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OSI MODEL
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OSI Reference Model: 7 Layers
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Provides physical interface for transmission of information.
Defines rules by which bits are passed from one system to
another on a physical communication medium.
Covers all - mechanical, electrical, functional and procedural -
aspects for physical communication.
Such characteristics as voltage levels, timing of voltage
changes, physical data rates, maximum transmission
distances, physical connectors, and other similar attributes are
defined by physical layer specifications.
Physical Layer
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Data Link Layer
Data link layer attempts to provide reliable communication over
the physical layer interface.
Breaks the outgoing data into frames and reassemble the
received frames.
Create and detect frame boundaries.
Handle errors by implementing an acknowledgement and
retransmission scheme.
Implement flow control.
Supports points-to-point as well as broadcast communication.
Supports simplex, half-duplex or full-duplex communication.
OSI Model
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Implements routing of frames (packets) through the network.
Defines the most optimum path the packet should take from
the source to the destination
Defines logical addressing so that any endpoint can be
identified.
Handles congestion in the network.
Facilitates interconnection between heterogeneous networks
(Internetworking).
The network layer also defines how to fragment a packet into
smaller packets to accommodate different media.
Network Layer
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Transport Layer
Purpose of this layer is to provide a reliable mechanism for the
exchange of data between two processes in different computers.
Ensures that the data units are delivered error free.
Ensures that data units are delivered in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
Provides for the connection management.
Multiplex multiple connection over a single channel.
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Session Layer
Session layer provides mechanism for controlling the dialogue between
the two end systems. It defines how to start, control and end
conversations (called sessions) between applications.
This layer requests for a logical connection to be established on an end-
users request.
Any necessary log-on or password validation is also handled by this layer.
Session layer is also responsible for terminating the connection.
This layer provides services like dialogue discipline which can be full
duplex or half duplex.
Session layer can also provide check-pointing mechanism such that if a
failure of some sort occurs between checkpoints, all data can be
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Presentation Layer
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Application Layer
Application layer interacts with application programs and is the
highest level of OSI model.
Application layer contains management functions to support
distributed applications.
Examples of application layer are applications such as file
transfer, electronic mail, remote login etc.
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OSI Model
Data unit Layer Function
Host
layers
Data
7. Application Network process to application
6. Presentation Data representation,encryption and decryption
5. Session Interhost communication
Segments 4. Transport End-to-end connections and reliability,Flow control
Media
layers
Packet 3.Network Path determination and logical addressing
Frame 2. Data Link Physical addressing
Bit 1. Physical Media, signal and binary transmission
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Shortcomings of OSI Model
Just because someone says it is a model/standard doesnot mean you have to follow
All layers do not have the same size and importance-session and presentation layers seldom present
-data link, network, and transport layers oftenvery full
Little agreement on where to place various featuresEncryption, network management
Large number of layers increases overheads
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UNIFIES VIEW OF LAYERS , PROTOCOLS,
SERVICES
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PDUs are relevant in relation to each of the first 4 layers of the
OSI model as follows:
The Layer 1 (Physical Layer) PDU is the bit
The Layer 2 (Data Link Layer) PDU is the frame
The Layer 3 (Network Layer) PDU is the packet
The Layer 4 (Transport Layer) PDU is the segment (e.g. TCP
segment)
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The PDU at any given layer, layer 'n', is the SDU of the layer
below, layer 'n-1'.
In effect the SDU is the 'payload' of a given PDU. That is, the
process of changing a SDU to a PDU, consists of an
encapsulation process, performed by the lower layer.
All the data contained in the SDU becomes enapsulated within
the PDU. The layer n-1 adds headers or footers, or both, to the
SDU, transforming it it into a PDU of layer n-1.
The added headers or footers are part of the process used to
make it possible to get data from a source to a destination.
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A layer may impose a limit on size of data block that can be
transferred due to implementation or due to other reasons.
The layer n SDUmay be too large to be handled as a single
unit by layer (n-1)
Sender side : SDU is segmented into multiple PDUs.
Receiver side : SDU is reassembled from sequence of PDUs
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Multiplexing: sharing of n service by a multiple layer n+1 users
Multiplexing a tag or ID required in each PDU to determine which
user an SDU belongs to
Eg port number in TCP/IP
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Application Layer
the services that user applications use to communicate over the
network, and it is the layer in which user-access network processes
reside
process user data, such as data encryption and decryption and
compression and decompression.
manages the sessions (connections) between cooperating
applications.
responsible for standardizing the presentation of data.
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Transport Layer
It is responsible for end-to-end data integrity
two most important protocols employed at this layer are the
Transmission Control Protocol(TCP) and User Datagram Protocol
(UDP).
TCP provides reliable, full-duplex connections and reliable service by
ensuring that data is resubmitted when transmission results in an
error (end-to-end error detection and correction). Also, TCP enables
hosts to maintain multiple, simultaneous connections.
When error correction is not required, UDP provides unreliable
datagram service (connectionless) that enhances network throughput
at the host-to-host transport layer
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Internetwork Layer
the basic packet delivery service for all TCP/IP networks
Define the datagram, which is the basic unit of transmission in
the Internet
Move data between the Network Access Layer and the Host-to-
Host Transport Layer
Fragment and reassemble datagram
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Network Access Layer
it is concerned with getting data across a specific type of
physical network
encapsulating the IP datagram intoframes that are
transmitted by the network
maps the IP addresses to the physical addresses used by the
network
responsible for routing and must add its routing informationto the data
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