1.3 OSI and TCP/IP Model

The Open Systems Interconnection model (OSI model) defined by Open Systems Interconnection, while the Transmission Control Protocol/Internet Protocol (TCP/IP) model was created by Departement of Defense (DoD).

Lets take a look at OSI 7 layer model and comparison to TCP/IP model.

Table 1 – OSI Model Summary

Layer	Function Description	Sample Protocols/Technologies
7. Application	Interfacing between communications software and any applications that need to communicate outside the computer. It also define proccess for user authentication.	HTTP, Telnet, DNS,DHCP, SNMP, FTP, TFTP, SMTP, POP3, NTP
6. Presentation	Data representation (define and negotiate data formats),encryption and decryption.	SSL, Shells and Redirectors, MIME
5. Session	Interhost communication. How to start, control, and end conversations (called session). An application can be notified if some of series of messages are completed. Allow presentation layer to have view of an incoming stream of data.	NetBIOS, Sockets, RPC
4. Transport	End-to-end connections and reliability,error recovery, and Flow control	TCP and UDP, SPX, NetBEUI/NBF
3. Network	Three main feature : logical addressing, routing (forwarding), and path determination. IP Address, routing protocols, and routing table belong to this layer	IPv4, IPv6, IP NAT, IPsec, ICMP, IPX, Routing protocols
2. Data Link	Defines how data is formatted for transmission and how access to the network is controlled. This layer has been divided by the IEEE 802 standards committee into two sublayers: media access control (MAC) and logical link control (LLC).	IEEE 802.2 LLC, Ethernet Family; Token Ring; FDDI, IEEE 802.11 (WLAN, Wi-Fi),ATM, SLIP and PPP
1. Physical	Media, signal and binary transmission. Refer to standard of physical characterstics of transmission medium such as connector, pins, use of pins, electrical currents, encoding, light modulation, and other rules about physical medium	Physical layers technologies (RS-232, Full duplex, RJ45, V.35,T1, E1, 10BASE-T, 100BASE-TX, POTS, SONET, SDH, DSL, 802.11a/b/g/n, etc

Table 2 will show you comparison of OSI model and TCP/IP model, also encapsulation at each layers.

Table 2 - OSI vs TCP/IP Model, and encapsulation at each layers

OSI Layer	TCP/IP Layer	Encapsulation Units	Encapsulation Proccess
7. Application	4. Application	Data				Data
6. Presentation		Data				Data
5. Session		Data				Data
4. Transport	3. Transport	Segments			TCP/UDP Header	Data
3. Network	2. Internet	Packets		IP Header	TCP/UDP Header	Data
2. Data Link	1. Network Access	Frames	MAC Header	IP Header	TCP/UDP Header	Data	CRC
1. Physical		Bits	Transmit into Physical Media (Bit Format)

Talking about encapsulation, bits transmitted from sender over physical media, at the receiving device will de-encapsulated from bits into frames, and so on untill arrived at Application layer.

Please find Sample CCNA exam (OSI and TCP/IP Model) to check if you already understand this material.