CN UNIT1 TO UNIT 5.pdf

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.

UNIT II
DATA LINK LAYER & MEDIA ACCESS
Introduction – Link-Layer Addressing – Block Coding –
Cyclic Coding – Checksum – DLC Services – Data-link Layer
Protocols – HDLC – Media Access Control – Wired LANs:
Ethernet – Wireless LANs: IEEE 802.11, Bluetooth –
Connecting Devices.

2.1)Introduction
 The main responsibility of the Data Link Layer is to transfer the
datagram across an individual link.
 When a packet is travelling, the data-link layer of a node (host or router)
is responsible for delivering a datagram to the next node in the path.
 Sending node needs to encapsulate the datagram and the data-link layer
of the receiving node needs to decapsulate the datagram.
 The new frame is logically transported from the router to the destination
host
 The frame is logically transported from the source host to the router.

Nodes and Links
 Communication at the data-link layer is node-to-node.
 A data unit from one point in the Internet needs to pass through many networks (LAN
and WAN) to reach another point. Theses LANs and WANs are connected by routers.
 The two end hosts and the routers are nodes and the networks in- between are links.
 The first node is the source host; the last node is the destination host.
 The other four nodes are four routers.
 The first, the third, and the fifth links represent the three LANs; the second and the
fourth links represent the two WANs.

Sublayers in Data Link layer
• The data link control sublayer deals with all issues common to
both point to-point and broadcast links

2.2) Link-Layer Addressing
 A link-layer address is sometimes called a link address or physical
address, or MAC address.
 When a datagram passes from the network layer to the data-link layer,
the datagram will be encapsulated in a frame and two data-link addresses
are added to the frame header.
 These two addresses are changed every time the frame moves from one
link to another.
 Types
 Unicast Address-It means one-to-one communication. A frame with a
unicast address destination is destined only for one entity in the link.
 Multicast Address-It means one-to-many Communication but not all.
 Broadcast Address-It means one to- all communication. A frame with a
destination broadcast address is sent to all entities in the link.

Address Resolution Protocol (ARP)
 Used to convert logical address to physical address.
 It uses ARP request and ARP response message.
 ARP maintains a cache table in which MAC addresses are mapped to IP
Addresses
 If a host wants to send an IP datagram to a host,it first checks for a mapping
in the cache table.
 If no mapping is found, it needs to invoke the Address Resolution Protocol
over the network.
 Every host or router on the network receives and processes the ARP query
packet, but only the intended recipient recognizes its IP address and sends
back an ARP response packet.
 The response packet contains the recipient's IP and physical addresses.

ARP operation
• The system (A) has a packet that needs to be delivered to system (B) with IP address
141.23.56.23.
• System A not know the physical address of B. It uses ARP protocol (broadcast ARP)
request packet to ask the physical address of a system which has an IP address of
141.23.56.23.
• This packet is received by every system in network, but only system B will answer it.
• System B sends an ARP reply packet that includes its physical address.
• System A can send all the packets to B by using the physical address.

 Hardware type. It is a 16-bit field defining the type of the network on which
 ARP is running. Each LAN has been assigned an integer based on its type.
 For example, Ethernet is given type 1.
 Protocol type. It is a 16-bit field defining the protocol. For example, the value
 of this field for the IPv4 protocol is 080016.
 Hardware length. It is an 8-bit field defining the length of the physical
address in bytes.
 Protocol length. It is an 8-bit field defining the length of the logical address
in bytes.
 Operation. It is a 16-bit field defining the type of packet(ARP request (1) and
ARP reply (2))
 Sender hardware address. It define physical address of the sender.
 Sender protocol address. It define logical address of the sender.
 Target hardware address. It define physical address of the receiver.
 Target protocol address. It define logical address of the receiver.

2.3)Datalink Layer-Services
 Framing - Divides the stream of bits received into data units
called frames.
 Physical addressing –DL 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 control mechanism.
 Error control- Used for detecting and retransmitting damaged
or lost frames and to prevent duplication of frames.
 Medium Access control - Used to determine which device has
control over the link at any given time.

2.3.1) Framing
 Framing in the data-link layer separates a message from one source to a
destination by adding a sender address and a destination address.
 The destination address defines where the packet is to go; the sender address
helps the recipient acknowledge the receipt.
 When a message is carried in one very large frame, even a single-bit error would
require the retransmission of the whole frame.
 When a message is divided into smaller frames, a single-bit error affects only
that small frame.
 Frame Size
 Fixed size framingsize of the frame is fixed and No need for defining the
boundaries of the frames and also Length of the frame itself acts as
delimiter
 Variable-size framing Size of each frame to be transmitted may be
different and Need a flag to define the boundary of two frames.and
Flag(Delimiter) special character or bit pattern

2.3.2)Character-Oriented Framing
 In this method data to be carried are 8-bit characters.
 To separate one frame from the next, an 8-bit (1-byte) flag is added at the
beginning and the end of a frame.
 The flag, composed of protocol-dependent special characters, signals the start
or end of a frame.
 Any character used for the flag could also be part of the information.
 If this happens, when it encounters this pattern in the middle of the data, the
receiver thinks it has reached the end of the frame.
 To fix this problem, a byte-stuffing strategy was added to character oriented
framing.
 Byte stuffing is the process of adding one extra byte whenever there is a flag or
escape character in the text.

2.3.3)Bit-Oriented Framing
 In this method ,the data section of a frame is a sequence of bits to be
interpreted by the upper layer as text, graphic, audio, video, and so on.
 Need a delimiter to separate one frame from the other( define the beginning
and the end of the frame)
 If the flag pattern appears in the data, the receiver must be informed that this
is not the end of the frame.
 This is done by stuffing 1 single bit (instead of 1 byte) to prevent the pattern
from looking like a flag. The strategy is called bit stuffing
 Bit stuffingIt is the process of adding one extra 0 whenever five consecutive
1s follow a 0 in the data, so that the receiver does not mistake the pattern

Example-1
Apply bit stuffing to the following data
0101111000111110101
Solution:
Data= 0101111000111110101
After bit stuffing,
Data= 01011110001111100101
15

Example-2
Apply bit stuffing to the following data 110101111101
0111111010
Solution:
Data= 110101111101 0111111010
After bit stuffing,
Data= 11010111110 01011111010 10
16

Point-to-Point Protocol (PPP)
PPP is a byte-oriented protocol.
 It is used to transmit multiprotocol data between two directly connected
(point-to-point) computers.
 Flag field: Flag field marks the beginning and end of the PPP frame.
Flag byte is 01111110. (1 byte).
 Address field: This field is of 1 byte and is always 11111111. This address
is the broadcast address i.e. all the stations accept this frame.

 Control field: This field is also of 1 byte. This field uses
the format of the U-frame (unnumbered) in HDLC.
The value is always 00000011 to show that the frame
does not contain any sequence numbers and there is
no flow control or error control
 Protocol field: This field specifies the kind of packet in
the data field i.e. what is being carried in data field.
 Data field: Its length is variable. If the length is not
negotiated using LCP during line set up, a default
length of 1500 bytes is used. It carries user data or
other information.
 FCS field: The frame checks sequence. It is either of 2
bytes or 4 bytes. It contains the checksum.

Transition Phases in PPP
 Dead: In dead phase the link is not used. There is no active
carrier and the line is quiet.
 Establish: Connection goes into this phase when one of the
nodes start communication.
 In this phase, two parties negotiate the options. If negotiation is
successful, the system goes into authentication phase or directly
to networking phase. LCP packets are used for this purpose.
 Authenticate: This phase is optional. The two nodes may decide
during the establishment phase, not to skip this phase.
 However if they decide to proceed with authentication, they
send several authentication packets.
 If the result is successful, the connection goes to the networking
phase; otherwise, it goes to the termination phase.

 Network: In network phase, negotiation for the
network layer protocols takes place. PPP specifies that
two nodes establish a network layer agreement before
data at the network layer can be exchanged.
 This is because PPP supports several protocols at
network layer.
 If a node is running multiple protocols simultaneously
at the network layer, the receiving node needs to know
which protocol will receive the data.
 Open: In this phase, data transfer takes place. The
connection remains in this phase until one of the
endpoints wants to end the connection.
 Terminate: In this phase connection is terminated.

2.4)Block Coding
 Block coding provides redundancy to ensure synchronization and
inherent error detection.
 It is normally referred to as mB/nB coding.
 It replaces each m-bit group with an n-bit group.
 Divide the message into blocks, each of k bits, called data words.
 Add r redundant bits to each block to make the length n = k + r.
 The resulting n-bit blocks are called codewords.

Process of error detection
 Sender creates codewords out of data words.
 Each codeword sent to the receiver may change during transmission.
 If the received codeword is the same as one of the valid codewords, then data word is
extracted for use.
 If the received codeword is not valid, it is discarded or if the codeword is corrupted
during transmission but the received word still matches a valid codeword, the error
remains undetected.
 This method can detect only single errors. Two or more errors may remain
undetected.
 If the following two conditions are met, the receiver can detect .
1. The receiver has (or can find) a list of valid codewords.
2. The original codeword has changed to an invalid one.

Process of error correction
 In error correction the receiver needs to find ( guess) the
original codeword sent.
 Need more redundant bits for error correction than for error
detection.

Types of errors
 Single-bit error
 The term Single-bit error means that only one bit of a given data unit (such as
byte, character, data unit or packet) is changed from 1 to 0 or from 0 to 1.
 Burst error
 The term Burst Error means that two or more bits in the data unit have changed
from 1 to 0 or from 0 to 1.

LINEAR BLOCK CODES
 In this method, the parity bits and message bits have a linear combination, which
means that the resultant codeword is the linear combination of any two code
words.
 In a (n,k) LBC, n is length of the codeword and k is length of the message word.
{mi} ---- LBC Encoder ---- {xi}
message word ‘m’ = {m1, m2, m3, …………… mk}
code word ‘c’ = {c1, c2, c3, …………… cn}
parity word ‘b’ = {b1, b2, b3, …………… bn-k}
 Structure of a systematic codeword
C = {bi, mi}
 Code rate Ratio of message bits(k) and the encoder output (n).
 r=(k/n) ; 0<r<1

Minimum Distance for Linear Block Codes
 It is simple to find the minimum Hamming distance for a linear block code.
 The minimum Hamming distance is the number of Is in the nonzero valid
codeword with the smallest number of Is.
 Simple Parity-Check Code
 A simple parity-check code is a single-bit error-detecting code in which n =k + 1
with dmin =2.

Example- Assume the sender sends the dataword 1011. The codeword created
from this dataword is 10111, which is sent to the receiver.
 1 .No error occurs; the received codeword is 10111. The syndrome is O. The dataword
1011 is created.
 2. One single-bit error changes aI' The received codeword is 10011. The syndrome is
1. No dataword is created.
 3. One single-bit error changes roo The received codeword is 10110. The syndrome is
1. No dataword is created.
 4. An error changes ro and a second error changes a3' The received codeword is
00110. The syndrome is O. The dataword 0011 is created at the receiver. The
dataword is wrongly created due to the syndrome value. The simple parity-check
decoder cannot detect an even number of errors. The errors cancel each other out
and give the syndrome a value of O.
 5. Three bits-a3, az, and aI-are changed by errors. The received codeword is 01011.
The syndrome is 1. The dataword is not created. This shows that the simple parity
check, guaranteed to detect one single error, can also find any odd number of
errors.

Error detection techniques / methods

PARITY CHECK
 One bit, called parity bit is added to every data unit so that the total
number of 1’s in the data unit becomes even (or) odd.
 The source then transmits this data via a link, and bits are checked and
verified at the destination.
 Data is considered accurate if the number of bits (even or odd)
matches the number transmitted from the source.
 This techniques is the most common and least complex method.
 1. Even parity – Maintain even number of 1s
 E.g., 1011 → 1011 1
 2. Odd parity – Maintain odd number of 1s
 E.g., 1011 → 1011 0

Cyclic Redundancy Check
 Cyclic codes refers to encoding messages by adding a fixed-length check value.
 Steps Involved :
 Consider the original message (dataword) as M(x) consisting of ‘k’ bits and the
divisor as C(x) consists of ‘n+1’ bits.
 The original message M(x) is appended by ‘n’ bits of zero’s. Let us call this zero-
extended message as T(x).
 Divide T(x) by C(x) and find the remainder.
 The division operation is performed using XOR operation.
 The resultant remainder is appended to the original message M(x) as CRC and
sent by the sender(codeword).

Example 1:
 Consider the Dataword / Message M(x) = 1001
 Divisor C(x) = 1011 (n+1=4)
 Appending ‘n’ zeros to the original Message M(x).
 The resultant messages is called T(x) = 1001 000. (here n=3)
 Divide T(x) by the divisor C(x) using XOR operation.
 Sender Side :

Receiver Side: (For Both Case – Without Error and With Error)

Hamming Codes
 The Hamming distance between two words (of the same size) is the number of
differences between the corresponding bits.
 The Hamming distance can easily be found if apply the XOR operation on the
two words and count the number of Is in the result.
 Hamming distance is a value greater than zero.
 Example
 1. The Hamming distance d(000 011) is 2 because 000 (XOR)011 is 011 (two 1s).
 2. The Hamming distance d(10101, 11110) is 3 because 10101 (XOR) 11110 is 01011
(three 1s).

CYCLIC CODES
 Cyclic codes are special linear block codes with one extra property. In a
cyclic code, if a codeword is cyclically shifted (rotated), the result is
another codeword.
 For example, if 1011000 is a codeword and we cyclically left-shift, then
0110001 is also a codeword.
 The bits in the first word ao to a6' and the bits in the second word bo to
b6, we can shift the bits by using the following:
 In the rightmost equation, the last bit of the first word is wrapped
around and becomes the first bit of the second word.

Checksum
 It is a calculated value that is used to determine the integrity of data.
 Sender side:
 1. The message is divided into 16-bit words.
 2. The value of the checksum word is set to O.
 3. All words including the checksum are added ushtg one's complement
addition.
 4. The sum is complemented and becomes the checksum.
 5. The checksum is sent with the data.
 Receiver side:
 1. The message (including checksum) is divided into 16-bit words.
 2. All words are added using one's complement addition.
 3. The sum is complemented and becomes the new checksum.
 4. If the value of checksum is 0, the message is accepted; otherwise, it is
rejected

 Let the message to be transmitted be 7,11,12,0,6.

Data-link Layer Protocols
 Flow control refers to a set of procedures used to restrict the amount of data
that the sender can send before waiting for acknowledgment.
 The receiving device has limited speed and limited memory to store the data.
 Therefore, the receiving device must be able to inform the sending device to
stop the transmission temporarily before the limits are reached.
 It requires a buffer, a block of memory for storing the information until they
are processed.
 Error control based on automatic repeat request, which is the retransmission
of data.

SIMPLE PROTOCOL
 It is a simple protocol with neither flow nor error control.
 The receiver can immediately handle any frame it receives or receiver can never be
overwhelmed with incoming frames.
 The data-link layers of the sender and receiver provide transmission services for
their network layers.
 The data-link layer at the sender gets a packet from its network layer, makes a
frame out of it, and sends the frame.
 The data-link layer at the receiver receives a frame from the link, extracts the
packet from the frame, and delivers the packet to its network layer.

Stop-and-Wait ARQ
 Stop-and-wait ARQ is a technique used to retransmit the data in
case of damaged or lost frames.
 This technique works on the principle that the sender will not
transmit the next frame until it receives the acknowledgement of
the last transmitted frame.

Damaged Frame
 When the receiver receives a damaged
frame, then it returns the NAK frame.
 when the frame DATA 1 is sent, and
then the receiver sends the ACK 0
frame means that the data 1 has arrived
correctly.
 The sender transmits the next frame:
DATA 0.
 It reaches undamaged, and the
receiver returns ACK 1.
 The sender transmits the third frame:
DATA 1.
 The receiver reports an error and
returns the NAK frame.
 The sender retransmits the DATA 1
frame.

Lost Frame
 Sender is equipped with the timer and
starts when the frame is transmitted.
 Sometimes the frame has not arrived at
the receiving end so that it cannot be
acknowledged either positively or
negatively.
 The sender waits for acknowledgement
until the timer goes off. If the timer goes
off, it retransmits the last transmitted
frame.

GO-BACK-N ARQ
 In Go-Back-N ARQ protocol, if one
frame is lost or damaged, then it
retransmits all the frames after which
it does not receive the positive ACK.
 In this three frames (Data 0,1,2) have
been transmitted before an error
discovered in the third frame.
 The receiver discovers the error in
Data 2 frame, so it returns the NAK 2
frame.
 All the frames including the damaged
frame (Data 2,3,4) are discarded as it
is transmitted after the damaged
frame.
 Therefore, the sender retransmits the
frames (Data2,3,4).

SELECTIVE-REJECT(REPEAT) ARQ
 In this technique, only those
frames are retransmitted for
which negative
acknowledgement (NAK) has
been received.
 The receiver storage buffer keeps
all the damaged frames on hold
until the frame in error is
correctly received.
 The receiver must have an
appropriate logic for reinserting
the frames in a correct order.
 The sender must consist of a
searching mechanism that
selects only the requested frame
for retransmission.

2.5)HDLC (HIGH-LEVEL DATA LINK CONTROL)
 It is a bit-oriented protocol
 It is used for communication over point-to-point and
multipoint links.
 HDLC implements the Stop-and-Wait protocol.
 It provides two common transfer modes
 1. Normal response mode (NRM)
 2. Asynchronous balanced mode (ABM).

Normal response mode (NRM)
 In normal response mode (NRM), the station configuration is unbalanced.
 One primary station and multiple secondary stations.
 A primary station can send commands; a secondary station can only
 respond.
 The NRM is used for both point-to-point and multipoint links

Asynchronous balanced mode (ABM)
 In this mode the configuration is balanced.
 The link is point-to-point, and each station can function as a primary
and a secondary (acting as peers).
 This is the common mode today.

HDLC FRAMES
 HDLC defines three types of frames:
 1. Information frames (I-frames) - used to carry user data
 2. Supervisory frames (S-frames) - used to carry control information
 3. Unnumbered frames (U-frames) – reserved for system management

 Flag fieldThis field contains synchronization pattern 01111110, which
identifies both the beginning and the end of a frame.
 Address fieldThis field contains the address of the secondary station.If
a primary station created the frame, it contains a ‘to’ address. If a
secondary station creates the frame, it contains a ‘from’ address. The
address field can be one byte or several bytes long, depending on the
needs of the network.
 Control fieldThe control field is one or two bytes used for flow and
error control.
 Information fieldThis field contains the user’s data from the network
layer or management information. Its length can vary from one network to
another.
 FCS fieldFrame check sequence (FCS) is the HDLC error detection
field. It can contain either a 16- bit or 32-bit CRC.

Control Field for I-Frames
 I-frames are designed to carry user data from the network layer.
 In addition,they can include flow-control and error-control
information
 If the first bit of the control field is 0indicate an I-frame.
 The next 3 bits, called N(S), define the sequence number of the frame.
 The last 3 bits, called N(R), correspond to the acknowledgment
number when piggybacking is used.
 The single bit between N(S) and N(R) is called the Poll/Final bit.
 If P/F=1 it means poll ( frame is sent by a primary station to a
secondary).
 If P/F=0 it means final(the frame is sent by a secondary to a Primary).

Control Field for S-Frames
 Supervisory frames are used for flow and error control .
 If the first 2 bits of the control field are 10indicates an S frame.
 The last 3 bits, called N(R),correspond to the acknowledgment number
(ACK) or negative acknowledgment number (NAK),
 The 2 bits called code are used to define the type of S-frame
itself.(Receive ready (RR), Receive not ready (RNR), Reject (REJ) and
Selective reject (SREJ).

Control Field for U-Frames
 Unnumbered frames are used to exchange session management and
control information between connected devices.
 If the first 2 bits of the control field are 11indicates an U-frame.
 U-frame codes are divided into two sections: a 2-bit prefix before the
P/F bit and a 3-bit suffix after the P/F bit.

2.6)Media Access Control
 When two or more nodes transmit data at the same time, their frames will collide
and the link bandwidth is wasted during collision.
 To coordinate the access of multiple sending/receiving nodes to the shared link,
need a protocol to coordinate the transmission, These protocols are called Medium
or Multiple Access Control

2.6.1)RANDOM ACCESS
 In this method Each station has the right to the medium without being
controlled by any other station.
 If more than one station tries to send, there is an access conflict-
collision-and the frames will be either destroyed or modified.
 To resolve it when it happens, each station follows a procedure
 When can the station access the medium?
 What can the station do if the medium is busy?
 How can the station determine the success or failure of the
transmission?
 What can the station do if there is an access conflict?

2.6.1a)Pure ALOHA
 Stations can transmit the frame whenever they have data to be sent.
 So, there will be collision , frames  damaged.
 A collision involves two or more stations. If all these stations try to
resend their frames after the time-out, the frames will collide again

Pure ALOHA- Vulnerable Time
 Vulnerable Time-Find the length of time, the vulnerable time, in
which there is a possibility of collision.
 Assume that the stations send fixed-length frames with each frame
taking Tfr S to send,
 The vulnerable time, during which a collision may occur in pure
ALOHA, is 2 times the frame transmission time.
 Pure ALOHA vulnerable time = 2 x Tfr

Pure ALOHA- Vulnerable Time
• Station A sends a frame at time t. Now imagine station B has already sent a
frame between t - Tfr and t. This leads to a collision between the frames from
station A and station B.
• The end of B's frame collides with the beginning of A's frame. On the other
• hand, suppose that station C sends a frame between t and t + Tfr .
• Here, there is a collision between frames from station A and station C. The
beginning of C's frame collideswith the end of A's frame.

Slotted ALOHA
 Divide time up into discrete intervals, each corresponding to one
packet.
 The stations can only transmit data in one of the time slots only.
 Station is allowed to send only at the beginning of the synchronized
time slot, if a station misses this moment, it must wait until the
beginning of the next time slot.
 This means that the station which started at the beginning of this slot
has already finished sending its frame.
 If two stations try to send at the beginning of the same time slot.
 However, the vulnerable time is now reduced to one-half, equal to Tfr
 Slotted-ALOHA vulnerable time = Tfr

Slotted ALOHA -Vulnerable Time
Vulnerable Time =Tfr

2.6.1b)Carrier Sense Multiple Access(CSMA)
 To minimize the chances of collision, so as to improve the
performance(efficiency).
 CSMA protocol is based on the principle of 'carrier sense'.
 The station senses the carrier or channel before
transmitting a frame.
 It means the station checks the state of channel, whether it
is idle or busy.
 CSMA can reduce the possibility of collision, but it cannot
eliminate it.

• At time tI' station B senses the medium and finds it idle, so it sends a
frame.
• At time t2 (t2> tI)' station C senses the medium and finds it idle
because, at this time, the first bits from station B have not reached
station C.
• Station C also sends a frame. The two signals collide and both frames are
destroyed.

CSMA -Persistence methods
1) I-persistent CSMA
2) Non- Persistent CSMA
3) p-persistent CSMA

1) I-persistent CSMA
 In this method, after the station finds the line idle, it sends its frame
immediately (with probability I).
 This method has the highest chance of collision because two or more
stations may find the line idle and send their frames immediately

2)Non- Persistent CSMA
 In the nonpersistent method, a station that has a frame to send senses the line.
 If the line is idle, it sends immediately.
 If the line is not idle, it waits a random amount of time and then senses the
line again.
 If two or more stations will wait the same amount of time and retry to send
simultaneously.
 This method reduces the efficiency of the network because the medium
remains idle when there may be stations with frames to send.

3) p-persistent CSMA
 If the channel has time slots with a slot duration equal to or greater
than the maximum propagation time.
 The p-persistent approach combines the advantages of the other two
strategies.
 It reduces the chance of collision and improves efficiency.

Carrier Sense Multiple Access with Collision
Detection (CSMA/CD)
 In this method, a station monitors the medium after it sends a frame to see if
the transmission was successful.
 If so, the station is finished. If, however, there is a collision, the frame is sent
again.
 If the first bits transmitted by the two stations involved in the collision.
 Although each station continues to send bits in the frame until it detects the
collision.

Collision and abortion in CSMA/CD
 At time t1, station A has executed its
persistence procedure and starts sending
the bits of its frame.
 At time t2, station C has not yet sensed
the first bit sent by A.
 Station C executes its persistence
procedure and starts sending the bits in
its frame, which propagate both to the
left and to the right.
 The collision occurs sometime after time
t2' Station C detects a collision at time t3
when it receives the first bit of A's frame.
 Station C immediately aborts
transmission.
 Station A detects collision at time t4
when it receives the first bit of C's frame;
it also immediately aborts transmission.

Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA)
 It was invented for wireless networks.
 It Prevents collisions prior to their occurrence.
 A station Wait until the link becomes idle before transmitting and back
off should a collision occur.
 It follows three strategies: the interframe space, the contention window,
and acknowledgments

 Interframe Space (IFS)
 First, collisions are avoided by deferring transmission even if the channel is
found idle.
 When an idle channel is found, the station does not send immediately. It waits
for a period of time called the interframe space or IFS.
 Contention Window
 The contention window is an amount of time divided into slots.
 A station that is ready to send chooses a random number of slots as its wait
time.
 The number of slots in the window changes according to the binary
exponential backoff strategy.
 This means that it is set to one slot the first time and then doubles each time
the station cannot detect an idle channel after the IFS time.
 Acknowledgment
 In addition, the data may be corrupted during the transmission.
 The positive acknowledgment and the time-out timer can help guarantee that
the receiver has received the frame.

2.6.2)Controlled Access
 In controlled access, the stations consult one another to find which
station has the right to send.
 A station cannot send unless it has been authorized by other stations.
 Methods
1) Reservation
2) Polling
3) Token Passing

2.6.2a)Reservation
 In the reservation method, a station needs to make a reservation before
sending data.
 Time is divided into intervals. In each interval, a reservation frame
precedes the data frames sent in that interval.
 The figure shows a situation with five stations and a five-minislot
reservation frame.
 In the first interval, only stations 1, 3, and 4 have made reservations.
 In the second interval, only station 1 has made a reservation

2.6.2b)Polling
 Polling works with topologies in which one device is designated as a primary
station and the other devices are secondary stations.
 All data exchanges must be made through the primary device even when the
ultimate destination is a secondary device.
 The primary device controls the link; the secondary devices follow its
instructions.
 It is up to the primary device to determine which device is allowed to use the
channel at a given time.

Select-function
 If the primary wants to send data, it tells the secondary to get ready to
receive; this is called select function.
 The primary must alert the secondary to the upcoming transmission and
wait for an acknowledgment of the secondary's ready status.
 Before sending data, the primary creates and transmits a select (SEL) frame,
one field of which includes the address of the intended secondary.

Poll-function
 If the primary wants to receive data, it asks the secondaries if they have
anything to send; this is called poll function.
 When the first secondary is approached, it responds either with a NAK frame
if it has nothing to send or with data if it does.
 If the response is negative (a NAK frame), then the primary polls the next
secondary in the same manner until it finds one with data to send.
 When the response is positive , the primary reads the frame and returns an
acknowledgment (ACK frame), verifying its receipt.

2.6.2c)Token Passing
 In the token-passing method, the stations in
a network are organized in a logical ring.
 For each station, there is a predecessor and a
successor.
 The predecessor is the station which is
logically before the station in the ring.
 The successor is the station which is after
the station in the ring.
 The current station is the one that is
accessing the channel now.
 The right to this access has been passed
from the predecessor to the current station.
 The right will be passed to the successor
when the current station has no more data
to send.

2.6.3) Channelization
 Channelization is a multiple-access method in which the
available bandwidth of a link is shared in time, frequency,
or through code, between different stations.
 Channelization methods
 Frequency-Division Multiple Access (FDMA)
 Time-Division Multiple Access (TDMA)
 Code-Division Multiple Access (CDMA)

2.6.3a)FDMA
 In frequency-division multiple access (FDMA), the available
bandwidth is divided into frequency bands.
 Each station is allocated a band to send its data.
 Each band is reserved for a specific station, and it belongs to
the station all the time.
 Each station also uses a bandpass filter to confine the
transmitter frequencies.
 To prevent station interferences, the allocated bands are
separated from one another by small guard bands.

2.6.3b)TDMA
 The stations share the bandwidth of the channel in time.
 Each station is allocated a time slot during which it can send data.
 Each station transmits its data in is assigned time slot.

2.6.3c)CDMA
 CDMA differs from FDMA because only one channel occupies
the entire bandwidth of the link.
 It differs from TDMA because all stations can send data
simultaneously; there is no timesharing.
 In CDMA, one channel carries all transmissions
simultaneously.
 Every user will be allocated the entire spectrum all of the time

2.7)Wired LANs: Ethernet
 Ethernet was developed in the mid-1970’s .
 IEEE controls the Ethernet standards.
 The Ethernet is the most successful local area networking technology, that
uses bus topology.
 The Ethernet is multiple-access networks that is set of nodes send and
receive frames over a shared link.
 Ethernet uses the CSMA / CD ( Carrier Sense Multiple Access with
 Collision Detection) mechanism.

 Preamble It contains 7 bytes (56 bits) of alternating 0s and 1s that alerts the
receiving system to the coming frame and enables it to synchronize its input timing.
 Start frame delimiter (SFD)It is a 1 byte (10101011) indicates the beginning of the
frame.
 Destination address (DA)It is 6 bytes and contains the physical address of the
destination station or stations to receive the packet.
 Source address (SA) It is 6 bytes and contains the physical address of the sender of
the packet.
 Length or typeIt is defined as a type field or length field.
 MAC framedefine the number of bytes in the data field.
 Data This field carries data encapsulated from the upper-layer protocols. It is a
 minimum of 46 and a maximum of 1500 bytes.
 CRCThe last field contains error detection information
Ethernet MAC frame format

10Base5-Thick Ethernet
 10Base5 was the first Ethernet specification to use a bus topology with
an external transceiver(transmitter/receiver) connected via a tap to a
thick coaxial cable.
 The maximum length of the coaxial cable must not exceed 500 m.
 If a length of more than 500 m is needed, then use repeaters
87

10Base2-Thin Ethernet
 It also uses a bus topology, but the cable is much thinner and more
flexible.
 In this case, the transceiver is normally part of the network interface
card (NIC), which is installed inside the station.
 The length of each segment cannot exceed 185 m (close to 200 m) due
to the high level of attenuation in thin coaxial cable.

10BaseT-Twisted-Pair Ethernet
 It uses a physical star topology.
 The stations are connected to a hub via two pairs of twisted cable.
 Two pairs of twisted cable create two paths (one for sending and one for
receiving) between the station and the hub.
 The maximum length of the twisted cable is 100 m, to minimize the effect
of attenuation in the twisted cable.

10BaseF-Fiber Ethernet
 It uses a star topology to connect stations to a hub.
 The stations are connected to the hub using two fiber-optic cables
 Supporting data rate 10Mbps

Fast Ethernet
 Upgrade the data rate to 100 Mbps.
 Make it compatible with Standard Ethernet.
 Keep the same 48-bit address.
 Keep the same frame format.
 Keep the same minimum and maximum frame lengths.
91

 100Base-TX uses two pairs of twisted-pair cable either UTP or STP. A
100Base-TX network can provide a data rate of 100 Mbps.
 100Base-T4 was designed to use four pairs of UTP for transmitting 100
Mbps.
 100Base-FX uses two pairs of fiber-optic cables. Optical fiber can easily
handle high bandwidth requirements

Gigabit Ethernet
 Supporting data rate to 1 Gbps(1000 Mbps).
 It can be categorized as either a two-wire or a four-wire
implementation.
 The two-wire implementations use fiber-optic cable (1000Base-SX,
shortwave, or 1000Base-LX, long-wave), or STP (1000Base-CX).
 The four-wire version uses category 5 twisted-pair cable (1000Base-T).
93

Ten-Gigabit Ethernet
 Upgrade the data rate to 10 Gbps.
 Make it compatible with Standard, Fast, and Gigabit Ethernet.
 Use the same 48-bit address.
 Use the same frame format.
 Keep the same minimum and maximum frame lengths.
 Allow the interconnection of existing LANs into a metropolitan area
network (MAN) or a wide area network (WAN).
 Make Ethernet compatible with technologies such as Frame Relay and
ATM

2.8)Wireless LAN
 It is an IEEE 802.11 standard
 Users connected by wireless LANs can move around within this limited
area.
 It can be found on college campuses, in office buildings, and in many
public areas.
 WLAN Configurations
1)Infrastructure based WLAN
2) Ad hoc based WALN

Architecture-Infrastructre Based

Architecture-Components
 Station (STA) - Mobile node ,Laptop ,PC with Wireless NIC, smart phone
 Access Point (AP)- Stations are connected to access point. Bridging/Routing function
 Basic Service Set (BSS) –It is the building blocks of a wireless LAN. It is made of
stationary or mobile wireless stations and an optional central base station, known as the
access point (AP).
 Extended Service Set (ESS) –It is made up of two or more BSSs with APs.
 BSSs are connected through a distribution system, which is a wired or a wireless
network.
 The distribution system connects the APs in the BSSs.
 The extended service set uses two types of stations: mobile and stationary.
 The mobile stations are normal stations inside a BSS.
 The stationary stations are AP stations that are part of a wired LAN.
 Station Types
 1. No-transition - A station with no-transition mobility is either stationary (not
moving) or moving only inside a BSS.
 2. BSS-transition - A station with BSS-transition mobility can move from one BSS
to another, but the movement is confined inside one ESS
 ESS-transition - A station with ESS-transition mobility can move from one ESS to
another.

2) Ad hoc based WLAN
•Mesh topology
•No access point or central controller
•Source station Destination station (stations communicate directly with one
another)

Frame Format
 Frame Control −It is a 2 bytes starting field composed of 11 subfields. It contains
control information of the frame. The 11 subfields are −
 Protocol version − The first sub-field is a two – bit field set to 00. It has been
included to allow future versions of IEE 802.11 to operate simultaneously.
 Type − It is a two-bit subfield that specifies whether the frame is a data frame,
control frame or a management frame.
 Subtype − it is a four – bit subfield states whether the field is a Request to Send
(RTS) or a Clear to Send (CTS) control frame. For a regular data frame, the value is
set to 0000.
 To DS − A single bit subfield indicating whether the frame is going to the access
point (AC), which coordinates the communications in centralised wireless systems.
 From DS − A single bit subfield indicating whether the frame is coming from the
AC.

 More Fragments − It is a 1bit subfield which when set to 1 indicates that more fragments
would follow.
 Retry − It is a 1bit subfield which when set to 1 specifies a retransmission of a previous
frame.
 Power Management − A single bit subfield indicating that the sender is adopting power-
save mode.
 More Data − A single bit subfield showing that sender has further data frames for the
receiver.
 Protected Frame − A single bit subfield indicating that this is an encrypted frame.
 Order − The last subfield, of one – bit, informs the receiver that to the higher layers the
frames should be in an ordered sequence.
 Duration − It is a 2-byte field that specifies the time period for which the frame and its
acknowledgement occupy the channel.
 Address fields: There are three 6-byte address fields containing addresses of source,
immediate destination and final endpoint respectively.
 Sequence − It a 2 bytes field that stores the frame numbers. It detects duplicate frames
and determines the order of frames for higher layers. Among the 16 bits, the first 4 bits
provides identification to the fragment and the rest 12 bits contain the sequence number
that increments with each transmission.
 Data − This is a variable sized field that carries the payload from the upper layers. The
maximum size of data field is 2312 bytes.
 Frame Check Sequence (FCS) − It is a 4-byte field containing error detection
information.

Collision Avoidance in WLAN / 802.11
 Hidden station problem
 Station B has a transmission range shown by the
left oval shape. Every station in this range can
hear any signal transmitted by station B.
 Station C has transmission range shown by the
right oval shape. Every station located in this
range can hear any signal transmitted by C.
 Station C is outside the transmission range of
B,and station B is outside the transmission
range of C.
 Station A, is in the area covered by both Band C;
it can hear any signal transmitted by B or C.
 Assume station B is sending data to station A.
 During this transmission, station C also has
data to send to station A, which results in a
collision at A because this station is receiving
data from both B and C.
 In this case, stations Band C are hidden from
each otherwith respect to A.

Solution to the hidden station problem
 Using of the handshake frames RTS-CTS(Request To Send and Clear To Send)
 RTS message from B reaches A, but not C. Both B and C are within the range of A,
the
 CTS message, which contains the duration of data transmission from B to A reaches
C.
 Station C knows that some hidden station is using the channel and refrains from
transmitting until that duration is over.
Station C doesn’t hear RTS from B, but it does hear CTS from A,
so it knows something is up.

Exposed Station Problem
 Station A is transmitting to station B.
 Station C has some data to send to station D, which can be sent without
interfering with the transmission from A to B.
 Station C is exposed to transmission from A, it hears what A is sending
and thus refrains from sending.
 In other words, C is too conservative and wastes the capacity of the
channel.
C wants to send to D, but hears A talking to B, so assumes
the medium is (incorrectly) busy.

Solution to the Exposed station problem
 Station C hears the RTS from A, but
does not hear the CTS from B.
 Station C after hearing the RTS from A,
can wait for a time so that the CTS from
B reaches A; it then sends an RTS to D
to show that it needs to communicate
with D.
 Both stations B and A may hear this
RTS, but station A is in the sending
state, not the receiving state.
 Station B, responds with a CTS.
 If station A has started sending its
data, station C cannot hear the CTS
from station D because of the collision.
 It cannot send its data to D. It remains
exposed until A finishes sending its
data

2.9)Bluetooth
 It is a wireless LAN technology designed to connect devices of different
functions such as telephones, notebooks, computers, cameras, printers, coffee
makers, and so on.
 It is an IEEE 802.15 standard.It defines a wireless personal-area network (PAN)
 Operating frequency 2.4 GHz Unlicensed ISM band.
 The range for Bluetooth communication is 0-30 feet (10 meters).
 This distance can be increased to 100 meters by amplifying the power.
 Data rate around 1 to 3 Mbps.
 Upto eight devices can be connected through Bluetooth.
 One device will function as a Master and the other seven devices will function as
slaves.
 Bluetooth uses Frequency Hopping Spread Spectrum (FHSS) to avoid any
interference.
 It defines two types of networks:
1) Piconet
2) scatternet

Architecture-Piconet
 The basic Bluetooth network configuration is called a Piconet
 A Piconet is a collection of eight bluetooth devices which are synchronized.
 One device in the piconet can act as Primary (Master), all other devices
connected to the master act as Secondary (Slaves).
 All the secondary stations synchronize their clocks and hopping sequence with
the primary.
 Only eight stations can be active in a piconet, activating a station from the
parked state means that an active station must go to the parked state.

 Active Device / State
1. Connected to the piconet and participates in the communication.
2. Can be a Master or a Slave device.
3. All active devices are assigned a 3-bit address (AMA).
 Parked Device / State
1. Connected to the piconet, but does not actively articipate in the
communication.
2. More than 200 devices can be parked.
3. All parked devices use an 8-bit parked member address (PMA).
 Stand-by Device / State
1. Not connected to the piconet.
2. They do not participate in the piconet currently but may take part at a
later time.
3. Devices in stand-by do not need an address.

Architecture-Scatternet
 Piconets can be combined to form what is called a scatternet.
 Many piconets with overlapping coverage can exist simultaneously,called
Scatternet.
 A secondary station in one piconet can be the primary in another piconet.
 This station can receive messages from the primary in the first piconet (as a
 secondary) and, acting as a primary, deliver them to secondaries in the second
Piconet.
 A station can be a member of two piconets

Bluetooth Protocol/LayersStack

Core System Protocols
• Radio (RF) protocol : Specifies details of the air interface, the use
of frequency hopping, modulation scheme, and transmit power.
• Baseband protocol : Concerned with connection establishment
within a Piconet, addressing, packet format, timing, and power
control.
• Link Manager protocol (LMP) : Responsible for link setup
between Bluetooth devices and ongoing link management.
• Logical link control and adaptation protocol (L2CAP)
L2CAP provides both connectionless and connection-oriented
services.
• Service discovery protocol (SDP) : Device information, services,
and the characteristics of the services can be queried to enable
the establishment of a connection between two or more
Bluetooth devices

 Radio Layer
 Bluetooth uses the frequency-hopping spread spectrum (FHSS) method in the
physical layer to avoid interference from other devices or other networks.
 Bluetooth hops 1600 times per second, which means that each device changes
its modulation frequency 1600 times per second.
 To transform bits to a signal, uses a sophisticated version of FSK, called GFSK.
 Baseband Layer
 The access method is TDMA.
 The primary and secondary stations communicate with each other using
 time slots. The length of a time slot is exactly 625 μs.
 During that time, a primary sends a frame to a secondary, or a secondary sends
a frame to the primary.
 L2CAP
 It is used for data exchange on an ACL link.
 SCO channels do not use L2CAP.
 The L2CAP functions are : multiplexing, segmentation and reassembly,
 quality of service (QoS), and group managemen.

2.10)Connecting Devices
 Connecting devices are used to connect hosts together to make a
network or to connect networks together to make an internet.
 Connecting devices can operate in different layers of the Internet
model.
 Connecting devices are divided into five different categories on the
basis of layers in which they operate in the network.

 Devices which operate below the
physical layer - Passive hub.
 Devices which operate at the
physical layer - Repeater.
physical and data link layers -
Bridge.
physical layer, data link layer and
network layer –Router.
 Devices which operate at all five
layers - Gateway.

Hubs
 Several networks need a central location to
connect media segments together. These central
locations are called as hubs.
 The hub organizes the cables and transmits
incoming signals to the other media segments.
 i) Passive hub
 It is a connector, which connects wires coming
from the different branches.
 By using passive hub, each computer can receive
the signal which is sent from all other
computers connected in the hub.
 ii) Active Hub
 It is a multiport repeater, which can regenerate
the signal.
 It is used to create connections between two or
more stations in a physical star topology.
 iii) Intelligent Hub
 Intelligent hub contains a program of network
management and intelligent path selection.

Repeaters
 A repeater receives the signal and it regenerates the signal in original bit pattern
before the signal gets too weak or corrupted.
 It is used to extend the physical distance of LAN.
 It works on physical layer.
 A repeater has no filtering capability.
 It repropagate a weak or broken signal and or service remote nodes.
 Amplify the received/input signal to a higher frequency domain so that it is reusable,
scalable and available.
 Repeaters are also known as signal boosters or range extender.
 A repeater cannot connect two LANs, but it connects two segments of the same LAN.

Bridges
 Bridges operate in physical layer as well as data link layer.
 As a physical layer device, they regenerate the receive signal.
 As a data link layer, the bridge checks the physical (MAC) address (of
the source and the destination) contained in the frame.
 The bridge has a filtering feature.
 It can check the destination address of a frame and decides, if the
frame should be forwarded or dropped.
 Bridges are used to connect two or LANs working on the same protocol.

 Transparent Bridges
 1. Frames must be forwarded from one station to another.
 2. The forwarding table is automatically made by learning frame movements
in the network.
 3. Loops in the system must be prevented.
 spanning tree is a graph in which there is no loop(creating a topology in
which each LAN can be reached from any other LAN through one path only )
 Source Routing Bridges
 In these bridges, routing operation is performed by source station and the
frame specifies which route to follow
 Translation Bridges
 These bridges connect networks with different architectures, such as
Ethernet and Token Ring. These bridges appear as:
 – Transparent bridges to an Ethernet host
 – Source-routing bridges to a Token Ring host

Switches
 It is a small hardware device , used to join multiple
computers together with one local area network.
 Allows us to interconnect links to form a large Network
 Switch is data link layer device.
 A switch is a multi port bridge with a buffer .
 Used to forward the packets based on MAC addresses.
 It is operated in full duplex mode.
 Packet collision is minimum as it directly
communicates between source and destination.
 It does not broadcast the message as it works with
limited bandwidth.
 A switch’s primary job is to receive incoming packets
on one of its links and to transmit them on some other
link.
 A Switch is used to transfer the data only to the device
that has been addressed.

Types of Switch
 1) Two- Layer Switch
 The two-layer switch performs at the physical and the data link layer.
 It is a bridge with many ports and design allows faster performs.
 A bridge is used to connect different LANs together.
 The two- layer switch can make a filtering decision bases on the MAC
address of the received frame. However, two- layer switch has a buffer
which holds the frame for processing.
 2) Three- Layer Switch
 The three-layer switch is a router.
 The switching fabric in a three-layer allows a faster table lookup and
forwarding mechanism.

Routers
 A router is a three-layer device.
 It operates in the physical, data-link, and network layers.
 As a physical-layer device, it regenerates the signal it receives.
 As a link-layer device, the router checks the physical addresses (source and
destination) contained in the packet.
 As a network-layer device, a router checks the network-layer addresses.
 A router is a device like a switch that routes data packets based on their IP
addresses.
 A router can connect networks. A router connects the LANs and WANs on the
internet.
 A router is an internetworking device.
 It connects independent networks to form an internetwork
 The key function of the router is to determine the shortest path to the
destination.
 Router has a routing table, which is used to make decision on selecting the
route.

Gateway
 It is a device, which operates in all five layers of the internet or seven layers of
OSI model.
 It is usually a combination of hardware and software.
 It connects two independent networks
 It is more complex than switch or router.
 It works as the messenger agents that take data from one system, interpret it,
and transfer it to another system.
 It also called protocol converters
 A gateway accepts a packet formatted for one protocol and converts it to a
packet formatted to another protocol before forwarding it.
 It adjust the data rate, size and data format.

Brouter
 It is a hybrid device. It combines the features of both bridge and router.
 It is a combination of Bridge and Router.
 Functions as a bridge for non-routable protocols and a router for
routable protocols.
 As a router, it is capable of routing packets across networks.
 As a bridge, it is capable of filtering local area network traffic.
 Provides the best attributes of both a bridge and a router
 Operates at both the Data Link and Network layers and can replace
separate bridges and routers.

UNIT III
NETWORK LAYER
Network Layer Services – Packet switching – Performance –
IPV4 Addresses – Forwarding of IP Packets – Network Layer
Protocols: IP, ICMP v4 – Unicast Routing Algorithms –
Protocols – Multicasting Basics – IPV6 Addressing – IPV6
Protocol.

CN UNIT1 TO UNIT 5.pdf

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