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Internet working
Models
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When networks first came into being, computer could typically communicate
only with computers from the same manufacturer. eg: companies ran either a
complete IBM solution or an DECnet solution, not both together. In the late
1970s, the OSI (Open System Interconnection) model was created by the
International Organization for Standardization to break this barrier. The OSI
model was meant to help the vendors to create the standard network
devices.
The OSI model is the Primary architectural model for networks. It
describes how data and network information are communicated from application on
one computer, through the network media, to an application on another computer.
The OSI reference model breaks this approach into Layers.
The OSI Reference
Model
The reference model was created in the late 1970s to help facilitate data
transfer between network nodes. A reference model is a conceptual blueprint of
how communication should take place. It addresses all the process required for
effective communication and divides these processes into logical groupings
called layers. When a communication system is designed in this manner, it’s
known as layered architecture. One of the greatest functions of the OSI
specifications is to assist in data transfer between disparate hosts. This means
you can transfer data between a UNIX host and a PC, for
example.
The OSI is not a physical; rather, it is a set of guidelines that
application developers can use to create and implement applications that run on
a network. It is provides a framework for creating and implementing networking
standards, devices, and internetworking schemes.
The OSI has seven different layers, the principles that we applied to
arrive at the seven layers are
The seven layers are divided into
two groups. The top three layers define how the applications within the end
stations will communicate with each other and with users. The bottom four layers
define how data is transmitted end-to-end.
Fig 1: The seven layers of
OSI
|
Application |
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Presentation |
|
Session |
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Transport |
|
Network |
|
Data
Link |
|
Physical |
The top three layers define how
the applications within the end stations will communicate with each other and
with the users. The user interface with the computer at the application layer,
and also that the upper layers are responsible for application communicating
between hosts. Remember that none of the upper layers know anything about
networking or network addresses. That is the responsibility of the four bottom
layers.
|
Transport |
|
Network |
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Data
Link |
|
Physical |
Fig
2:
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Application |
|
Presentation |
|
Session |
The above figure (Fig 2), shows the two groups of an OSI layers, the
column on the right side, they are bottom four layers of OSI which define how
data is transferred through a physical wire or through switches and routers, and
how to rebuild a data stream from a transmitting host to a destination host’s
application.
The OSI
Layers
The International Organization for Standardization (ISO) is the Emily
Post of the network protocol world. Defining the etiquette of communication
models, it remains today the most popular means of comparison for protocol
suites. The OSI reference model has seven layers:
Layer
Functions
|
Application |
Provides a user interface,
such has File, Print, Message, Database, and Application
services. |
|
Presentation |
Presents Data, Handles
processing such as encryption, compression, and Translation
services. |
|
Session |
Keeps different applications
data separate, Dialog Control |
|
Transport |
Provides reliable or
unreliable delivery, performs error correction before retransmit.
End-to-end Connection. |
|
Network |
Provides logical addressing
which routers use for path determination, Routing. |
|
Data
Link |
Combines packets into bytes
and bytes into frames, provides access to media using MAC
address. |
|
Physical |
Moves bits between
devices. |
The Application
Layer
The Application Layer is the top layer of the reference model. It
provides a set of interfaces for applications to obtain access to networked
services as well as access to the kinds of network services that support
applications directly, including services such as networked file transfer,
message handling, and database query processing.
The application layer, where
users communicate to the computer. The layer is responsible for identifying and
establishing the availability of the intended communication partner and
determining if sufficient resources for the intended communication
exist.
The application layer contains a variety of protocols that are commonly
needed.
For example, there are hundreds of
incompatible terminal types in the world. To handle each terminal type, a piece of software must
be written to map the function of the network virtual terminal onto the real
terminal.
Some
characteristics
The Presentation
layer
The presentation layer gets its name from is purpose: It presents data to
the application layer. The Presentation Layer handles data format information
for networked communications. For outgoing messages, it converts data into a
generic format that can survive the rigors of network transmission; for incoming
messages, it converts data from its generic networked representation into a
format that will make sense to the receiving application. A successful data
transfer technique is to adapt the data into a standard format before
transmission.
The presentation layer performs certain functions that are requested
sufficiently often to warrant finding a general solution for them, rather than
letting each user slove the problems. In particular, unlike all the lower
layers, which are just interested in moving bits reliably from here to there,
the presentation layer is concerned with the syntax and semantics of the
information transmitted. In order to make it possible for computers with
different representation to communicate, the data structures to be exchanged can
be defined in an abstract way, along with a standard encoding to be used “on the
wire”. The presentation layer manages these abstract data structures and
converts from the representation used inside the computer to the network
standard representation and back.
The presentation layer is also concerned with other aspects of
information representation. The OSI has protocol standards that define how
standard data should be formatted. For example, data compression can be used to
reduce the number of bits that have to be transmitted and cryptography is
frequently required for privacy and authentication.
The Session
layer
The Session Layer permits two parties to hold ongoing communications
called a session across a network. These applications on either end of the
session can exchange data or send packets to another for as long as the session
lasts. The Session layer handles session setup, data or message exchanges, and
tear-down when the session ends. It also monitors session identification so only
designated parties can participate and security services to control access to
session information. It coordinates communication between system and serves to
organize their communication by offering three modes: simplex, half-duplex and
full-duplex.
The session layer allows users on different machines to establish session
between them. A session allows ordinary data transport, as does the transport
layer, but it also provides enhanced services useful in some applications. One
of the services of the session layer is to manage dialogue control. Session can
allow traffic to go in both direction at the same time, or in only one direction
at a time. The session layer basically keeps different application’s data
separate from other application’s data.
The Transport
layer
The basic function of the transport layer is to accept data from the
session layer, split it up into smaller units if need be, pass these to the
network layer, and ensure that the pieces all arrive correctly at the other end.
Furthermore, all this must be done efficiently, and in a way that isolates the
upper layers from the inevitable changes in the hardware
technology.
The transport layer creates a distinct network connection for each
transport connection required by the session layer. If the transport connection
requires a high throughput, however, the transport layer might create multiple
network connections, dividing the data among the network connections to improve
throughput. The transport layer also determines what type of service to provide
the session layer, and ultimately, the users of the network. The most popular
type of transport connection is an error-free point-to-point channel that
delivers messages or byte in the order in which they were sent. The transport
layer is a true end-to-end layer. From source to destination. The difference
between layers 1 through 3, which are chained, and layers 4 through 7, which are
end-to-end.
The Network
layer
The Network layer is responsible for routing through an internet works
and for network addressing. This means that the Network layer is responsible for
transporting traffic between devices that are not locally
attached.
When a packet is received on a router interface, the destination IP
address is checked. If the packet is not destined for the router, then the
router will look up the destination network address in the routing table. Once
an exist interface is chosen, the packet will be sent to the interface to be
framed and sent out on the local network. If the entry for the destination
network is not found in the routing table, the router drops the
packet.
The Data Link
layer
The Data Link layer ensure that messages are delivered to the proper
devices and translates message from the Network layer into bits for the Physical
layer to transmit. It formats the messages into data frames and adds a
customized header containing the hardware destination and source address. In
another words, the Data Link Layer transforms a stream of raw bits (0s and 1s)
from the physical into a data frame and provides an error-free transfer from one
node to another, allowing the layers above it to assume virtually error-free
transmission.
The Physical
layer
The Physical layer has two responsibilities: i) to sends bits ii) to
receives bits. Bits come only in values of 1 and 0’s. The Physical layer
communicates directly with the various types of actual communication media.
Different kinds of media represent these bit values in different ways. Some use
audio tones, while others employ state transitions changes in voltage from high
to low and low to high. Basically, this means that the typical role of the
physical layer is to transform bits in a computer system into electromagnetic
(or equivalent) signals for a particular transmission medium (wire, fiber,
ether, etc.).
The physical layer is concerned with transmitting raw bits over a
communication channel. The design issues have to do with making sure that when
one side sends a 1 bit, it is received by the other side as a 1 bit, not as 0
bit. It should also note that how many volts should be used to represent a one
and how many for zero, how many microseconds a bit lasts, whether transmission
may proceed simultaneously in both direction, how the initial connection is
established and how it is torn down when both sides are finished, and how many
pins the network connector has and what each pin is used for. The design issue
here largely deals with mechanical, electrical, and procedural interface, and
the physical transmission medium, which lies below the physical
layer.
That all for now. Thanks a
lot
With
regards
VijayAnand