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

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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

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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.
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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".
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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. 

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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. 
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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.
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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.
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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