The
Open Systems Interconnection model (OSI model) is a conceptual
model that characterizes and standardizes the communication functions of a
telecommunication or computing system without regard to their underlying
internal structure and technology. Its goal is the interoperability of diverse
communication systems with standard protocols. The model partitions a
communication system into abstraction layers. The original version of the model
defined seven layers.
Layer 1 Physical
Layer: It conveys the bit stream to
electrical impulse, light or radio signal through the network at the electrical
and mechanical level. It provides the hardware means of sending and receiving
data on a carrier, including defining cables, cards and physical aspects. Fast
Ethernet, RS232, and ATM are protocols with physical layer components.
Layer 2 Data Link
Layer: At OSI Model, Layer 2, data
packets are encoded and decoded into bits. It furnishes transmission protocol
knowledge and management and handles errors in the physical layer, flow control
and frame synchronization. The data link layer is divided into two sub layers:
The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer.
The MAC sub layer controls how a computer on the network gains access to the
data and permission to transmit it. The LLC layer controls frame
synchronization and flow control. The FCS (Frequency Check Sequence) extra
error-detecting code added to a frame in a communications protocol.
Layer 3 Network
Layer: Layer 3 provides switching and
routing technologies, creating logical paths, known as virtual circuits, for
transmitting data from node to node. Routing and forwarding are functions of
this layer, as well as addressing, internetworking, error handling, congestion
control and packet sequencing.
Layer 4 Transport
Layer: OSI Model, Layer 4, provides
transparent transfer of data between end systems, or hosts, and is responsible
for end-to-end error recovery and flow control. It ensures complete data
transfer.
Layer 5 Session
Layer: This layer establishes, manages
and terminates connections between applications. The session layer sets up,
coordinates, and terminates conversations, exchanges, and dialogues between the
applications at each end. It deals with session and connection coordination.
Layer 6
Presentation Layer: This layer provides
independence from differences in data representation (e.g., encryption) by
translating from application to network format, and vice versa. The
presentation layer works to transform data into the form that the application
layer can accept. This layer formats and encrypts data to be sent across a
network, providing freedom from compatibility problems. It is sometimes called
the syntax layer.
Layer 7
Application Layer: OSI Model, Layer 7,
supports application and end-user processes. Communication partners are
identified, quality of service is identified, user authentication and privacy
are considered, and any constraints on data syntax are identified. Everything
at this layer is application-specific. This layer provides application services
for file transfers, e-mail, and other network software services. Telnet and FTP
are applications that exist entirely in the application level. Tiered
application architectures are part of this layer.
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