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The Open System Interconnection Model



The Open System Interconnection Model
The Open System Interconnection (OSI) model specifies how dissimilar computing devices such as Network Interface Cards (NICs), bridges and routers exchange data over a network by offering a networking framework for implementing protocols in seven layers. Beginning at the application layer, control is passed from one layer to the next. The following describes the seven layers as defined by the OSI model, shown in the order they occur whenever a user transmits information.
Layer 7: Application
This layer supports the application and end-user processes. Within this layer, user privacy is considered and communication partners, service and constraints are all identified. File transfers, email, Telnet and FTP applications are all provided within this layer.
Layer 6: Presentation (Syntax)
Within this layer, information is translated back and forth between application and network formats.  This translation transforms the information into data the application layer and network recognize regardless of encryption and formatting.
Layer 5: Session
Within this layer, connections between applications are made, managed and terminated as needed to allow for data exchanges between applications at each end of a dialogue.
Layer 4: Transport
Complete data transfer is ensured as information is transferred transparently between systems in this layer. The transport layer also assures appropriate flow control and end-to-end error recovery.
Layer 3: Network
Using switching and routing technologies, this layer is responsible for creating virtual circuits to transmit information from node to node. Other functions include routing, forwarding, addressing, internet working, error and congestion control, and packet sequencing.
Layer 2: Data Link
Information in data packets are encoded and decoded into bits within this layer. Errors from the physical layer flow control and frame synchronization are corrected here utilizing transmission protocol knowledge and management. This layer consists of two sub layers: the Media Access Control (MAC) layer, which controls the way networked computers gain access to data and transmit it, and the Logical Link Control (LLC) layer, which controls frame synchronization, flow control and error checking.
Layer 1: Physical
This layer enables hardware to send and receive data over a carrier such as cabling, a card or other physical means. It conveys the bitstream through the network at the electrical and mechanical level. Fast Ethernet, RS232, and ATM are all protocols with physical layer components.
This order is then reversed as information is received, so that the physical layer is the first and application layer is the final layer that information passes through.
Standard Ethernet Code
In order to understand standard Ethernet code, one must understand what each digit means. Following is a guide:
Guide to Ethernet Coding
10
at the beginning means the network operates at 10Mbps.
BASE
means the type of signaling used is baseband.
2 or 5
at the end indicates the maximum cable length in meters.
T
the end stands for twisted-pair cable.
X
at the end stands for full duplex-capable cable.
FL
at the end stands for fiber optic cable.
For example: 100BASE-TX indicates a Fast Ethernet connection (100 Mbps) that uses a
twisted pair cable capable of full-duplex transmissions.

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