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Tuesday, August 17, 2010

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
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.
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
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 HeaderData
3. Network 2. Internet Packets IP HeaderTCP/UDP HeaderData
2. Data Link 1. Network Access Frames MAC HeaderIP HeaderTCP/UDP HeaderDataCRC
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.

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Edited by CCNA Study Guide August 2010.