TCP/IP. We now know the end of the story: TCP/IP wins. However, the OSI Model is still used today as an excellent
way to describe and fully understand network communication. You will find that a deep understanding
of the OSI Model is critical to your networking success in the Cisco world. This is not one of those models that
you learn in order to pass the exam and then never use again. With that foundation, let’s begin.
The Layers of the OSI Model

The OSI Model is comprised of 7 layers, each of which describes a specific aspect of network communication:

OSI Layer

Once you’ve got the layers down, you now need to know what each of them accomplishes. I’ll present this to you in two ways. First, we’ll look at the cold, hard facts about each layer, and then we’ll look at a practical example of how the OSI Model is used in real-world network communication. So, here are the facts:
Application Layer: This layer interfaces directly with the network-aware application, 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.
Physical Layer: Defines the physical standards used for network communication.

(copy from : ) PrepLogic CCNA

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