7 OSI Layer Model

OSI reference model of open network or OSI Reference Model for open networking is a network architectural model developed by the International Organization for Standardization (ISO) in Europe in 1977. OSI itself is an abbreviation of the Open Systems Interconnection. This model is also called "model OSI seven layer Model" (OSI seven layer model).Before the advent of the OSI reference model, a computer network system is very dependent on the supplier (vendor). OSI seeks to form a common standard computer networks to support interoperability between different suppliers. In a large network there are usually many different network protocols. The absence of a similar protocol, makes a lot of devices can not communicate with each other.This reference model was originally intended as a basis for developing network protocols, despite the fact that this initiative failed. The failure was caused by several factors:

    The standard reference model, if compared with the reference model DARPA (Internet Model) developed by the Internet Engineering Task Force (IETF), is very close. DARPA model is the base model of the TCP / IP is popularly used.

    This reference model is considered to be very complex. Some functions (such as connectionless communication method) is considered less good, while the other functions (such as flow control and error correction) are repeated on multiple layers.

    The growth of the Internet and TCP / IP (a network protocol real world) create OSI Reference Model becomes less attractive.The U.S. government tried to support the protocol in the OSI Reference Model network solutions government in the 1980s, by implementing some standard called the Government Open Systems Interconnection Profile (gossip). Yet. This effort was eventually abandoned in 1995, and the implementation of networks that use the OSI Reference Model are rarely found outside of Europe.OSI Reference Model was eventually seen as an ideal model of a logical connection that have to happen for data communication in the network can take place. Some of the protocols used in the real world, a kind of TCP / IP, DECnet and IBM Systems Network Architecture (SNA) to map the protocol stack (protocol stack) them to the OSI Reference Model. OSI Reference Model was used as a starting point to study how multiple network protocols in a collection protocol can function and interact.The structure of the seven layers of the OSI model, along with a protocol data unit at each layerOSI Reference Model has seven layers:

1. Layer 7 Application layer functions as the interface to the applications with the functionality of the network, manage how applications can access the network, and then make the error messages. Protocols that are in this layer are HTTP, FTP, SMTP, and NFS. 
2. Layer 6 Presentation Layer Function to translate the data to be transmitted by the application into a format that can be transmitted over the network. Protocols that are in this level are software redirektor (redirector software), such as the Workstation service (in Windows NT) and also Network shell (like Virtual Network Computing (VNC) or Remote Desktop Protocol (RDP)). 
3. Layer 5 Session Layer Functions to define how connections can be made, maintained, or destroyed. In addition, at this level also do name resolution. 
4. Layer 4 Transport Layer Functions for breaking data into packets of data and provide the serial number of the packages so that can be rearranged on the purpose of receipt. In addition, at this level also makes a sign that the packet received successfully (acknowledgment), and terhadp retransmit packets lost in the middle of the road. 
5. Layer 3 Network Layer Functions to define IP addresses, making header for the packets, and then do the routing through internetworking using routers and layer-3 switches. 
6. Layer 2 data-link layer Function to determine how the data bits are grouped into a format called a frame. In addition, at this level occurs error correction, flow control, addressing hardware (such as Media Access Control address (MAC address)), and determine how the network devices such as hubs, bridges, repeaters, and Layer 2 switches operate. IEEE 802 specification, dividing it into two levels level children, the layer Logical Link Control (LLC) layer and Media Access Control (MAC).
7.  Layer 1 Physical Layer Functions for defining network transmission media, signaling method, bit synchronization, network architectures (such as Ethernet or Token Ring), network topology and cabling. In addition, this level also defines how the Network Interface Card (NIC) can interact with cable or radio media. 

Sumber : http://id.wikipedia.org/wiki/Model_OSI