OSI Layers - Tutorial 2

As we can see in OSI (Open Systems Interconnection) layers, the information and data flow enable a diverse communication system to communicate using standard protocols. In computer networking, one computer sends data to another computer and a user can view the data. Thus, OSI layers show the process of displaying data that has been sent by one computer and displaying the data in the user's monitor. When data is sending through the network, data comes through OSI layers. These layers are divided for a specific purpose and let's see how it works.

The layers architecture shows how data is transferred from one system to another. OSI layers allow vendors to perform standardized operations. Hense. different vendors can work together to create communication. We can see there are 7 layers in the OSI model and can be divided into two layers as an upper and lower layer. These layers also can divide into TCP/IP layers. Now you may realize, learning OSI layers might little hard. Don't get confused, let's discuss one by one.

The OSI layers can be divided into TCP/IP layers as follows: TCP/IP layers have four layers. 

When you consider those 7 layers, you may now confuse about the numbering system because the numbering starting by 7 and going down. This number system is useful when talking about data encapsulation and decapsulation (Let's see what these in later). Now we can jump into the next chapter to find out how each layer behavior when sending data across a network. 

7. Application Layer

The user interface (desktop) belongs to this layer. It allows the programs which need for network services such as user authentication and privacy. User authentication means, giving username and password to login to a system to verify you are a real user and you have the ability or privilege to access. Privacy means private. File transfer, email, telnet, FTP are using in the application layer. If you have any doubts about these words, put a comment below. We can explain more to you. Ther are a lot of protocols use in this layer such as Telnet, HTTP, FTP, SMTP, SNMP, TFTP, POP3, VOIP. These protocol sets are very important and you should remember the port numbers as well. You can click here and see other protocols used under the layer. 

6. Presentation Layer

Data is converted to different data formats. Let's say a video file comes through the network. Then this layer identifies the video file and puts it into the correct video file format (MPEG, WMA which it belongs to and verifies the file can be view by a user in application layer. Data compression and encryption happens under this layer. Users can view encrypted data if they have the encryption key.

5. Session Layer

This layer managed, maintained and terminated the communication between network devices and services. The session layer determines the simplex, half-duplex or full-duplex in the data communication.

4. Transport Layer

Data packet becomes a segment in this layer (Let's see more about this in data encapsulation). When the data is flowing between sending and receiving devices, the transport layer is managing the controllability. The flow control uses buffering, windowing and congestion avoidance for efficient communication. This layer also can identify errors and correct them before data flow to the next layer. When sending a network across the network, this layer keeps a buffer (some kind of a method used to store data as in cache memory) and due to that buffering, the data can be sent to the user at a fast rate and reducing the damage done to the data.

Windowing is a mechanism used in conjunction with a TCP protocol (TCP is also in this layer). The special feature of the TCP protocol is that it is always expected to reply from other end after sending it to the data. This means that when the device receives a data packet from the source, it sends a message from the receiver to the source to ensure that the data is received. Only after receiving that message does send the next data packet from source to destination.

 

This is called the three types of handshaking communication in TCP which is done by ICMP (Internet Control Message Protocol). Therefore we can verify the TCP protocol is a reliable protocol to transmit data across the network. Actually, TCP is a collection of protocols. Let's back to the windowing phrase again. When transmitting data across TCP, it reduces the number of replies from source to destination and able to transmit data at high speed. 

The congestion avoidance controls the traffic in the network from source to destination when data is transmitting through TCP. In this layer, besides the flow control, there are two functions called sequencing and acknowledgment when transmitting data. 

TCP and UDP protocols are using under the transport layer. UDP (User Datagram Protocol - doesn't wait for replies from the source. Hence UDP is faster than TCP). However, when data communication, UDP is an unreliable protocol. SPX (Sequenced Packet Exchange) protocol also is used in the transport layer.

3. Network Layer

This layer takes the entire responsibility to send data. Routers belong to this layer. Now you may imagine a better picture of how the data flow across a network. The router selects the best path to send data from source to destination. Routing tables and IP addresses also belong to this layer. IP (Internet Protocol), IPX (Internet Packet Exchange), ARP (Address Resolution Protocol), ICMP, RIP, OSPF, and other routing protocols are used under this layer. We will discuss more about these protocols in our routing lesson. (Once we publish the article, we will mention the link here).

2. Data Link Layer

MAC addresses are using in this layer. If you cannot remember the basics, go to our article Basic components of the networking - Tutorial 1. If data transmission having any issues, the data link layer will trigger notifications.  The data transmission through the LAN under this layer because switches, bridges, NIC are used in the data link layer. STP, VTP, and CDP protocols are used as well under the data link layer.

1. Physical Layer

Here is the final layer. No matter the data comes from what layer. Eventually, that data will go either wired connection or wireless as bits. Bit means 1 or 0. The computer can understand that 1, 0 language only. As per that data sending and receiving happens through this layer. Hub, repeaters, cables, and connectors belong to physical layers.

If you have any doubts, please don't hesitate to let us know. The comment section is open for everyone. Let's see you in the next lesson >> Data encapsulation and decapsulation.

Cheers!