LESSON 1
What is a network?
A computer network can be described as a system of interconnected devices that can communicate using some common standard (called protocol). These devices communicate to exchange resources (e.g. files and printers) and services.
Here is an example network consisting of two computers connected together:
in the example above, the two computers are directly connected using a cable. This small network can be used to exchange data between just these two computers.
What if we want to expand our network? Then we can use a network device, either a switch or a hub, to connect more than two computers together:
 |
| A network with a hub |
Now all of the devices on the network can communicate with each other.
We’ll talk more about hubs and switches in just a moment. For now, just remember that these devices serve as a central point to which all of the computers connect to.
LESSON 2
OSI & TCP/IP Models
OSI Model
OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO), an international standard-setting body. It was designed to be a reference model for describing the functions of a communication system. It has seven layers, with each layer describing a different function of data traveling through a network.
Here is the graphical representation of these layers:
| Application |
| Presentation |
| Session |
| Transport |
| Network |
| Data Link |
| Physical |
The layers are usually numbered from the last one, meaning that the Physical layer is consider to be the first layer. It is good to learn these layers, since there will certainly be a couple of questions on the CCNA exam regarding them. Most people learn the mnemonic "Please Do Not Throw Sausage Pizza Away":
| Application | Away |
| Presentation | Pizza |
| Session | Sausage |
| Transport | Throw |
| Network | Not |
| Data Link | Do |
| Physical | Please |
So, what is the purpose of these layers? They are most commonly used by vendors. They enable them to implement some functionality into a networking device, which then enables easier interoperability with devices from other vendors.
Here is a brief description of each of these layers.
Physical – defines how to move bits from one device to another. It details how cables, connectors and network interface cards are going to work and how to send and receive bits.
Data Link – encapsulates a packet in a frame. A frame contains a header and a trailer that enable devices to communicate. A header, most commonly, contains a source and a destination MAC address. A trailer contains the Frame Check Sequence field, which is used to detect transmission errors.
The data link layer has two sublayers:
1. Logical Link Control – used for flow control and error detection
2. Media Access Control – used for hardware addressing and controlling the access method
Network – defines device addressing, routing, and path determination. Device (logical) addressing is used to identify a host on a network (e.g. by its IP address).
Transport – segments great chunks of data received from the upper layer protocols. Establishes and terminates connections between two computers. Used for flow control and data recovery.
Session – defines how to establish and terminate a session between the two systems.
Presentation – defines data formats. Compression and encryption are defined at this layer.
Application – this layer is the closest to the user. It enables network applications to communicate with other network applications.
The following table shows which protocols reside on which layer:
| Application | HTTP |
| Presentation | MIME |
| Session | SSL, NetBIOS |
| Transport | TCP, UDP |
| Network | IP, ICMP |
| Data Link | PPP, HDLC |
| Physical | Ethernet |
TCP/IP Model
The TCP/IP model was created in the 1970s by the Defense Advance Research Project Agency (DARPA). Like the OSI model, it describes general guidelines for designing and implementing computer protocols. It consists of four layers: Network Access, Internet, Transport, and Application.
| Application |
| Transport |
| Internet |
| Net Access |
The following picture show the comparison between the TCP/IP model and OSI model:
| TCP/IP Model | OSI Model |
|---|---|
| Application | Application |
| Presentation | |
| Session | |
| Transport | Transport |
| Internet | Network |
| Network Access | Data Link |
| Physical |
As you can see, the TCP/IP model has fewer layers than the OSI model. The Application, Presentation, and Session layers of the OSI model are merged in only one layer, Application layer, in the TCP/IP model. Also, Physical and Data Link layers are called Network Access layer in the TCP/IP model.
Differences between OSI and TCP/IP Model
There are some other differences between these two models, besides the obvious difference in the number of layers. OSI model prescribes the steps needed to transfer data over a network and it is very specific in it, defining which protocol is used at each layer and how. The TCP/IP model is not that specific. It can be said that the OSI model prescribes and TCP/IP model describes.
LESSON 3
Encapsulation
The term “encapsulation” is used to describe a process of adding headers and trailers around some data. For example, when you send an email using your favourite email program (like Outlook or Thunderbird) that email is sent from the Application layer to the Transport layer. The Transport layer encapsulates the data and adds its own header (with its own information, such as which port will be used) and passes the data to the Internet layer, which again encapsulates the received data and adds its own header, usually with information about the source and destination IP addresses. The Internet layer than passes the data to the Network Access layer. This layer is the only layer that adds both a header and a trailer. The data is then sent through a physical network link.
Each layer adds its own information:
| Frame Header | IP Header | TCP Header | Data | Network | Frame Trailer |
The term “decapsulation” refers to the process of removing headers and trailers as data passes from lower to upper layers. This process happens on a computer that is receiving data.
Frame, packet, segment
Frame – the term “frame” refers to the encapsulated data defined by the Network Access layer. A frame can have a header and a trailer that encapsulate a data section.
Packet – the term “packet” is used to describe the encapsulated data defined by the Internet layer. A packet can have a header with the source and destination IP addresses.
Segment – the term “segment” describes encapsulated data defined by the Transport layer. A segment can have a header with informations such as source and destionation port numbers, sequence and acknowledgment numbers, etc.