CCNA v3.0 OSI - TCP/IP Model

APPLICATION LAYER -
	Data Unit: Message;
	Communicates with 'Network Aware' Applications
	SMTP, HTTP, FTP...

PRESENTATION LAYER -
	Generifying data; data conversion; encryption.

SESSION LAYER -
	Creates and maintains session.

TRANSPORT LAYER -
	Data Unit: Segment;
	When the information comes from the upper layers to the transport layer, transport layer breaks it into manageable SEGMENTS. The
 	transport layer adds a header to each segment to create enncapsulation.
	Two critical decisions of the transport layer are:
		1) Whether to use Reliable communication (TCP-Transmission Control Protocol) or Unreliable (UDP-User Datagram Protocol); and
		2) Create Port Numbers.
	Reliable communication has to get an acknowledgement for every packet that is sent. In unreliable, there is no  acknowledgement.
 	UDP is also faster. Realtime apps use UDP.
	Port number is a number that is attached with the IP address to identify which application this info is coming from. For ex. 80;
  	The transport layer creates the socket. Socket is the IP address with the port number ex. 185.23.1.92:80

NETWORK LAYER -
	Data Unit: Packet for TCP, Datagram for UDP;
	When the network layer gets a segment from the transport layer, the network layer adds its own header to this segment. When a
 	nework layer header is added, this information is called a Packet.
	One of the critical functions of the Network Layer is IP Addressing (also known as Logical addressing). This is where IP Address is
 	added. It also finds the best path.

DATA LINK LAYER -
	Data Unit: Frame;
	When the packet comes from the Network layer to the Data Link Layer, the Data link layer adds a header to it and it now becomes a
 	Frame.
	The Data link layer is responsible for MAC (Media Access Controller) Addressing.
	This is also layer where Error checking happens. When the information comes from the Physical layer to the Data link layer, the
 	Data link layer has the ability to check if there were any errors that happened during transmission. So we can use Cyclic
 	Redundancy Check (CRC) or Parity check or other error checking mechanisms.

PHYSICAL LAYER -
	Data Unit: Bit;
	The layer where the actual data transfer happens. This is the layer that deals with wires and cables, hardware etc.
	When troubleshooting network, we start from Physical and go to top layers.