CN-UNIT-1 09.01.23.ppt

1. Behrouz A. Forouzan, "Data Communications
and Networking", Fifth Edition TMH 2013

UNIT I
INTRODUCTION
Data Communications - Data Flow - Networks - Networks
types - The Internet - Standards - Network Models- TCP/IP
Protocol Suite - The OSI Model -Physical Layer: Performance
-Transmission media - Switching - Circuit-switched
Networks - Packet Switching.

1.1)Introduction-Data Communications
 Data Communication  Sharing of information
(exchange of data between two devices via some form
of transmission medium such as a wire cable)
 Data may be Text, Number, Image, Audio and Video
 Networks Group of Devices(Computers)

Fundamental characteristics of Data
Communication
1) Correct Delivery
The system must deliver data to the correct destination.
2) Accuracy(Accurate Delivery)
The system must deliver the data accurately.
3) Timely Delivery (Timeliness)
 The system must deliver data in a timely manner without delay.
 Data delivered late are useless.
4) Jitter
 It refers to the variation in the packet arrival time.
 It is the uneven delay in the delivery of audio or video packets.

Components of data communication

1)Message- It is the Information (data) to be communicated.
(text, numbers, pictures, audio, and video)
2) Sender-It is the device that sends the message.(Computer,
workstation, telephone handset, video camera)
3)Receiver-It is the device that receives the message.
(Computer, workstation, telephone handset, video camera)
4) Transmission medium-It is the physical path by which the
message travels form sender to receiver.(twisted-pair wire,
coaxial cable, fiber-optic cable, and radio waves)
5) Protocol-It is a set of rules that govern data
communication. It represents an agreement between the
communicating devices.

Data Representation
1) Text-Text is represented as a bit pattern, a sequence
of bits (0s or 1s).
2) Number -Number is directly converted to a binary
number to simplify mathematical operations.
3) Image- Represented by bit patterns.(Pixel-2bit
pattern-Black-00 and white-11)
4) Audio-Continuous sound or music represented by
digital format
5) Video- Represented by digital format

1.2)Data Flow or Transmission Mode

1)Simplex
 Data flow in one direction
 One device can only send the data and other can only receive it.
 Ex(The keyboard can only introduce input; the monitor can only accept
output)
2)Half-Duplex
 Data flow in both the directions, but not at the same time.
 Device can transmit and receive the data as well.
 Ex(Walkie-talkies and CB (citizens band) radios are both half-duplex
systems)
3)Full Duplex
 Data flow in both the directions.
 Both devices can send and receive the message simultaneously.
 Ex(Telephone ,Mobile Communication)

1.3) NETWORKS
 A network is a set of devices or nodes connected by communication
links.
 A node can be a computer, printer, or any other device capable of
sending and/or receiving data
 Network Criteria
1) Performance-The performance of a network depends on the
number of users, the type of transmission medium, the capabilities
of the connected hardware, and the efficiency of the software.
2) Reliability -Measured by the frequency of failure, the time it takes a
link to recover from a failure
3) Security -protecting data from unauthorized access, protecting
data from damage and development, and implementing policies
and procedures for recovery from breaches and data losses

1)Point-to-point
 Dedicated link between two devices.
 Link is wired(cable) or Wireless (microwave, satellite).
 The entire capacity of the link is reserved for transmission between
those two devices.
1) Point-to-point
2) Multipoint
1.4)Networks types

2) Multipoint
 A multipoint connection is one in which 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 (timeshared connection)

1.4a) Physical Topology
 It is 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
Types

1.4.1)Mesh topology
• Every device has a dedicated point-to-point link to every other
device.
• The link carries traffic only between the two devices it connects.
• Number of links= n(n- 1)/2

 Advantages
 Traffic problems eliminated, because dedicated links guarantees
that each connection can carry its own data load.
 Robust Topology, If one link becomes unusable, it does not
affect the entire system.
 High privacy and high security
 Disadvantages
 Installation and reconnection are difficult because every device
must be connected to every-other device.
 Bulk of the wiring can be greater than the available space (in
walls, ceilings, or floors) can accommodate.
 Hardware required to connect each link (I/O ports and cable)
can be prohibitively expensive.

1.4.2)Star topology
 Each device has a dedicated point-to-point link only to a
central controller(Hub).
 No direct link between devices.
 If one device want to send data to another device, it send it
to the hub, which send it to other device.

 Advantages
 Less expensive than a mesh topology.
 If one link fails, only that link is affected but other links remain
active.
 only one link and one I/O port to connect it to any number of
others.
 Easy to install and reconfigure and requires less cabling
 Disadvantages
 Dependency of the whole topology on one single point, the
hub.(If the hub goes down, the whole system is dead)

1.4.3)Bus topology
 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.
 Drop line is a connection running between the device and the main
cable
 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.

 Advantages
 Easy to install with the help of long cable by drop lines.
 Requires less cabling than Mesh and Star topology.
 Disadvantages
 Difficult reconnection and fault isolation.
 Difficult to add new devices.
 If a fault or break in the bus cable stops all transmission,
even between devices on the same side of the problem
 The damaged area reflects signals back in the direction of
origin, creating noise in both directions.

1.4.4)Ring topology
 Each device has a dedicated point-to-point connection with only the
two devices on either side of it.
 Data is passed along the ring in one direction, from device to device,
until it reaches its destination.
 When a device receives a signal intended for another device, its
repeater regenerates the bits and passes them along.
 Each device in the ring incorporates a repeater.

 Advantages
 Easy to install and reconfigurable.
 Fault isolation is simple because 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.
 Disadvantages
 Unidirectional traffic.(If ring will break itself ,can disable the
entire network)

1.4.5)Hybrid Topology
 It is an interconnection of two or more basic network topologies, each
of which contains its own nodes.
 The resulting interconnection allows the nodes in a given basic
topology to communicate with other nodes in the same basic topology
as well as those in other basic topologies within the hybrid topology
Integration of Star topology & Bus topology

 Advantages
 Any topology can be combined to make a new network
 Reliable to use
 Scalable and very effective
 Disadvantages
 Difficult to install
 Hardware requirements are more.
 Installation cost is very expensive

1.4.6)Tree Topology (Hierarchy)
 There is one central node (the “trunk”), and each node is connected to
the central node through a single path.
 Nodes can be thought of as branches coming off of the trunk.

 Advantages
 Easy maintenance and easy fault identification can be done.
 Tree Topology is highly secure and reliable.
 It is a callable topology. Leaf nodes can hold more nodes.
 Supported by several hardware and software vendors.
 Disadvantages
 Very difficult to configure as compared to the other network
topologies.
 Due to the presence of a large number of nodes, the network
performance of tree topology becomes a bit slow.
 Requires a large number of cables compared to star and ring
topology.
 Establishment cost is high

1.5)Networks Types or Networks Models

1.5.1)LAN (Local Area Network)
 LAN is a group of computers connected to each other in a small area
such as building, school, college office etc.
 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.
 LAN topologies are Star, Ring and Bus topologies.
 Distance coverage100 meters
 Supporting Data rate at 100 to 1000 Mbps

IAN connecting 12 computers to a hub in a closet

1.5.2) WAN(Wide Area Network)
 A Wide Area Network is a network that extends over a large
geographical area such as state, country or continent.
 Distance coverage is 100Km to 1000Km.
 WAN Provides long distance transmission of data, voice, image
and video information over large areas.
 A Wide Area Network is quite bigger network than the
LAN,MAN.
 It connects large geographical area through a telephone line,
fibre optic cable or satellite links.
 The internet is one of the biggest WAN in the world.(>1000Km)

1.5.3) MAN (Metropolitan Area Network)
 MAN is a network that covers a larger geographic area by
interconnecting a different LAN to form a larger network.
 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.
 Distance coverage is larger than LANs and smaller than WANs.
 Distance coverage is 1Km to 100Km.
 It has a higher range than Local Area Network(LAN).

1.6) The Internet
 Internet is a world-wide global system of interconnected computer
networks.
 Internet is accessible to every user all over the world.
 Internet is a collaboration of more than 100 of 1000 interconnected
network.
 History
 1960 ARPA (Advanced Research Projects Agency)-Connect
computers-research lab purpose
 1967 ARPANET, small network of connected computers.
 1969 ARPANET was reality-NCP protocol
 1972 ARPANET group
 1973ARPANET-TCP/IP protocol

Internet Service Providers
 International Internet Service Providers- connect nations together
 National Internet Service Providers- are backbone networks created and
maintained by specialized companies.
 Regional Internet Service Providers- connected to one or more national
ISPs
 Local Internet Service Providers- provide direct service to the end users

1.6) Protocols and Standards
 Protocol-A protocol is a set of rules that govern data
communications.
 A protocol defines what is communicated, how it is
communicated, and when it is communicated.
 Key elements
 Syntax
 Semantic
 Timing

 Syntax: It refers to the structure or format of the data.
 This refers the order in which the data are presented.
 Example
 The first 8 bits of data to be the address of the sender.
The second 8 bits to be the address of the receiver.
 The rest of the stream may be the message itself

 Semantics: It refers to the meaning of each section of
bits.
 Example
 An address specifies the route to be taken or the final
destination of the message.
 Timing: It refers When data should be sent and how
fast they can be sent.
 Example
 If a sender produces data at 100 Mbps and the receiver
process data at only 1 Mbps, it will overload the receiver
and data will be lost.

Standards
Why do we need standards?
 To create and maintain an open and competitive market for
equipment manufacturers
 To guarantee national and international
interoperability of data, telecommunication
technology and process
 To give a fixed quality and product to the customer.
 To provide guidelines to manufacturers, vendors, government
agencies and other service providers to ensure kind of
interconnectivity.
 To allow the same product to be re used again elsewhere

Standards organizations
 ITU - International Telecommunications Union formerly the (CCITT)
 It a standard for telecommunication in general and data systems in
particular.
 ISO - International Standards Organization
 It is active in developing cooperation in the realms of scientific,
technological and economic activity.
 ANSI - American National Standards Institute
 It is a private & nonprofit corporation and affiliated with the U.S federal
government.
 IEEE - Institute of Electrical and Electronics Engineers
 It aims to advance theory, creativity, and product quality in the fields of
electrical engineering , electronics radio and in all related branches.
 EIA - Electronic Industries Association
 It is a nonprofit organization, its activities include public awareness
education and also by defining physical connection interfaces and electronic
signaling specifications for data communication.


1.7) OSI(Open Systems Interconnection) Model
 An open system is a set of protocol that allows any two different systems to
communicate regardless of their underlying architectures.
 It was designed by ISO-International Organization for Standardization in
late 1970s.
 It is a seven-layer model.

APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
DATA LINK
PHYSICAL
APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
DATA LINK
PHYSICAL
Application to Application
Process to Process
Hop to Hop
Switch
Hop to Hop
Physical Medium
Hub and Repeater
Router
Source to Destination
Source to Destination
OSI Model's 7 Layers

TCP/IP Suit
44
OSI Layers
Application
Telnet, FTP, SMTP, HTTP, DNS, SNMP, Specific address etc…
Session
Presentation
Transport
SCTP, TCP, UDP, Sockets and Ports address
Network
IP, ARP/RARP, ICMP, IGMP, Logical address
Data Link
IEEE 802 Standards, TR, FDDI, PPP, Physical address
Application
Session
Presentation
Transport
Network
Data Link
Activities
To allow access to network resources
To establish, manage, and terminate
session
To Translate, encrypt, and compress data
To Provide reliable process-to-process
Message delivery and error recovery
To move packets from source to
destination; to provide internetworking
To organize bits into frames; to provide
Hop-to-hop delivery
Data, Protocol & Activities
Physical
Medium, Coax, Fiber, 10base, Wireless
Physical
To Transmit bits over a medium; to provide
Mechanical and electrical specifications

 The physical layer is responsible for transmitting individual bits
from one node to the next
 It Deals with the mechanical and electrical specifications of the
interface and transmission medium.
 Data rate defines the duration of a bit, which is how long it lasts.
 Representation of bits Type of encoding (how os and 1s are
changed to signals).
 Line configurationDefine the type of Connection of devices to the
media.(Point to point or multipoint configuration)
 Transmission modeDefines the direction of transmission between
two devices( simplex, half-duplex, or full-duplex)
 Physical topologyDefines how devices are connected to make a
network(Bus,Star,Mesh,ring)

 It is responsible for transmitting frames from one node to the
next.
 Framing Divides the stream of bits received into data units
called frames.
 Physical addressing Adds a header to the frame to define the
sender and receiver.
 Flow control If the rate at which the data are absorbed by the
receiver is less than the rate produced in the sender, the Data link
layer imposes a flow ctrl mechanism.
 Error control Used for detecting and retransmitting damaged or
lost frames and to prevent duplication of frames.
 Access controlUsed to determine which device has control over
the link at any given time.

1.7.1C) Network Layer
•It is responsible for the delivery of packets from the original source to
the final destination.
•Logical addressing It adds a header to the packet coming from the
upper layer , includes the logical addresses of the sender and receiver.
•Routing The devices which connects various networks called routers
are responsible for delivering packets to final destination.

 It is responsible for process-to-process delivery of the entire message.
 Port addressing The header in this must therefore include a address
called port address. This layer gets the entire message to the correct
process on that computer. (FTP-21,TELNET-23,SMTP-25,DNS-
53,HTTP-80,POP3-110,DHCP-67)
 Segmentation and reassembly The message is divided into segments
and each segment is assigned a sequence number. These numbers are
arranged correctly on the arrival side by this layer.
 Flow and error control Flow control at this layer is performed end to
end rather than across a single link.
 Connection control This can either be connectionless or connection-
oriented.
 The connectionless treats each segment as a individual packet and
delivers to the destination.
 The connection-oriented makes connection on the destination side
before the delivery. After the delivery the termination will be
terminated.

1.7.1e) Session Layer
•The session layer is the network dialog controller.
• It establishes, maintains and synchronizes the interaction between
communicating systems
•Dialog control This session allows two systems to enter into a dialog
either in half duplex or full duplex.
•SynchronizationThis allows to add checkpoints into a stream of data
synchronization points or to a stream of data

 It is concerned with the syntax and semantics of the information
exchanged between two systems
 Encryption and decryptionIt means that sender transforms the
original information to another form and sends the resulting message
over the network and Decryption reverses the original process to
transform the message back to its original form
 Compression and expansion Compression reduces the number of
bits contained in the information particularly in text, audio and video.
 Translation Different computers use different encoding system, this
layer is responsible for interoperability between these different
encoding methods. It will change the message into some common
format.

1.7.1g) Application Layer
 It is responsible enables the user, whether human or software, to
access the network.
 It provides user interfaces and support for services.
 FTAM (file transfer, access, mgmt) Allows user to access files in a
remote host.
 Mail services Provides email forwarding and storage.
 Directory services Provides database sources to access information
about various sources and objects.
 Network virtual terminal It is a software version of a physical
terminal, and it allows a user to log on to a remote host

1.8)TCP/IP Protocol suite
 TCP/IP Reference Model is a four-layered suite of communication
protocols.
 It was developed by the DoD (Department of Defence) in the 1960s.

 Host-to- Network Layer −It is the lowest layer that is concerned with
the physical transmission of data. TCP/IP does not specifically
define any protocol here but supports all the standard protocols.
 Internet Layer −It defines the protocols for logical transmission of
data over the network. The main protocol in this layer is Internet
Protocol (IP) and it is supported by the protocols ICMP, IGMP,
RARP, and ARP.
 Transport Layer − It is responsible for error-free end-to-end delivery
of data. The protocols defined here are Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP).
 Application Layer − This is the topmost layer and defines the
interface of host programs with the transport layer services. This
layer includes all high-level protocols like Telnet, DNS, HTTP, FTP,
SMTP, etc.

 The physical layer deals with bit-level transmission between
different devices and supports electrical or mechanical interfaces
connecting to the physical medium for synchronized
communication.
1.10) Physical Layer: Performance

Data & Signals
 Data refers to information that conveys some meaning based on some
mutually agreed up rules.
 Data can be of two types; analog and digital.
 Analog data take on continuous values on some interval. Typical
examples of analog data are voice and video.
 Digital data take on discrete values. Text or character strings can be
considered as examples of digital data.
 Signal is electrical, electronic or optical representation of data, which
can be sent over a communication medium.

Physical Layer - Standards
 The International Organization for Standardization (ISO)
 The Institute of Electrical and Electronics Engineers (IEEE)
 The American National Standards Institute (ANSI)
 The International Telecommunication Union (ITU)
 The Electronics Industry Alliance/Telecommunications
Industry Association (EIA/TIA)
 National telecommunications authorities such as the Federal
Communication Commission (FCC) in the USA.

Physical Layer
• Describe the role of bits in representing a frame as it is transported
across the local media

1.10) Transmission Media
 A transmission medium can be broadly defined as anything that can
carry information from a source to a destination.
 The transmission medium is usually free space, metallic cable, or
fiber-optic cable.

1.10.1) Guided Media
 Guided media, which are those that provide a conduit from one
device to another.
 A signal traveling along any of these media is directed and
contained by the physical limits of the medium (twisted-pair cable,
coaxial cable, and fiber-optic cable)
 Twisted-pair and coaxial cable use metallic (copper) conductors
that accept and transport signals in the form of electriccurrent
 Optical fiber is a cable that accepts and transports signals in the
form of light.

 Twisted pairs can be used for transmitting either analog or digital signals.
 The wires are twisted together in a helical form, just like a DNA molecule.
 When the wires are twisted, the waves from different twists cancel out, so
the wire radiates less effectively.
 It is used for telephone communications and most modern Ethernet
networks.
 Types shielded twisted-pair (STP) and unshielded
twisted-pair (UTP).
1.10.1) Twisted Pair cable

 The most common twisted-pair cable used in communications is referred
to as unshielded twisted-pair (UTP).
 This cable consists of 4 twisted pairs of metal wires (that means there are
8 wires in the cable).
 Each pair is twisted with a different number of twists per inch to help
eliminate interference from adjacent pairs and other electrical devices.
 Each twisted pair consists of two metal conductors that are insulated
separately with their own coloured plastic insulation.
1.10.1) Unshielded Twisted-Pair (UTP)

 This cable has a metal foil or braided-mesh covering that covers each pair
of insulated conductors.
 The metal foil is used to prevent infiltration of electromagnetic noise and
also helps to eliminate crosstalk.
 It is suited for environments with electrical interference and also provides
higher data rates.
1.10.1) Shielded Twisted-Pair (STP)

 A coaxial cable consists of a stiff copper wire as the core, surrounded by an
insulating material.
 The insulator is encased by a cylindrical conductor, often as a closely-
woven braided mesh.
 The outer conductor is covered in a protective plastic sheath.
 It can support greater cable lengths between network devices and greater
bandwidth than twisted-pair cable.
 Supporting data at a fast rate of 10Mbps.
 Thicknet and Thinnet are two varieties of coaxial cable, but rarely used.
Ethernet can run approx 100mts (328 feet) with UTP, while coaxial cable
increases this distance to 500mts (1640 feet).
 The RG numbering system used with coaxial cables refers to cables
approved by U.S. Department of Defense (DoD).
1.10.1) Co-axial Cable

 To connect coaxial cable to devices, it is necessary to use coaxial
connectors.
 The most common type of connector is the Bayone-Neill-
Concelman, or BNC, connectors.
 BNC connectors are sometimes referred to as bayonet mount, as
they can be easily twisted on or off.
 There are three types: the BNC connector, the BNC T connector,
the BNC terminator
1.10.1) Connectors

 Fiber-optic cable or optical fiber consists of thin glass fibers that can carry
information in the form of visible light.
 The typical optical fiber consists of a very narrow strand of glass or plastic
called the core.
 Around the core is a concentric layer of less dense glass or plastic called the
cladding, whose refractive index is less than that of the core.
 The outer most layer of the cable is known as the jacket, which shields the
cladding and the core from moisture, crushing and abrasion.
1.10.1) Optical Fiber

Fiber Optic Cable
 A fiber optic cable is made of glass or plastic and transmits signals
in the form of light.
 Properties of light
 Light travels in a straight line as long as it moves through a single
uniform substance.
 If array traveling through one substance suddenly enters another
the ray changes direction.
 Refraction
 If the angle of incidence is less than the critical angle the ray
refracts and moves closer to the surface.
 Reflection
 If the angle of incidence is greater than the critical angle the ray
reflects and travels again in the denser substance.

 Optical fibers use reflection to guide light through a channel.
 A glass or plastic core is surrounded by a cladding of less dense glass
or plastic.
 When a light beam from a source enters the core, the core refracts
the light and guides the light along its path.
 The cladding reflects the light back into the core and prevents it from
escaping through the medium.

 Multimode
 Multiple beams from a light source move
through the core in different paths. Multimode
can be implemented in two forms
1)Multimode Step –index fiber
 In Multimode Step –index fiber the density of
the fiber remains constant from the center to the
edges and light density not varies.
2)Multimode Graded-index fiber
 Light Density is highest at the center of the core
and decreases gradually to its lowest at the edge.
 Single-Mode
 Light that limits beams to a small range of
angles, all close to the horizontal.
 All the beams arrive at the destination "together"
and can be recombined with little distortion to
the signal


Single mode Vs Multimode
• In multimode, many beams from a light source traverse along multiple
paths and at multiple angles.
• In single mode, the beams propagate almost horizontally.

Laser Connectors
 LED or LASER acts as the source converting electric pulse to light
pulses and photodiode acts as receiver doing vice versa.
 SC (Subscriber Connector)- used to connect cableTV
 ST (Straight Tip)- to connect networkdevice
 MT-RJ (Mechanical Transfer-Registered Jack)- for network
applications

Unguided Media: Wireless Transmission
 Unguided media transport electromagnetic waves without using
a physical conductor.
 Signals are normally broadcast through free space and thus are
available to anyone who has a device capable of receiving them.

Propagation methods
Unguided signals travels from the source to destination in several ways
it is known as propagation.
They are three types:
Ground propagation
Sky propagation
Line-of-Sight Propagation

Ground propagation:
 Radio waves travel through the
lowest portion of the atmosphere
 Touching the earth.
Sky propagation:
 Radio wavesradiate to the
ionosphere then they are reflected
back to earth.
Line-of-Sight Propagation:
 In straight lines directly from
antenna to antenna.

Wireless Transmission
 Radio Waves
 Frequencies between 3 kHz and 1 GHz
 When an antenna transmits radio waves, they are
propagated in all directions.
 Both sending and receiving antennas do not have to
be aligned.
 Radio waves use omnidirectional antennas that send
out signals in all directions.
 Based on the wavelength, strength, and the purpose
of transmission

 Micro Waves
 Frequencies between 1and 300 GHz.
 Microwaves are unidirectional.
 When an antenna transmits microwave waves, they can be
narrowly focused.
 Both the sending and receiving antennas need to be
aligned.
 Microwave propagation is line-of-sight
 Very high-frequency microwaves cannot penetrate walls
 Infrared Waves
 Frequencies from 300 GHz to 400 THz (wavelengths from 1
mm to 770 nm), can be used for short-range
communication.
 Cannot penetrate walls

1.11)Switching
 A switch is a small hardware device
which is used to join multiple
computers together with one local area
network (LAN).
 Capable of creating temporary
connections between two or more
devices linked to the switch.
 used to forward the packets based on
MAC addresses.
 It is used to transfer the data only to
the device that has been addressed.
 It verifies the destination address to
route the packet appropriately.
 It is operated in full duplex mode.

1.11.1)Circuit Switched Networks
 Circuit switching is a switching technique that establishes a dedicated path
between sender and receiver.
 Once the connection is established then the dedicated path will remain to
exist until the connection is terminated.
 The network is made of a set of switches connected by physical links, in
which each link is divided into n channels.
 When any user wants to send the data, voice, video, a request signal is
sent to the receiver then the receiver sends back the acknowledgment
to ensure the availability of the dedicated path.
 After receiving the acknowledgment, dedicated path transfers the data.
 It is implemented in physical layer

Circuit Switched Network
 The end systems, such as computers or telephones, are directly connected to a switch.
 Connection Setup-Phase
 When end system A needs to communicate with end system M, system A needs to
request a connection to M that must be accepted by all switches as well as by M itself.
 Data transfer-Phase
 After the dedicated path made of connected circuits (channels) is established, data
transfer can take place.
 Circuit Disconnect-Phase
 After all data have been transferred, the circuits are tom down.

Circuit Switching -Types
Space Division Switches:
 Space Division Switching is a circuit switching technology in
which a single transmission path is accomplished in a switch by
using a physically separate set of cross points.
 Space Division Switching can be achieved by using crossbar
switch. A crossbar switch is a metallic cross point or
semiconductor gate that can be enabled or disabled by a control
unit.
 The Crossbar switch is a switch that has n input lines and n
output lines. The crossbar switch has n2 intersection points
known as cross points.
Disadvantage of Crossbar switch:
 The number of cross points increases as the number of stations
is increased. Therefore, it becomes very expensive for a large
switch. The solution to this is to use a multistage switch.

Multistage Switch
 Multistage Switch is made by splitting the crossbar
switch into the smaller units and then interconnecting
them.
 It reduces the number of cross points.
 If one path fails, then there will be an availability of
another path.

 Advantages
 communication channel is dedicated.
 It has fixed bandwidth.
 Disadvantages:
 Once the dedicated path is established, therefore no other data
can be transferred even if the channel is free.
 It takes a long time to establish a connection approx 10 seconds
during which no data can be transmitted.
 It is more expensive than other switching techniques as a
dedicated path is required for each connection.
 It is inefficient to use because once the path is established and
no data is transferred, then the capacity of the path is wasted.
 In this case, the connection is dedicated

1.11.2) Message Switching
 Message Switching is a switching technique in which a message is
transferred as a complete unit and routed through intermediate nodes at
which it is stored and forwarded.
 In Message Switching technique, there is no establishment of a
dedicated path between the sender and receiver.
 The destination address is appended to the message.
 Message Switching provides a dynamic routing as the message is routed
through the intermediate nodes based on the information available in
the message.
 Message switches are programmed in such a way so that they can
provide the most efficient routes.
 Each and every node stores the entire message and then forward it to the
next node.
 This type of network is known as store and forward network.
 Message switching treats each message as an independent entity.

Advantages Of Message Switching
 Data channels are shared among the communicating
devices that improve the efficiency of using available
bandwidth.
 Traffic congestion can be reduced because the message
is temporarily stored in the nodes.
 Message priority can be used to manage the network.
 The size of the message which is sent over the network
can be varied.

1.11.3)Packet Switching
 The packet switching is a switching technique in which the message is
sent in one go, but it is divided into smaller pieces, and they are sent
individually.
 The message splits into smaller pieces known as packets and packets
are given a unique number to identify their order at the receiving end.
 Every packet contains some information in its headers such as source
address, destination address and sequence number.
 Packets will travel across the network, taking the shortest path as
possible.
 All the packets are reassembled at the receiving end in correct order.
 If any packet is missing or corrupted, then the message will be sent to
resend the message.
 If the correct order of the packets is reached, then the acknowledgment
message will be sent.

Approaches of Packet Switching
 1)Datagram Packet switching
 In Datagram Packet Switching technique, the path is not fixed.
 Intermediate nodes take the routing decisions to forward the packets.
 It is also known as connectionless switching.
 It is implemented in Network layer

Approaches of Packet Switching
 2) Virtual Circuit Switching
 It is also known as connection-oriented switching.
 As in a circuit-switched network, there are setup and teardown phases in
addition to the data transfer phase
 Resources can be allocated during the setup phase,or on demand
 Preplanned route fixed path is established before the messages are sent.
 All packets follow the same path established during the connection
 It is normally implemented in the data link layer.

Source-to-destination data transfer in a VCN

 Source A sends a setup frame to switch 1.
 Switch 1 receives the setup request frame. It knows that a frame
going from A to B goes out through port 3.
 Switch 2 receives the setup request frame. The same events
happen here as at switch 1; three columns of the table are
completed: in this case, incoming port (l),incoming VCI (66),
and outgoing port (2).
 Switch 3 receives the setup request frame. Again, three columns
are completed: incoming port (2), incoming VCI (22), and
outgoing port (3).
 Destination B receives the setup frame, and if it is ready to
receive frames from A, it assigns a VCI to the incoming frames
that come from A, in this case 77.
 This VCI lets the destination know that the frames come from
A, and not other sources

Differences b/w Datagram approach and Virtual Circuit approach
Datagram approach Virtual Circuit approach
Node takes routing decisions to
forward the packets.
Node does not take any routing
decision.
Congestion cannot occur as all
the packets travel in different
directions.
Congestion can occur when the
node is busy, and it does not allow
other packets to pass through.
It is more flexible as all the
packets are treated as an
independent entity.
It is not very flexible.

Advantages of Packet Switching
 In packet switching technique, switching devices do not require
massive secondary storage to store the packets, so cost is minimized to
some extent.
 If any node is busy, then the packets can be rerouted. This ensures
reliable communication.
Disadvantages Of Packet Switching
 Packet Switching technique cannot be implemented in low delay and
high-quality services.
 The protocols used in a packet switching technique are very complex
and requires high implementation cost.
 If the network is overloaded or corrupted, then it requires
retransmission of lost packets.

CN-UNIT-1 09.01.23.ppt

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