#1 Physical Layer.pdf

by
Dr. S. SELVAGANESAN
Professor and HoD / IT
sselvaganesan@gmail.com
CS8591 Computer Networks
1
UNIT I
Introduction and
Physical Layer

CS8591 COMPUTER NETWORKS - Syllabus
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CS8591 COMPUTER NETWORKS - Books
3

UNIT I
Introduction and Physical
Layer
Networks – Network Types – Protocol
Layering – TCP/IP Protocol suite –
OSI Model – Physical Layer:
Performance – Transmission media –
Switching – Circuit-switched
Networks – Packet Switching. 4

Data Communication
 Data Communication is the
exchange of data between two
devices via some form of
transmission medium (such as a wire
cable).
5

Data Communication
 Characteristics:
Effectiveness of a data communication system depends on:
 Delivery: The system must deliver data to the correct
destination.
 Accuracy: The system must deliver the data accurately
(without any change/alteration).
 Timeliness: The system must deliver the data in a timely
manner. In the case of video and audio, timely delivery
means delivering the data as they are produced, in the same
order, that they are produced, and without significant delay.
This kind of delivery is called real-time transmission.
 Jitter: Jitter refers to the variation in the packet arrival time.
It is the uneven delay in the delivery of audio or video
packets 6

Components of Data communication
 Components
 A Data Communication system has five components.
Figure 1: Five Components of Data Communication System
7

 A Data Communication system consists of five components. They are
 1. Sender: The sender is a device that sends the message. It can be a
computer, workstation, telephone handset, video camera, and so on.
 2. Receiver: The receiver is a device that receives the message sent by the
sender. It can be a computer, workstation, telephone handset, television, and
so on.
 3. Message: The message is the information or data to be communicated.
(Text, number, images, audio and video)
 4. Medium: The transmission medium is the physical path by which a
message travels from sender to receiver. Some examples of transmission
media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio
waves.
 5. Protocol: A protocol is a set of rules that govern data communications. It
represents an agreement between the communicating devices. Without a
protocol, two devices may be connected but not communicating, just as a
person speaking French cannot be understood by a person who speaks only
Japanese.
Components of Data communication
8

 Communication between two devices can be Simplex,
Half-duplex, and full-duplex.
Figure 2: Data flow (Simplex, Half-duplex, and full-duplex)
Data Flow (Transmission mode)
9

 Communication between two devices can be of three types:
1. Simplex: The communication is unidirectional. Only one of
the two stations on a link can transmit and other can only
receive.
 Keyboards and traditional monitors are examples of
simplex devices. 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.
2. Half-duplex: Each station can both transmit and receive; but
not at the same time. The entire capacity of the channel is
taken by the station which transmits the data.
 In a half-duplex transmission, the entire capacity of a
channel is taken over by whichever of the two devices is
transmitting at the time. Walkie-talkies and CB
(Citizens Band) radios are both half-duplex systems.
Data Flow
10

 Communication between two devices can be of three
types:
3. Full-duplex: Both stations can transmit and receive the
data at the same time. The capacity of the channel is
divided between the signals traveling in the opposite
direction.
 One common example of full-duplex communication is the
telephone network. When two people are communicating by a
telephone line, both can talk and listen at the same time. The
full-duplex mode is used when communication in both
directions is required all the time. The capacity of the channel,
however, must be divided between the two directions
Data Flow
11

NETWORKS
 A network is a set of devices (also referred to as
nodes) connected by communication links.
 A node can be a computer, printer or any other
device capable of sending and/or receiving data
generated by other nodes on the network. 12

Computer NETWORKS
 A Computer network is a group of
interconnected computers.
Why we need Networks?
 It allows computers to communicate with
each other and to share resources and
information.
 The best known computer network is the
Internet.
13

NETWORKS
Distributed Processing
 Most networks use distributed processing in
which a task is divided among multiple
computers.
 Instead of one single large machine being
responsible for all aspects of a process, separate
computers (usually a personal computer or
workstation) handle a subset.
14

NETWORKS
Network Criteria
 A network must be able to meet a certain
number of criteria. The most important of
these criteria are
 Performance
 Reliability
 Security
15

NETWORKS
Performance
 Performance can be measured in many ways,
including transit time and response time.
 Transit time is the amount of time required for a
message to travel from one device to another.
 Response time is the elapsed time between an
inquiry and a response.
 Performance of a network depends on a number
of factors including number of users, type of
transmission medium, capabilities of connected
hardware, and efficiency of software.
16

NETWORKS
Performance
 Performance is often evaluated by two
networking metrics: throughput and delay.
 We often need more throughput and less delay.
 However, these 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.
17

NETWORKS
Reliability
 In addition to accuracy of delivery, network
reliability is measured by the frequency of
failure, the time it takes a link to recover from
a failure, and the network’s robustness in a
catastrophe.
Security
 Network security issues include protecting
data from unauthorized access, protecting data
from damage and, development and
implementing policies and procedures for
recovery from breaches and data losses.
18

Physical Structures
Type of Connection
Before discussing networks, we need to define some
network attributes.
 Direction of Data Flow
 A network is a set of two or more devices connected
through links.
 A Link is the physical communication pathway that
transfers data from one device to another.
 Line Configuration refers to the way two or more
communication devices attach to a link.
 Two types of connections (Line Configuration):
1. Point-to-Point
2. Multipoint
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 Point-to-Point Line Configuration provides a dedicated link
between two devices. The entire capacity of the channel is
reserved for transmission between those two devices.
 Most point-to-point line configuration 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: 1. Point-to-Point connection between remote control
and Television for changing the channels (through infrared ray)
 2. A computer connected by a telephone line.
Figure 3: Point-to-Point Line Configuration
Physical Structures
Type of Connection
20

 Multipoint Line Configuration is one in which more than two
specific devices share a single link.
 In a multipoint environment, the capacity of the channel is
shared, either spatially or temporally.
 If several devices can use the link simultaneously, it is a spatially shared
line configuration. (Example: internet communication, Telephony
communication.)
 If users must take turns, it is a time-shared line configuration.
(temporally)
Figure 4: Multipoint Line Configuration
Physical Structures
Type of Connection
21

 Topology: The term topology refers to the way a network is
laid out physically.
 Two or more devices connect to a link; two or more links
form a topology.
 The topology of a network is the geometric representation of
the relationship of all the links and linking devices (usually
called nodes) to each other.
 Four basic topologies:
1. Mesh
2. Star
3. Bus
4. Ring
(In addition, Hybrid)
Physical Structures
Physical Topology
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 Mesh Topology:
 Every device has a dedicated point-to-point link to every other device.
 In a mesh (topology) network with n nodes, there are
direct links;
 number of I/O ports (for each device) required is (n-1).
Advantages:
1. Mesh topology is robust
2. Better privacy and security
3. Failure of one link will not disturb
other links
4. Helps the network manager to find
the fault location and solution
Disadvantages:
1. Large amount of cabling and I/O
ports are required.
2. Installation and reconnection are
difficult.
Physical Structures
Physical Topology
Fig.5: A fully connected mesh topology
(five devices)
23

Exercise
In a mesh (topology) network with n nodes, there are direct
links.
No. of ports on each device = (n-1)
Problem:
Assume five devices are arranged in a mesh
topology. How many cables are needed? How
many ports are needed for each device?
Ans:
No. of cables = = = 10
No. of ports for each device = (n-1) = 5-1
= 4
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 Star Topology:
 Every device has a dedicated point-to-point link to a central
controller (HUB) only.
 In a Star topology network with n nodes, there are direct n links;
Advantages:
1. Star topology is robust.
2. Less expensive.
3. Fault identification and fault
isolation are easy.
4. Modification of star network
is easy.
Disadvantages:
1. If the central hub fails, the
whole network will not work.
2. Communication is possible
through the hub.
Physical Structures
Physical Topology
Fig.6: A star topology connecting five stations
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Hub
Physical Structures
Physical Topology
Fig.7: A star topology connecting four stations
26

 Bus Topology:
 One long cable acts as a backbone to link all the devices in the
network
 Nodes are connected to the backbone by taps and drop lines.
 Drop line is establishing the connection between the devices and the
cable. The taps are used as connectors.
Advantages:
1. Easy Installation.
2. Less cabling and less number
of I/O ports is required.
3. Less cost.
Disadvantages:
1. Network traffic is high.
2. Fault isolation and reconnection is
difficult.
3. Adding new device is difficult.
4. A break in the bus cable stops all
transmissions
Physical Structures
Physical Topology
Fig.8: A bus
topology
connecting
three stations
27

 Ring Topology:
 Each device has a dedicated point-to-point link with only the two
devices on either side of it.
 A signal is travelling along a ring in only one direction from device
to device until it reaches its destination.
 The repeater is used to regenerate the signals during transmission.
Advantages:
1. Easy to install and reconfigure.
2. Link failure can be easily found
out.
Disadvantages:
1. Maximum length of ring and
number of devices are limited.
2. Failure of one node on the ring
affects the entire network.
3. Addition of nodes or removal of
nodes disrupts the network.
4. Signal traffic is unidirectional.
Physical Structures
Physical Topology
Fig. 9: A ring topology connecting six stations
28

 Ring Topology:
 In a Ring topology network with n nodes,
there are direct n links;
Physical Structures
Physical Topology
Fig. 9: A ring topology connecting six stations
29

 Hybrid Topology:
 A hybrid topology is a type of network topology that uses two or
more other network topologies, including bus topology, mesh
topology, ring topology, star topology, and tree topology.
Physical Structures
Physical Topology
Fig.10: A hybrid topology: a star backbone with three bus networks
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 Hybrid Topology:
Physical Structures
Physical Topology
Fig.11: A hybrid topology: a star backbone with three bus networks
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Types of Networks
 Categories of Network
32

LAN, MAN, and WAN
 Categories of Network
33

Fig 12: LAN
Local Area Networks (LAN)
34

Figure 13 An isolated LAN connecting 12 computers to a hub in a closet
35

 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 (up to 10 kms.)
41

 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.
42

 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. (10 km
– 100 km)
 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.
Metropolitan Area Networks (MAN)
43

 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 or as simple as a dial-up line that connects a home computer
to the Internet.
 We normally refer to the first as a switched WAN and to the second
as a point-to-point WAN (Figure 17).
 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.
Wide Area Networks (WAN)
44

Figure 17 WANs: a switched WAN and a point-to-point WAN
45

Interconnection of Networks:
Internet
 Today, it is very rare to see a LAN, a MAN, or a LAN in isolation; they are
connected to one another. When two or more networks are connected, they
become an internetwork, or internet.
 As an example, assume that an organization has two offices, one on the east
coast and the other on the west coast. The established office on the west
coast has a bus topology LAN; the newly opened office on the east coast has
a star topology LAN. 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 as
shown in Figure 1.12. 46

Figure 18 A heterogeneous network made of four WANs and two LANs 47

The Internet
 A network is a group of connected devices such as
computers and printers.
 An internet is two or more networks that can
communicate with each other.
 The most notable internet is called the Internet.
 The Internet is a collaboration of more than
hundreds of interconnected networks.
48

The Internet
 In 1969, a project was funded by the Advanced Research
Project Agency, an arm of the U.S. Department of
Defense.
 ARPA established a packet-switching network, called
Advanced Research Project Agency Network
(ARPANET).
 In 1972, Vint Cerf and Bob Kahn, collaborated on what
they called the Internetting Project.
49

Figure 19 Hierarchical organization of the Internet
50

Protocols and Standards
 Protocol is synonymous with rule.
 Standards are agreed upon rules.
51

 Protocols
 A protocol is a set of rules that governs data
communications.
 A protocol defines what is communicated, how it is
communicated, and when it is communicated.
 The important elements of a protocol are: Syntax,
Semantics and timing.
 Syntax: It refers to the structure or format of the data (the
order in which they are presented).
 Semantics: It refers to the meaning for each section of
bits, how the data is going to be interpreted and the action
to be taken based on the interpretation.
 Timing: It indicates when the data should be sent and how
fast the data can be sent. 52

 Standards
 De facto: The standards that have not been approved
by an organization body but have been adopted as
standards through widespread use are called De
facto Standard.
 De jure: Standards that have been approved by an
organized body.
Standards Organization
International Organization for Standardization (ISO),
International Telecommunication Union-Telecommunication
Standards Sector (ITU-T),
American National Standards Institute (ANSI),
Institute of Electrical and Electronics Engineers (IEEE),
Electronic Industries Association (EIA)
53

Building a Network
 A Computer Network must provide a general, cost
effective, fair and robust connectivity among a large
number of computers.
 Networks must evolve to accommodate changes in both
underlying technologies upon which they are based as
well as changes in the demands placed on them by
application programs.
 To deal with this complexity, network designers have
developed general blueprint called Network
Architecture.

Building a Network
 A Network Architecture is defined as which identifies
the available hardware and software components and
shows how they can be arranged to form a complete
network.
 To build a network, we must know the following
things.
 Discover the requirements that different applications and
different communities of people place on the network.
 Network Architecture on which the applications are going to
be developed.
 Key elements in the implementation of computer networks.
 Identifying key metrics that are used to evaluate the
performance of computer networks.

Building a Network
 Networking is a planned, and ongoing effort.
 We set goals, develop strategies for achieving them,
take action, evaluate how well our plan is working, and
make changes as necessary.
 How to build a network in five steps:
1. Make a Networking Plan
2. Make contact
3. Organize our network
4. Take action; and
5. Practice networking etiquette

Building a Network
 Applications
 Some applications of the computer networks are:
 World Wide Web (WWW)
 Email
 Streaming audio and video
 Chat rooms
 Music (file) sharing

Requirements
 Requirements
 For building a computer network, we must identify
the set of constraints and requirements that influence
network design and the expectations we have for a
network depend on our perspective:
 An Application Programmer would list the services that
the application needs.
 A Network Designer would list the properties of a cost-
effective design.
 A Network Provider would list the characteristics of a
system that is easy to administer and manage.

Layered Tasks
 We use the concept of layers in our daily life.
 As an example, let us consider two friends who
communicate through postal mail.
 The process of sending a letter to a friend would
be complex if there were no services available from
the post office.
 Figure shows the steps in this task.

Figure - Tasks involved in sending a letter
Layered Tasks

Sender, Receiver, and Carrier
In Figure, we have a sender, a receiver, and a carrier that transports the letter. There is a
hierarchy of tasks.
At the Sender Site
Let us first describe, in order, the activities that take place at the sender site.
o Higher layer. The sender writes the letter, inserts the letter in an envelope, writes
the sender and receiver addresses, and drops the letter in a mailbox.
o Middle layer. The letter is picked up by a letter carrier and delivered to the post
office.
o Lower layer. The letter is sorted at the post office; a carrier transports the letter.
On the Way
The letter is then on its way to the recipient. On the way to the recipient's local post
office, the letter may actually go through a central office. In addition, it may be trans
ported by truck, train, airplane, boat, or a combination of these.
At the Receiver Site
o Lower layer. The carrier transports the letter to the post office.
o Middle layer. The letter is sorted and delivered to the recipient's mailbox.
o Higher layer. The receiver picks up the letter, opens the envelope, and reads it.
Layered Tasks

Layering and Protocols
 Layering is the technique for organizing the protocols into an
ordered series of distinct abstractions.
 The services provided by a layer depend only on the services
provided by the previous less abstract layer.
 The layer immediately above the hardware might provide host-
to-host connectivity.
 The next layer up provides the support for process-to-process
channels.
Figure: Example of a layered network system

Layering and Protocols
 Layering provides two nice features:
1. It decomposes the problem of building a network into more manageable
components.
2. It provides a more modular design.
 If we want to add a new service, we have to modify the functionality at
one layer, reusing the functions provided at all other layers.
 In the above figure, one channel provides a request/reply service, and
another channel provides a message stream service.
 The abstract objects that make up the layers of a network system are
called protocols. That is, a protocol provides a communication service
that higher-level objects use to exchange messages.
Figure: Layered systems with alternative abstractions

OSI Model
 Open Systems Interconnection (OSI) Model
 An ISO standard that covers all aspects of network
communications is the Open Systems Interconnection (OSI)
model.
 It was first introduced in the late 1970s.
 An open system is a model that allows any different systems to
communicate regardless to their underlying architecture.
 The purpose of OSI model is to open communications between
different systems without requiring changes to the logic of the
underlying hardware and software.
 The OSI model is not a protocol; it is a model for understanding
and designing a network architecture that is flexible, robust, and
interoperable.

OSI Model
 Open Systems Interconnection (OSI)
 The Open Systems Interconnection model is a layered framework for the
design of network systems that allows for communication across all types of
computer systems.
 It consists for seven separate but related layers, each of which defines a
segment of the process of moving information across a network.

OSI Model
 The figure gives an overall view of the OSI layers.
Figure – The Interaction between layers in the OSI model

Layered Architecture (OSI Model)
 Open Systems Interconnection (OSI).
 Figure shows the layers involved when a message is sent from
device A to device B.
 As the message travels from A to B, it may pass through many
intermediate nodes.
 These intermediate nodes usually involve only the first three
layers of the OSI model
 Each layer calls upon the services of the layers just below it.
This is done with the help of protocols.
 The processes on each machine that communicate at a given
layer are called Peer-to-Peer Processes.
 The passing of data and network information between the layers
are carried out with the help of interfaces. Interface is used to
define the information and services to be provided by each layer.

Peer-to-Peer Process
 At the physical layer, communication is direct.
 In Figure, device A sends a stream of bits to device B
(through intermediate nodes).
 At the higher layers, however, communication must move
down through the layers on device A, over to device B, and
then back up through the layers.
 Each layer in the sending device adds its own information to
the message it receives from the layer just above it and passes
the whole package to the layer just below it.
 At layer 1, the entire package is converted to a form that can
be transmitted to the receiving device.
 At the receiving machine, the message is unwrapped layer by
layer, with each process receiving and removing the data
meant for it.

 The passing of the data and network information down
through the layers of the sending device and back up
through the layers of the receiving device is made possible
by an interface between each pair of adjacent layers.
 Each interface defines the information and services a layer
must provide for the layer above it.
 Well-defined interfaces and layer functions provide
modularity to a network.
 As long as a layer provides the expected services to the
layer above it, the specific implementation of its functions
can be modified or replaced without requiring changes to
the surrounding layers.
Interfaces Between Layers

OSI Model
 The figure gives an overall view of the
OSI layers.
Figure – The Interaction between layers in the OSI
model
The seven layers are
organized into three
subgroups.
# Layers 1, 2 and 3
(Physical, Data Link and
Network layers) are
Network Support Layers.
# Layers 5, 6 and 7
(Session, Presentation and
Application layers) are
User Support Layers.
# Layer 4 (Transport Layer)
links the two subgroups
and ensures that what
lower layers have
transmitted is in a form that
the upper layers can use.

OSI Model
 Network support Layer deal with the physical aspects of moving
data from one device to another such as electrical specifications,
physical connections, physical addressing and transport timing and
reliability.
 User support Layers allow interoperability among unrelated
software systems.
 The upper OSI layers are always implemented in software. Lower
layers are a combination of hardware and software, except for the
physical layer, which is mostly hardware.

OSI Model
Figure – An Exchange using the OSI model

OSI Model
• The process starts at the application layer;
then moves from layer to layer in
descending sequential order.
• At each layer, a header, can be added to the
data unit.
• The trailer is added only at layer 2.
• When the formatted data unit passes
through the physical layer, it is changed into
an electromagnetic signal and transported
along the physical link.
• Upon reaching its destination, the signal
passes into physical layer and is transformed
back into digital form.
•The data units are then moved back up
through the OSI layers.
•When the block of data reaches the next
higher layer, the headers and trailers attached
by the sending layer are removed.
•When the data unit reaches the application
layer, the message is again in form
appropriate to the application and is made
available to the recipient.
Figure – An Exchange using the OSI model

Encapsulation
 Figure 2.3 reveals another aspect of data communications in
the OSI model: encapsulation.
 A packet (header and data) at level 7 is encapsulated in a
packet at level whole packet at level 6 is encapsulated in a
packet at level 5, and so on.
 In other words, the data portion of a packet at level N - 1
carries the whole (data and header and maybe trailer) from
level N. The concept is called encapsulation; level N - 1 is not
aware of which part of the encapsulated packet is data and
which is the header or trailer.
 For level N - 1, the whole packet coming from level N as one
integral unit.

Physical Layer
 Physical Layer
 The physical layer is responsible for movements of individual bits
from one hop (node) to the next.
 The physical layer coordinates the functions required to carry a bit
stream over a physical medium.
 It deals with the mechanical and electrical specifications of the
interface and the transmission medium.

 Functions of Physical Layer
1. Physical Characteristics of Interfaces and Media:
• It defines the electrical and mechanical characteristics of the interface and the media.
• It defines the types of transmission medium.
2. Representation of Bits
• To transmit the streams of bits they must be encoded into signal.
• It defines the type of encoding whether electrical or optical.
3. Data rate
• It defines the transmission rate, i.e., the number of bits sent per second.
4. Synchronization of Bits
• The sender and receiver must be synchronized at bit level.
5. Line Configuration
• It defines the type of connection between the devices
• Two types of connection are : Point-to-Point; Multipoint
6. Physical Topology
• The sender and receiver must be synchronized at bit level.
7. Transmission Mode
• The physical layer also defines the direction of transmission between two devices:
Simplex, Half-duplex, Full-duplex.
Physical Layer

2.79
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
Note

 Data Link Layer
 The data link layer is responsible for moving frames from one hop
(node) to the next.
Data Link Layer

 Functions of Data Link Layer
1. Framing: It divides the stream of bits received from network layer into
manageable data units called frames.
2. Physical Addressing: It adds a header to the frame to define the sender
and receiver of the frame.
3. Flow Control: The data link layer imposes a flow control mechanism to
avoid overwhelming the receiver.
4. Error Control
• It adds reliability by adding mechanisms to detect and retransmit
damaged or lost frames.
• It also uses a mechanism to recognize duplicate frames by adding the
trailer to the end of the frame
5. Access Control
• It determines which device has control over the link at any given
time when two or more devices are connected to the same link.
Data Link Layer

 Sub-layers of Data Link Layer
1. Logical Link Control (LLC) Layer
• The Logical Link Control (LLC) layer is one of two sub-layers
that make up the Data Link Layer of the OSI model.
• The Logical Link Control layer controls frame
synchronization, flow control and error checking.
2. Media Access Control (MAC) Layer
• The Media Access Control Layer is one of two sub-layers that
make up the Data Link Layer of the OSI model.
• The MAC layer is responsible for moving data packets to and
from one Network Interface Card (NIC) to another across a
shared channel.
Data Link Layer

The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

2.84
Figure 2.7 Hop-to-hop delivery

 Network Layer
 The network layer is responsible for the delivery of individual packets
from source host to the destination host.
Network Layer

 Other responsibilities of Network Layer
1. Logical Addressing: When a packet passes the network
boundary, the network layer adds the logical addresses of
the sender and receiver.
2. Routing: When independent networks or links are
connected to create internetworks, the connecting devices
(called routers or switches) route or switch the packets to
their final destination.
Network Layer

2.89
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

2.90
Figure 2.9 Source-to-destination delivery

 Transport Layer
 The transport layer is responsible for the delivery of a message from
one process to another.
Transport Layer

Transport Layer
 Other responsibilities of Transport Layer
1. Service-point addressing: The transport layer gets the entire
message to the correct process on the destination systems by adding a
type of address called a service-point address (or port address).
2. Segmentation and reassembly: A message is divided into
transmittable segments, with each segment containing a sequence
number. These numbers are used to reassemble the message at the
destination and to identify and replace packets that were lost in
transmission.
3. Connection Control: In a connectionless service, each segment is
treated as independent packet and in connection oriented service,
each segment is treated as dependent packet. After all the data are
transferred, the connection is terminated.
4. Flow Control: Flow control is performed from end to end rather than
across a single link.
5. Error Control: At this layer, the error control is performed in a
process-to-process rather than across a single link.

2.94
The transport layer is responsible for the delivery
of a message from one process to another.
Note

2.95
Figure 2.11 Reliable process-to-process delivery of a message

Session Layer
 Session Layer

Session Layer
 Specific responsibilities of Session Layer
1. Dialog Control: 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 or full-
duplex mode.
2. Synchronization: The session layer allows a process to add
checkpoints, or synchronization points, to a stream of data.
• E.g. If a system is sending a file of 100 pages, it is advisable to
insert checkpoints after every 10 pages to ensure that each 10-
page unit is received and acknowledged independently. In this
case, if a crash happens during the transmission of page 23, the
only pages that need to be resent after system recovery are
pages 21 to 30.

2.100
The session layer is responsible for dialog
control and synchronization.
Note

Presentation Layer
 Presentation Layer
 The presentation layer is responsible for translation, compression, and
encryption.

Presentation Layer
 Specific responsibilities of Presentation Layer
1. Translation: Presentation layer is responsible for the
interoperability between different encoding methods.
2. Encryption: To carry sensitive information, a system must
be able to ensure privacy. Encryption means that the sender
transforms the original information to another form and send
the resulting message out over the network. Decryption
reverses the original process to transform the message back
to its original form.
3. Compression: Data compression reduces the number of bits
contained in the information. Data compression is important
in the transmission of multimedia such as text, audio and
video.

2.103
The presentation layer is responsible for translation,
compression, and encryption.
Note

Application Layer
 Application Layer: is responsible for providing services to the
user.

Application Layer
 Specific services provided by Application Layer:
1. Network Virtual Terminal: is a software version of a
physical terminal and it allows a user to log on to a remote
host.
2. File transfer, access, and management: The application
allows a user to access files in a remote host, to retrieve files
from a remote computer for use in the local computer, and to
manage or control files in a remote computer locally.
3. Mail Services: This application provides the basis for e-mail
forwarding and storage.
4. Directory Services: This application provides distributed
sources and access for global information about various
objects and services.

2.106
The application layer is responsible for
providing services to the user.
Note

Data Link Layer also adds a trailer
- Trailer contains additional information that
deals with error correction.

Internet Architecture
 Internet architecture is a meta-network, a constantly
changing collection of thousands of individual networks
intercommunicating with a common protocol.
 Internet architecture is described in its name, the short form
of ‘inter-networking’.
 This architecture is based on the very specification of the
standard TCP/IP protocol, designed to connect any two
networks which may be very different in internal hardware,
software and technical design.
 Once two networks are interconnected, communication with
TCP/IP is enabled end-to-end, so that any node on the
Internet has the ability to communicate with any other node
(on anywhere). The Internet architecture is also sometimes
called the TCP/IP architecture.

TCP/IP Protocol Suite
 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.
 The original TCP/IP protocol suite was defined as having
four layers: host-to-network, internet, transport, and
application.
 The TCP/IP protocol suite is made of five layers: Physical,
Data link, Network, Transport, and Application.
 The first four 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, however, are
represented in TCP/IP by a single layer called the
application layer (see Figure).

2.115
Figure 2.16 TCP/IP and OSI model

1. Physical and Data Link Layers:
 At the physical and data link layers, TCP/IP does
not define any specific protocol.
 It supports all the standard and proprietary
protocols.
 A network in a TCP/IP internetwork can be a
local-area network or a wide-area network.

2. Network Layer:
At the network layer (or, more accurately, the internetwork layer),
TCP/IP supports the Internetworking Protocol. IP, in turn, uses four
Supporting protocols: ARP, RARP, ICMP, and IGMP.
 a. Internetworking Protocol (IP)
The Internetworking Protocol (IP) is the transmission
mechanism used by the TCP/IP protocols. The IP layer provides
an unreliable, connectionless delivery system. The reason why it
is unreliable is that IP provides no error checking or tracking.
 b. Address Resolution Protocol
The Address Resolution Protocol (ARP) is used to associate a
logical address with a physical address. On a typical physical
network, such as a LAN, each device on a link is identified by a
physical or station address, usually imprinted on the network
interface card (NIC). ARP is used to find the physical address of the
node when its Internet address is known.

2. Network Layer: (Contd)
 c. Reverse Address Resolution Protocol
 The Reverse Address Resolution Protocol (RARP) allows a
host to discover its Internet address when it knows only its
physical address. It is used when a computer is connected to a
network for the first time or when a diskless computer is
booted.
 d. Internet Control Message Protocol
 The Internet Control Message Protocol (ICMP) is a mechanism
used by hosts and gateways to send notification of datagram
problems back to the sender. ICMP sends query and error
reporting messages.
 e. Internet Group Message Protocol
 The Internet Group Message Protocol (IGMP) is used to
facilitate the simultaneous transmission of a message to a
group of recipients.

3. Transport Layer:
 Traditionally the transport layer was represented in TCP/IP by
two protocols: TCP and UDP.
 IP is a host-to-host protocol, meaning that it can deliver a packet
from one physical device to another.
 UDP and TCP are transport level protocols responsible for
delivery of a message from a process (running program) to
another process. A new transport layer protocol, SCTP, has been
devised to meet the needs of some newer applications.
 a. User Datagram Protocol
 The User Datagram Protocol (UDP) is the simpler of the two
standard TCP/IP transport protocols.
 It is a process-to-process protocol that adds only port
addresses, checksum error control, and length information to
the data from the upper layer.

4. Transport Layer: (Contd)
b. Transmission Control Protocol (TCP)
 The Transmission Control Protocol (TCP) provides full transport-
layer services to applications.
 TCP is a reliable stream transport protocol. The term stream, in
this context, means connection-oriented: A connection must be
established between both ends of a transmission before either can
transmit data.
 At the sending end of each transmission, TCP divides a stream of
data into smaller units called segments. Each segment includes a
sequence number for reordering after receipt, together with an
acknowledgment number for the segments received. Segments
are carried across the internet inside of IP datagrams. At the
receiving end, TCP collects each datagram as it comes in and
reorders the transmission based on sequence numbers.

4. Transport Layer: (Contd)
c. Stream Control Transmission Protocol (SCTP)
 The Transmission Control Protocol (TCP) provides
full transport-layer services to applications. A new
transport layer protocol, SCTP, has been devised to
meet the needs of some newer applications.
 The Stream Control Transmission Protocol (SCTP)
provides support for newer applications such as voice
over the Internet. The applications that derive the
most benefit from the use of SCTP are in the voice
and video communications area.
 It is a transport layer protocol that combines the best
features of UDP and TCP.

5. Application Layer:
 The application layer in TCP/IP is equivalent to
the combined session, presentation and
application layers in the OSI model. Many
protocols are defined at this layer.

ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical, logical, port,
and specific.
Figure: Addresses in TCP/IP

2.124
Figure: Relationship of layers and addresses in TCP/IP

The Internet Assigned Numbers Authority (IANA) is responsible for the global
coordination of the DNS Root, IP addressing, and other Internet protocol resources.
The port numbers are divided into three ranges: the well-known ports, the registered
ports, and the dynamic or private ports.
The well-known ports (also known as system ports) are those from 0 through 1023.
The requirements for new assignments in this range are stricter than for other
registrations,[2]examples include:
21: File Transfer Protocol (FTP)
22: Secure Shell (SSH)
23: Telnet remote login service
25: Simple Mail Transfer Protocol (SMTP)
53: Domain Name System (DNS) service
80: Hypertext Transfer Protocol (HTTP) used in the World Wide Web
110: Post Office Protocol (POP3)
119: Network News Transfer Protocol (NNTP)
123: Network Time Protocol (NTP)
143: Internet Message Access Protocol (IMAP)
161: Simple Network Management Protocol (SNMP)
194: Internet Relay Chat (IRC)
443: HTTP Secure (HTTPS)
The registered ports are those from 1024 through 49151. IANA maintains the official
list of well-known and registered ranges. The dynamic or private ports are those from
49152 through 65535. One common use for this range is for ephemeral ports.

Performance
 Network Performance Monitoring
 The goal of network performance monitoring tools is to provide a
depiction of operations, so potential problems can be avoided, and
anomalies that occur can be detected, isolated and resolved with a
minimum mean-time-to-repair.
 Bandwidth and Latency
 Network performance is measured in two fundamental ways:
 Bandwidth
 Latency (also called delay)
 The Bandwidth of a network is given by the number of bits that
can be transmitted over the network in a certain period of time.
 The Latency of a network is given by time, taken by a message
to travel from one end of a network to other.

Performance
 Throughput is a measure of how fast we
can actually send data through a network.
 Although bandwidth in bits per second and
throughput seem the same, they are
different.

A network with bandwidth of 10 Mbps can pass only an average of 12,000 frames per
minute with each frame carrying an average of 10,000 bits. What is the throughput of
this network?
Solution
We can calculate the throughput as
Example 3.44
The throughput is almost one-fifth of the bandwidth in this case.

Performance
 The Latency or delay defines how long it takes for an entire message
to completely arrive at the destination from the time the first bit is sent
out from the source.
 Latency is made of four components: Propagation time,
Transmission time, Queuing time and Processing delay.
We can define the total latency as
Latency=Propagation time + Transmission time + Queuing time + Processing delay
Propagation Time: measures the time required for a bit to travel
from the source to the destination.
Propagation time = Distance / Propagation Speed
The propagation speed of electromagnetic signals depends on the
medium and on the frequency of the signal.
Light travels at 3 × 108 m/s in a vacuum; 2.3 × 108 m/s in a
cable; 2 × 108 m/s in a fiber.

What is the propagation time if the distance between the two points is 12,000 km?
Assume the propagation speed to be 2.4 × 108 m/s in cable.
Solution
We can calculate the propagation time as
Example 3.45
The example shows that a bit can go over the Atlantic Ocean in only 50 ms if there
is a direct cable between the source and the destination.

Performance
Transmission Time:
Transmission time = Message size / Bandwidth

What are the propagation time and the transmission time for a 2.5-kbyte message (an
e-mail) if the bandwidth of the network is 1 Gbps? Assume that the distance between
the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.
Solution
We can calculate the propagation and transmission time as shown on the next slide:
Example 3.46
Propagation time = Distance / Propagation Speed
Transmission time = Message size / Bandwidth

What are the propagation time and the transmission time for a 5-Mbyte message
(an image) if the bandwidth of the network is 1 Mbps? Assume that the distance
between the sender and the receiver is 12,000 km and that light travels at 2.4 ×
108 m/s.
Solution
We can calculate the propagation and transmission times as shown on the next
slide.
Example 3.47

Note that in this case, because the message is very long and the bandwidth is not
very high, the dominant factor is the transmission time, not the propagation time.
The propagation time can be ignored.
Example 3.47 (continued)

3.142
The bandwidth-delay product defines the
number of bits that can fill the link.
Note

3.143
Figure 3.33 Concept of bandwidth-delay product

Performance
 Delay × Bandwidth Product
 It is useful to talk about the product of these two metrics, often
called the delay × bandwidth product.
 Now, we will think about a channel between a pair of processes
as a hollow pipe, where the latency corresponds to the length of
the pipe and the bandwidth gives the diameter of the pipe, then
the delay × bandwidth product gives the volume of the pipe –
the maximum number of bits that could be in transmit through
the pipe at any given instant.

Performance
 Delay × Bandwidth Product
 For example, a transcontinental channel with one-way latency of
50 ms and a bandwidth of 45 Mbps is able to hold
= 50 × 10-3 sec × 45 × 106 bits/sec
= 2.25 × 106 bits
= 275 KB of data

Figure Transmission medium and physical layer
• Transmission media are actually located below the physical layer
and are directly controlled by the physical layer.
• Figure shows the position of transmission media in relation to the
physical layer.
Transmission Media

Figure : Classes of transmission media
•A transmission medium can be broadly defined as anything
that can carry information from a source to a destination.
•In telecommunications, transmission media can be divided into
two broad categories: guided and unguided.
•Guided media include twisted-pair cable, coaxial cable, and
fiber-optic cable. Unguided medium is free space.
Classes of Transmission Media

GUIDED MEDIA
Guided media, which are those that provide a conduit
from one device to another, include twisted-pair cable,
coaxial cable, and fiber-optic cable.
1. Twisted-Pair Cable
2. Coaxial Cable
3. Fiber-Optic Cable

Figure: Twisted-pair cable
1. Twisted-Pair Cable
 A twisted pair consists of two conductors (normally
copper), each with its own plastic insulation, twisted
together.
 One of the wires is used to carry signals to the
receiver, and the other is used only as a ground
reference.
Twisted-Pair Cable

Figure: UTP and STP cables
Unshielded Versus Shielded Twisted-Pair Cable
 The most common twisted-pair cable used in communications is referred to as
unshielded twisted-pair (UTP).
 IBM has also produced a version of twisted-pair cable for its use called shielded
twisted-pair (STP). STP cable has a metal foil or braided mesh covering that
encases each pair of insulated conductors. Although metal casing improves the
quality of cable by preventing the penetration of noise or crosstalk, it is bulkier
and more expensive.
 Figure shows the difference between UTP and STP.
Twisted-Pair Cable

7.151
Table : Categories of unshielded twisted-pair cables

Figure UTP connector
Connectors
The most common UTP connector is RJ45 (RJ stands for registered jack), as shown in
Figure. The RJ45 is a keyed connector, meaning the connector can be inserted in only one
way.
Twisted-Pair Cable

Applications
1. Twisted-pair cables are used in telephone lines.
2. TP used in telephone network.
3. In LAN, TP wires are mainly used for low cost, low
performance applications.
Twisted-Pair Cable

Figure Coaxial cable
Coaxial Cable
 Coaxial cable (or coax) carries signals of higher frequency ranges than
those in twisted pair cable.
It has a central core conductor of solid or stranded wire (usually copper)
enclosed in an insulating sheath, which is, in turn, encased in an outer
conductor of metal foil, braid, or a combination of the two.
The outer conductor is also enclosed in an insulating sheath, and the
whole cable is protected by a plastic cover (see Figure)

Table: Categories of coaxial cables
Coaxial Cable

Figure BNC connectors
Coaxial Cable

Figure: Coaxial cable performance

Propagation modes
Figure: Modes
1. Single-mode fiber
Carries light pulses along single path.
2. Multimode fiber
Many pulses of light travel at different angles
Fiber-optic Cable

In multimode step-index fiber, the density of the core remains constant from
the center to the edges. A beam of light moves through this constant density
in a straight line until it reaches the interface of the core and the cladding. At
the interface, there is an abrupt change due to a lower density; this alters the
angle of the beam's motion. The term step index refers to the suddenness of
this change, which contributes to the distortion of the signal as it passes
through the fiber.
In multimode graded-index fiber, decreases this distortion of the signal
through the cable. The word index here refers to the index of refraction. As
we saw above, the index of refraction is related to density. A graded-index
fiber, therefore, is one with varying densities. Density is highest at the center
of the core and decreases gradually to its lowest at the edge. Figure 7.13
shows the impact of this variable density on the propagation of light beams.
Fiber-optic Cable

Applications: Fiber-optic cable is often found in backbone networks because its
wide bandwidth is cost-effective. Today, with wavelength-division multiplexing
(WDM), we can transfer data at a rate of 1600 Gbps.
Advantages Fiber-optic cable has several advantages over metallic cable
(twisted- pair or coaxial).
•
1. Higher bandwidth. 2. Less signal attenuation. 3. Immunity to
electromagnetic interference.
4. Resistance to corrosive materials. 5. Light weight. 6. Greater immunity
to tapping.
Disadvantages There are some disadvantages in the use of optical fiber.
1. Installation and maintenance. Fiber-optic cable is a relatively new technology.
Its installation and maintenance require expertise that is not yet available
everywhere.
2.
•Unidirectional light propagation. Propagation of light is unidirectional. If we
need bidirectional communication, two fibers are needed.
3.
•
Cost. The cable and the interfaces are relatively more expensive than those of
other guided media. If the demand for bandwidth is not high, often the use of
optical fiber cannot be justified.
Fiber-optic Cable

Figure: Fiber-optic cable connectors

Figure: Optical fiber performance

UNGUIDED MEDIA: WIRELESS
Unguided media transport electromagnetic waves
without using a physical conductor. This type of
communication is often referred to as wireless
communication.
Radio Waves
Microwaves
Infrared
Topics discussed in this section:

Figure: Electromagnetic spectrum for wireless communication
UNGUIDED MEDIA: WIRELESS
• Unguided media transport electromagnetic waves without using a physical
conductor. This type of communication is often referred to as wireless
communication.
• Figure shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900
THz, used for wireless communication.

1.Ground Propagation:
 Radio waves travel through the lowest portion of the atmosphere, hugging the earth.
 The low frequency signal follow the curvature of the planet.
 Distance depends on the amount of the power.
Unguided signal can travel from the source to destination in several ways:
2.Sky Propagation:
 Higher frequency radio radiate upward into the ionosphere where they are reflected back to
the earth.
 Sky propagation allow for greater distance with lower power output.
3.line-of-sight Propagation: Very high frequency signals are transmitted in straight lines directly
from antenna to antenna.
Figure Propagation methods

Figure: Wireless transmission waves

Radio waves are used for multicast communications, such as radio and
television, and paging systems. They can penetrate through walls.
Highly regulated. Use omni directional antennas
Note
Radio Waves

Microwaves are used for unicast communication such as cellular telephones,
satellite networks,
and wireless LANs.
Higher frequency ranges cannot penetrate walls.
Use directional antennas - point to point line of sight communications.
Note
Microwaves

Infrared signals can be used for short-range communication in a closed area
using line-of-sight propagation.
Note
Infrared

Wireless Channels
 Are subject to a lot more errors than guided
media channels.
 Interference is one cause for errors, can be
circumvented with high SNR.
 The higher the SNR the less capacity is
available for transmission due to the
broadcast nature of the channel.
 Channel also subject to fading and no
coverage holes.

Switching
 Connectivity
 Whenever we have multiple devices, we have the problem of how to
connect them to make one-on-one communication possible.
 One solution is to install a point-to-point connection between each pair
of devices (mesh topology) or between a central device (hub) and
every other device (star topology).
 However, these methods are impractical and wasteful when applied to
very large networks.
 The number and length of the links require too much infrastructure to
be cost efficient, and the majority of those links would be idle most of
the time.
 In Bus topology, the distances between devices and the total number
of devices increase beyond the capacities of the media and equipment.
 A better solution is switching.

 Switched Network
 A Switched network consists of a series of interlinked nodes, called
Switches.
 Switches are devices capable of creating temporary connections
between two or more devices linked to the switch.
 In a switched network, some of these nodes are connected to the
communicating devices (e.g. telephones). Others are used only for
routing.
Switched Network

Switched Network
Switched Network
 Long distance transmission between devices is typically
done over a network of switching nodes.

Circuit Switched Network
 Circuit-switched Network
 Circuit switching creates a direct physical connection between two
devices such as phones or computers.
 A circuit switch is a device with n inputs and m outputs that creates a
temporary connection between an input link and an output link

Circuit Switched Network
Advantages of Circuit Switching:
 The dedicated path/circuit established between sender and receiver
provides a guaranteed data rate.
 Once the circuit is established, data is transmitted without any delay as
there is no waiting time at each switch.
 Since a dedicated continuous transmission path is established, the
method is suitable for long continuous transmission.
Disadvantages of Circuit Switching:
 As the connection is dedicated it cannot be used to transmit any other
data even if the channel is free.
 It is inefficient in terms of utilization of system resources. As resources
are allocated for the entire duration of connection, these are not
available to other connections.
 Dedicated channels require more bandwidth.
 Prior to actual data transfer, the time required to establish a physical
link between the two stations is too long.

Packed Switched Network
Packet Switching
 Circuit switching was designed for voice communication. In a telephone
conversation, for example, once a circuit is established, it remains connected for
the duration of the session.
1. Circuit switching is less well suited to data and other non-voice transmissions.
 Non-voice transmissions tend to be bursty; meaning that data come in spurt
with idle gaps between them. When circuit-switched links are used for data
transmission, therefore, the line is often idle and its facilities wasted.
2. A second weakness of circuit-switched connections for data transmission is in its
data rate. A circuit-switched link creates the equivalent of a single cable between
two devices and thereby assume a single data rate for both devices.
 This assumption limits the flexibility and usefulness of a circuit-switched
connection for networks interconnecting a variety of digital devices.
3. Third, circuit switching is inflexible. Once a circuit has been established, that
circuit is the path taken by all parts of the transmission whether it remains the
most efficient / available or not.
 Finally, circuit switching sees all transmission as equal.

Packet Switched Network
 Packet-switched Network
 When a computer attempts to send a file to another computer, the file
is broken into packets so that it can be sent across the network in the
most efficient way.

 Connectionless Packet-switched Network
 Each packet contains complete addressing or routing
information (Destination Address, Source Address, Total
number of pieces, Sequence number - - written in the header
section of packet)

 Connection-oriented Packet-switched Network
 Data packets are sent sequentially over a predefined route. (Fixed path
between a source and destination is established prior to transfer of packets.)
 Packets are assembled, given a sequence number and then transported over
the network to a destination in order.
 In this mode, address information is not required. This is also known as
virtual circuit switching.
The Process is
completed in 3 phases.
i. Connection
Establishment
Phase
ii. Data Transfer
Phase
iii. Connection
Release Phase

Advantages of Packet Switching:
 Efficient use of Network.
 Easily get around broken bits or packets.
 Circuit Switching charges user on the distance and duration of connection but
Packet Switching charges users only on the basis of duration of connectivity.
 High Data Transmission in a Packet Switching is very easy.
 All the packets need not follow same route in Packet Switching but in Circuit
Switching all the packets follow same route.
 Packet Switching use digital network and enables digital data to be directly
transmitted toward destination.
Disadvantages of Packet Switching:
 In Packet Switching Packets arriving in wrong order.
 Takes Transmission delay.
 Requires Large amount RAM (Random Access Memory) to handle large
amount of data communication in packets.
 Switching Nods required more procession power to reconstruct packets
 Packets may be lost on their route, so sequence numbers are required to
identify missing packets.

Message Switched Network
 Message switched Network
 Message switching is a method in which the whole
message is stored in a switch and forwarded when a
route is available.

Message Switching
Advantages of Message Switching:
 Efficient traffic management
 Reduces network traffic congestion
 Efficient use of transmission control
Disadvantages of Message Switching:
 Because of store and forward, transmission delay
 Each node requires large capacity for storing.

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