Hello Mr.Jitendera Kuamr Sinha, how is every thing going on? Thanks for your r

Hello Mr.Jitendera Kuamr Sinha, how is every thing going
on? Thanks for your reply. Dear Friend if you can Explain
it little bit deeper i will be realy thanks full. Wish you
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Hello Mr.Jitendera Kuamr Sinha, how is every thing going on? Thanks for your reply. Dear Friend if..

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Introduction
When computers were first linked together into networks,
moving information between different types of computers was
a very difficult task.
In the early 1980s, the International Standards
Organization (ISO) recognized the need for a standard
network model. This would help vendors to create
interpretable network devices. The Open Systems
Interconnection (OSI) reference model, released in 1984,
addressed this need.
The OSI model describes how information makes its way from
application programs through a network medium to another
application program in another computer. It divides this
one big problem into seven smaller problems.
Each of these seven problems is reasonably self-contained
and therefore more easily solved without excessive reliance
on external information. Each problem is addressed by one
of the seven layers of the OSI model. The seven layers of
the OSI model are:-

• application
• presentation
• session
• transport

• network
• data-link
• physical
The acronym used to remember these layers is: All People
Seem To Need Data Processing.
The lower two OSI model layers are implemented with
hardware and software.
The upper five are generally implemented only in software.

Advantages of Layered Approach
The layered approach to network communications provides the
following benefits:

• reduced complexity
• improved teaching and learning
• modular engineering
• accelerated evolution
• interoperable technology
• standard interfaces
As the information to be sent descends through the layers
of a system it looks less and less like human language and
more and more like the 1s and 0s that a computer
understands.

Let's look at an example of OSI-type communication. Assume
that System A has information to send to System B. System
A's application program communicates with System A's layer
seven (Application Layer). Layer seven communicates with
layer six which communicates with layer five and so on
until System A's layer one is reached. The information
traverses the physical medium and is received by System B's
layer one.
It then ascends through System B's layers in reverse order
until it finally reaches System B's application program.

Each of System A's layers has certain tasks it must
perform. Each layer communicates directly with its adjacent
layers. However, its primary concern in carrying out its
tasks is to communicate with its peer layer in System B.
For example, the primary concern of layer six in System A
is to communicate with layer six in System B. It does this
using its own layer protocol. Each layer's protocol
exchanges information, called protocol data units (PDUs),
between peer layers. Each layer uses a specific term for
its PDU.

For example, in TCP/IP the transport layer, TCP
communicates to the peer TCP function using "segments".
Each layer in System A must rely on services provided by
its lower layers for it to communicate with its System B
peer. The upper layer is said to be the service user while
the lower layer is the service provider. The lower layer
services are provided to the upper layer at a service
access point (SAP).

Layers - Functions - Devices

The application layer
The application layer of the OSI model is the layer that is
closest to the user. Instead of providing services to other
OSI layers, it provides services to application programs
outside the scope of the OSI model. It's services are often
part of the application process. Main functions are:-

• identifies and establishes the availability of the
intended communication partner.
• synchronizes the sending and receiving applications.
• establishes agreement on procedures for error recovery
and control of data integrity.
• determines whether sufficient resources for the intended
communications exist.

Devices:-

• Browsers
• Search engines
• E-mail programs
• Newsgroup and chat programs
• Transaction services
• Audio/video conferencing

• Telnet

• SNMP

The presentation layer
It ensures that information sent by the application layer
of one system will be readable by the application layer of
another system. It provides a common format for
transmitting data across various systems, so that data can
be understood, regardless of the types of machines involved.
The presentation layer concerns itself not only with the
format and representation of actual user data, but also
with data structure used by programs. Therefore, the
presentation layer negotiates data transfer syntax for the
application layer.
Devices:-

• Encryption

• EBCDIC and ASCII

• GIF & JPEG

Top

The Session Layer

The main function of the OSI model's session layer is to
control "sessions", which are logical connections between
network devices. A session consists of a dialog, or data
communications conversation, between two presentation
entities. Dialogs can be

• simplex (one-way)
• half-duplex (alternate)
• full-duplex (bi-directional)
Simplex conversations are rare on networks. Half-duplex
conversations require a good deal of session layer control,
because the start and end of each transmission need to be
monitored.
Most networks are of course capable of full-duplex
transmission, but in fact many conversations are in
practice half-duplex.
Devices:-

Some examples of session layer protocols and interfaces are:

• Network File System (NFS)
• Concurrent database access
• X-Windows System

• Remote Procedure Call (RPC)

• SQL

• NetBIOS Names
• AppleTalk Session Protocol (ASP)
• Digital Network Architecture

Top

The Transport Layer
You can think of the transport layer of the OSI model as a
boundary between the upper and lower protocols. The
transport layer provides a data transport service that
shields the upper layers from transport implementation
issues such as the reliability of a connection.
The transport layer provides mechanisms for:-

• multiplexing upper layer applications
• the establishment, maintenance, and orderly termination
of virtual circuits
• information flow control
• transport fault detection and recovery

Devices:-

• TCP, UDP, SPX and Sliding Windows.

Multiplexing & De-multiplexing

The transport layer uses a technique called multiplexing to
segment and reassemble data from several upper layer
applications onto the same transport layer data stream.
When data is being sent, the source machine includes extra
bits with the data that encode the message type,
originating application, and protocols used.
The destination machine de-multiplexes the data stream, and
reassembles the data so that it can be passed up to the
destination peer application.

The transport layer data stream provides end-to-end
transport services.

It constitutes a logical connection between the end points
of an internetwork, that is, the originating host and the
destination host.
Before data transfer can begin, both the sending and
receiving applications inform their respective operating
systems that a connection is going to be initiated.
In essence, one machine places a call that must be accepted
by the other.
Protocol software modules in the two operating systems
communicate by sending messages across the network to
verify that the transfer is authorized and that both sides
are ready.
After all the synchronization has occurred, a connection is
said to be established and data transfer can begin.
Top

Sequencing - Acknowledgements - Flow Control (Windowing)

During a transfer using TCP, the two machines continue to
communicate with their protocol software to verify that
data is received correctly. Once data transfer is in
progress, congestion can occur for two reasons.
First, the sending device might be able to generate traffic
faster than the network can transfer it.
Second, if multiple devices need to send data through the
same gateway, or to the same destination, the gateway or
destination may experience congestion.
When datagrams arrive too quickly for a device to process,
it temporarily stores them in memory and the process being
called as buffering. If the datagrams are part of a small
burst, this buffering solves the problem.
However, if the traffic continues to arrive at this rate,
the device eventually exhausts its memory and must discard
additional datagrams that arrive. Instead of losing data,
the transport function can issue a "not ready" indicator to
the sender. This acts like a stop sign and signals the
sender to discontinue sending segment traffic to its peer.
After the receiving device has processed sufficient
segments to free space in its buffers, the receiver sends a
ready transport indicator - which is like a go signal. When
it receives this indicator, the sender can resume segment
transmission.

The transport layer may provide a reliable service
regardless of the quality of the underlying network. One
technique that is used to guarantee reliable delivery is
called "positive acknowledgement with retransmission".
This requires the receiver to issue an acknowledgement
message to the sender when it receives data. The sending
device keeps a record of each packet it sends and it waits
for an acknowledgement before sending another packet. The
sender also starts a timer when it sends a packet. It
retransmits the packet if the timer expires before an
acknowledgement is received.

Acknowledging every data segment, however, has its
drawbacks. If the sender has to wait for an acknowledgement
of each data segment, the throughput will be very low.
A technique called "windowing" is used to increase the
throughput. Time is available after the sender finishes
transmitting the data segment, but before the sender
finishes processing any received acknowledgement. This is
used for transmitting more data. The number of data
elements the sender is allowed to have outstanding is known
as the "window".
For example, with a window size of three the sender can
transmit three data segments before expecting an
acknowledgement.
In reality, the acknowledgements and data segments will
intermix as they communicate across the network. This is
known as "piggyback acknowledgement".
Top

The Network Layer
Layer three of the OSI model is the network layer.

• The network layer sends packets from source network to
destination network.

• It provides consistent end-to-end packet delivery
services to its user, the transport layer.
In wide area networking a substantial geographic distance
and many networks can separate two end systems that wish to
communicate. Between the two end systems the data may have
to be passed through a series of widely distributed
intermediary nodes. These intermediary nodes are normally
routers.
Routers are special stations on a network, capable of
making complex routing decisions.
• The network layer is the domain of routing.
Routing protocols select optimal paths through the series
of interconnected networks.
Network layer protocols then move information along these
paths.

• One of the functions of the network layer is "path
determination".
Path determination enables the router to evaluate all
available paths to a destination and determine which to
use. It can also establish the preferred way to handle a
packet.
After the router determines which path to use it can
proceed with switching the packet.
It takes the packet it has accepted on one interface and
forwards it to another interface or port that reflects the
best path to the packet's destination.

Devices:-

• IP, IPX, Routers, Routing Protocols (RIP, IGRP, OSPF,
BGP etc), ARP, RARP, ICMP.

Top

The Data-Link Layer
Layer two of the OSI reference model is the data-link
layer. This layer is responsible for providing reliable
transit of data across a physical link. The data-link layer
is concerned with

• physical addressing; Bridges, Transparent Bridges, Layer
2 Switches
• network topology; CDP
• line discipline (how end systems will use the network
link)
• error notification
• ordered delivery of frames
• flow control

• Frame Relay, PPP, SDLC, X.25, 802.3, 802.3, 802.5/Token
Ring, FDDI.

At the data-link layer, the bits that come up from the
physical layer are formed into data frames, using any of a
variety of data-link protocols. Frames consist of fields,
containing bits.
The data-link layer is subdivided into two sub layers:

• the logical link control (LLC) sub layer
• the media access control (MAC) sub layer

The LLC sub layer provides support for

• connections between applications running on a LAN
• flow control to the upper layer by means of ready/not
ready codes
• sequence control bits.

The LLC sub layer rests on top of other media access
protocols to provide interface flexibility.
Because the LLC sub layer operates independently of
specific media access protocols, upper layer protocols, for
example IP at the network layer, can operate autonomously
without concern as to the specific type of LAN media. The
LLC sub layer can depend on lower layers to provide access
to the media. It provides Service Access Points (SAP's) and
flow control. This layer puts 1's & 0's into a logical
frame.

The MAC sub layer provides orderly access to the LAN
medium. For multiple stations to share the same medium and
still uniquely identify each other, the MAC sub layer
defines a hardware, or data-link address called the "MAC
address". The MAC address is unique for each LAN interface.
On most LAN interface cards the MAC address is burned into
ROM.
The ROM MAC address is sometimes known as the burned-in
address (BIA).

The MAC address is a 48-bit address expressed as 12
hexadecimal digits written in three groups of four digits.
The first six hexadecimal digits (the first 24 bits)
represent a vendor code known as the organizationally
unique identifier (OUI). To ensure vendor uniqueness, the
IEEE administers OUIs. The last six hexadecimal digits are
administered by the vendor and often represent the
interface serial number.
Top

Process of Finding Hosts on the Same Network Segment- ARP

Before a frame is exchanged with a device on the same LAN,
the sending device needs to have a MAC address it can use
as a destination address.
The sending device may use an address resolution protocol
(such as TCP/IP's address resolution protocol (ARP)) to
discover the destination's MAC address. In other protocols
the MAC address can be determined directly from the network
address.
For example, assume that host Y and host Z are on the same
LAN. Host Y broadcasts an ARP request onto the LAN looking
for host Z. Because it is a broadcast message all devices
on the LAN, including host Z, process the request. However,
host Z is the only device to respond and it does so with
its MAC address. Host Y receives host Z's reply and stores
the MAC address in local memory. This is often called
an "ARP cache". The next time host Y needs to communicate
with host Z it recalls host Z's stored MAC address.

Process of Finding Hosts on the Different Network Segment-
ARP + Router

Let's look at how host Y communicates with host X on a
different LAN, which it can access via router A.
As before host Y broadcasts its ARP request. Router A,
along with all the other devices on the LAN, processes the
request. It knows that host X will not see the request
because it is on another LAN, and that any packets destined
for host X will have to be relayed. So instead, router A
provides its own MAC address to host Y as a "proxy" reply
to the ARP request. Host Y receives the router's response
and saves the MAC address in its ARP cache memory. The next
time host Y needs to communicate with host X, it recalls
the stored MAC address of router A.
Top

The Physical Layer
Layer one of the OSI model is the physical layer. The
physical layer is concerned with the interface to the
transmission medium. At the physical layer, data is
transmitted onto the medium (e.g. coaxial cable or optical
fiber) as a stream of bits.
So, the physical layer is concerned, not with networking
protocols, but with the transmission media on the network.
The physical layer defines the electrical, mechanical,
procedural, and functional specifications for activating,
maintaining, and deactivating the physical link between end
systems. This layer puts 1's & 0's onto the wire.
Characteristics specified by the physical layer include

• voltage levels
• timing of voltage changes
• physical data rates
• maximum transmission distances
• physical connectors

Devices:-

• Hubs, FDDI Hardware, Fast Ethernet, Token Ring Hardware.
Top

Example of Layered Transmission
Let's look at the transport layer in TCP/IP as an example.
The transport layer must use the services of the network
layer in order to communicate to the peer TCP function on
another system. Each lower layer in turn takes upper layer
information as part of the PDUs it exchanges with its peer
layer.

Each lower layer adds whatever headers and trailers it
requires to perform its functions. This is called "data
encapsulation".
The transport layer's segments become part of the network
layer's "packets" exchanged between IP peers. Network layer
packets are also known as "datagrams".
The network layer adds to the start of the PDU, a header to
the data that identifies the source and destination logical
addresses. These addresses help network devices send the
packets across the network along a chosen path.
The Host-to-network layer takes the IP packet and adds a
header to form a "frame". The header contains information
required to complete the data-link functions. For example,
the frame header contains a physical address which allows
the network device to communicate over its interface to the
next directly connected network device on the link.
Ultimately, these frames must be converted into electrical
pulses as the data is finally transmitted by the physical
layer protocol across the wire or other physical medium
used by the networ

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