UNIT- II

Network Models And Services

OSI Reference Model:

OSI Model
o OSI stands for Open System Interconnection is a reference model that describes how
information from a software application in one computer moves through a physical
medium to the software application in another computer.
o OSI consists of seven layers, and each layer performs a particular network function.
o OSI model was developed by the International Organization for Standardization (ISO) in
1984, and it is now considered as an architectural model for the inter-computer
communications.
o OSI model divides the whole task into seven smaller and manageable tasks. Each layer is
assigned a particular task.
o Each layer is self-contained, so that task assigned to each layer can be performed
independently.

Characteristics of OSI

o The OSI model is divided into two layers: upper layers and lower layers.

o The upper layer of the OSI model mainly deals with the application related issues, and they
are implemented only in the software. The application layer is closest to the end user. Both
the end user and the application layer interact with the software applications. An upper
layer refers to the layer just above another layer.
 o The lower layer of the OSI model deals with the data transport issues. The data link layer
and the physical layer are implemented in hardware and software. The physical layer is the
lowest layer of the OSI model and is closest to the physical medium. The physical layer is
mainly responsible for placing the information on the physical medium.

7 Layers of OSI Model

There are the seven OSI layers. Each layer has different functions. A list of seven layers are
given below:

1. Physical Layer
2. Data-Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

1) Physical layer
 o The main functionality of the physical layer is to transmit the individual bits from one node
to another node.
o It is the lowest layer of the OSI model.
o It establishes, maintains and deactivates the physical connection.
o It specifies the mechanical, electrical and procedural network interface specifications.

Functions of a Physical layer:

o Line Configuration: It defines the way how two or more devices can be connected
physically.
o Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or
full-duplex mode between the two devices on the network.
o Topology: It defines the way how network devices are arranged.
o Signals: It determines the type of the signal used for transmitting the information.

2) Data-Link Layer

o This layer is responsible for the error-free transfer of data frames.

o It defines the format of the data on the network.
o It provides a reliable and efficient communication between two or more devices.
o It is mainly responsible for the unique identification of each device that resides on a local
network.
o It contains two sub-layers:
o Logical Link Control Layer
 o It is responsible for transferring the packets to the Network layer of the
receiver that is receiving.
o It identifies the address of the network layer protocol from the header.
o It also provides flow control.
o Media Access Control Layer
o A Media access control layer is a link between the Logical Link Control layer
and the network's physical layer.
o It is used for transferring the packets over the network.

Functions of the Data-link layer

o Framing: The data link layer translates the physical's raw bit stream into packets known
as Frames. The Data link layer adds the header and trailer to the frame. The header which
is added to the frame contains the hardware destination and source address.

o Physical Addressing: The Data link layer adds a header to the frame that contains a
destination address. The frame is transmitted to the destination address mentioned in the
header.
o Flow Control: Flow control is the main functionality of the Data-link layer. It is the
technique through which the constant data rate is maintained on both the sides so that
no data get corrupted. It ensures that the transmitting station such as a server with higher
processing speed does not exceed the receiving station, with lower processing speed.
o Error Control: Error control is achieved by adding a calculated value CRC (Cyclic
Redundancy Check) that is placed to the Data link layer's trailer which is added to the
message frame before it is sent to the physical layer. If any error seems to occurr, then the
receiver sends the acknowledgment for the retransmission of the corrupted frames.
 o Access Control: When two or more devices are connected to the same communication
channel, then the data link layer protocols are used to determine which device has control
over the link at a given time.

3) Network Layer

o It is a layer 3 that manages device addressing, tracks the location of devices on the
network.
o It determines the best path to move data from source to the destination based on the
network conditions, the priority of service, and other factors.
o The Data link layer is responsible for routing and forwarding the packets.
o Routers are the layer 3 devices, they are specified in this layer and used to provide the
routing services within an internetwork.
o The protocols used to route the network traffic are known as Network layer protocols.
Examples of protocols are IP and Ipv6.

Functions of Network Layer:

o Internetworking: An internetworking is the main responsibility of the network layer. It
provides a logical connection between different devices.
o Addressing: A Network layer adds the source and destination address to the header of
the frame. Addressing is used to identify the device on the internet.
o Routing: Routing is the major component of the network layer, and it determines the best
optimal path out of the multiple paths from source to the destination.
o Packetizing: A Network Layer receives the packets from the upper layer and converts
them into packets. This process is known as Packetizing. It is achieved by internet protocol
(IP).
4) Transport Layer

o this case receiver does not send any acknowledgment when the packet is received,
the sender does not wait for any acknowledgment. Therefore, this makes a protocol
unreliable.

Functions of Transport Layer:

o Service-point addressing: Computers run several programs simultaneously due to this
reason, the transmission of data from source to the destination not only from one
computer to another computer but also from one process to another process. The
transport layer adds the header that contains the address known as a service-point address
or port address. The responsibility of the network layer is to transmit the data from one
computer to another computer and the responsibility of the transport layer is to transmit
the message to the correct process.
o Segmentation and reassembly: When the transport layer receives the message from the
upper layer, it divides the message into multiple segments, and each segment is assigned
with a sequence number that uniquely identifies each segment. When the message has
arrived at the destination, then the transport layer reassembles the message based on their
sequence numbers.
o Connection control: Transport layer provides two services Connection-oriented service
and connectionless service. A connectionless service treats each segment as an individual
packet, and they all travel in different routes to reach the destination. A connection-
oriented service makes a connection with the transport layer at the destination machine
before delivering the packets. In connection-oriented service, all the packets travel in the
single route.
 o Flow control: The transport layer also responsible for flow control but it is performed end-
to-end rather than across a single link.
o Error control: The transport layer is also responsible for Error control. Error control is
performed end-to-end rather than across the single link. The sender transport layer
ensures that message reach at the destination without any error.

5) Session Layer

The session layer is Layer 5 layer from the bottom in the OSI model. The job
of the session layer is to control and maintain connections between systems
to share data. It establishes, maintains, and ends sessions across all channels.
In case of a network error, it checks the authenticity and provides recovery
options for active sessions. It manages sessions and synchronizes data flow.

Basically, this layer regulates when computers can send data and how much
data they can send. Essentially it coordinates communication between
devices.

o It is a layer 3 in the OSI model.

o The Session layer is used to establish, maintain and synchronizes the interaction between
communicating devices.
Functions of Session layer:
o Dialog control: Session layer acts as a dialog controller that creates a dialog between two
processes or we can say that it allows the communication between two processes which
can be either half-duplex or full-duplex.
o Synchronization: Session layer adds some checkpoints when transmitting the data in a
sequence. If some error occurs in the middle of the transmission of data, then the
transmission will take place again from the checkpoint. This process is known as
Synchronization and recovery.

6) Presentation Layer

o A Presentation layer is mainly concerned with the syntax and semantics of the information
exchanged between the two systems.
o It acts as a data translator for a network.
o This layer is a part of the operating system that converts the data from one presentation
format to another format.
o The Presentation layer is also known as the syntax layer.

Functions of Presentation layer:

o Translation: The processes in two systems exchange the information in the form of
character strings, numbers and so on. Different computers use different encoding
methods, the presentation layer handles the interoperability between the different
encoding methods. It converts the data from sender-dependent format into a common
 format and changes the common format into receiver-dependent format at the receiving
end.
o Encryption: Encryption is needed to maintain privacy. Encryption is a process of
converting the sender-transmitted information into another form and sends the resulting
message over the network.
o Compression: Data compression is a process of compressing the data, i.e., it reduces the
number of bits to be transmitted. Data compression is very important in multimedia such
as text, audio, video.

7) Application Layer

o An application layer serves as a window for users and application processes to access
network service.
o It handles issues such as network transparency, resource allocation, etc.
o An application layer is not an application, but it performs the application layer functions.
o This layer provides the network services to the end-users.

Functions of Application layer:

o File transfer, access, and management (FTAM): An application layer allows a user to
access the files in a remote computer, to retrieve the files from a computer and to manage
the files in a remote computer.
o Mail services: An application layer provides the facility for email forwarding and storage.
o Directory services: An application provides the distributed database sources and is used
to provide that global information about various objects.
The TCP/IP Reference Model

What is TCP/IP Model?

The TCP/IP model refers to the Transmission Control Protocol/Internet Protocol Model.
This model is a part of the network domain designed specifically for overseeing
efficient and error-free transmission of data.

The model works on a four-layered architecture model, where each layer implicit the
required network protocols on the data to be transmitted, which remodels the data to
the most optimum structure for efficient transmission over the network.

The TCP/IP model is divided into four different layers:

• Application layer

• Transport layer

• Internet layer

• Network Access layer

Each layer performs a specific task on the data that is being transmitted over the
network channel, and data moves from one layer to another in a preset pattern as
mentioned below:
Application Layer
o An application layer is the topmost layer in the TCP/IP model.

o It is responsible for handling high-level protocols, issues of representation.

o This layer allows the user to interact with the application.

o When one application layer protocol wants to communicate with another

application layer, it forwards its data to the transport layer.

o There is an ambiguity occurs in the application layer. Every application cannot

be placed inside the application layer except those who interact with the
communication system. For example: text editor cannot be considered in
application layer while web browser using HTTP protocol to interact with the
network where HTTP protocol is an application layer protocol.

Following are the main protocols used in the application layer:

o HTTP: HTTP stands for Hypertext transfer protocol. This protocol allows us to

access the data over the world wide web. It transfers the data in the form of
plain text, audio, video. It is known as a Hypertext transfer protocol as it has
the efficiency to use in a hypertext environment where there are rapid jumps
from one document to another.

o SNMP: SNMP stands for Simple Network Management Protocol. It is a

framework used for managing the devices on the internet by using the TCP/IP
protocol suite.

o SMTP: SMTP stands for Simple mail transfer protocol. The TCP/IP protocol that

supports the e-mail is known as a Simple mail transfer protocol. This protocol
is used to send the data to another e-mail address.

o DNS: DNS stands for Domain Name System. An IP address is used to identify

the connection of a host to the internet uniquely. But, people prefer to use the
names instead of addresses. Therefore, the system that maps the name to the
address is known as Domain Name System.
 o TELNET: It is an abbreviation for Terminal Network. It establishes the

connection between the local computer and remote computer in such a way
that the local terminal appears to be a terminal at the remote system.

o FTP: FTP stands for File Transfer Protocol. FTP is a standard internet protocol

used for transmitting the files from one computer to another computer.

Transport Layer
The transport layer is responsible for the reliability, flow control, and
correction of data which is being sent over the network.

The two protocols used in the transport layer are User Datagram protocol and
Transmission control protocol.

o User Datagram Protocol (UDP)

o It provides connectionless service and end-to-end delivery of

transmission.

o It is an unreliable protocol as it discovers the errors but not specify the

error.

o User Datagram Protocol discovers the error, and ICMP protocol reports
the error to the sender that user datagram has been damaged.

o UDP consists of the following fields:

Source port address: The source port address is the address of the

application program that has created the message.

Destination port address: The destination port address is the address of

the application program that receives the message.

Total length: It defines the total number of bytes of the user datagram

in bytes.
Checksum: The checksum is a 16-bit field used in error detection.

o UDP does not specify which packet is lost. UDP contains only checksum;
it does not contain any ID of a data segment.
 o Transmission Control Protocol (TCP)

o It provides a full transport layer services to applications.

o It creates a virtual circuit between the sender and receiver, and it is

active for the duration of the transmission.

o TCP is a reliable protocol as it detects the error and retransmits the

damaged frames. Therefore, it ensures all the segments must be
received and acknowledged before the transmission is considered to be
completed and a virtual circuit is discarded.

o At the sending end, TCP divides the whole message into smaller units
known as segment, and each segment contains a sequence number
which is required for reordering the frames to form an original message.

o At the receiving end, TCP collects all the segments and reorders them
based on sequence numbers.

Network Access Layer

o A network layer is the lowest layer of the TCP/IP model.
o A network layer is the combination of the Physical layer and Data Link layer defined in the
OSI reference model.
o It defines how the data should be sent physically through the network.
o This layer is mainly responsible for the transmission of the data between two devices on
the same network.
o The functions carried out by this layer are encapsulating the IP datagram into frames
transmitted by the network and mapping of IP addresses into physical addresses.
o The protocols used by this layer are ethernet, token ring, FDDI, X.25, frame relay.

Internet Layer
o An internet layer is the second layer of the TCP/IP model.
o An internet layer is also known as the network layer.
o The main responsibility of the internet layer is to send the packets from any network, and
they arrive at the destination irrespective of the route they take.
 o Following are the protocols used in this layer are
o IP Protocol: IP protocol is used in this layer, and it is the most significant part of
the entire TCP/IP suite.
o Following are the responsibilities of this protocol
o IP Addressing: This protocol implements logical host addresses known as IP addresses.
The IP addresses are used by the internet and higher layers to identify the device and to
provide internetwork routing.
o Host-to-host communication: It determines the path through which the data is to be
transmitted.
o Data Encapsulation and Formatting: An IP protocol accepts the data from the transport
layer protocol. An IP protocol ensures that the data is sent and received securely, it
encapsulates the data into message known as IP datagram.
o Fragmentation and Reassembly: The limit imposed on the size of the IP datagram by
data link layer protocol is known as Maximum Transmission unit (MTU). If the size of IP
datagram is greater than the MTU unit, then the IP protocol splits the datagram into
smaller units so that they can travel over the local network. Fragmentation can be done by
the sender or intermediate router. At the receiver side, all the fragments are reassembled
to form an original message.
o Routing: When IP datagram is sent over the same local network such as LAN, MAN, WAN,
it is known as direct delivery. When source and destination are on the distant network,
then the IP datagram is sent indirectly. This can be accomplished by routing the IP
datagram through various devices such as routers.

Differences between OSI and TCP/IP Reference Model −

Model −

OSI TCP/IP

OSI represents Open System TCP/IP model represents the Transmission

Interconnection. Control Protocol / Internet Protocol.

OSI is a generic, protocol independent TCP/IP model depends on standard

standard. It is acting as an interaction protocols about which the computer
network has created. It is a connection
 OSI TCP/IP

gateway between the network and the protocol that assigns the network of hosts
final-user. over the internet.

The OSI model was developed first, and The protocols were created first and then
then protocols were created to fit the built the TCP/IP model.
network architecture’s needs.

It provides quality services. It does not provide quality services.

The OSI model represents defines It does not mention the services, interfaces,
administration, interfaces and and protocols.
conventions. It describes clearly which
layer provides services.

The protocols of the OSI model are better The TCP/IP model protocols are not
unseen and can be returned with another hidden, and we cannot fit a new protocol
appropriate protocol quickly. stack in it.

It is difficult as distinguished to TCP/IP. It is simpler than OSI.

It provides both connection and It provides connectionless transmission in

connectionless oriented transmission in the network layer and supports connecting
the network layer; however, only and connectionless-oriented transmission
connection-oriented transmission in the in the transport layer.
transport layer.

It uses a vertical approach. It uses a horizontal approach.

The smallest size of the OSI header is 5 The smallest size of the TCP/IP header is
bytes. 20 bytes.
 OSI TCP/IP

Protocols are unknown in the OSI model In TCP/IP, returning protocol is not difficult.
and are returned while the technology
modifies.

Types of Protocols and Standards

There are several types of protocols and standards used in computer networks,
including Transmission Control Protocol (TCP), Internet Protocol (IP), User Datagram
Protocol (UDP) and network standards such as TCP/IP, HTTP, FTP, among others.

Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP) is a fundamental component of computer
networks, designed to ensure accurate and reliable data transmission between devices.
As one of the core protocols in the Internet Protocol (IP) suite, TCP guarantees that all
information packets arrive at their intended destination in the correct order and without
errors.
TCP's well-established rules help facilitate smooth communication across various
network architectures while maintaining high standards of security and performance.
For example, when downloading a large file from a website or conducting online
banking transactions, TCP ensures that your connection remains stable and secure
throughout the entire process.

Internet Protocol (IP)

Internet Protocol (IP) is a fundamental network protocol that enables data
communication across the internet. It is responsible for routing and forwarding data
packets from one device to another, based on their unique addressing scheme.
IP addresses are used to identify devices that are connected to a network, allowing them
to communicate with each other.
One of the key features of IP is its ability to work in conjunction with other protocols
such as TCP or UDP, forming the backbone of the internet's infrastructure. Without it,
we wouldn't be able to send emails or browse web pages online.
User Datagram Protocol (UDP)
User Datagram Protocol (UDP) is a connectionless protocol that operates on top of the
Internet Protocol (IP). It is faster than Transmission Control Protocol (TCP) because it
does not guarantee the delivery of packets or in order delivery, making it less reliable.
UDP is commonly used for time-sensitive applications such as online gaming, video
streaming, and voice-over-IP (VoIP), where speed and efficiency are more important
than reliability.
It should be noted that while UDP does not provide flow control or error correction like
TCP, it has several benefits. For one thing, it requires fewer resources than TCP since
no state tracking is required at either end of the communication channel.
Professionals who work with computer networks must understand which protocols to
use for specific applications to optimize network performance effectively.

Network Standards (TCP/IP, HTTP, FTP, Etc.)

There are different types of network standards used in computer networks that are
essential for data communication. These include −
• Transmission Control Protocol/Internet Protocol (TCP/IP) − TCP/IP is the
most commonly used protocol and standard in computer networks. It is
responsible for ensuring reliable data transmission over the internet by
breaking down data into smaller packets that can be sent across different
network devices.
• Hypertext Transfer Protocol (HTTP) − HTTP is a standard protocol used
for transmitting web pages and other content on the World Wide Web. It
defines how web clients such as browsers communicate with servers to
retrieve information.
• File Transfer Protocol (FTP) − FTP is another protocol used to transfer
files between devices on a network. It provides a simple way of sending
and receiving files, making it an essential tool for businesses that need to
transfer large amounts of data.
• Simple Mail Transfer Protocol (SMTP) − SMTP is a standard protocol
used for sending email messages between different email clients and
servers on the internet.
• Domain Name System (DNS) − DNS is a set of protocols that translates
human-readable domain names into IP addresses used by computers to
locate websites and other resources on the internet.
Domain Name System (DNS) in Application Layer:

An application layer protocol defines how the application processes running on different
systems, pass the messages to each other.

o DNS stands for Domain Name System.

o DNS is a directory service that provides a mapping between the name of a host on the
network and its numerical address.
o DNS is required for the functioning of the internet.
o Each node in a tree has a domain name, and a full domain name is a sequence of symbols
specified by dots.
o DNS is a service that translates the domain name into IP addresses. This allows the users
of networks to utilize user-friendly names when looking for other hosts instead of
remembering the IP addresses.
o For example, suppose the FTP site at EduSoft had an IP address of 132.147.165.50, most
people would reach this site by specifying ftp.EduSoft.com. Therefore, the domain name
is more reliable than IP address.

DNS is a TCP/IP protocol used on different platforms. The domain name space is divided
into three different sections: generic domains, country domains, and inverse domain.

Generic Domains
o It defines the registered hosts according to their generic behavior.
 o Each node in a tree defines the domain name, which is an index to the DNS
database.

o It uses three-character labels, and these labels describe the organization type.

o Label Description

aero Airlines and aerospace

companies

biz Businesses or firms

com Commercial Organizations

coop Cooperative business

Organizations

edu Educational institutions

gov Government institutions

info Information service providers

int International Organizations

mil Military groups

museum Museum & other nonprofit

organizations
 name Personal names

net Network Support centers

org Nonprofit Organizations

pro Professional individual

Organizations

Country Domain
The format of country domain is same as a generic domain, but it uses two-
character country abbreviations (e.g., us for the United States) in place of
three character organizational abbreviations.

Inverse Domain
The inverse domain is used for mapping an address to a name. When the
server has received a request from the client, and the server contains the files
of only authorized clients. To determine whether the client is on the authorized
list or not, it sends a query to the DNS server and ask for mapping an address
to the name.

Working of DNS
o DNS is a client/server network communication protocol. DNS clients send requests to the.
server while DNS servers send responses to the client.
o Client requests contain a name which is converted into an IP address known as a forward
DNS lookups while requests containing an IP address which is converted into a name
known as reverse DNS lookups.
o DNS implements a distributed database to store the name of all the hosts available on the
internet.
o If a client like a web browser sends a request containing a hostname, then a piece of
software such as DNS resolver sends a request to the DNS server to obtain the IP address
of a hostname. If DNS server does not contain the IP address associated with a hostname,
 then it forwards the request to another DNS server. If IP address has arrived at the resolver,
which in turn completes the request over the internet protocol.

HTTP
o HTTP stands for HyperText Transfer Protocol.
o It is a protocol used to access the data on the World Wide Web (www).
o The HTTP protocol can be used to transfer the data in the form of plain text, hypertext,
audio, video, and so on.
o This protocol is known as HyperText Transfer Protocol because of its efficiency that allows
us to use in a hypertext environment where there are rapid jumps from one document to
another document.
o HTTP is similar to the FTP as it also transfers the files from one host to another host. But,
HTTP is simpler than FTP as HTTP uses only one connection, i.e., no control connection to
transfer the files.
o HTTP is used to carry the data in the form of MIME-like format.
o HTTP is similar to SMTP as the data is transferred between client and server. The HTTP
differs from the SMTP in the way the messages are sent from the client to the server and
from server to the client. SMTP messages are stored and forwarded while HTTP messages
are delivered immediately.

Features of HTTP:
o Connectionless protocol: HTTP is a connectionless protocol. HTTP client initiates a
request and waits for a response from the server. When the server receives the request,
the server processes the request and sends back the response to the HTTP client after
which the client disconnects the connection. The connection between client and server
exist only during the current request and response time only.
o Media independent: HTTP protocol is a media independent as data can be sent as long
as both the client and server know how to handle the data content. It is required for both
the client and server to specify the content type in MIME-type header.
o Stateless: HTTP is a stateless protocol as both the client and server know each other only
during the current request. Due to this nature of the protocol, both the client and server
do not retain the information between various requests of the web pages.
SMTP
o SMTP stands for Simple Mail Transfer Protocol.
o SMTP is a set of communication guidelines that allow software to transmit an electronic
mail over the internet is called Simple Mail Transfer Protocol.
o It is a program used for sending messages to other computer users based on e-mail
addresses.
o It provides a mail exchange between users on the same or different computers, and it also
supports:
o It can send a single message to one or more recipients.
o Sending message can include text, voice, video or graphics.
o It can also send the messages on networks outside the internet.
o The main purpose of SMTP is used to set up communication rules between servers. The
servers have a way of identifying themselves and announcing what kind of communication
they are trying to perform. They also have a way of handling the errors such as incorrect
email address. For example, if the recipient address is wrong, then receiving server reply
with an error message of some kind.

Components of SMTP

o First, we will break the SMTP client and SMTP server into two components such as user
agent (UA) and mail transfer agent (MTA). The user agent (UA) prepares the message,
creates the envelope and then puts the message in the envelope. The mail transfer agent
(MTA) transfers this mail across the internet.
 o SMTP allows a more complex system by adding a relaying system. Instead of just having
one MTA at sending side and one at receiving side, more MTAs can be added, acting either
as a client or server to relay the email.

Working of SMTP
1. Composition of Mail: A user sends an e-mail by composing an electronic mail message
using a Mail User Agent (MUA). Mail User Agent is a program which is used to send and
receive mail. The message contains two parts: body and header. The body is the main part
of the message while the header includes information such as the sender and recipient
address. The header also includes descriptive information such as the subject of the
message. In this case, the message body is like a letter and header is like an envelope that
contains the recipient's address.
2. Submission of Mail: After composing an email, the mail client then submits the
completed e-mail to the SMTP server by using SMTP on TCP port 25.
3. Delivery of Mail: E-mail addresses contain two parts: username of the recipient and
domain name. For example, vivek@gmail.com, where "vivek" is the username of the
recipient and "gmail.com" is the domain name.
If the domain name of the recipient's email address is different from the sender's domain
name, then MSA will send the mail to the Mail Transfer Agent (MTA). To relay the email,
the MTA will find the target domain. It checks the MX record from Domain Name System
to obtain the target domain. The MX record contains the domain name and IP address of
the recipient's domain. Once the record is located, MTA connects to the exchange server
to relay the message.
4. Receipt and Processing of Mail: Once the incoming message is received, the exchange
server delivers it to the incoming server (Mail Delivery Agent) which stores the e-mail
where it waits for the user to retrieve it.
5. Access and Retrieval of Mail: The stored email in MDA can be retrieved by using MUA
(Mail User Agent). MUA can be accessed by using login and password.
Telnet
o The main task of the internet is to provide services to users. For example, users want to
run different application programs at the remote site and transfers a result to the local
site. This requires a client-server program such as FTP, SMTP. But this would not allow us
to create a specific program for each demand.
o The better solution is to provide a general client-server program that lets the user access
any application program on a remote computer. Therefore, a program that allows a user
to log on to a remote computer. A popular client-server program Telnet is used to meet
such demands. Telnet is an abbreviation for Terminal Network.
o Telnet provides a connection to the remote computer in such a way that a local terminal
appears to be at the remote side.

There are two types of login:

Local Login
 o When a user logs into a local computer, then it is known as local login.
o When the workstation running terminal emulator, the keystrokes entered by
the user are accepted by the terminal driver. The terminal driver then passes
these characters to the operating system which in turn, invokes the desired
application program.
o However, the operating system has special meaning to special characters.
For example, in UNIX some combination of characters have special
meanings such as control character with "z" means suspend. Such situations
do not create any problem as the terminal driver knows the meaning of such
characters. But, it can cause the problems in remote login.

Remote login

o When the user wants to access an application program on a remote computer,

then the user must perform remote login.

At the local site

The user sends the keystrokes to the terminal driver, the characters are then sent
to the TELNET client. The TELNET client which in turn, transforms the characters to
a universal character set known as network virtual terminal characters and delivers
them to the local TCP/IP stack

At the remote site

The commands in NVT forms are transmitted to the TCP/IP at the remote machine.
Here, the characters are delivered to the operating system and then pass to the
TELNET server. The TELNET server transforms the characters which can be
understandable by a remote computer. However, the characters cannot be directly
 passed to the operating system as a remote operating system does not receive the
characters from the TELNET server. Therefore it requires some piece of software
that can accept the characters from the TELNET server. The operating system then
passes these characters to the appropriate application program.

FTP
o FTP stands for File transfer protocol.
o FTP is a standard internet protocol provided by TCP/IP used for transmitting the files from
one host to another.
o It is mainly used for transferring the web page files from their creator to the computer that
acts as a server for other computers on the internet.
o It is also used for downloading the files to computer from other servers.

Objectives of FTP
o It provides the sharing of files.
o It is used to encourage the use of remote computers.
o It transfers the data more reliably and efficiently.

There are two types of connections in FTP:

o Control Connection: The control connection uses very simple rules for communication.
Through control connection, we can transfer a line of command or line of response at a
time. The control connection is made between the control processes. The control
connection remains connected during the entire interactive FTP session.
o Data Connection: The Data Connection uses very complex rules as data types may vary.
The data connection is made between data transfer processes. The data connection opens
when a command comes for transferring the files and closes when the file is transferred.

Introduction Of Presentation Layer and Services of the

presentation layer :

The presentation layer is one of the seven layers in the Open Systems Interconnection
(OSI) model of networking. It is responsible for formatting and presenting data in a way
that is both understood by the sender and can be interpreted by the receiver. In essence,
the presentation layer ensures that data transmitted between systems is in a compatible
format, meaning that the data is encoded and properly formatted based on the standards
of both sender and receiver.

• The main functions of the presentation layer are as follows −

o It encodes the messages from the user dependent format to the common
format and vice versa, for communication among dissimilar systems.

o It is responsible for data encryption and decryption of sensitive data before

they are transmitted over common channels.

o It is also responsible for data compression. Data compression is done at the

source to reduce the number of bits to be transmitted. It reduces the storage
space and increases the file transfer rate. It is particularly useful for
transmission of large multimedia files.

More specifically, the presentation layer provides a set of services to ensure that
information is intelligible to an application or user interface. This includes:

1. Data Translation: The presentation layer serves as a data translator, converting data from
one format to another, if necessary. For example, it can convert data from ASCII to Unicode
formats.

2. Data Encryption: The presentation layer can encrypt data to provide access control and
privacy during transmission, securing the data between the sender and receiver. This is
especially important for sensitive or confidential information.
 3. Data Compression: Data compression reduces the amount of data that must be
transmitted from one system to another. The presentation layer provides data
compression services to minimize the data transmitted during communication.

4. Data Formatting: The presentation layer reformats data according to the requirements
of the application. For example, it might convert a spreadsheet into a graph or a chart for
a user interface.

5. Error detection and correction: The presentation layer provides error detection and
correction services to ensure that any data transmission errors are detected and corrected
quickly, to ensure that the correct data is transmitted.

6. Session Management: The presentation layer manages multiple application sessions,

ensuring that the information being transmitted is associated with the correct application
session.

In summary, the presentation layer provides services that ensure that data sent between
two systems is accurately represented, secured, and formatted for the receiving system. It
also provides management of multiple sessions between applications and provides error
detection and correction to ensure that the correct data is transmitted.

• The session layer

The session layer is Layer 5 layer from the bottom in the OSI model. The job
of the session layer is to control and maintain connections between systems
to share data. It establishes, maintains, and ends sessions across all channels.
In case of a network error, it checks the authenticity and provides recovery
options for active sessions. It manages sessions and synchronizes data flow.

Basically, this layer regulates when computers can send data and how much
data they can send. Essentially it coordinates communication between
devices.

The session layer is responsible for establishing, maintaining, and

terminating communication sessions between two devices. It is
responsible for managing the flow of data between the devices to
ensure that data is transmitted efficiently and accurately.

• Data flow control is an important function of the session layer. It ensures that
data is transmitted at a rate that the receiver can handle. If data is transmitted
too quickly, the receiver may not be able to process it all, leading to data loss
and retransmissions. If data is transmitted too slowly, it may lead to delays in
the communication session.

The session layer uses several techniques to manage data flow control. One
of the techniques is called buffering, where data is temporarily stored in the
sender’s device until it can be transmitted to the receiver. Another technique
is called windowing, where the sender and receiver agree on a window size
that determines how much data can be transmitted at a time.

Simplex mode

o In Simplex mode, the communication is unidirectional, i.e., the data flow in one direction.
o A device can only send the data but cannot receive it or it can receive the data but cannot
send the data.
o This transmission mode is not very popular as mainly communications require the two-
way exchange of data. The simplex mode is used in the business field as in sales that do
not require any corresponding reply.
o The radio station is a simplex channel as it transmits the signal to the listeners but never
allows them to transmit back.
o Keyboard and Monitor are the examples of the simplex mode as a keyboard can only
accept the data from the user and monitor can only be used to display the data on the
screen.
o The main advantage of the simplex mode is that the full capacity of the communication
channel can be utilized during transmission.
Advantage of Simplex mode:
o In simplex mode, the station can utilize the entire bandwidth of the communication
channel, so that more data can be transmitted at a time.

Disadvantage of Simplex mode:

o Communication is unidirectional, so it has no inter-communication between devices.

Half-Duplex mode

o In a Half-duplex channel, direction can be reversed, i.e., the station can transmit and
receive the data as well.
o Messages flow in both the directions, but not at the same time.
o The entire bandwidth of the communication channel is utilized in one direction at a time.
o In half-duplex mode, it is possible to perform the error detection, and if any error occurs,
then the receiver requests the sender to retransmit the data.
o A Walkie-talkie is an example of the Half-duplex mode. In Walkie-talkie, one party speaks,
and another party listens. After a pause, the other speaks and first party listens. Speaking
simultaneously will create the distorted sound which cannot be understood.

Advantage of Half-duplex mode:

o In half-duplex mode, both the devices can send and receive the data and also can utilize
the entire bandwidth of the communication channel during the transmission of data.

Disadvantage of Half-Duplex mode:

o In half-duplex mode, when one device is sending the data, then another has to wait, this
causes the delay in sending the data at the right time.
Full-duplex mode

o In Full duplex mode, the communication is bi-directional, i.e., the data flow in both the
directions.
o Both the stations can send and receive the message simultaneously.
o Full-duplex mode has two simplex channels. One channel has traffic moving in one
direction, and another channel has traffic flowing in the opposite direction.
o The Full-duplex mode is the fastest mode of communication between devices.
o The most common example of the full-duplex mode is a telephone network. When two
people are communicating with each other by a telephone line, both can talk and listen at
the same time.

Advantage of Full-duplex mode:

o Both the stations can send and receive the data at the same time.

Disadvantage of Full-duplex mode:

o If there is no dedicated path exists between the devices, then the capacity of the
communication channel is divided into two parts.

Token management is an essential process in the Session Layer of the OSI (Open
Systems Interconnection) model. The Session Layer is responsible for establishing,
maintaining, and synchronizing sessions between two communicating devices over
a network.

Token Management :-

Token management in the Session Layer involves the use of tokens, which are
pieces of data that are passed between devices to control communication access.
Tokens are used to prevent two devices from transmitting data at the same time,
which can lead to data collisions and loss.
The process of token management starts with one device obtaining a token and
being granted access to the network. The device can then transmit data until it has
completed its transmission or the token has expired. Once the device has
completed its transmission or the token has expired, the token is released, and
other devices can request access to the network and obtain a token.

In a token ring network, for example, the token is passed around the network in a
predefined direction. Only the device that holds the token can transmit data,
ensuring that no two devices try to communicate at the same time. Since token
management is essential for network performance and reliability, it is important to
have robust mechanisms for granting and releasing tokens.

• Synchronization: The session maintains proper connectivity between

systems, and if any error occurs, then it provides a recovery option which
is called a known state. The session layer adds synchronization bits to the
message to use the known state in the event of an error. These bits can be
used as checkpoints. It adds synchronization points or checkpoints to the
data stream for longer communication. It ensures that the data streams
are successfully received and acknowledged up to the checkpoints. In case
of any failure, only the stream needs to be retransmitted after the
checkpoints.

It synchronizes communication. It adds synchronization points or checkpoints in data

streams for long communications. This ensures that data streams up to the
checkpoints are successfully received and acknowledged. In case of any failures, only
the streams after the checkpoints have to be re-transmitted.