OSI LAYER MODEL

OSI Layer Model

To start explaining the OSI layer model we must show the layers and their numbers.

• Application layer (seventh layer)
• Presentation layer (sixth layer)
• Session layer (fifth layer)
• Transport layer (fourth layer)
• Network layer (third layer)
• Data link layer (second layer)
• Physical layer (first layer)

A-Sender computer

"the proccess at this point is called encapsulation" 

⦁ Application layer (Layer 7): - 

This layer is the closest to the end user and it provides services to the user and decides which protocol we should follow Let us say that the lecturer will send an email through the app and 

This layer will decide to use SMTP as a protocol (simple mail transfer protocol)

And data will go through firewall at this layer.

PDU = data

1.1 (application layer for sender)

⦁ Presentation layer (Layer 6)

This layer formats data in a way that the receiving device will understand (this layer is responsible for the syntax and semantics that we are going to exchange, In our case its most probably going to be ASCII (American Standard Code for Information Interchange) and this layer can add encryption to sensitive data , and since we are sending an email we will encrypt using (AES)

PDU = data

1.2 (presentation layer for sender)

⦁ Session layer (Layer 5): - 

A session with the receiving UTEM server is started at this layer which will allow both sides to enter a dialog.

PDU = data

1.3 (session layer for sender)

⦁ Transport layer (Layer 4): -

At this layer we decide to use TCP or UDP and for this case its going to be TCP to make sure every packet gets delivered also the source and distention ports are added to the data

PDU = segment

1.4 (Transport layer for sender)

⦁ Network layer (Layer 3): - 

The IP address of the UTEM server (logical address) is added as the destination 

And the source IP address is also added to the data 

The data will go through the router at this layer.

PDU = packet

1.5 (network layer for sender)

⦁ Data link layer (Layer 2): -

The mac address of the router and the source mac address of the host are added to the data 

Data will go through switches at this layer

PDU = frame

1.6 (data link layer for sender)

⦁ Physical layer (Layer 1): -

The data is sent on the network using ethernet 

PDU = Bits

1.7 (physical layer for sender)

B-Receiving server

"the process at this point is called decapsulation"

⦁  Physical layer (Layer 1): -

The data is received from the network using ethernet 

PDU = Bits

2.1 (physical layer for receiver)

⦁ Data link layer (Layer 2): -

The mac address of the router and the source mac address of the host are removed from the data

PDU = frame

2.2 (data link layer for receiver)

⦁ Network layer (Layer 3): -

The IP address of the UTEM server is removed as will as the source IP address

PDU = packet

2.3 (network layer for receiver)

⦁ Transport layer (Layer 4): -

this layer recognizes if UDP OR TCP IS Used and gather the packets into segments accordingly.

PDU = segment

2.4 (transport layer for receiver)

⦁ Session layer (Layer 5): -

The session is closed at this layer and no more dialog is allowed between both sides, if further dialog is needed there must be another request done to open a new dialog session.

PDU = data

2.5 (session layer for receiver)

⦁ Presentation layer (Layer 6): -

This layer undoes the changes done at the presentation layer of the lecturer computer and decrypt the data using the same encryption method done at the sending side.

PDU = data

2.6 (presentation layer for receiver)

⦁ Application layer (Layer 7): -

Reads the data and waits for a request to process, read, or send the data to another destination.

Data will pass through firewall at this layer

PDU = data

2.7 (Application layer for receiver)