TWO TYPES OF TRANSMISSION TECHNOLOGY :( 5 MARK)

Or

NETWORK HARDWARE

The transmission technologies are divided into two types −

 Broadcast networks and

 Point-to-point networks.

Let us begin by learning about broadcast networks.

Broadcast Networks

 Transmitting data from one source host to all other hosts present in the same or other
network is called broadcast. It is called a one to all transmission.

Types of broadcast

 Broadcast is classified into two types, which are as follows −

Limited Broadcast

 Transmitting data from one source host to all other hosts present in the same network
is called a limited broadcast. Given below is the diagram of limited broadcast −

 In Limited Broad casting if the destination address is 255.255.255.255 then the packet
will be sent to all the hosts in the network.
 Limited Broadcast address of any network=255.255.255.255 =
11111111.11111111.11111111.11111111
 For example: If the source IP address is 12.23.2.5 sending data to all other hosts
present in the same network, then the destination address is 255.255.255.255.

Direct Broadcast

Transmitting data from source host to all other hosts present in different networks then it is
called as direct broadcast. Given below is the diagram of direct broadcast –

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  In direct broadcast Host ID bits are all set to 1, Network ID is the IP address where all
destination hosts are present.
 For example: Source IP address is 12.34.5.6 sending data to all other nodes present at
different network having IP address 24.0.0.0

Therefore source address= 12.34.5.6

Destination address= 24.255.255.255.

Point-to-point networks:

 Point-to-point networks consist of many connections between individual pairs of

machines. Generally, the packets need to follow multiple routes, of different lengths.
So, routing algorithms are very important in point-to-point networks.
 Point-to-point networking is also called point-to-point protocol (PPP). Point to point
is a data link layer which is layer 2 communication protocol. It is present between two
routers without any host or any networking layers in between
 It also provides properties like connection, authentication, transmission, encryption
and the data compression. It is used over many physical layers like serial cables,
phone lines, and radio links.
 The two derivatives of PPP are Point to Point Protocol over Ethernet (PPPoE) and
another one is Point to Point Protocol over ATM (PPPoA). This is mainly used to
provide internet connection between the customers
 Point to Point Protocol is widely used by ISPs that are Internet Service Providers to
provide dial up connections to their internet. It mainly facilitates the transmission of
data packets between the point to point links.
 The importance of using PPP is to provide serial connections that are to provide
internet connections. Given below is the diagram of point to point network

Local Area Network (LAN) –(5 MARK)

 LAN or Local Area Network connects network devices in such a way that personal
computers and workstations can share data, tools, and programs. The group of
computers and devices are connected together by a switch, or stack of switches, using
a private addressing scheme as defined by the TCP/IP protocol. Private addresses are
unique in relation to other computers on the local network. Routers are found at the
boundary of a LAN, connecting them to the larger WAN.

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 A Communication medium used for LAN has twisted-pair cables and coaxial cables.
It covers a short distance, and so the error and noise are minimized.
 A LAN is a network that is used for communicating among computer devices, usually
within an office building or home.
 LAN’s enable the sharing of resources such as files or hardware devices that may be
needed by multiple users
 Is limited in size, typically spanning a few hundred meters, and no more than a mile
 Is fast, with speeds from 10 Mbps to 10 Gbps
 Requires little wiring, typically a single cable connecting to each device
 Has lower cost compared to MAN’s or WAN’s

 5 Local Area Network (LAN)

 LAN’s can be either wired or wireless. Twisted pair, coax or fiber optic cable can be
used in wired LAN’s.
 Every LAN uses a protocol – a set of rules that governs how packets are configured
and transmitted.
 Nodes in a LAN are linked together with a certain topology. These topologies include:
Bus, Ring, Star
 LANs are capable of very high transmission rates (100s Mb/s to G b/s).
Metropolitan Area Network (MAN) –(5 mark)
 MAN or Metropolitan area Network covers a larger area than that of a LAN and
smaller area as compared to WAN. It connects two or more computers that are apart
but reside in the same or different cities. It covers a large geographical area and may
serve as an ISP (Internet Service Provider). MAN is designed for customers who need
high-speed connectivity. Speeds of MAN range in terms of Mbps. It’s hard to design
and maintain a Metropolitan Area Network.

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  A metropolitan area network (MAN) is a large computer network that usually spans a
city or a large campus.
 A MAN is optimized for a larger geographical area than a LAN, ranging from several
blocks of buildings to entire cities.
 A MAN might be owned and operated by a single organization, but it usually will be
used by many individuals and organizations.
 A MAN often acts as a high speed network to allow sharing of regional resources.
 A MAN typically covers an area of between 5 and 50 km diameter.
Examples of MAN: Telephone company network that provides a high speed DSL to
customers and cable TV network.
 The fault tolerance of a MAN is less and also there is more congestion in the network.
It is costly and may or may not be owned by a single organization. The data transfer
rate and the propagation delay of MAN are moderate. Devices used for transmission
of data through MAN are Modem and Wire/Cable. Examples of a MAN are the part
of the telephone company network that can provide a high-speed DSL line to the
customer or the cable TV network in a city.

WAN: Wide Area Network ( 5 MARK)

WAN  stands for Wide Area Network. WAN is a computer network that extends over a
wide geographical area. WAN can contain multiple smaller networks, such as Local Area
Network (LAN) and Metropolitan Area Network (MAN).
Typically, a WAN consists of two or more local-area networks (LANs). Computers
connected to a wide-area network are often connected through public networks, such as the
telephone system. They can also be connected through leased lines or satellites.

Advantages of WAN

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 It covers large geographical area which enhances the reach of organisation to transmit data
quickly and cheaply
 The data can be stored in centralised manner because of remote access to data provided by
WAN
 The travel charges that are needed to cover the geographical area of work can be
minimised
 WAN enables a user or organisation to connect with the world very easily and allows to
exchange data and do business at global level.

LOW EARTH ORBIT (LEO) : (5 MARK)

 Low earth orbit (LEO) satellites systems orbit below 2000 km from the earth’s
surface, i.e. below the lower Van Allen belt. They move at very high speeds and may
not have any fixed space with respect to the earth.
 The following diagram depicts LEO satellites in their orbits.

Features of LEO Satellites

 A network of LEO satellites are needed for global coverage as their orbits are not
geostationary.
 These satellites are not as powerful as the MEO and GEO satellites.
 Due to their high speeds, satellites move in and out of the earth station’s range from
time to time. So, data is handed off from one satellite to the other to achieve
uninterrupted data communication.
 They are very much energy efficient. It takes much less energy to place the LEO
satellites in their orbits, in comparison to MEOs and GEOs. Also, their amplifiers
consume less power.
 They are quite cheap in comparison with other data communication modes. So, they
can be used as a more economic way of communication for underdeveloped areas.
 They can be used for establishing networks in remote terrains where it is not feasible
to lay land lines.
Types of LEO Satellites and their Uses
 Communication Satellites −   they are used for low cost data communication.

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  Earth Monitoring Satellites −  they are used for monitoring ground features. They
are better than satellites placed far away since they have a clearer view of the earth’s
surface.
 International Space Station −  they provide research laboratory to conduct
experiments within space environment. It is suited for testing spacecraft systems.

Message switching techniques (5 MARK)

Switched communication networks are those in which data transferred from source to
destination is routed between various intermediate nodes. Switching is the technique by
which nodes control or switch data to transmit it between specific points on a network. There
are 3 common switching techniques:
1. Circuit Switching
2. Packet Switching
3. Message Switching
Message Switching – 
Message switching was a technique developed as an alternative to circuit switching before
packet switching was introduced. In message switching, end-users communicate by sending
and receiving messages that included the entire data to be shared. Messages are the smallest
individual unit. 
Also, the sender and receiver are not directly connected. There are a number of intermediate
nodes that transfer data and ensure that the message reaches its destination. Message switched
data networks are hence called hop-by-hop systems.
They provide 2 distinct and important characteristics:
1. Store and forward – The intermediate nodes have the responsibility of transferring the
entire message to the next node. Hence, each node must have storage capacity. A message
will only be delivered if the next hop and the link connecting it are both available,
otherwise, it’ll be stored indefinitely. A store-and-forward switch forwards a message
only if sufficient resources are available and the next hop is accepting data. This is called
the store-and-forward property. 

2. Message delivery – This implies wrapping the entire information in a single message and
transferring it from the source to the destination node. Each message must have a header
that contains the message routing information, including the source and destination.
Message switching network consists of transmission links (channels), store-and-forward
switch nodes, and end stations as shown in the following picture:

Characteristics of message switching – 

Message switching is advantageous as it enables efficient usage of network resources. Also,
because of the store-and-forward capability of intermediary nodes, traffic can be efficiently

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regulated and controlled. Message delivery as one unit, rather than in pieces, is another
benefit.
However, message switching has certain disadvantages as well. Since messages are stored
indefinitely at each intermediate node, switches require a large storage capacity. Also, these
are pretty slow. This is because at each node, first there is a wait till the entire message is
received, then it must be stored and transmitted after processing the next node and links to it
depending on availability and channel traffic. Hence, message switching cannot be used for
real-time or interactive applications like a video conference.
Advantages of Message Switching – 
Message switching has the following advantages:
1. As message switching is able to store the message for which communication channel is
not available, it helps in reducing the traffic congestion in the network.
2. In message switching, the data channels are shared by the network devices.
3. It makes traffic management efficient by assigning priorities to the messages.
4. Because the messages are delivered via a store and forward method, it is possible to
include priority in them.
5. It allows for infinite message lengths.
6. Unlike circuit switching, it does not necessitate the actual connection of source and
destination devices.
Disadvantages of Message Switching – 
Message switching has the following disadvantages:
1. Message switching cannot be used for real-time applications as storing messages causes
delay.
2. In message switching, the message has to be stored for which every intermediate device
in the network requires a large storing capacity.
3. Because the system is so intricate, people are frequently unaware of whether or not
messages are correctly conveyed. This could cause problems in social relationships.
4. The type of message switching does not create a dedicated path between the devices. It is
not dependable communication because there is no direct relationship between sender and
receiver.
CRYPTOGRAPHY: (5 MARK)
Cryptography refers to the science and art of transforming messages to make them secure
and immune to attacks. It is a method of storing and transmitting data in a particular form so
that only those for whom it is intended can read and process it. Cryptography not only
protects data from theft or alteration but can also be used for user authentication.

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Components
There are various components of cryptography which are as follows −

Plaintext and Ciphertext

The original message, before being transformed, is called plaintext. After the message is
transformed, it is called ciphertext. An encryption algorithm transforms the plaintext into
ciphertext; a decryption algorithm transforms the ciphertext back into plaintext. The sender
uses an encryption algorithm, and the receiver uses a decryption algorithm.

Cipher
We refer to encryption and decryption algorithms as ciphers. The term cipher is also used to
refer to different categories of algorithms in cryptography. This is not to say that every
sender-receiver pair needs their very own unique cipher for secure communication. On the
contrary, one cipher can serve millions of communicating pairs.

Key
A key is a number (or a set of numbers) that the cipher, as an algorithm, operates on. To
encrypt a message, we need an encryption algorithm, an encryption key, and plaintext. These
create the cipher text. To decrypt a message, we need a decryption algorithm, a decryption
key, and the cipher text. These reveal the original plaintext.

Types
There are two types of cryptography which are as follows −

Symmetric Key Cryptography

In symmetric-key cryptography, the same key is used by both parties. The sender uses this
key and an encryption algorithm to encrypt data; the receiver uses the same key and the
corresponding decryption algorithm to decrypt the data.

Asymmetric-Key Cryptography
In asymmetric or public-key cryptography, there are two keys: a private key and a public
key. The private key is kept by the receiver. The public key is announced to the public.
In public-key encryption/decryption, the public key that is used for encryption is different
from the private key that is used for decryption. The public key is available to the public, and
the private key is available only to an individual.

Sliding Window Protocol: (5 MARK)

The sliding window is a technique for sending multiple frames at a time. It controls the data
packets between the two devices where reliable and gradual delivery of data frames is
needed. It is also used in TCP (Transmission Control Protocol).

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In this technique, each frame has sent from the sequence number. The sequence numbers are
used to find the missing data in the receiver end. The purpose of the sliding window
technique is to avoid duplicate data, so it uses the sequence number.

Types of Sliding Window Protocol

Sliding window protocol has two types:

1. Go-Back-N ARQ

2. Selective Repeat ARQ

Go-Back-N ARQ

Go-Back-N ARQ protocol is also known as Go-Back-N Automatic Repeat Request. It is a

data link layer protocol that uses a sliding window method. In this, if any frame is corrupted
or lost, all subsequent frames have to be sent again.

The size of the sender window is N in this protocol. For example, Go-Back-8, the size of the
sender window, will be 8. The receiver window size is always 1.

If the receiver receives a corrupted frame, it cancels it. The receiver does not accept a
corrupted frame. When the timer expires, the sender sends the correct frame again. The
design of the Go-Back-N ARQ protocol is shown below.

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The example of Go-Back-N ARQ

Selective Repeat ARQ

Selective Repeat ARQ is also known as the Selective Repeat Automatic Repeat Request. It is
a data link layer protocol that uses a sliding window method. The Go-back-N ARQ protocol
works well if it has fewer errors. But if there is a lot of error in the frame, lots of bandwidth
loss in sending the frames again. So, we use the Selective Repeat ARQ protocol. In this
protocol, the size of the sender window is always equal to the size of the receiver window.
The size of the sliding window is always greater than 1.

If the receiver receives a corrupt frame, it does not directly discard it. It sends a negative
acknowledgment to the sender. The sender sends that frame again as soon as on the receiving
negative acknowledgment. There is no waiting for any time-out to send that frame. The
design of the Selective Repeat ARQ protocol is shown below.

Difference between Datagram switching & Virtual circuit switching :

(5 MARK)
Datagram Switching Virtual Circuit
It is connection less service. There is Virtual circuits are connection-oriented, which
no need for reservation of resources means that there is a reservation of resources
as there is no dedicated path for a like buffers, bandwidth, etc. for the time
connection session. during which the new setup VC is going to be
used by a data transfer session.
All packets are free to use any The first sent packet reserves resources at each
available path. As a result, server along the path. Subsequent packets will
intermediate routers calculate routes follow the same path as the first sent packet for
on the go due to dynamically the connection time.
changing routing tables on routers.
Data packets reach the destination in Packets reach in order to the destination as

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 random order, which means they need data follows the same path.
not reach in the order in which they
were sent out.
Every packet is free to choose any All the packets follow the same path and hence
path, and hence all the packets must a global header is required only for the first
be associated with a header packet of connection and other packets will not
containing information about the require it.
source and the upper layer data.
Datagram networks are not as reliable Virtual Circuits are highly reliable.
as Virtual Circuits.
Efficiency high, delay more Efficiency low and delay less
But it is always easy and cost- Implementation of virtual circuits is costly as
efficient to implement datagram each time a new connection has to be set up
networks as there is no need of with reservation of resources and extra
reserving resources and making a information handling at routers.
dedicated path each time an
application has to communicate.
A Datagram based network is a true A virtual circuit network uses a fixed path for
packet switched network. There is no a particular session, after which it breaks the
fixed path for transmitting data. connection and another path has to be set up
for the next session.
Widely used in Internet Used in X.25, ATM(Asynchronous Transfer
Mode)

TRANSPOSITION CIPHER: (5 MARK)

Transposition Cipher is a cryptographic algorithm where the order of alphabets in the
plaintext is rearranged to form a cipher text. In this process, the actual plain text alphabets are
not included.

Example
A simple example for a transposition cipher is columnar transposition cipher where each
character in the plain text is written horizontally with specified alphabet width. The cipher is
written vertically, which creates an entirely different cipher text.
Consider the plain text hello world, and let us apply the simple columnar transposition
technique as shown below

The plain text characters are placed horizontally and the cipher text is created with vertical
format as : holewdlo lr. Now, the receiver has to use the same table to decrypt the cipher text
to plain text.

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Code
The following program code demonstrates the basic implementation of columnar
transposition technique −
def split_len(seq, length):
return [seq[i:i + length] for i in range(0, len(seq), length)]
def encode(key, plaintext):
order = {
int(val): num for num, val in enumerate(key)
}
ciphertext = ''

for index in sorted(order.keys()):

for part in split_len(plaintext, len(key)):
try:ciphertext += part[order[index]]
except IndexError:
continue
return ciphertext
print(encode('3214', 'HELLO'))
Explanation
 Using the function split_len(), we can split the plain text characters, which can be
placed in columnar or row format.
 encode method helps to create cipher text with key specifying the number of columns
and prints the cipher text by reading characters through each column.

Output
The program code for the basic implementation of columnar transposition technique gives the
following output
Protocol Hierarchies : (5 MARK)
Prerequisite – 
A protocol is simply defined as a set of rules and regulations for data communication. Rules
are basically defined for each and every step and process at time of communication among
two or more computers. Networks are needed to follow these protocols to transmit data
successfully. All protocols might be implemented using hardware, software, or combination
of both of them. There are three aspects of protocols given below :
 Syntax –
It is used to explain data format that is needed to be sent or received.

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  Semantics –
It is used to explain exact meaning of each of sections of bits that are usually
transferred.
 Timings –
It is used to explain exact time at which data is generally transferred along with speed
at which it is transferred.
NETWORK SOFTWARE: (5 MARK)
 The first computer networks were designed with the hardware as the main concern
and the software as an afterthought. This strategy no longer works. Network software
is now highly structured. In the following sections we examine the software
structuring technique in some detail. The method described here forms the keystone
of the entire book and will occur repeatedly later on.
ProtocolHierarchies
Generally, Computer networks are comprised of or contain a large number of pieces of
hardware and software. To just simplify network design, various networks are organized and
arranged as a stack of layers of hardware and software, one on top of another. The number,
name, content, and function of each layer might vary and can be different from one network
to another. The main purpose of each of layers is just to offer and provide services to higher
layers that are present. Each and every layer has some particular task or function. In
programming, this concept is very common. The networks are organized and arranged as
different layers or levels simply to reduce and minimize complexity of design of network
software.
Example
Below is diagram representing a five-layer network. The diagram shows communication
between Host 1 and Host 2. The data stream is passed through a number of layers from one
host to other. Virtual communication is represented using dotted lines between peer layers.
Physical communication is represented using solid arrows between adjacent layers. Through
physical medium, actual communication occurs. The layers at same level are commonly
known as peers. The peer basically has a set of communication protocols. An interface is
present between each of layers that are used to explain services provided by lower layer to
higher layer.
Advantages :
 The layers generally reduce complexity of communication between networks
 It increases network lifetime.
 It also uses energy efficiently.
 It does not require overall knowledge and understanding of network.
ERROR CORRECTING CODES: (5 Mark)
Error-correcting codes are widely used on wireless links, which are notoriously
noisy and error prone when compared to copper wire or optical fibers. Without
error-correcting codes, it would be hard to get anything through. However, over
copper wire or fiber, the error rate is much lower, so error detection and
retransmission is usually more efficient there for dealing with the occasional
error.

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 Error-correcting codes (ECC) are a sequence of numbers generated by specific
algorithms for detecting and removing errors in data that has been transmitted over
noisy channels.
When bits are transmitted over the computer network, they are subject to get
corrupted due to interference and network problems. The corrupted bits leads to
spurious data being received by the receiver and are called errors.
Error correcting codes ascertain the exact number of bits that has been corrupted and
the location of the corrupted bits, within the limitations in algorithm. This method of
correcting errors at the receiver’s end is called forward error correction.
Types of Error Correcting Codes
ECCs can be broadly categorized into two types, block codes and convolution codes.
 Block codes − The message is divided into fixed-sized blocks of bits, to which
redundant bits are added for error detection or correction.
 Convolutional codes − The message comprises of data streams of arbitrary length
and parity symbols are generated by the sliding application of a Boolean function to
the data stream.
Common Error Correcting Codes
There are four popularly used error correction codes.

 Hamming Codes − It is a block code that is capable of detecting up to two

simultaneous bit errors and correcting single-bit errors.
 Binary Convolution Code − Here, an encoder processes an input sequence of bits of
arbitrary length and generates a sequence of output bits.
 Reed - Solomon Code − They are block codes that are capable of correcting burst
errors in the received data block.
 Low-Density Parity Check Code − It is a block code specified by a parity-check
matrix containing a low density of 1s. They are suitable for large block sizes in very
noisy channels.

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 What is Multiplexing? (1 mark)
Multiplexing is a technique used to combine and send the multiple
data streams over a single medium. The process of combining the
data streams is known as multiplexing and hardware used for
multiplexing is known as a multiplexer.

Frequency-division Multiplexing (FDM) (5MARK)

o It is an analog technique.
o Frequency Division Multiplexing is a technique in which the available bandwidth of
a single transmission medium is subdivided into several channels.

o In the above diagram, a single transmission medium is subdivided into several

frequency channels, and each frequency channel is given to different devices. Device
1 has a frequency channel of range from 1 to 5.
o The input signals are translated into frequency bands by using modulation techniques,
and they are combined by a multiplexer to form a composite signal.
o The main aim of the FDM is to subdivide the available bandwidth into different
frequency channels and allocate them to different devices.
o Using the modulation technique, the input signals are transmitted into frequency
bands and then combined to form a composite signal.
o The carriers which are used for modulating the signals are known as sub-carriers.
They are represented as f1,f2..fn.
o FDM is mainly used in radio broadcasts and TV networks.

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 Advantages Of FDM:
o FDM is used for analog signals.
o FDM process is very simple and easy modulation.
o A Large number of signals can be sent through an FDM simultaneously.
o It does not require any synchronization between sender and receiver.
Disadvantages Of FDM:
o FDM technique is used only when low-speed channels are required.
o It suffers the problem of crosstalk.
o A Large number of modulators are required.
o It requires a high bandwidth channel.
Applications Of FDM:
o FDM is commonly used in TV networks.
o It is used in FM and AM broadcasting. Each FM radio station has different
frequencies, and they are multiplexed to form a composite signal. The multiplexed
signal is transmitted in the air.

COMPUTER NETWORK: ( 2 MARK)

 A computer network is a system that connects numerous independent computers

in order to share information (data) and resources. The integration of computers
and other different devices allows users to communicate more easily.
 A computer network is a collection of two or more computer systems that are
linked together. A network connection can be established using either cable or
wireless media. Hardware and software are used to connect computers.

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  A computer network consists of various kinds of nodes. Servers, networking
hardware, personal computers, and other specialized or general-purpose hosts
can all be nodes in a computer network. Host names and network addresses are
used to identify them.
Goal of Networking: ( 2 MARK)

 Programs do not have to execute on a single system because of resource and load
sharing.

 Reduced costs – Multiple machines can share printers, tape drives, and other
peripherals.

 Reliability – If one machine fails, another can take its place.

 Scalability (it’s simple to add more processors or computers)

 Communication and mail (people living apart can work together)

 Information Access (remote information access, access to the internet, e-mail,

video conferencing, and online shopping)

 Entertainment that is interactive (online games, videos, etc.)

 Social Networking

OSI Reference Models : (10 MARK)

Now that we have discussed layered networks in the abstract, it is time to look at some
examples. In the next two sections we will discuss two important network architectures, the
OSI reference model and the TCP/IP reference model. Although the protocols associated with
the OSI model are rarely used any more, the model itself is actually quite general and still
valid, and the features discussed at each layer are still very important. The TCP/IP model has
the opposite properties: the model itself is not of much use but the protocols are widely used.
For this reason we will look at both of them in detail. Also, sometimes you can learn more
from failures than from successes.

OSI:

 OSI stands for Open Systems Interconnection

 Created by International Standards Organization (ISO)
 Was created as a framework and reference model to explain how different
 networking technologies work together and interact
 It is not a standard that networking protocols must follow
 Each layer has specific functions it is responsible for
All layers work together in the correct order to move data around a network

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Physical Layer

 Deals with all aspects of physically moving data from one computer to the next
 Converts data from the upper layers into 1s and 0s for transmission over media
 Defines how data is encoded onto the media to transmit the data
 Defined on this layer Cable standards, wireless standards, and fiber optic standards.
Copper wiring, fiber optic cable, radio frequencies, anything that can be used to
transmit data is defined on the Physical layer of the OSI Model
 Device example: Hub
 Used to transmit data
 Data Link Layer
 Is responsible for moving frames from node to node or computer to computer
 Can move frames from one adjacent computer to another, cannot move frames across
routers
 Encapsulation = frame
 Requires MAC address or physical address
 Protocols defined include Ethernet Protocol and Point-to-Point Protocol (PPP)
 Device example: Switch
 Two sub layers: Logical Link Control (LLC) and the Media Access Control (MAC) o
Logical Link Control (LLC)

 Data Link layer addressing, flow control, address notification, error control o Media
Access Control (MAC)
Network Layer

 Responsible for moving packets (data) from one end of the network to the other,
called end-to-end communications
 Requires logical addresses such as IP addresses
 Device example: Router
 –Routing is the ability of various network devices and their related software to move
data packets from source to destination

Transport Layer

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  Takes data from higher levels of OSI Model and breaks it into segments that can be
sent to lower-level layers for data transmission
 Conversely, reassembles data segments into data that higher-level protocols and
applications can use
 Also puts segments in correct order (called sequencing) so they can be reassembled in
correct order at destination
 Concerned with the reliability of the transport of sent data
 May use a connection-oriented protocol such as TCP to ensure destination received
segments
 May use a connectionless protocol such as UDP to send segments without assurance
of delivery
 Uses port addressing

Session Layer

 Responsible for managing the dialog between networked devices

 Establishes, manages, and terminates connections
 Provides duplex, half-duplex, or simplex communications between devices
 Provides procedures for establishing checkpoints, adjournment, termination, and
restart or recovery procedures

Presentation Layer

 Concerned with how data is presented to the network

 Handles three primary tasks: –Translation , –Compression , –Encryption

Application Layer

 Contains all services or protocols needed by application software or operating system

to communicate on the network
 Examples o –Firefox web browser uses HTTP (Hyper-Text Transport Protocol) o –E-
mail program may use POP3 (Post Office Protocol version 3) to read e-mails and
SMTP (Simple Mail Transport Protocol) to send e-mails.

TRANPORT LAYER Addressing (5 MARK)

o According to the layered model, the transport layer interacts with the functions of the session
layer. Many protocols combine session, presentation, and application layer protocols into a
single layer known as the application layer. In these cases, delivery to the session layer means
the delivery to the application layer. Data generated by an application on one machine must
be transmitted to the correct application on another machine. In this case, addressing is
provided by the transport layer.
o The transport layer provides the user address which is specified as a station or port. The port
variable represents a particular TS user of a specified station known as a Transport Service
access point (TSAP). Each station has only one transport entity.

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 o The transport layer protocols need to know which upper-layer protocols are communicating.

GUIDED TRANSMISSION MEDIA: (10 MARK)

Twisted-pair cable, coaxial cable, and fiber-optic cable are examples of guided
transmission media that offer a channel from one device to another. The physical
boundaries of the medium direct and contain a signal flowing via any of these
mediums.
Metallic (copper) conductors in twisted-pair and coaxial cable accept and convey
signals in the form of electric current. Optical fiber is a type of cable that receives
and transmits light signals.

Types of Guided Transmission Media:

1. Twisted Pair Cable:
This cable is the most often used and the least expensive. It is lightweight,
inexpensive, and simple to install, and it supports a wide range of network types.
Twisted pairs are made up of 2 conductors (mostly copper), each with its own
insulation, that are twisted together. One of these lines carries signals to the
receiver, while the other serves merely as a ground reference. The difference
between the two is used by the receiver to interpret signals.
In addition to the sender’s signal on one of the lines, interference may impact both
wires and generate undesired signals. Because the two wires are in different
positions relative to the noise sources, the effect of these undesirable signals is not
the same in both wires if they are parallel. As a result, there is a difference at the
receiver.

20
Types of Twisted Pair Cables:
a. Unshielded Twisted Pair Cable:
When compared to Shielded Twisted Pair Cable, which comprises two
conductors, generally copper, each with its own color plastic insulator,
Unshielded Twisted Pair is the most prevalent kind of cable used in
telecommunication.
UTP cables are made up of two or four pairs of twisted cables. RJ-11
connectors are used for two-pair cables, and RJ-45 connectors are used for
four-pair cables.
Advantages of Unshielded twisted pair cable:
Simple to set up.
Flexible
Cheap
Has a high speed capacity and a range of 100 meters.
Higher grade UTP is utilized in LAN technologies such as Ethernet.
Disadvantages of unshielded twisted pair cable:
Bandwidth is low when compared with the bandwidth of Coaxial Cable
Provides less protection from interference.
b. Shielded twisted pair cable:
Each pair of insulated conductors is encased in a metal foil or braided-mesh
coating on this cable. Metal enclosure prevents electromagnetic noise from
penetrating. Shielding also reduces cross talk.
This exhibits the same attenuation as an unshielded twisted pair. It
outperforms unshielded and coaxial cable in terms of speed. It costs more than
coaxial and unshielded twisted pair.
Advantages of shielded twisted pair cable:
Installation is simple, and performance is sufficient.
It may be used for both analogue and digital transmission.
Increases the pace of signaling
Unshielded twisted pair has a higher capacity than protected twisted pair.
Cross talk is eliminated.
Disadvantages of shielded twisted pair cable:
Manufacturing is difficult.
Heavy
Uses of Shielded Twisted Pair:

21
 Voice and data channels are provided in telephone lines. The telephone
companies’ DSL lines, which enable high-data-rate connections, also make use
of the high-bandwidth capabilities of unshielded twisted-pair cables.
Twisted-pair cables are also used in Local Area Networks such as 10Base-T
and 100Base-T.
2. Coaxial Cable:
Coaxial is so named because it has two conductors that are parallel to each
other. Copper is utilized as the center conductor in this, which can be a solid
wire or a normal wire. It is encircled by a PVC installation, a sheath, and an
outside conductor made of metal foil, metal braid, or both.
The outer copper wrapping serves as a noise barrier as well as the second
conductor that completes the circuit. An insulating layer surrounds the outside
conductor as well. The plastic cover on the outside covers the whole cable.
Types of Coaxial Cable:

a. Base band Coaxial cable:

This is a coaxial cable with a resistance of 50 ohms () that is used for digital
transmission. It is mostly used for LAN. Base band transmits a single signal at a
time at a high rate. The main disadvantage is that it requires amplification every
1000 feet.
b. Broadband Coaxial Cable:
This is accomplished through the use of analogue transmission over conventional
cable television wiring. It sends several signals at the same time at various
frequencies. When compared to Base band Coaxial Cable, it covers a larger region.
Advantages of broadband coaxial cable:
The bandwidth is really large.
Long-distance telephone lines make use of this term.
Digital signals are sent at a very high rate of 10Mbps.
Significantly improved noise immunity
Transmission of data with no distortion.
Because they have greater insulation than twisted pair cable, they can traverse
longer distances at higher speeds.
Disadvantages of broadband coaxial cable:
A single cable failure may bring the entire network to a halt.
When compared to twisted pair, it is more difficult to install and more costly.

22
 If the shield is not flawless, it might result in a grounded loop.
Uses of Coaxial Cable:
Coaxial cable is used in analog telephone networks, with a single coaxial
network capable of carrying 10,000 voice transmissions.
These are also used in cable TV networks. Coaxial cable was utilized across
the conventional cable TV network. Cable television employs RG-59 coaxial
cable.
In standard Ethernet LANs. Coaxial cable was selected for digital
transmission in early Ethernet LANs due to its large bandwidth and, as a result,
high data rate. 10Base-2, also known as Thin Ethernet, uses RG-58 coaxial
cable with BNC connections to transport data at 10Mbps over a distance of
185 meters.
3. Fiber Optic Cable:
Fiber optic is a cable that contains optical fibers coated in plastic and is used to
transmit data via light pulses.
The plastic covering shields the optical fibers from heat, cold, and electromagnetic
interference caused by other types of wiring.
Fiber optics transmit data at a quicker rate than copper cables.
Parts of Fiber Optic Cable:
a. Core:
The optical fiber is made up of a thin strand of glass or plastic known as the core.
A core is the portion of the fiber that transmits light. The larger the core area, the
more light will be transferred into the fiber.
b. Cladding:
Cladding refers to the concentric layer of glass. The primary and foremost function
of the cladding is to create a lower refractive index at the core interface, thereby
causing reflection within the core and allowing light waves to pass through the
fiber.
c. Jacket:
A jacket is a type of protective covering made of plastic. The primary function of a
jacket is to retain fiber strength, absorb stress, and provide further fibre protection.
Advantages of fiber optic cable:

23
Greater Bandwidth: When compared to copper, fiber optic cable delivers
greater bandwidth. As a result, fiber optic cable can carry more data than
copper wire.
Faster data transmission: Fiber optic cable transmits data in the form of
light. This enables the fiber optic cable to transport signals at a faster rate.
Larger lengths: When compared to copper cable, fiber optic cable transports
data across longer distances.
Better reliability: Fiber optic cable is more dependable than copper cable
since it is resistant to temperature fluctuations, which can create obstructions in
copper cable communication.
Thinner and more robust: Because fiber optic cable is thinner and lighter in
weight, it can resist more draw pressure than copper wire.

Telephone system Structure of the Telephone System ( 10 MARK)

The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and
Multiplexing Switching (DIAGRAM SEE IN THE BOOK PG.NO 93 )

 Public Switched Telephone Network

The PSTN (Public Switched Telephone Network), was designed many years ago, with
one goal – to transmit the human voice in a more-or-less recognizable form Its suitability
for use in computer-computer communication is often marginal at best, but the situation
is rapidly changing with the introduction of fiber optics and digital technology Still, the
telephone system is tightly intertwined with (wide area) computer networks

Cable versus dial-up lines

A cable running between two computers can transfer data at 109 bps, maybe more A
dial-up line has a maximum data rate of 56 kbps, a difference of a factor of almost
20,000With an ADSL connection, there is still a factor of 1000–2000 difference

 Structure of the Telephone System

The initial market was for the sale of telephones, which came in pairs. It was up to the
customer to string a single wire between them Then came the single switching office
Then came the need to connect the switching offices Second-level switching offices
were invented and after a while, multiple second-level offices were needed - the
hierarchy grew to five levels

 Structure of the Telephone System

(a) Fully-interconnected network.(b) Centralized switch.(c) Two-level hierarchy.

 The Telephone System Each telephone has two copper wires coming out of it that go
directly to the telephone company's nearest end office (also called a local central office)
– distance 1 to 10 km, being shorter in cities than in rural areas In the United States
alone there are about 22,000 end offices The two-wire connections between each
subscriber's telephone and the end office are known in the trade as the local loop

24
The Telephone System If a subscriber attached to a given end office calls another
subscriber attached to the same end office, the switching mechanism within the office
sets up a direct electrical connection between the two local loops and it remains intact
for the duration of the call Each end office has a number of outgoing lines to one or more
nearby switching centers, called toll offices The toll, primary, sectional, and regional
exchanges communicate with each other via high-bandwidth Interpol trunks

 Structure of the Telephone System

A typical circuit route for a medium-distance call

The Telephone System Local loops consist of category 3 twisted pairs

Between switching offices, coaxial cables, microwaves, and especially fiber optics are
widely used In the past, transmission throughout the telephone system was analog, with
the actual voice signal being transmitted as an electrical voltage from source to
destination Nowadays all the trunks and switches are digital, leaving the local loop as the
last piece of analog technology in the system

The Telephone System Digital transmission is preferred because it is able to correctly

distinguish a 0 from 1 which makes digital transmission more reliable than analog; it is
also cheaper and easier to maintain In summary, the telephone system consists of three
major components: Local loops (analog twisted pairs going into houses and
businesses).Trunks (digital fiber optics connecting the switching offices).Switching
offices (where calls are moved from one trunk to another).

The Local Loop: Modems, ADSL, and Wireless

The use of both analog and digital transmissions for a computer to computer call.
Conversion is done by the modems and codecs (compressor/decompressor)

 The Local Loop When a computer wishes to send digital data over an analog dial-up
line, the data must first be converted to analog form for transmission over the local loop
The conversion is done by modem - modulator-demodulator – a device that accepts a
serial stream of bits as input and produces a carrier modulated by one (or more) methods
(or vice versa – accepts modulated carrier and produces serial stream of bits)At the
telephone company end office the data are converted to digital form for transmission
over the long-haul trunks

 The Local Loop Transmission lines suffer from three major problems: attenuation, delay
distortion, and noise Attenuation - the loss of energy of the signal during its propagation
Delay distortion - the different Fourier components propagate at different speeds in the
wire Noise - unwanted energy from sources other than the transmitter

Modems and modulations

As the square waves used in digital signals have a wide frequency spectrum and are
subject to strong attenuation and delay distortion, DC (Direct Current) signaling is
unsuitable except at slow speeds and over short distances, so AC (Alternating Current)
signaling is used With AC signaling, a continuous tone in the 1000 to 2000-Hz range,
called a sine wave carrier, is introduced. Its amplitude, frequency, or phase can be
modulated to transmit information

Modems and modulations

In amplitude modulation, two different amplitudes are used to represent 0 and 1,
respectively In frequency modulation, two (or more) different tones are used .In the
25
simplest form of phase modulation, the carrier wave is systematically shifted 0 or 180
degrees at uniformly spaced intervals. A better scheme is to use shifts of 45, 135, 225, or
315 degrees to transmit 2 bits of information per time interval.

Modulations (a) A binary signal (c) Frequency modulation

(b) Amplitude modulation(c) Frequency modulation(d) Phase modulation

Other modulations For higher speeds, it is not possible to just keep increasing the
sampling rate - even with a perfect 3000-Hz line (NY Quist theorem), there is no point in
sampling faster than 6000 Hz Most modems sample 2400 times/sec and focus on getting
more bits per sample The number of samples per second is measured in baud - for each
baud, one symbol is sentn-baud line transmits n symbols/sec- for example, a 2400-baud
line sends one symbol about every µsec

 Baud rate and bit rate Baud rate is the number of time a line changes per second
Example :If Baud rate = 4 – this means that there will be 4 changes per second If the
number of bits per line change are 2=> the bit rate = 8bpsIf we have amplitude
modulation – 4 levels – 00, 01, 10 and 11 level (similar for phase mod), or 4 tones
(frequency modulation)

Multiple Access Protocols ( 10 mark)

The Data Link Layer is responsible for transmission of data between two nodes. Its
main functions are- 

 Data Link Control

 Multiple Access Control

Data Link control – 

The data link control is responsible for reliable transmission of message over
transmission channel by using techniques like framing, error control and flow control.
For Data link control refer to – Stop and Wait ARQ 

Multiple Access Control – 

If there is a dedicated link between the sender and the receiver then data link control
layer is sufficient, however if there is no dedicated link present then multiple stations
can access the channel simultaneously. Hence multiple access protocols are required
to decrease collision and avoid crosstalk. For example, in a classroom full of students,
when a teacher asks a question and all the students (or stations) start answering
simultaneously (send data at same time) then a lot of chaos is created( data overlap or
data lost) then it is the job of the teacher (multiple access protocols) to manage the
students and make them answer one at a time. Thus, protocols are required for sharing

26
 data on non dedicated channels. Multiple access protocols can be subdivided further
as –

1. Random Access Protocol: In this, all stations have same superiority that is no
station has more priority than another station. Any station can send data depending on
medium’s state (idle or busy). It has two features: 

1. There is no fixed time for sending data

2. There is no fixed sequence of stations sending data

The Random access protocols are further subdivided as: 

(a) ALOHA – It was designed for wireless LAN but is also applicable for shared
medium. In this, multiple stations can transmit data at the same time and can hence
lead to collision and data being garbled. 

 Pure Aloha: 
When a station sends data it waits for an acknowledgement. If the acknowledgement
doesn’t come within the allotted time then the station waits for a random amount of
time called back-off time (Tb) and re-sends the data. Since different stations wait for
different amount of time, the probability of further collision decreases. 

Vulnerable Time = 2* Frame transmission time

Throughput = G exp {-2*G}

Maximum throughput = 0.184 for G=0.5

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 Slotted Aloha: 
It is similar to pure aloha, except that we divide time into slots and sending of data is
allowed only at the beginning of these slots. If a station misses out the allowed time, it
must wait for the next slot. This reduces the probability of collision. 

Vulnerable Time = Frame transmission time

Throughput = G exp {-*G}

Maximum throughput = 0.368 for G=1

For more information on ALOHA refer – LAN Technologies 

(b) CSMA – Carrier Sense Multiple Access ensures fewer collisions as the station is
required to first sense the medium (for idle or busy) before transmitting data. If it is
idle then it sends data, otherwise it waits till the channel becomes idle. However there
is still chance of collision in CSMA due to propagation delay. For example, if station
A wants to send data, it will first sense the medium.If it finds the channel idle, it will
start sending data. However, by the time the first bit of data is transmitted (delayed
due to propagation delay) from station A, if station B requests to send data and senses
the medium it will also find it idle and will also send data. This will result in collision
of data from station A and B. 

CSMA access modes- 

 1-persistent: The node senses the channel, if idle it sends the data, otherwise it
continuously keeps on checking the medium for being idle and transmits
unconditionally(with 1 probability) as soon as the channel gets idle.

 Non-Persistent: The node senses the channel, if idle it sends the data, otherwise it
checks the medium after a random amount of time (not continuously) and transmits
when found idle. 

 P-persistent: The node senses the medium; if idle it sends the data with p probability.
If the data is not transmitted ((1-p) probability) then it waits for some time and checks
the medium again, now if it is found idle then it send with p probability. This repeat
continues until the frame is sent. It is used in Wi-Fi and packet radio systems. 

28
 O-persistent: Superiority of nodes is decided beforehand and transmission occurs in
that order. If the medium is idle, node waits for its time slot to send data. 

(c) CSMA/CD – Carrier sense multiple access with collision detection. Stations can
terminate transmission of data if collision is detected. For more details refer
– Efficiency of CSMA/CD 

(d) CSMA/CA – Carrier sense multiple access with collision avoidance. The process
of collisions detection involves sender receiving acknowledgement signals. If there is
just one signal(its own) then the data is successfully sent but if there are two
signals(its own and the one with which it has collided) then it means a collision has
occurred. To distinguish between these two cases, collision must have a lot of impact
on received signal. However it is not so in wired networks, so CSMA/CA is used in
this case. 

CSMA/CA avoids collision by: 

1. Interframe space – Station waits for medium to become idle and if found idle it does
not immediately send data (to avoid collision due to propagation delay) rather it waits
for a period of time called Interframe space or IFS. After this time it again checks the
medium for being idle. The IFS duration depends on the priority of station.

2. Contention Window – It is the amount of time divided into slots. If the sender is
ready to send data, it chooses a random number of slots as wait time which doubles
every time medium is not found idle. If the medium is found busy it does not restart
the entire process, rather it restarts the timer when the channel is found idle again.

3. Acknowledgement – The sender re-transmits the data if acknowledgement is not

received before time-out.

2. Controlled Access: 
In this, the data is sent by that station which is approved by all other stations. For
further details refer – Controlled Access Protocols 

3. Channelization: 
In this, the available bandwidth of the link is shared in time, frequency and code to
multiple stations to access channel simultaneously. 

29
  Frequency Division Multiple Access (FDMA) – The available bandwidth is divided
into equal bands so that each station can be allocated its own band. Guard bands are
also added so that no two bands overlap to avoid crosstalk and noise. 

 Time Division Multiple Access (TDMA) – In this, the bandwidth is shared between
multiple stations. To avoid collision time is divided into slots and stations are allotted
these slots to transmit data. However there is a overhead of synchronization as each
station needs to know its time slot. This is resolved by adding synchronization bits to
each slot. Another issue with TDMA is propagation delay which is resolved by
addition of guard bands. 
For more details refer – Circuit Switching 

 Code Division Multiple Access (CDMA) – One channel carries all transmissions
simultaneously. There is neither division of bandwidth nor division of time. For
example, if there are many people in a room all speaking at the same time, then also
perfect reception of data is possible if only two person speak the same language.
Similarly, data from different stations can be transmitted simultaneously in different
code languages.

INTERNET ( 10 OR 5 MARK )

The Internet, sometimes called simply "the Net," is a worldwide system of computer
networks -- a network of networks in which users at any one computer can, if they have
permission, get information from any other computer (and sometimes talk directly to users at
other computers). It was conceived by the Advanced Research Projects Agency (ARPA) of
the U.S. government in 1969 and was first known as the ARPANET. The original aim was to
create a network that would allow users of a research computer at one university to "talk to"
research computers at other universities. A side benefit of ARPANet's design was that,
because messages could be routed or rerouted in more than one direction, the network could
continue to function even if parts of it were destroyed in the event of a military attack or other
disaster.

30
Today, the Internet is a public, cooperative and self-sustaining facility accessible to hundreds
of millions of people worldwide. It is used by many as the primary source of information
consumption, and fueled the creation and growth of its own social ecosystem through social
media and content sharing. Furthermore, e-commerce, or online shopping, has become one of
the largest uses of the Internet.

How the Internet works

Physically, the Internet uses a portion of the total resources of the currently existing
public telecommunication networks. Technically, what distinguishes the Internet is its use of
a set of protocols called Transmission Control Protocol/Internet Protocol (TCP/IP). Two
recent adaptations of Internet technology, the Intranet and the extranet, also make use of the
TCP/IP protocol.

The Internet can be seen as having two major components: network protocols and hardware.
The protocols, such as the TCP/IP suite, present sets of rules that devices must follow in
order to complete tasks. Without this common collection of rules, machines would not be
able to communicate.

The protocols are also responsible for translating the alphabetic text of a message into
electronic signals that can be transmitted over the Internet, and then back again into legible,
alphabetic text.

Hardware, the second major component of the Internet, includes everything from the
computer or smartphone that is used to access the Internet to the cables that carry information
from one device to another. Additional types of hardware include satellites, radios, cell phone
towers, routers and servers.

These various types of hardware are the connections within the network. Devices such as
computers, smartphones and laptops are end points, or clients, while the machines that store
the information are the servers. The transmission lines that exchange the data can either be
wireless signals from satellites or 4G and cell phone towers, or physical lines, such as cables
and fiber optics.

The process of transferring information from one device to another relies on packet
switching. Each computer connected to the Internet is assigned a unique IP address that

31
allows the device to be recognized. When one device attempts to send a message to another
device, the data is sent over the Internet in the form of manageable packets. Each packet is
assigned a port number that will connect it to its endpoint.

A packet that has both a unique IP address and port number can be translated from alphabetic
text into electronic signals by travelling through the layers of the OSI model from the
top application layer to the bottom physical layer. The message will then be sent over the
Internet where it is received by the Internet service provider's (ISP) router. The router will
examine the destination address assigned to each packet and determine where to send it.

Eventually, the packet reaches the client and travels in reverse from the bottom physical layer
of the OSI model to the top application layer. During this process, the routing data -- the port
number and IP address -- is stripped from the packet, thus allowing the data to be translated
back into alphabetic text and completing the transmission process.

Uses of the internet

In general, the Internet can be used to communicate across large or small distances, share
information from any place in the world and access information or answers to almost any
question in moments.

Some specific examples of how the Internet is used include:

 Social media and content sharing;

 E-mail and other forms of communication, such as Internet Relay Chat (IRC), Internet
telephony, instant messaging, video conferencing

 education and self-improvement through access to online degree programs, courses and
workshops and

 searching for jobs -- both the employer and applicant use the Internet to post open
positions, apply for jobs and recruit individuals found on social networking sites
like LinkedIn.

Other examples include:

 Online discussion groups and forums

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 Online dating

 Online gaming

 Research

 Reading electronic newspapers and magazines

 Online shopping, or e-commerce.

Social impact of the Internet

 The social impact of the Internet can be seen as both positive and negative. On one
side, people argue that the Internet has increased the risk of isolation, alienation and
withdrawal from society, pointing to increases in an emotional response
called FOMO, or the fear of missing out. On the other side, people believe the Internet
to have had the opposite effect on society, arguing that the Internet increases civic
engagement, sociability and the intensity of relationships.

Benefits of the Internet

Benefits of the Internet include:

 Access to endless information, knowledge and education.

 An increased ability to communicate, connect and share.

 The ability to work from home, collaborate and access a global workforce.

 The chance to sell and make money as a business or individual.

 Access to an unlimited supply of entertainment sources, such as movies, music, videos

and games.

 The ability to amplify the impact of a message, allowing charities and other organizations
to reach a wider audience and increase the total amount of donations.

 Access to the internet of things (IoT), which allows home appliances and devices to
connect and be controlled from a computer or smartphone.

 The ability to save data and easily share files with cloud storage.

 The ability to monitor and control personal accounts instantly, such as bank accounts or
credit card bills.

33
Channel Allocation Problem in Computer Network ( 10 mark)

Channel allocation is a process in which a single channel is divided and allotted to

multiple users in order to carry user specific tasks. There are user’s quantity may vary
every time the process takes place. If there are N number of users and channel is divided
into N equal-sized sub channels, Each user is assigned one portion. If the number of users
are small and don’t vary at times, than Frequency Division Multiplexing can be used as it
is a simple and efficient channel bandwidth allocating technique. 

Channel allocation problem can be solved by two schemes: Static Channel Allocation
in LANs and MANs, and Dynamic Channel Allocation. 

 These are explained as following below. 

1. Static Channel Allocation in LANs and MANs: 

It is the classical or traditional approach of allocating a single channel among multiple
competing users Frequency Division Multiplexing (FDM). if there are N users, the
bandwidth is divided into N equal sized portions each user being assigned one portion.
since each user has a private frequency band, there is no interface between users. 

It is not efficient to divide into fixed number of chunks. 

 T = 1/(U*C-L)

T(FDM) = N*T(1/U(C/N)-L/N)

Where, 
 T = mean time delay,

C = capacity of channel,

L = arrival rate of frames,

1/U = bits/frame,

N = number of sub channels,

34
 T(FDM) = Frequency Division Multiplexing Time

2.Dynamic Channel Allocation: ( 5MARK)

Possible assumptions include: 

1. Station Model: 
Assumes that each of N stations independently produce frames. The probability of
producing a packet in the interval IDt where I is the constant arrival rate of new
frames. 

2. Single Channel Assumption: 

In this allocation all stations are equivalent and can send and receive on that channel. 

3. Collision Assumption: 
If two frames overlap in time-wise, then that’s collision. Any collision is an error, and
both frames must re transmitted. Collisions are only possible error. 

4. Time can be divided into Slotted or Continuous. Stations can sense a channel is busy

before they try it. 

Protocol Assumption: 

 N independent stations.

 A station is blocked until its generated frame is transmitted.

 Probability of a frame being generated in a period of length Dt is IDt where I is the

arrival rate of frames.

 Only a single Channel available.

 Time can be either: Continuous or slotted.

 Carrier Sense: A station can sense if a channel is already busy before transmission.

 No Carrier Sense: Time out used to sense loss data.

TCP/IP Model (Transmission Control Protocol/Internet Protocol) ( 10 mark)

35
A protocol suite is a large number of related protocols that work together to allow networked
computers to communicate.

Application Layer
 Application layer protocols define the rules when implementing specific network
applications
 Rely on the underlying layers to provide accurate and efficient data delivery
 Typical protocols: o FTP – File Transfer Protocol
 For file transfer o Telnet – Remote terminal protocol
 For remote login on any other computer on the network o SMTP – Simple Mail
Transfer Protocol
 For mail transfer o HTTP – Hypertext Transfer Protocol
 For Web browsing
 encompasses same functions as these OSI Model layers Application Presentation
Session
Transport Layer TCP &UDP
TCP is a connection-oriented protocol o does not mean it has a physical connection
between sender and receiver o TCP provides the function to allow a connection
virtually exists – also called virtual circuit
 UDP provides the functions: o dividing a chunk of data into segments of Reassembly
segments into the original chunk o Provide further the functions such as reordering
and data resend
 Offering a reliable byte-stream delivery service.
 Functions the same as the Transport layer in OSI
 Synchronize source and destination computers to set up the session between the
respective computers.
Internet Layer
The network layer also called the internet layer, deals with packets and connects
independent networks to transport the packets across network boundaries.

36
 The network layer protocols are the IP and the Internet Control Message Protocol
(ICMP) which is used for error reporting. Host-to-network layer The Host-to-network
layer is the lowest layer of the TCP/IP reference model. It combines the link layer and
the physical layer of the ISO/OSI model. At this layer, data is transferred between
adjacent network nodes in a WAN or between nodes on the same LAN.
What is Transmission Control Protocol (TCP)?
 TCP (Transmission Control Protocol) is one of the main protocols of the
Internet protocol suite. It lies between the Application and Network Layers
which are used in providing reliable delivery services. It is a connection-
oriented protocol for communications that helps in the exchange of messages
between the different devices over a network.

Working of TCP
To make sure that each message reaches its target location intact, the TCP/IP model
breaks down the data into small bundles and afterward reassembles the bundles into
the original message on the opposite end. Sending the information in little bundles of
information makes it simpler to maintain efficiency as opposed to sending everything
in one go. 
After a particular message is broken down into bundles, these bundles may travel
along multiple routes if one route is jammed but the destination remains the same.
For example, When a user requests a web page on the internet, somewhere in the
world, the server processes that request and sends back an HTML Page to that user.
The server makes use of a protocol called the HTTP Protocol. The HTTP then
requests the TCP layer to set the required connection and send the HTML file.
Now, the TCP breaks the data into small packets and forwards it towards the Internet
Protocol (IP) layer. The packets are then sent to the destination through different
routes.
The TCP layer in the user’s system waits for the transmission to get finished and
acknowledges once all packets have been received.
Features of TCP/IP
Some of the most prominent features of Transmission control protocol are
1. Segment Numbering System
 TCP keeps track of the segments being transmitted or being received by assigning
numbers to each and every single one of them.
 A specific Byte Number is assigned to data bytes that are to be transferred while
segments are assigned sequence numbers.

37
 Acknowledgment Numbers are assigned to received segments.
2. Flow Control
 Flow control limits the rate at which a sender transfers data. This is done to ensure
reliable delivery.
 The receiver continually hints the sender on how much data can be received (using a
sliding window)
3. Error Control
 TCP implements an error control mechanism for reliable data transfer
 Error control is byte-oriented
 Segments are checked for error detection
 Error Control includes – Corrupted Segment & Lost Segment Management, Out-of-
order segments, Duplicate segments, etc.
4. Congestion Control
 TCP takes into account the level of congestion in the network
 Congestion level is determined by the amount of data sent by a sender
Advantages
 It is a reliable protocol
 It provides an error-checking mechanism as well as one for recovery
 It gives flow control
 It makes sure that the data reaches the proper destination in the exact order that it was
sent
 Open Protocol, not owned by any organization or individual
 It assigns an IP address to each computer on the network and a domain name to each
site thus making each device site to be distinguishable over the network.
Disadvantages
 TCP is made for Wide Area Networks, thus its size can become an issue for small
networks with low resources
 TCP runs several layers so it can slow down the speed of the network
 It is not generic in nature. Meaning, it cannot represent any protocol stack other than
the TCP/IP suite. E.g., it cannot work with a Bluetooth connection.
 No modifications since their development around 30 years ago.

 What are the elements of Transport Protocol? (10 mark)

 To establish a reliable service between two machines on a network, transport protocols are
implemented, which somehow resembles the data link protocols implemented at layer 2. The
major difference lies in the fact that the data link layer uses a physical channel between two
routers while the transport layer uses a subnet.

 Following are the issues for implementing transport protocols−

Types of Service
 The transport layer also determines the type of service provided to the users from the
session layer. An error-free point-to-point communication to deliver messages in the
order in which they were transmitted is one of the key functions of the transport layer.

38
 Error Control
 Error detection and error recovery are an integral part of reliable service, and
therefore they are necessary to perform error control mechanisms on an end-to-end
basis. To control errors from lost or duplicate segments, the transport layer enables
unique segment sequence numbers to the different packets of the message, creating
virtual circuits, allowing only one virtual circuit per session.

 Flow Control
 The underlying rule of flow control is to maintain a synergy between a fast process
and a slow process. The transport layer enables a fast process to keep pace with a
slow one. Acknowledgements are sent back to manage end-to-end flow control. Go
back N algorithms are used to request retransmission of packets starting with packet
number N. Selective Repeat is used to request specific packets to be retransmitted.

 Connection Establishment/Release
 The transport layer creates and releases the connection across the network. This
includes a naming mechanism so that a process on one machine can indicate with
whom it wishes to communicate. The transport layer enables us to establish and
delete connections across the network to multiplex several message streams onto one
communication channel.

 Multiplexing/De multiplexing
 The transport layer establishes a separate network connection for each transport
connection required by the session layer. To improve throughput, the transport layer
establishes multiple network connections. When the issue of throughput is not
important, it multiplexes several transport connections onto the same network
connection, thus reducing the cost of establishing and maintaining the network
connections.
 When several connections are multiplexed, they call for DE multiplexing at the
receiving end. In the case of the transport layer, the communication takes place only
between two processes and not between two machines. Hence, communication at the
transport layer is also known as peer-to-peer or process-to-process communication.

 Fragmentation and re-assembly

 When the transport layer receives a large message from the session layer, it breaks the
message into smaller units depending upon the requirement. This process is called
fragmentation. Thereafter, it is passed to the network layer. Conversely, when the
transport layer acts as the receiving process, it reorders the pieces of a message before
reassembling them into a message.

 Addressing
 Transport Layer deals with addressing or labeling a frame. It also differentiates
between a connection and a transaction. Connection identifiers are ports or sockets
that label each frame, so the receiving device knows which process it has been sent
from. This helps in keeping track of multiple-message conversations. Ports or sockets
address multiple conservations in the same location.

39
IP ADDRESS: ( 5 MARK)

IP Addresses Every host and router on the Internet has an IP address, which encodes
its network number and host number. The combination is unique: in principle, no two
machines on the Internet have the same IP address. All IP addresses are 32 bits long
and are used in the Source address and Destination address fields of IP packets. It is
important to note that an IP address does not actually refer to a host. It really refers to
a network interface, so if a host is on two networks, it must have two IP addresses.
However, in practice, most hosts are on one network and thus have one IP address.
For several decades, IP addresses were divided into the five categories listed in Fig. 5-
55. This allocation has come to be called classful addressing.Itisno longer used, but
references to it in the literature are still common. We will discuss the replacement of
classful addressing shortly. Figure 5-55. IP address formats. 332
(PG.NO :333 DIAGRAM)
The class A, B, C, and D formats allow for up to 128 networks with 16 million hosts
each, 16,384 networks with up to 64K hosts, and 2 million networks (e.g., LANs)
with up to 256 hosts each (although a few of these are special). Also supported is
multicast, in which a datagram is directed to multiple hosts. Addresses beginning with
1111 are reserved for future use. Over 500,000 networks are now connected to the
Internet, and the number grows every year. Network numbers are managed by a
nonprofit corporation called ICANN (Internet Corporation for Assigned Names and
Numbers) to avoid conflicts. In turn, ICANN has delegated parts of the address space
to various regional authorities, which then dole out IP addresses to ISPs and other
companies. Network addresses, which are 32-bit numbers, are usually written in
dotted decimal notation. In this format, each of the 4 bytes is written in decimal, from
0 to 255. For example, the 32-bit hexadecimal address C0290614 is written as
192.41.6.20. The lowest IP address is 0.0.0.0 and the highest is 255.255.255.255. The
values 0 and -1 (all 1s) have special meanings, as shown in Fig. 5-56. The value 0
means this network or this host. The value of -1 is used as a broadcast address to
mean all hosts on the indicated network. Figure 5-56. Special IP addresses. (334
P.NO)

The IP address 0.0.0.0 is used by hosts when they are being booted. IP addresses with
0 as network number refer to the current network. These addresses allow machines to
refer to their own network without knowing its number (but they have to know its
class to know how many 0s to include). The address consisting of all 1s allows
broadcasting on the local network, typically a LAN. The addresses with a proper
network number and all 1s in the host field allow machines to send broadcast packets
to distant LANs anywhere in the Internet (although many network administrators
disable this feature). Finally, all addresses of the form 127.xx.yy.zz are reserved for
loopback testing. Packets sent to that address are not put out onto the wire; they are

40
processed locally and treated as incoming packets. This allows packets to be sent to
the local network without the sender knowing its number.
Error Detection & Correction ( 10 mark)
There are many reasons such as noise, cross-talk etc., which may help data to get
corrupted during transmission. The upper layers work on some generalized view of
network architecture and are not aware of actual hardware data processing. Hence, the
upper layers expect error-free transmission between the systems. Most of the
applications would not function expectedly if they receive erroneous data.
Applications such as voice and video may not be that affected and with some errors
they may still function well.
Data-link layer uses some error control mechanism to ensure that frames (data bit
streams) are transmitted with certain level of accuracy. But to understand how errors
is controlled, it is essential to know what types of errors may occur.
Types of Errors
There may be three types of errors:
Single bit error

In a frame, there is only one bit, anywhere though, which is corrupt.

Multiple bits error

Frame is received with more than one bits in corrupted state.

Burst error

Frame contains more than1 consecutive bits corrupted.

Error control mechanism may involve two possible ways:
Error detection
Error correction
Error Detection
Errors in the received frames are detected by means of Parity Check and Cyclic
Redundancy Check (CRC). In both cases, few extra bits are sent along with actual
data to confirm that bits received at other end are same as they were sent. If the
counter-check at receiver’ end fails, the bits are considered corrupted.
Parity Check
One extra bit is sent along with the original bits to make number of 1s either even in
case of even parity, or odd in case of odd parity.
The sender while creating a frame counts the number of 1s in it. For example, if even
parity is used and number of 1s is even then one bit with value 0 is added. This way

41
number of 1s remains even .if the number of 1s is odd, to make it even a bit with
value 1 is added.

The receiver simply counts the number of 1s in a frame. If the count of 1s is even and
even parity is used, the frame is considered to be not-corrupted and is accepted. If the
count of 1s is odd and odd parity is used, the frame is still not corrupted.
If a single bit flips in transit, the receiver can detect it by counting the number of 1s.
But when more than one bits are erroneous, then it is very hard for the receiver to
detect the error.
Cyclic Redundancy Check (CRC)
CRC is a different approach to detect if the received frame contains valid data. This

technique involves binary division of the data bits being sent. The divisor is generated
using

polynomials. The sender performs a division operation on the bits being sent and
calculates the remainder. Before sending the actual bits, the sender adds the remainder
at the end of the actual bits. Actual data bits plus the remainder is called a codeword.
The sender transmits data bits as code word.
At the other end, the receiver performs division operation on code words using the
same CRC divisor. If the remainder contains all zeros the data bits are accepted,
otherwise it is considered as there some data corruption occurred in transit.
Error Correction

42
 In the digital world, error correction can be done in two ways:
Backward Error Correction When the receiver detects an error in the data
received; it requests back the sender to re transmit the data unit.
Forward Error Correction When the receiver detects some error in the data
received; it executes error-correcting code, which helps it to auto-recover and to
correct some kinds of errors.
The first one, Backward Error Correction, is simple and can only be efficiently used
where retransmitting is not expensive. For example, fiber optics. But in case of
wireless transmission retransmitting may cost too much. In the latter case, Forward
Error Correction is used.
To correct the error in data frame, the receiver must know exactly which bit in the
frame is corrupted. To locate the bit in error, redundant bits are used as parity bits for
error detection. For example, we take ASCII words (7 bits data), then there could be 8
kind of information we need: first seven bits to tell us which bit is error and one more
bit to tell that there is no error.
For m data bits, r redundant bits are used. r bits can provide 2r combinations of
information. In m+r bit code word, there is possibility that the r bits themselves may
get corrupted. So the number of r bits used must inform about m+r bit locations plus

no-error information, i.e. m+r+1

Congestion Control in Datagram Subnets (10 MARK)

 we will discuss different approaches to Congestion control in the data-gram
subnet. Also, we will discuss the drawback and will explain each approach in
detail. Let’s discuss it one by one.
Pre-requisite –Congestion control 
Congestion control in data-gram and sub-nets :
Some congestion Control approaches which can be used in the datagram Subnet (and
also in virtual circuit subnets) are given under.
1. Choke packets
2. Load shedding
3. Jitter control.
Approach-1: Choke Packets :
 This approach can be used in virtual circuits as well as in the data gram sub-nets. In
this technique each router associates a real variable with each of its output line. 
 This real variable say u has a value between 0 and 1, and it indicates the percentage
utilization of that line .If the value of the variable goes above the threshold then the
output line will enter into a warning state. 
 The router will check each of newly arriving packet to see if its output line is in the
warning state .if it is in the warning state then router will send back a choke packets.
Several variations on the congestion control algorithm have been proposed depending
on the value of thresholds.

43
 Depending upon the threshold value, the choke packets can contain a mild warning a
stern warning, or an ultimatum. Another variation can be in terms of queue lengths or
buffer utilization instead of using the line utilization as a deciding factor
Drawback –
The problem with choke packet technique is that the action to be taken by the source
host on receiving a choke packet is voluntary and not compulsory.
Approach-2: Load Shedding :
 Admission control, choke packets, fair queuing are the techniques suitable for
congestion control. But if these techniques cannot make the congestion to disappear,
then load shedding technique is to be used. 
 The principle of load shedding states that when the router is being inundated by the
packets that they cannot handle, they should simply through packets away. 
 A router flooding with packets due to congestion can drop any packet at random. But
there are better ways of doing this. 
 The policy for dropping a packet depends on the type of packet. For file transfer, an
old packet is more important than a new packet In contrast, for multimedia a new
packet is more important than an old one So.the policy for file transfer called wine
(old is better than new) and that for the multimedia is called milk (new is better than
old). 
 An intelligent discard policy can be decided depending on the applications. To
implement such an intelligent discard policy, co-operation from the sender is
essential. 
 The application should mark their packets in priority classes to indicate how
important they are.
  If this is done then when the packets are to be discarded the routers can first drop
packets from the lowest class (i.e. the packets which are least important). Then the
routers will discard the packets from next lower class and so on. One or more header
bits are required to put the priority tor making the class of a packet. In every ATM
cell, 1 bit is reserved in the header for marking the priority. Every ATM cell is labeled
either as a low priority or high priority.
Approach-3: Jitter control :
 Jitter may be defined as the variation in delay for the packet belonging to the same
flow. The real time audio and video cannot tolerate jitter on the other hand the jitter
doesn’t matter if the packets are carrying an information contained in a file.
 For the audio and video transmission if the packets take 20 ms to 30 ms delay to each
the destination, it doesn’t matter, provided that the delay remains constant. 
 The quality of sound and visuals will be hampered of the delays associated with
different packets have different values. Therefore, practically we can say that 99% of
packets should be delivered with a delay ranging from 24.5 ms to 25.5 ms.
 When а packet arrives at a router, the router will check to see whether the packet is
behind or ahead and by what time. 
 This information is stored in the packet and updated at every hop. If packet is ahead
of the schedule then router will hold it for a slight longer time and if the packet is

44
 behind schedule, then the router will try to send it out as quickly as possible. This will
help in keeping the average delay per packet constant and will avoid time jitter.

Third-Generation (3G) Mobile Phones ( 5 MARK)

Third generation mobile phones, or “3G Internet” mobile phones, is a set of standards
for wireless mobile communication systems, that promises to deliver quality
multimedia services along with high quality voice transmission.
Features
 3G systems comply with the International Mobile Telecommunications-2000 (IMT-
2000)
 specifications by the International Telecommunication Union (ITU).
 The first 3G services were available in 1998.
 It provides high speed transmission having data transfer rate more than 0.2Mbps.
 Global roaming services are available for both voice and data.
 It offers advanced multimedia access like playing music, viewing videos, television
services etc.
 It provides access to all advanced Internet services, for example surfing webpages
with audio and video.
 It paved the way for the increased usage of smartphones with wide screens as they
provided better viewing of mobile webpages, videos and mobile televisions.
Specifications for 3G
3G specifications are laid down by two groups, 3GPP and 3GPP2.
 3GPP (Third Generation Partnership Project) − These specifications are based
upon Global System for Mobile (GSM) communications, and are known as Universal
Mobile Telecommunications Systems (UMTS). The technologies includes in it are −
o Universal Terrestrial Radio Access (UTRA)
o General Packet Radio Service (GPRS)
o Enhanced Data rates for GSM Evolution (EDGE)
 3GPP2 − These specifications are based upon Code Division Multiple Access
(CDMA). Two main specifications under this are −
o Wideband CDMA (WCDMA)
o CDMA2000
Areas of Application
 Wireless voice telephony
 Fixed wireless Internet access
 Mobile Internet access
 Video calls
 Mobile TV technologies
 Video-on-demand
 Video conferencing
 Tele-medicine
 Global Positioning System (GPS)
 Location-based services

45
BASEBAND TRANSMISSION ( 5 MARK)
In baseband transmission, the data bits are directly converted into signals. Generally a
higher voltage level represents the bit 1, while a lower voltage level represents bit 0.

The different encoding schemes are shown in the diagram. Among these, the first
three are come in the category of polar encoding. In polar signaling, one logical state
is represented by only one voltage state. In bipolar schemes, two voltage levels may
be used to represent a logical state.

NRZ (Non – Return to Zero)

NRZ is an unipolar coding scheme. Here, a high voltage represents 1, while a low
voltage represents 0. Non-return to zero implies that the signal does not return to zero
at the middle of the bit.

NRZ-I (NRZ Invert)

NRZ-I is an polar coding scheme. In NRZI, bit 1 is represented by a transition in
voltage, while bit 0 is represented by no such transitions. It has an average signal rate
of N/2 baud.

Manchester Encoding
Manchester encoding is a biphase coding scheme. Bit 1 is represented by a voltage
transition from high to low, while bit 0 is represented by a voltage transition from low
to high.

Bipolar Encoding
It is also called Alternate Mark Inversion or AMI. Three voltage levels are used here.
Here, bit 0 is represented by no line signal, while bit 1 is represented by a positive or
negative voltage level, alternating for successive ones.
Example
Let there be a bit stream 01001101. The following diagram plots the different
encoding schemes –

46
7

Store – and – Forward Packet Switching ( 5 mark)

In telecommunications, store − and − forward packet switching is a technique where
the data packets are stored in each intermediate node, before they are forwarded to the
next node. The intermediate node checks whether the packet is error−free before
transmitting, thus ensuring integrity of the data packets. In general, the network layer
operates in an environment that uses store and forward packet switching.

Working Principle
The node which has a packet to send, delivers it to the nearest node, i.e. router. The
packet is stored in the router until it has fully arrived and its checksum is verified for
error detection. Once, this is done, the packet is transmitted to the next router. The
same process is continued in each router until the packet reaches its destination.
The following scenario exemplifies the mechanism −
In the above diagram, we can see that the Internet Service Provider (ISP) has six
routers (A to F) connected by transmission lines shown in blue lines. There are two
hosts, host H1 is connected to router A, while host H2 is connected to router D.
Suppose that H1 wants to send a data packet to H2. H1 sends the packet to router A.
The packet is stored in router A until it has arrived fully. Router A verifies the
checksum using CRC (cyclic redundancy check) code. If there is a CRC error, the
packet is discarded, otherwise it is transmitted to the next hop, here router F. The
same process is followed by router F which then transmits the packet to router D.
Finally router D delivers the packet to host H2.
Advantages and Disadvantages
Store − and forward packet switching ensures high quality data packet transmission.
Since erroneous packets are discarded at each router, bad packets or invalid packets in
the network are mostly eliminated.

47
 However, error − free packet transmission is achieved by compromising on the
overall speed of transmission. Switch latency is introduced due to waiting for entire
packet to arrive as well as computation of CRC. Though the latency at each router
may seem small enough, the cumulative latency at all routers make it inappropriate
for time − critical online applications.
Network Layer Services- Packetizing, Routing and Forwarding ( 5 MARK)
Network layer is the third layer in the OSI model of computer networks. It’s main
function is to transfer network packets from the source to the destination. It is
involved both at the source host and the destination host. At the source, it accepts a
packet from the transport layer, encapsulates it in a datagram and then deliver the
packet to the data link layer so that it can further be sent to the receiver. At the
destination, the datagram is decapsulated, the packet is extracted and delivered to the
corresponding transport layer. 
Features :  
1. Main responsibility of Network layer is to carry the data packets from the source to
the destination without changing or using it. 
2. If the packets are too large for delivery, they are fragmented i.e., broken down into
smaller packets. 
3. It decides the route to be taken by the packets to travel from the source to the
destination among the multiple routes available in a network (also called as routing). 
4. The source and destination addresses are added to the data packets inside the network
layer. 
The services which are offered by the network layer protocol are as follows: 
1. Packetizing – 
The process of encapsulating the data received from upper layers of the network(also
called as payload) in a network layer packet at the source and decapsulating the
payload from the network layer packet at the destination is known as packetizing. 
The source host adds a header that contains the source and destination address and
some other relevant information required by the network layer protocol to the payload
received from the upper layer protocol, and delivers the packet to the data link layer. 
The destination host receives the network layer packet from its data link layer,
decapsulates the packet, and delivers the payload to the corresponding upper layer
protocol. The routers in the path are not allowed to change either the source or the
destination address. The routers in the path are not allowed to decapsulate the packets
they receive unless they need to be fragmented. 
 
2. Routing and Forwarding – 
These are two other services offered by the network layer. In a network, there are a
number of routes available from the source to the destination. The network layer
specifies has some strategies which find out the best possible route. This process is
referred to as routing. There are a number of routing protocols which are used in this
process and they should be run to help the routers coordinate with each other and help
in establishing communication throughout the network. 

48
 Forwarding is simply defined as the action applied by each router when a packet
arrives at one of its interfaces. When a router receives a packet from one of its
attached networks, it needs to forward the packet to another attached network (unicast
routing) or to some attached networks(in case of multicast routing). 
Advantages of Network Layer Services : 
 Packetization service in network layer provides an ease of transportation of the data
packets. 
 Packetization also eliminates single points of failure in data communication systems. 
 Routers present in the network layer reduce network traffic by creating collision and
broadcast domains. 
 With the help of Forwarding, data packets are transferred from one place to another in
the network. 
Disadvantages of Network Layer Services : 
 There is a lack of flow control in the design of the network layer. 
 Congestion occurs sometimes due to the presence of too many datagrams in a network
which are beyond the capacity of network or the routers. Due to this, some routers
may drop some of the datagrams and some important piece of information maybe
lost. 
 Although indirectly error control is present in network layer, but there is a lack of
proper error control mechanisms as due to presence of fragmented data packets, error
control becomes difficult to implement. 

A Utopian Simplex Protocol ( 5 mark)

The Simplex protocol is data link layer protocol for transmission of frames over computer
network. It is hypothetical protocol designed for unidirectional data transmission over an
ideal channel, i.e. a channel through which transmission can never go wrong.
It is assumed that both the sender and the receiver are always ready for data processing and
both of them have infinite buffer. The sender simply sends all its data available onto the
channel as soon as they are available its buffer. The receiver is assumed to process all
incoming data instantly. It is does not handle flow control or error control. Since this
protocol is totally unrealistic, it is often called Utopian Simplex protocol.
The significance of this protocol lies in the fact that it shows the basic structure on which the
usable protocols are built upon.

Design
 Sender Site: The data link layer in the sender site waits for the network layer to send a
data packet. On receiving the packet, it immediately processes it and sends it to the
physical layer for transmission.
 Receiver Site: The data link layer in the receiver site waits for a frame to be available.
When it is available, it immediately processes it and sends it to the network layer.

49
Algorithm of Simplex Protocol for Sender Site
begin
while (true) //check repeatedly
do
Wait_For_Event(); //wait for availability of packet
if ( Event(Frame_Available)) then
Get_Data_From_Network_Layer();
Make_Frame();
Send_Frame_To_Physical_Layer();
end if
end while
end

Algorithm of Simplex Protocol for Receiver Site

begin
while (true) //check repeatedly
do
Wait_For_Event(); //wait for arrival of frame
if ( Event(Frame_Arrival)) then
Receive_Frame_From_Physical_Layer();
Extract_Data();
Deliver_Data_To_Network_Layer();
end if
end while

50
end

Flow Diagram
The following flow diagram depicts communication via simplex protocol.

Wireless Networks ( 5 MARK)

Computer networks that are not connected by cables are called wireless networks. They
generally use radio waves for communication between the network nodes. They allow
devices to be connected to the network while roaming around within the network coverage.

Types of Wireless Networks

 Wireless LANs − Connects two or more network devices using wireless distribution
techniques.
 Wireless MANs − Connects two or more wireless LANs spreading over a
metropolitan area.

51
  Wireless WANs − Connects large areas comprising LANs, MANs and personal
networks.
Advantages of Wireless Networks
 It provides clutter-free desks due to the absence of wires and cables.
 It increases the mobility of network devices connected to the system since the devices
need not be connected to each other.
 Accessing network devices from any location within the network coverage or Wi-Fi
hotspot becomes convenient since laying out cables is not needed.
 Installation and setup of wireless networks are easier.
 New devices can be easily connected to the existing setup since they needn’t be wired
to the present equipment. Also, the number of equipment that can be added or
removed to the system can vary considerably since they are not limited by the cable
capacity. This makes wireless networks very scalable.
 Wireless networks require very limited or no wires. Thus, it reduces the equipment
and setup costs.
Examples of wireless networks
 Mobile phone networks
 Wireless sensor networks
 Satellite communication networks
 Terrestrial microwave networks

52
53