CN

1. Syllabus of unit 1:
Introduction: Importance of Computer Networks, Classifications
& Types. Layered Architecture: Protocol hierarchy, Interfaces and
Services, Connection Oriented &Connection less Services, ISO-
OSI Reference Model, TCP/IP model overview, comparison of
TCP/IP and ISO-OSI reference model.

2. Network Topologies:
How the computers are connected to each other is known as topology.
There are five types of topology – Mesh, Star, Bus, Ring and Hybrid.

3. Mesh topology:
In mesh topology each device is connected to every other device on the
network through a dedicated point-to-point link.

4. Star Topology:
In star topology each device in the network is connected to a central device
called hub. Unlike Mesh topology, star topology doesn’t allow direct
communication between devices, a device must have to communicate
through hub.

5. Bus Topology:
In bus topology there is a main cable and all the devices are connected to
this main cable through drop lines. There is a device called tap that
connects the drop line to the main cable. Since all the data is transmitted
over the main cable, there is a limit of drop lines and the distance a main
cable can have.

6. Ring Topology:
In ring topology each device is connected with the two devices on either
side of it. There are two dedicated point to point links a device has with the
devices on the either side of it. This structure forms a ring thus it is knowns
ring topology.

7. Hybrid topology:
A combination of two or more topology is known as hybrid topology. For
example a combination of star and mesh topology is known as hybrid
topology.

8. Computer Network:
A computer network is a group of computers connected with each other
through a transmission medium such as cable, wire etc.
Types of Computer Network

9. 1. Local Area Network (LAN):
Local area network is a group of computers connected with each other in
a small places such as school, hospital, apartment etc.

11. 2. Metropolitan Area Network (MAN):
MAN network covers larger area by connections LANs to a larger network
of computers. In Metropolitan area network various Local area networks
are connected with each other through telephone lines.

12. 3. Wide area network (WAN):
Wide area network provides long distance transmission of data. The size of
the WAN is larger than LAN and MAN. A WAN can cover country,
continent or even a whole world. Internet connection is an example of
WAN. Other examples of WAN are mobile broadband connections such as
3G, 4G etc.

13. Network Devices:
The devices which are used for communication between different
hardware’s used in the computer network are known as network devices.
Types of Network Devices
 Network Hub
 Network Switch
 Modem
 Network Router
 Bridge
 Repeater

14. Network Hub:
The network hub is one kind of networking device in a computer network,
used to communicate with various network hosts and also for data
transferring. The transferring of data in a computer network can be done in
the form of packets. Whenever the data processing can be done from a host
to a network hub, then the data can transmit to all the connected ports.

15. Network Switch:
Similar to a hub, this is also working at the layer in the LAN and a switch
is more clever compare with a hub. As the hub is used for data
transferring, whereas a switch is used for filtering & forwarding the data.
So this is the more clever technique to deal with the data packets.

16. Modem:
Modem is a hardware device that connects a computer or router to a
broadband network. when it receives a analog signal it changes into a
digital signal . it stands modulator and demodulator .it enables a computer
to transfer the data.

17. Network Router:
A network router is one kind of network device in a computer network and
it is used for routing traffic from one network to another. These two
networks could be private to a public company network. For example, here
a router is considered as traffic police at the junction, he directs dissimilar
traffic networks to dissimilar directions.

18. Bridge
 Bridge stores MAC address of pc in a network.
 Bridge is used to reduce the network traffic.
 It is basically used in bus topology, in this topology if pc 1 wants to
send data to pc 8 ,bus topology broadcast the data to all the pc
available in the network so that it simply create the lots of traffic.so
for overcoming this problem we are using a networking device
called bridge. Bridge divide a LAN into two segemnet and it stores
all the pc MAC address. If pc 1 want to send dta to pc 8 it first reach
to bridge and read MAC address and decide where to send data so it
reduce the network traffic.

20. Repeater
The operating of a repeater can be done at the physical layer. The main
function of this device is to reproduce the signal on a similar network
before the signal gets weak otherwise damaged. The significant point to be
noted regarding these devices is that they do not strengthen the signal.
Whenever the signal gets weak, then they reproduce it at the actual
strength. A repeater is a two-port device.

21. Gateway
Generally, a gateway performs at the session & transport layers in the OSI
model. Gateways offer conversion between networking technologies like
OSI (Open System Interconnection) & TCP/IP. Because of this, these are
connected to two or many autonomous networks, where each network has
its own domain name service, routing algorithm, topology, protocols, and
procedures of network administration & policies.
gateway-device
Gateways execute all the functions of routers. Actually, a router with
additional conversion functionality is a gateway, so the conversion between
various network technologies is known as a protocol converter.

22.  To understand how the data is actually transferred and
received at a computer level, we use computer network model.
 A simple transmission of data consists several steps at various
layers of computer network.

23. Layers of a Computer Network Models
 1. The main purpose of having several layers in a computer
network model is to divide a process of sending and receiving
data into small-small tasks.
 2. These layers are connected with each other, each layer
provide certain data to its immediate higher and immediate
lower layer and receives certain data from the same.
 3. Dividing a model in layers makes the structure quite simple
that makes it easy to identify the issue if it occurs.
 There are three main components of a computer network
model. Sender, receiver and carrier.

24. The most important computer network models are:
1. OSI Model
2. TCP/IP Model

25. OSI Model in Computer Network:
 OSI Model stands for Open System interconnection model.
 It was first introduced in the late 1970s
 Given by International Standards Organization (ISO).
 ISO is the organization

29.  In Figure which gives an overall view of the OSI layers.
 D7 means the data unit at layer 7, D6 means the data unit at layer 6, and so
on.
 The process starts at layer 7 (the application layer), then moves from layer
to layer in descending, sequential order.
 At each layer, a header, or possibly a trailer, can be added to the data
unit.
 Commonly, the trailer is added only at layer 2.
 When the formatted data unit passes through the physical layer (layer 1), it
is changed into an electromagnetic signal and transported along a physical
link.

30. Note:
 Transport layer converts the data into segments.
 Network layer converts the segments into packets and
 Data link layer converts the packets into frames.
 A frame is nothing but a sequence of bits such as
1001011.
 Physical layer converts these binary sequences into
signals and transfer it through a transmission media such
as cables etc.

31. Physical Layer:
 The physical layer coordinates the functions required to carry
a bit stream over a physical medium.

32. Main functions of Physical Layer:
 Physical characteristics of interfaces and medium: The
physical layer defines the characteristics of the interface
between the devices and the transmission medium. It also
defines the type of transmission medium.
 Representation of Bits: The physical layer data consists of a
stream of bits (sequence of Os or 1s). To be transmitted, bits
must be encoded into signals, electrical or optical. The
physical layer defines the type of encoding (how Os and 1s are
changed to signals).
 Data Rate: (The transmission rate)-the number of bits sent
each second . Data rate unit is bps(bits/second)

33. Main functions of Physical Layer:
 Synchronization of bits: The sender and receiver not only
must use the same bit rate but also must be synchronized at the
bit level. In other words, the sender and the receiver clocks
must be synchronized.
 Line configuration: The physical layer is concerned with the
connection of devices to the media. In a point-to-point
configuration, two devices are connected through a dedicated
link. In a multipoint configuration, a link is shared among
several devices.
 Physical Topology: The physical topology defines how
devices are connected to make a network. Devices can be
connected by using a mesh topology, a star topology, a ring
topology, a bus topology, or a hybrid topology.
 Transmission mode: The physical layer also defines the
direction of transmission between two devices: simplex, half-
duplex, or full-duplex.

34. Data Link Layer:
 The data link layer transforms the physical layer, a raw
transmission facility, to a reliable link. It makes the physical
layer appear error-free to the upper layer (network layer).
 Figure shows the relationship of the data link layer to the
network and physical layers.

35. Data Link Layer:
Other responsibilities of the data link layer include:
 Framing: The data link layer divides the stream of bits
received from the network layer into manageable data units
called frames.
 Physical addressing: If frames are to be distributed to
different systems on the network, the data link layer adds a
header to the frame to define the sender and/or receiver of the
frame. If the frame is intended for a system outside the
sender's network, the receiver address is the address of the
device that connects the network to the next one.

36. Data Link Layer:
 Flow control: If the rate at which the data are absorbed by the
receiver is less than the rate at which data are produced in the
sender, the data link layer imposes a flow control mechanism
to avoid overwhelming the receiver.
 Error control: The data link layer adds reliability to the
physical layer by adding mechanisms to detect and retransmit
damaged or lost frames. It also uses a mechanism to recognize
duplicate frames. Error control is normally achieved through a
trailer added to the end of the frame.
 Access control: When two or more devices are connected to
the same link, data link layer protocols are necessary to
determine which device has control over the link at any given
time.

37. Network layer:
 The network layer is responsible for the source-to-destination
delivery of a packet, possibly across multiple networks (links).
 Figure shows the relationship of the network layer to the data
link and transport layers.

38. The main functions of Network Layer:
 Logical addressing: If a packet passes the network boundary,
we need another addressing system to help distinguish the
source and destination systems. The network layer adds a
header to the packet coming from the upper layer that, among
other things, includes the logical addresses of the sender and
receiver.
 Routing: When independent networks or links are connected
to create internetworks (network of networks) or a large
network, the connecting devices (called routers or switches)
route or switch the packets to their final destination. One of
the functions of the network layer is to provide this
mechanism.

39. Transport layer:
 The transport layer is responsible for process-to-process
delivery of the entire message.
 A process is an application program running on a host.
 The transport layer, on the other hand, ensures that the whole
message arrives intact and in order, overseeing both error
control and flow control at the source-to-destination level.

42. The main functions of transport layer are:
 Service-point addressing: Computers often run several
programs at the same time. For this reason, source-to-
destination delivery means delivery not only from one
computer to the next but also from a specific process (running
program) on one computer to a specific process (running
program) on the other. The transport layer header must
therefore include a type of address called a service-point
address (or port address).
 Flow control: It controls the flow of data. It checks the
capability of the receiver device receiving capability before
transmitting data. For example a sender server can send the
data at a rate of 200Mbps but a receiving data can only receive
data at a rate of 10 Mbps then it controls the flow of data to
10Mbps so that the data doesn’t get lost during transmission.

43. The main functions of transport layer are:
 Error control: Transport layer also performs error control using
Automatic Repeat Request, if a data is lost during transmission, it is
send again using automatic repeat request. Transport layer also adds
a group of bits called checksum with each segment to check whether
the data received at receiver side is not corrupt.
 Segmentation and reassembly: A message is divided into
transmittable segments, with each segment containing a sequence
number.
 Connection control: The transport layer can be either
connectionless or connection oriented.
 A connectionless transport layer treats each segment as an
independent packet and delivers it to the transport layer at the
destination machine.
 A connection oriented transport layer makes a connection with the
transport layer at the destination machine first before delivering the
packets. After all the data are transferred, the connection is
terminated.

44. Session Layer:
 The session layer is responsible for dialog control and
synchronization.
 Dialog control: (identification of mode of communication) The session
layer allows two systems to enter into a dialog. It allows the
communication between two processes to take place in either
half duplex (one way at a time) or full-duplex (two ways at a
time) mode.
 Synchronization: The session layer allows a process to add
checkpoints, or synchronization points, to a stream of data.
 For example, if a system is sending a file of 2000 pages, it is
advisable to insert checkpoints after every 100 pages to ensure
that each 100-page unit is received and acknowledged
independently. In this case, if a crash happens during the
transmission of page 523, the only pages that need to be resent
after system recovery are pages 501 to 523. Pages previous to
501 need not be resent.

45. Main functions of session layer:
 Authentication: Before a computer can be connected to a
server, the computer has to provide user name and password
for the authentication. The function of authentication and
setting up a connection after authentication is performed by
session layer.
 Authorization: Once a connection is established, session
layer checks whether the connected computer is authorized to
access the data, this function of authorization checking is also
performed by session layer.
 Session management: Session layer also checks that the data
which is received from the server in form of data packets
belongs to which application. for example, when you access
Facebook profile through your browser, the data transferred
from the Facebook server is transferred to your web browser
application, thus the session layer helps in session
management.

46. Presentation Layer:
 The presentation layer is responsible for translation,
compression, and encryption.
 Presentation layer receives the data from top most layer which
is application layer.

47. Functions of Presentation layer:
 Translation: The data received from application layer is in form of
characters and numbers such as 1234, ERFF etc. The presentation layer
converts these characters and numbers into machine understandable format
which is known as binary format for example 100111101.
Because different computers use different encoding systems,
the presentation layer is responsible for interoperability between these
different encoding methods. The presentation layer at the sender changes
the information from its sender-dependent format into a common format.
The presentation layer at the receiving machine changes the common
format into its receiver-dependent format
 Encryption: To protect the sensitivity of data, presentation layer encrypts
the data at the sender side before the transmission and the receiver side this
data is decrypted by the presentation layer at the receiver side. Secure
sockets layer protocol (SSL) is used by the presentation layer for
encryption and decryption.
 Compression: Data compression reduces the number of bits contained
in the information. Data compression becomes particularly important in
the transmission of multimedia such as text, audio, and video.
Compress the data to small size so that it can be transferred faster
over a network. This compression can be lossy or lossless compression.

48.  The application layer enables the user, whether human
or software, to access the network.
 It provides user interfaces and support for services such
as electronic mail, remote file access and transfer,
shared database management, and other types of
distributed information services

49.  Network virtual terminal. A network virtual terminal is a software version of a
physical terminal, and it allows a user to log on to a remote host. The remote host
believes it is communicating with one of its own terminals and allows the user to
log on.
 File transfer, access, and management. This application allows a user to access
files in a remote host (to make changes or read data), to retrieve files from a remote
computer for use in the local computer.
 Mail services This application provides the basis for e-mail forwarding and
storage.
 Directory services. This application provides distributed database sources and
access for global information about various objects and services.
 Application layer defines the protocols that are used by computer applications. For
example: HTTP and HTTPS protocols are used by web browsers , SMTP protocol
is used for emails, Telnet is used for virtual terminals etc

50.  The TCP/IP protocol suite was developed prior to the OSI
model. Therefore, the layers in the TCP/IP protocol suite do
not exactly match those in the OSI model.
 TCP/IP protocol suite, was designed in 1970s by 2 DARPA
scientists—Vint Cerf and Bob Kahn
 The original TCP/IP protocol suite was defined as having four
layers:
 Host-to-network layer
 Internet layer
 Transport layer and
 Application layer.

51. TCP/IP Vs OSI:
 However, when TCP/IP is compared to OSI, we can say that:
 The host-to-network layer is equivalent to the combination of
the physical and data link layers.
 The internet layer is equivalent to the network layer.
 The transport layer in TCP/IP taking care of part of the duties
of the transport layer. and
 The application layer is roughly doing the job of the session,
presentation, and application layers.
 The first three layers provide physical standards, network
interfaces, internetworking, and transport functions that
correspond to the first four layers of the OSI model.
 The three topmost layers in the OSI model, are represented in
TCP/IP by a single layer called the application layer.

52. TCP/IP Model:

53. Internetworking protocol(IP) :
 In the network layer, the TCP/IP model supports
internetworking protocol (IP) for data transmission.
 IP protocol transmits the data in form of small packets known
as datagram's. Each of these datagram's are transmitted
separately, thus they can take different routes and sometime
duplicate datagram's can be reached to destination, also they
are reached in no particular order at the destination.

54. Internetworking protocol(IP) :
 The IP uses four protocols internally: ARP, RARP, ICMP &
IGMP.
Address Resolution Protocol (ARP)
 In a network, each device is known as physical address usually
imprinted on Network interface card (NIC).
 ARP protocol is used to find the physical address of a device
whose internet address (IP address) is known.
Reverse Address Resolution Protocol (RARP)
 RARP protocol helps to find the internet address of a device
whose physical address is known.

55. Internet Control Message Protocol (ICMP)
 IP in network layer sends data in form of small packets known
as datagram's. ICMP protocol sends the datagram's problems
back to sender.
 It is used for query and error reporting messages.
Internet Group Message Protocol (IGMP)
 This protocol is used for simultaneous transmission of a
message to a group of recipients.
TELecommunication NETwork(TELNET )
 This protocol is used a two-way communication protocol
which allows connecting to a remote machine and run
applications on it.

56. FTP(File Transfer Protocol)
 is a protocol,that allows File transfer amongst
computer users connected over a network. It is
reliable, simple and efficient.
SMTP(Simple Mail Transport Protocol)
 is a protocol, which is used to transport
electronic mail between a source and destination,
directed via a route.
DNS(Domain Name Server)
 an IP address into a textual address for Hosts
connected over a network.

57.  OSI model distinguishes well between the
services, interfaces and protocols.
 Protocols of OSI model are very well hidden.
 Protocols can be replaced by new protocols
as technology changes.
 Supports connection oriented services as
well as connectionless service.

58. Connection Oriented Service
 There is a sequence of operation to be followed by the users of
connection oriented service. These are:
 STEP-1-Connection is established
 STEP-2-Information is sent
 STEP-3-Connection is released
 In connection oriented service we have to establish a connection
before starting the communication. When connection is
established we send the message or the information and then we
release the connection.
 Connection oriented service is more reliable than connectionless
service. We can send the message in connection oriented service
if there is an error at the receivers end.
 Example of connection oriented is TCP (Transmission Control
Protocol) protocol.

59. Connectionless Services
 It is similar to the postal services, as it carries the full
address where the message (letter) is to be carried.
Each message is routed independently from source to
destination.
 The order of message sent can be different from the
order received.
 In connectionless the data is transferred in one
direction from source to destination without checking
that destination is still there or not or if it prepared to
accept the message.
 Authentication is not needed in this.
 Example of Connectionless service is UDP (User
Datagram Protocol) protocol.

60. Sr
no.
Connection
Oriented Services
Connectionless
Services
1 It needs authentication. It does not need authentication.
2 It guarantees a delivery It does not guarantee a delivery
3 It is more reliable It not that reliable
4 Connection Oriented is
stream based
Connectionless is message based.
5 Slow service Fast service
6 In connection-oriented
Service, Congestion is not
possible.
In connection-less Service,
Congestion is possible.
7 In connection-oriented
Service, Packets follow the
same route.
In connection-less Service, Packets
do not follow the same route.
8 Ex-TCP Ex-UDP

61. Sr. OSI TCP/IP
1 OSI model Developed after
TCP/IP
TCP/IP model Developed before OSI
2 OSI model has 7 layers TCP/IP has 4 layers
3 OSI differentiates services ,
protocols and interfaces.
TCP/IP doesn’t differentiate
them.
4 Connection oriented and
connectionless in network
layer and connection
oriented in transport layer
in OSI model
Connection oriented and
connectionless in transport layer and
connection oriented in network layer
in tcp/ip model
5 OSI represents Open System
Interconnection.
TCP/IP model represents the
Transmission Control Protocol /
Internet Protocol.
6 The smallest size of the OSI
header is 5 bytes.
The smallest size of the TCP/IP header
is 20 bytes.
7 Protocols in the OSI model are
hidden and can be easily replaced
when the technology changes.
In this model, the protocol cannot be easily
replaced.