PHYSICAL LAYER IN COMPUTER NETWORKS

1. VELAMMALENGINEERINGCOLLEGE
AnAutonomousInstitution,Affiliatedto AnnaUniversityChennai, &
Approvedby AICTEDelhi
COMPUTER NETWORKS
INTRODUCTIONTO
PHYSICALLA
YER
MRSP.APARNA
AP
,CSE

2. 1-1 DATA COMMUNICATIONS
Term
Telecommunication means
• communication at a distance
Data refers to
•information presented in whatever form is agreed
upon by the parties creating and using the data.
Data communications are the exchange of data
between two devices via some form of transmission
medium such as a wire cable.
Topics :
 Components of a data communications system
 Data Flow
Dept of CSE, VEC 1.2

3. Figure 1.1 Components of a data communication system
Dept of CSE, VEC 1.3

4. ⦁ Message: The information (data) to be communicated.
◦ Popular : text, numbers, pictures, audio, and video.
⦁ Sender: The sender is the device that sends the data message.
◦ It can be a computer, workstation, telephone handset, video camera, etc.
⦁ Receiver: The receiver is the device that receives the message.
◦ It can be a computer, workstation, telephone handset, television, etc.
⦁ Transmission medium: The physical path by which a message travels from
sender to receiver.
◦ Some examples : twisted-pair wire, coaxial cable, fiber-optic cable, and
radio waves.
⦁ Protocol: A protocol is a set of rules that govern data communications.
◦ It represents an agreement between the communicating devices.
Dept of CSE, VEC 1.4

5. ⦁ Today Information comes in different forms
◦ Text, Numbers, Images, Audio and Video
⦁ Text:
◦ Represented as a bit pattern,
◦ a sequence of bits 0s or 1s.
⦁ Different codes are used
◦ ASCII (7 bits per symbol)
◦ Extended ASCII (8 bits per symbol)
◦ Unicode (16 bits – supports different languages)
◦ ISO UTF (32 bits)
5

6. ⦁ Numbers
◦ Converted to a binary number – to simplify the mathematical
operations
⦁ Images
◦ Represented by bit patterns
◦ Each pixel is assigned a bit pattern
◦ Black and White Image – 1 bit per pixel
◦ Gray scale images – depends on number of levels in gray scale
◦ Colour images: Each pixel has 3 bit patterns (RGB)
⦁ Audio / Video
◦ Converted in to Analog/Digital
6

7. ⦁ Communication between two devices can be
◦ Simplex
◦ Half-duplex
◦ Full-duplex
1.9
Dept of CSE, VEC

8. Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)
1.1
0
Dept of CSE, VEC

9. The communication is unidirectional, as on a one-way street.
⦁ Only one of the two devices on a link can transmit;the other
can only receive
⦁ Examples : Keyboards and traditional monitors
⦁ The keyboard can only introduce input;
The monitor can only accept output.
⦁ The simplex mode can use the entire capacity of the channel
to send data in one direction.
1.1
1
Dept of CSE, VEC

10. ⦁ Each station can both transmit and receive, but not at
the same time
⦁ When one device is sending, the other can only receive,
and vice versa
⦁ The entire capacity of a channel is taken over by
whichever of the two devices is transmitting at the time
⦁ Examples : Walkie-talkies and radios
1.1
2
Dept of CSE, VEC

11. ⦁ Both stations can transmit and receive simultaneously
⦁ It is like a two-way street with traffic flowing in both
directions at the same time.
⦁ The link capacity can be shared in two ways:
◦ Either the link must contain two physically separate transmission
paths, one for sending and the other for receiving
◦ Or the capacity of the channel is divided between signals traveling
in both directions.
⦁ Example: The telephone network
◦ When two people are communicating by a telephone line, both can
talk and listen at the same time.
1.1
3
Dept of CSE, VEC

12. 14
⦁ Depends on 3 fundamental characteristics:
◦ Delivery :
🞄 The system must deliver data to the correct destination
🞄 Data must be received by the intended device or user and
only by that device or user
◦ Accuracy:
🞄 The system must deliver the data accurately.
🞄 Data that have been altered in transmission and left
uncorrected are unusable
◦ Timeliness:
🞄 The system must deliver data in a timely manner
🞄 Data delivered. late are useless
🞄 Delivering the data in the same order that they are produced
and without significant delay (real time transmission)

13. 1-2 NETWORKS
Anetwork
•A set of nodes (often referred to as devices) connected by
communication links.
•A node can be a computer, printer, or any other device capable
of sending and/or receiving data generated by the devices in the
network.
•A link can be a cable, air, optical fiber, or any medium which
can transport a signal carrying information.
 Network Criteria
 Physical Structures
 Categories of Networks
Topics :
Dept of CSE, VEC 5
1.1

14. Network Criteria
⦁ Performance
◦ Depends on Network Elements
◦ Measured in terms of Delay and Throughput
⦁ Reliability
◦ Failure rate of network components
◦ Measured in terms of availability/robustness
⦁ Security
◦ Data protection against corruption/loss of data due to:
🞄 Errors
🞄 Malicious users
Dept of CSE, VEC 6
1.1

15. ⦁ Performance can be measured by the following metrics:
◦ Transit time: Amount of time required for a message to travel
from one device to another
◦ Response time: The elapsed time between an inquiry and a
response
⦁ The performance of a network depends on the factors
such as:
◦ number of users
◦ type of transmission medium
◦ capabilities of the connected hardware
◦ efficiency of the software.
Dept of CSE, VEC 15

16. ⦁ Performance is often evaluated by two networking
metrics:
◦ Throughput
◦ Delay.
⦁ We often need more throughput and less delay.
⦁ Two criteria are often contradictory.
⦁ If we try to send more data to the network, we may
increase throughput
⦁ But we increase the delay because of traffic congestion
in the network
Dept of CSE, VEC 16

17. ⦁ Network reliability is measured by
◦ the frequency of failure,
◦ the time it takes a link to recover from a failure
◦ the network's robustness in a catastrophe.
Dept of CSE, VEC 17

18. ⦁ Network security issues include
◦ protecting data from unauthorized access
◦ protecting data from damage and development
◦ implementing policies and procedures for recovery from
breaches and data losses
Dept of CSE, VEC 18

19. Physical Structures
⦁ Type of Connection
◦ Point to Point - single transmitter and receiver
◦ Multipoint - multiple recipients of single transmission
⦁ Physical Topology
◦ Connection of devices
◦ Type of transmission –
🞄 Unicast,
🞄 Mulitcast,
🞄 Broadcast
1.2
1
Dept of CSE, VEC

20. Figure 1.3 Types of connections: point-to-point and multipoint
1.2
2
Dept of CSE, VEC

21. ⦁ A point-to-point connection provides a dedicated link between two
devices
⦁ The entire capacity of the link is reserved for transmission between those
two devices
⦁ Most point-to-point connections use an actual length of wire or cable to
connect the two ends, but other options, such as microwave or satellite
links, are also possible
⦁ Example :
change television channels by infrared remote control,
between the remote control and the television's control system.
Dept of CSE, VEC 21

22. ⦁ A multipoint also called multidrop connection
⦁ More than two specific devices share a single link
⦁ The capacity of the channel is shared, either spatially or temporally
⦁ If several devices can use the link simultaneously, it is a spatially shared
connection
⦁ If users must take turns, it is a timeshared connection.
Dept of CSE, VEC 22

23. ⦁ The geometric representation of the relationship of
all the links and linking devices (nodes) to one
another
⦁ Two or more devices connect to a link;
Two or more links form a topology
Dept of CSE, VEC 23

24. Figure 1.4 Categories of topology
1.2
Dept of CSE, VEC 6

25. ⦁ In a mesh topology, every device has a dedicated point-to-point link
to every other device.
⦁ The term dedicated means that the link carries traffic only between
the two devices it connects
⦁ To find the number of physical links in a fully connected mesh
network with n nodes, we first consider that each node must be
connected to every other node.
⦁ So we need n(n - 1) physical links.
🞄
Dept of CSE, VEC 27
However, if each physical link allows communication in both
directions (duplex mode), we can divide the number of links by 2
⦁ In other words, we can say that in a mesh topology, we need n(n-
1)/2 duplex mode links

26. Figure 1.5 A fully connected mesh topology (five devices)
1.2
Dept of CSE, VEC 8

27. ⦁ Dedicated links guarantees that each connection can carry its
own data load, thus eliminating the traffic problems that can
occur when links must be shared by multiple devices
⦁ Mesh topology is robust. If one link becomes unusable, it
does not incapacitate the entire system
⦁ Privacy or security – as message travels along a dedicated
line, only the intended recipient sees it
⦁ Point-to-point links make fault identification and fault
isolation easy
Dept of CSE, VEC 27

28. ⦁ Every device must be connected to every other device so installation and
reconnection are difficult
⦁ The sheer bulk of the wiring can be greater than the available space (in
walls, ceilings, or floors) can accommodate
⦁ The hardware required to connect each link (I/O ports and cable) can be
prohibitively expensive
⦁ For these reasons a mesh topology is usually implemented in a limited
fashion, for example, as a backbone connecting the main computers of a
hybrid network that can include several other topologies
⦁ Example: Connection of telephone regional offices in which each regional
office needs to be connected to every other regional office.
Dept of CSE, VEC 28

29. ⦁ Each device has a dedicated point-to-point link only to a
central controller, usually called a hub.
⦁ The devices are not directly linked to one another.
⦁ Unlike a mesh topology, a star topology does not allow
direct traffic between devices.
⦁ The controller acts as an exchange:
◦ If one device wants to send data to another, it sends the
data to the controller, which then relays the data to the
other connected device
Dept of CSE, VEC 29

30. Figure 1.6 A star topology connecting four stations
1.3
Dept of CSE, VEC 2

31. ⦁ Less expensive than a mesh topology: each device needs
only one link and one I/O port to connect it to any
number of others.
⦁ Far less cabling needs to be housed, only one
connection: between that device and the hub
⦁ Robustness: If one link fails, only that link is affected. All
other links remain active. This factor also lends itself to
easy fault identification and isolation
Dept of CSE, VEC 31

32. ⦁ Dependency of the whole topology on one single point,
the hub. If the hub goes down, the whole system is dead
⦁ Although a star requires far less cable than a mesh, each
node must be linked to a central hub. For this reason,
often more cabling is required in a star than in some
other topologies (such as ring or bus).
Example:
⦁ The star topology is used in local-area networks
(LANs)
Dept of CSE, VEC 32

33. ⦁ It follows a multipoint connection
⦁ One long cable acts as a backbone to link all the devices in a
network
⦁ Nodes are connected to the bus cable by drop lines and taps.
⦁ A drop line is a connection running between the device and
the main cable.
⦁ A tap is a connector that either splices into the main cable or
punctures the sheathing of a cable to create a contact with
the metallic core.
Dept of CSE, VEC 33

34. Figure 1.7 A bus topology connecting three stations
1.3
Dept of CSE, VEC 6

35. ⦁ Ease of installation
⦁ Bus uses less cabling than mesh or star topologies, as
backbone cable can be laid along the most efficient
path, then connected to the nodes by drop lines of
various lengths
Dept of CSE, VEC 35

36. ⦁ Difficult reconnection and fault isolation
⦁ Signal reflection at the taps can cause degradation in
quality. This can be controlled by limiting the number
and spacing of devices connected to a given length of
cable
⦁ A fault or break in the bus cable stops all transmission,
even between devices on the same side of the problem
Example:
⦁ Ethernet LANs can use a bus topology
Dept of CSE, VEC 36

37. Figure 1.8 A ring topology connecting six stations
1.3
Dept of CSE, VEC 9

38. ⦁ In a ring topology, each device has a dedicated point-to-
point connection with only the two devices on either
side of it.
⦁ A signal is passed in one direction along the ring, until it
reaches its destination from device to device.
⦁ Each device in the ring incorporates a repeater.
⦁ When a device receives a signal intended for another
device, its repeater regenerates the bits and passes them
along
Dept of CSE, VEC 38

39. ⦁ Easy to install and reconfigure: Each device is linked to
only its immediate neighbor. To add or delete a device
requires changing only two connections
⦁ Fault isolation is simplified: In a ring, a signal is
circulating at all times. If one device does not receive a
signal within a specified period, it can issue an alarm.
The alarm alerts the network operator to the problem
and its location.
Dept of CSE, VEC 39

40. ⦁ Unidirectional traffic: In a simple ring, a break in the ring
(such as a disabled station) can disable the entire
network.
⦁ This weakness can be solved by using a dual ring or a
switch capable of closing off the break.
Example:
⦁ Ring topology was prevalent when IBM introduced its
local-area network Token Ring. Today, the need for
higher-speed LANs has made this topology less popular.
Dept of CSE, VEC 40

41. ⦁ A network can be hybrid.
Example:
⦁ a main star topology with each branch connecting
several stations in a bus topology
Dept of CSE, VEC 41

42. Figure 1.9 A hybrid topology: a star backbone with three bus networks
1.4
4
Dept of CSE, VEC

43. Categories of Networks or Network Types
⦁ LocalArea Networks (LANs)
◦ Short distances
◦ Designed to provide local interconnectivity
⦁ WideArea Networks (W
ANs)
◦ Long distances
◦ Provide connectivity over large areas
⦁ MetropolitanArea Networks (MANs)
◦ Provide connectivity over areas such as a city, a campus
1.4
5
Dept of CSE, VEC

44. ⦁ A local area network (LAN) is usually privately owned and
links the devices in a single office, building, or campus
⦁ Depending on the needs of an organization and the type of
technology used, a LAN can be as simple as two PCs and a
printer in someone's home office; or it can extend throughout
a company and include audio and video peripherals.
⦁ Currently, LAN size is limited to a few kilometers.
Dept of CSE, VEC 46

45. Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet
1.4
Dept of CSE, VEC 7

46. ⦁ LANs are designed to allow resources to be shared between
personal computers or workstations.
⦁ The resources to be shared can include hardware (e.g., a
printer), software (e.g., an application program), or data.
⦁ A common example of a LAN, found in many business
environments, links a workgroup of task-related computers, for
example, engineering workstations or accounting PCs.
⦁ One of the computers may be given a large capacity disk drive
and may become a server to clients.
⦁ Software can be stored on this central server and used as
needed by the whole group.
Dept of CSE, VEC 46

47. ⦁ In addition to size, LANs are distinguished from other types of
networks by their transmission media and topology.
⦁ In general, a given LAN will use only one type of transmission
medium.
⦁ The most common LAN topologies are bus, ring, and star.
⦁ Early LANs had data rates in the 4 to 16 megabits per second
(Mbps) range.
⦁ Today, however, speeds are normally 100 or 1000 Mbps.
⦁ Wireless LANs are the newest evolution in LAN technology.
Dept of CSE, VEC 47

48. ⦁ A metropolitan area network (MAN) is a network with a size
between a LAN and a WAN.
⦁ It normally covers the area inside a town or a city.
⦁ It is designed for customers who need a high-speed connectivity,
normally to the Internet, and have endpoints spread over a city or
part of city.
⦁ A good example of a MAN is the part of the telephone company
network that can provide a high-speed DSL line to the customer.
⦁ Another example is the cable TV network that originally was
designed for cable TV, but today can also be used for high-speed
data connection to the Internet
Dept of CSE, VEC 48

49. 51
MAN
Designed to extend over an entire city – May be a single
network such as cable television network – may be a means
of connecting a number of LANS into a larger network

50. 1.5
server(s)
cable headend
cable distribution
network (simplified)
A MAN may be a single network such as cable television network
Dept of CSE, VEC 2

51. ⦁ A wide area network (WAN) provides long-distance transmission of
data, image, audio, and video information over large geographic
areas that may comprise a country, a continent, or even the whole
world.
⦁ A WAN can be as complex as the backbones that connect the
Internet (Switched WAN) or as simple as a dial-up line that
connects a home computer to the Internet (a point-to-point WAN )
⦁ The switched WAN connects the end systems, which usually
comprise a router (internetworking connecting device) that
connects to another LAN or WAN.
⦁ The point-to-point WAN is normally a line leased from a telephone
or cable TV provider that connects a home computer or a small LAN
to an Internet service provider (lSP).
⦁ This type of WAN is often used to provide Internet access.
Dept of CSE, VEC 53

52. Figure 1.11 WANs: a switched WAN and a point-to-point WAN
1.5
4
Dept of CSE, VEC

53. Example:
•Assume that an organization has two offices, one on the east coast
with star topology and the other on the west coast with bus topology
•The president of the company lives somewhere in the middle and
needs to have control over the company from her home
•To create a backbone WAN for connecting these three entities (two
LANs and the president's computer), a switched WAN (operated by a
service provider such as a telecom company) has been leased.
•To connect the LANs to this switched WAN, however, three point-to-
point WANs are required.
•These point-to-point WANs can be a high-speed DSL line offered by a
telephone company or a cable modern line offered by a cable TV
provider
Wide Area Network (WAN)
1.5
5
Dept of CSE, VEC

54. Figure 1.12 A heterogeneous network made of four WANs and two LANs
1.5
6
Dept of CSE, VEC

55. ⦁ An early example of a switched WAN is X.25, a network designed to
provide connectivity between end users
⦁ A good example of a switched WAN is the Asynchronous Transfer
Mode (ATM) network, which is a network with fixed-size data unit
packets called cells
⦁ Another example of WANs is the wireless WAN
Dept of CSE, VEC 57

56. s, scanners
⦁ Communicating using email,
video, instant messaging and
other methods
⦁ Sharing devices such as printer
and photocopiers
⦁ Sharing files
⦁ Sharing software and operating programs
on remote systems
⦁ Allowing network users to easily access
and maintain information
1.5
Dept of CSE, VEC 8

57. end systems (hosts):
59
run application programs
e.g. Web, email
at “edge of network”
client/server model
client host requests,
receives service from
always-on server
e.g. Web browser/server;
email client/server
Client/server model is applicable
in an intranet

58. 60
⦁ Peer-Peer model:
◦ No fixed clients or servers
◦ Each host can act as both client & server
⦁ Examples: Napster, Gnutella, KaZaA

59. 61
⦁ WWW
⦁ Instant Messaging (Internet chat, text
messaging on cellular phones)
⦁ Peer-to-Peer
⦁ Internet Phone
⦁ Video-on-demand
⦁ Distributed Games
⦁ Remote Login (Telnet)
⦁ File Transfer

60. 1-3 PROTOCOLS
Aprotocol is
•synonymous with rule.
•It consists of a set of rules that govern data
communications.
•It determines
•what is communicated,
•how it is communicated and
•when it is communicated.
•The key elements of a protocol are syntax, semantics
and timing
1.6
2
Dept of CSE, VEC

61. Elements of a Protocol
⦁ Syntax
◦ Structure or format of the data
◦ Indicates how to read the bits - field delineation
⦁ Semantics
◦ Interprets the meaning of the bits
◦ Knows which fields define what action
⦁ Timing
◦ When data should be sent and what
◦ Speed at which data should be sent or speed at which it is being
received.
1.6
3
Dept of CSE, VEC

62. ⦁ Standards
◦ Essential in creating and maintaining an open and competitive
market for equipment manufacturers and in guaranteeing
national and international interoperability of data and
telecommunications technology and processes
◦ De facto: Standards that have not been approved by an
organised body but have been adopted as standards through
widespread use
◦ De jure: legislated by an officially recognised body
62

63. ⦁ Standards Organisations
◦ International organisation for Standardisation (ISO)
◦ International Telecommunication Union –
Telecommunication Standards Sector (ITU-T)
◦ American National Standards Institute (ANSI)
◦ Institute of Electrical and Electronics Engineers (IEEE)
◦ Electronic Industries Assoctiation (EIA)
63

64. ⦁ Regulatory Agencies
◦ Federal Communication Commission (FCC)
⦁ Internet Standards
◦ Internet Draft
◦ Request for Comment (RFC)
64

65. ⦁ In data communication and networking, a protocol defines
Dept of CSE, VEC 65
the rules that both
intermediate devices
the sender and receiver
need to follow to be
and all
able to
communicate effectively.
⦁ When communication is simple, we
simple protocol
may need only one
⦁ When the communication is complex, we may need to divide
the task between different layers, in which case we need a
protocol at each layer or protocol layering.

66. ⦁ In the first scenario, communication is so simple that it can
occur in only one layer. Assume Maria and Ann are neighbors
with a lot of common ideas. Communication between Maria
and Ann takes place in one layer, face to face, in the same
language
Asingle-layer protocol
Dept of CSE, VEC 66

67. ⦁ In the second scenario, we assume that Ann is offered a
higher-level position in her company, but needs to move to
another branch located in a city very far from Maria.
⦁ The two friends still want to continue their communication
and exchange ideas because they have come up with an
innovative project to start a new business when they both
retire.
⦁ They decide to continue their conversation using regular mail
through the post office. However, they do not want their ideas
to be revealed by other people if the letters are intercepted.
They agree on an encryption/decryption technique.
Dept of CSE, VEC 67

68. Athree-layer protocol
Dept of CSE, VEC 68

69. ⦁ Protocol layering enables us to divide a complex task into several smaller
and simpler tasks
⦁ One of the advantages of protocol layering is that it allows us to separate
the services from the implementation
⦁ A layer needs to be able to receive a set of services from the lower layer
and to give the services to the upper layer
⦁ Protocol layering in the Internet, is that communication does not always
use only two end systems
⦁ There are intermediate systems that need only some layers, but not all
layers
⦁ If we did not use protocol layering, we would have to make each
intermediate system as complex as the end systems, which makes the
whole system more expensive
Dept of CSE, VEC 69

70. First Principle
⦁ The first principle dictates that if we want bidirectional
communication, we need to make each layer so that it is able to
perform two opposite tasks, one in each direction
Second Principle
⦁ The second principle that we need to follow in protocol layering is
that the two objects under each layer at both sites should be
identical
Dept of CSE, VEC 70

71. Logical Connections
⦁ Layer-to-layer communication
⦁ Logical (imaginary) connection at each layer through which they can
send the object created from that layer
Dept of CSE, VEC 71

72. Overview
72

73. THANK YOU