1. MCSE 514: Communication
System and Networking
Computer Science and Engineering
Islamic University, Kushtia.

2. 2
Chapter Syllabus
 Communication System: Band Width, Data transmission
method, Data transmission mode,
 Medium of data communication: Co-axial, Twisted pair,
Optical fiber, Wireless, Radio wave, Microwave, Wireless
communication System: Bluetooth, Wi-Fi, Wi-Max, Mobile
communication, Concept and Objectives of Computer
Networking, Functions of network,Types of network, Network
topology, Network Devices, NIC, Cloud computing.

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Telecommunications
 Tele (Far) + Communications
 Early telecommunications
 smoke signals and drums
 visual telegraphy (or semaphore in 1792)
 Telegraph and telephone
 Telegraph (1839)
 Telephone (1876)
 Radio and television
 Telephony
 Voice and Data

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Communications and Networks
 Basic components
 Communication technologies
 Communication devices
 Communication Channels
 Physical medias
 Wireless
 Communication software
 Networks of communication systems

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Communication Systems
 Process describing transfer of information, data,
instructions between one or more systems through some
media
 Examples: people, computers, cell phones, etc.
 Computer communication systems
 Signals passing through the communication channel can be
Digital, or analog
 Analog signals: continuous electrical waves
 Digital signals: individual electrical pulses (bits)
 Receivers and transmitters: desktop computers, mainframe
computers, etc.

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Communication Technologies
 Different technologies allowing us to communicate
 Voice mail, fax, email, instant message, chat rooms,
news groups, telephony, GPS, and more
 Voice mail: Similar to answering machine but digitized
 Fax: Sending hardcopy of text or photographs between
computers using fax modem
 Email: electronic mail – sending text, files, images
between different computer networks
 Must have email software
 More than 1.3 billion people send 244 billion messages
monthly!

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Communication Technologies
 Chat rooms: Allows communication in real time when
connected to the Internet
 voice chat, radio, etc.
 Telephony: Talking to other people over the Internet (also
called VoIP)
 Sends digitized audio signals over the Internet
 Requires Internet telephone software
 Groupware: Software application allowing a group of people
to communicate with each other (exchange data)
 A ddress book, appointment book, schedules, etc.
 GPS: consists of receivers connected to satellite systems
 Determining the geographical location of the receiver
 Used for cars, advertising, hiking, tracking, etc.

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Simplified Communications Model - Diagram

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A Communications Model
 Source
 generates data to be transmitted
 Transmitter
 Converts data into transmittable signals
 Transmission System
 Carries data
 Receiver
 Converts received signal into data
 Destination
 Takes incoming data

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Communication Devices
 Any type of hardware capable of transmitting data,
instructions, and information between devices
 Basic characteristics: How fast, how far, how much data!
 Functioning as receiver, transmitter, adaptor, converter
 Examples: Dial-up modems, ISDN, DSL modems, network
interface cards
 Dial-up modem: uses standard phone lines
 Converts digital information into analog
 Consists of a modulator and a demodulator
 Can be external, internal, wireless
 Special applications: fax machine

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Communication Devices
 ISDN and DSL Modem: Allows digital communication
between networks and computers
 Requires a digital model
 Digital is better than analog – why?
 Cable modem: a modem that transmits and receives data over
the cable television (CATV) network
 Also called broadband modem (carrying multiple signals)
 The incoming signal is split
 Requires a cable modem
 Network interface cards: Adaptor cards residing in the
computer to transmit and receiver data over the network (NIC)
 Operate with different network technologies (Ethernet,
wireless)

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Communication Protocol (OSI)
Application: Provides access to the OSI environment for users and also provides
distributed information services.
Presentation: Provides independence to the application processes from differences in
data representation (syntax).
Session: Provides the control structure for communication between applications;
establishes, manages, and terminates connections (sessions) between cooperating
applications.
Transport: Provides reliable, transparent transfer of data between end points; provides
end-to-end error recovery and flow control.
Network: Provides upper layers with independence from the data transmission and
switching technologies used to connect systems; responsible for establishing,
maintaining, and terminating connections.
Data Link: Provides for the reliable transfer of information across the physical link; sends
blocks of data (frames) with the necessary synchronization, error control, and flow
control.
Physical: Concerned with transmission of unstructured bit stream over physical medium;
deals with the mechanical, electrical, functional, and procedural characteristics to access
the physical medium.

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Communication Protocol (TCP/IP)

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OSI~TCP/IP

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Communication Software
 Examples of applications (Layer 7) take advantage of the
transport (Layer 4) services of TCP and UDP
 Hypertext Transfer Protocol (HTTP): A client/server
application that uses TCP for transport to retrieve HTML
pages.
 Domain Name Service (DNS): A name-to-address
translation application that uses both TCP and UDP
transport.
 Telnet: A virtual terminal application that uses TCP for
transport.
 File Transport Protocol (FTP): A file transfer application
that uses TCP for transport.

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Communication Software
 Examples of applications (Layer 7) take advantage of the
transport (Layer 4) services of TCP and UDP
 Trivial File Transfer Protocol (TFTP): A file transfer
application that uses UDP for transport.
 Network Time Protocol (NTP): An application that
synchronizes time with a time source and uses UDP for
transport.
 Border Gateway Protocol (BGP): An exterior gateway
routing protocol that uses TCP for transport. BGP is used
to exchange routing information for the Internet and is the
protocol used between service providers.

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Communication Channels
 A channel is a path between two communication devices
 Channel capacity: How much data can be passed through
the channel (bit/sec)
 Also called channel bandwidth
 The smaller the pipe the slower data transfer!
 Consists of one or more transmission media
 Materials carrying the signal
 Two types:
 Physical: wire cable
 Wireless: Air destination
network
server
T1
lines
T1
lines
T1
lines
T3
lines

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Physical Transmission Media
 A tangible media
 Examples: Twisted-pair cable, coaxial cable, Fiber-optics,
etc.
 Twisted-pair cable:
 One or more twisted wires bundled together
 Made of copper
 Coax-Cable:
 Consists of single copper wire surrounded by three layers of
insulating and metal materials
 Typically used for cable TV
 Fiber-optics:
 Strands of glass or plastic used to transmit light
 Very high capacity, low noise, small size, less suitable to
natural disturbances

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Physical Transmission Media
plastic outer
coating
woven or
braided metal
insulating
material
copper wire
twisted-pair cable twisted-pair wire
protective
coating
glass cladding
optical fiber
core

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Wireless Transmission Media
 Broadcast Radio
 Distribute signals through the air over long distance
 Uses an antenna
 Typically for stationary locations
 Can be short range
 Cellular Radio
 A form of broadcast radio used for mobile communication
 High frequency radio waves to transmit voice or data
 Utilizes frequency-reuse

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Wireless Transmission Media
 Microwaves
 Radio waves providing high speed transmission
 They are point-to-point (can’t be obstructed)
 Used for satellite communication
 Infrared (IR)
 Wireless transmission media that sends signals using
infrared light-

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 The bandwidth of a channel (medium) is defined to be the range of
frequencies that the medium can support. Bandwidth is measured
in Hz
 With each transmission medium, there is a frequency range of
electromagnetic waves that can be transmitted:
 Twisted pair cable: 0 to 109
Hz (BW: 109
Hz)
 Coax cable: 0 to 1010
Hz (BW: 1010
Hz)
 Optical fiber: 1014
to 1016
Hz (BW: 1016
-1014
= 9.9x1015
Hz)
 Optical fibers have the highest bandwidth (they can support
electromagnetic waves with very high frequencies, such as light
waves)
 The bandwidth of the channel dictates the information carrying
capacity of the channel
 This is calculated using Shannon’s channel capacity formula
Channel Bandwidth:

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Shannon’s Theorem
(Shannon’s Limit for Information Capacity)
 Claude Shannon at Bell Labs figured out how much information
a channel could theoretically carry:
I = B log2 (1 + S/N)
 Where I is Information Capacity in bits per second (bps)
 B is the channel bandwidth in Hz
 S/N is Signal-to-Noise ratio (SNR: unitless…don’t make into
decibel: dB)
 Signal-to-noise ratio (often abbreviated SNR or S/N) is a
measure used in science and engineering that compares the
level of a desired signal to the level of background noise. It is
defined as the ratio of signal power to the noise power, often
expressed in decibels.

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Transmission Media

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Twisted Pair Wires
 Consists of two insulated copper wires arranged in a regular
spiral pattern to minimize the electromagnetic interference
between adjacent pairs
 Often used at customer facilities and also over distances to
carry voice as well as data communications
 Low frequency transmission medium
 Types of Twisted Pair Wire
 STP (shielded twisted pair)
 the pair is wrapped with metallic foil or braid to insulate
the pair from electromagnetic interference
 UTP (unshielded twisted pair)
 each wire is insulated with plastic wrap, but the pair is
encased in an outer covering

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Twisted-pair cable

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UTP connector

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Twisted Pair Advantages and Limitations
Advantages
 Inexpensive and readily available
 Flexible and light weight
 Easy to work with and install
Limitations
 Susceptibility to interference and noise
 Attenuation problem
 For analog, repeaters needed every 5-6km
 For digital, repeaters needed every 2-3km
 Relatively low bandwidth (3000Hz)

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Coaxial Cable (or Coax)
 Used for cable television, LANs, telephony
 Has an inner conductor surrounded by a braided mesh
 Both conductors share a common center axial, hence the term “co-
axial”
 Advantages
 Higher bandwidth
 400 to 600Mhz
 up to 10,800 voice conversations
 Can be tapped easily (pros and cons)
 Much less susceptible to interference than twisted pair
 Disadvantages
 High attenuation rate makes it expensive over long distance
 Bulky

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Coax Layers
copper or aluminum
conductor
insulating material
shield
(braided wire)
outer jacket
(polyethylene)

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Coaxial cable
Categories of coaxial cables

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Fiber optics: Bending of light ray

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Fiber Optic Cable
 Relatively new transmission medium used by telephone
companies in place of long-distance trunk lines
 Also used by private companies in implementing local
data communications networks
 Require a light source with injection laser diode (ILD) or
light-emitting diodes (LED)

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plastic jacket glass or plastic
cladding
fiber core
Fiber Optic Layers
 consists of three concentric sections

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Fiber Optic Advantages and Disadvantages
Advantages
 greater capacity (bandwidth of up to 2 Gbps)
 smaller size and lighter weight
 lower attenuation
 immunity to environmental interference
 highly secure due to tap difficulty and lack of signal radiation
Disadvantages
 expensive over short distance
 requires highly skilled installers
 adding additional nodes is difficult

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Optical fiber
Propagation modes

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Fiber Optic Types
 multimode step-index fiber
 the reflective walls of the fiber move the light pulses to the
receiver
 multimode graded-index fiber
 acts to refract the light toward the center of the fiber by
variations in the density
 single mode fiber
 the light is guided down the center of an extremely narrow
core

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Modes

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Fiber construction

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UNGUIDED MEDIA: WIRELESS
 Unguided media transport electromagnetic waves without
using a physical conductor.
 Radio Waves
 Microwaves
 Infrared
 transmission and reception are achieved by means of an
antenna
 directional
 transmitting antenna puts out focused beam
 transmitter and receiver must be aligned
 omnidirectional
 signal spreads out in all directions
 can be received by many antennas

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Electromagnetic spectrum for wireless communication

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Bands

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Wireless transmission waves

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Wireless transmission waves
Radio
 radio is omnidirectional and microwave is directional
 Radio is a general term often used to encompass frequencies in
the range 3 kHz to 300 GHz.
 Mobile telephony occupies several frequency bands just under
1 GHz.
Infrared
 Uses transmitters/receivers (transceivers) that modulate
noncoherent infrared light.
 Transceivers must be within line of sight of each other
(directly or via reflection ).
 Unlike microwaves, infrared does not penetrate walls.

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Wireless transmission waves
 Radio waves are used for multicast communications, such as
radio and television, and paging systems.
 They can penetrate through walls.
 Highly regulated. Use omni directional antennas
 Microwaves are used for unicast communication such as
cellular telephones, satellite networks, and wireless LANs.
 Higher frequency ranges cannot penetrate walls.
 Use directional antennas - point to point line of sight
communications.
 Infrared signals can be used for short-range communication in
a closed area using line-of-sight propagation.

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Terrestrial Microwave
 used for long-distance telephone service
 uses radio frequency spectrum, from 2 to 40 Ghz
 parabolic dish transmitter, mounted high
 used by common carriers as well as private networks
 requires unobstructed line of sight between source and
receiver
 curvature of the earth requires stations (repeaters) ~30
miles apart

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dish
dish
uplink station downlink station
satellite
transponder
22,300 miles
Satellite Transmission Process

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Satellite Transmission Process
Satellite Microwave Transmission
 a microwave relay station in space
 can relay signals over long distances
 geostationary satellites
 remain above the equator at a height of 22,300 miles
(geosynchronous orbit)
 travel around the earth in exactly the time the earth takes to
rotate
Satellite Transmission Links
 earth stations communicate by sending signals to the satellite on
an uplink
 the satellite then repeats those signals on a downlink
 the broadcast nature of the downlink makes it attractive for
services such as the distribution of television programming

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Satellite Microwave
Satellite Transmission Bands
 C band: 4(downlink) - 6(uplink) GHz
 the first to be designated
 Ku band: 12(downlink) -14(uplink) GHz
 rain interference is the major problem
 Ka band: 19(downlink) - 29(uplink) GHz
 equipment needed to use the band is still very expensive
Applications
 Television distribution
 Long-distance telephone transmission
 Private business networks
Disadvantages
 line of sight requirement
 expensive towers and repeaters
 subject to interference such as passing airplanes and rain

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Fiber vs Satellite

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Advantages of Wireless Communication
 Mobility: Because radio waves travel freely through the air, wireless
modes of communication give you a great deal of mobility.
 Convenience: Wireless devices have no cables to connect; if the signal has
sufficient strength, the device will work. This is also true for mobile
computing devices; as long as user have the password for the local wireless
data network, wireless devices connects automatically. When you leave a
location, your mobile device automatically drops the connection and picks
up the next strong network signal it finds.
 Flexibility: A wired communications system is limited to the number of
physical connections on the equipment; if these run out, user must replace
the equipment to support more users. A typical Ethernet router for home
use, for example, offers only eight sockets, even though its network
software can handle 254 users.
 Lower Cost: Wireless communications networks are less expensive to
install and maintain than equivalent wired systems. Any changes to the
wiring plan add to these costs. Although even wireless systems need some
cabling, the amount involved is a small fraction of that needed for wired
equipment.

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Simplified Data Communications Model

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Transmission Modes
 We use the term transmission mode to refer to the manner in
which data is sent over the underlying medium
 Transmission modes can be divided into two fundamental
categories:
 Serial — one bit is sent at a time
 Serial transmission is further categorized according to
timing of transmissions
 Parallel — multiple bits are sent at the same time

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Taxonomy of Transmission Modes

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PARALLEL TRANSMISSION
 In parallel transmission, all the bits of data are transmitted
simultaneously on separate communication lines.
 Parallel transmission is used for short distance communication.
 In order to transmit n bit, n wires or lines are used.
 More costly.
 Faster than serial transmission.
 Data can be transmitted in less time.

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SERIAL TRANSMISSION
 In serial transmission , the various bits of data are transmitted
serially one after the other.
 It requires only one communication line rather than n lines to
transmit data from sender to receiver.
 Thus all the bits of data are transmitted
 on single lines in serial fashion.
 Less costly.
 Long distance transmission.

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SERIAL TRANSMISSION

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Timing of Serial Transmission
 Serial transmission mechanisms can be divided into three
broad categories (depending on how transmissions are spaced
in time):
 Asynchronous transmission can occur at any time
 with an arbitrary delay between the transmission of two
data items
 Synchronous transmission occurs continuously
 with no gap between the transmission of two data items
 Isochronous transmission occurs at regular intervals
 with a fixed gap between the transmission of two data items

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ASYNCHORONOUS TRANSMISSION
 Sends only one character at a time (one byte of data at a time)
 Synchronize two devices using Start Bit and Stop Bit.
 Start bit refers to the start of the data. Usually 0 is used for
start bit.
 Stop bit indicates the end of data.more than one bit can be
used for end.

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SYNCHRONOUS TRANSMISSION
 Data sent at one time multiple bytes.
 Start and stop bit not used.
 Gap between data units not present.
 Data transmission speed fast.
 Cost high.
 Transfer of data between two computer.
 Synchronization between sender and receiver required.

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SYNCHRONOUS TRANSMISSION

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Isochronous Transmission
 Isochronous transmission
 is designed to provide steady bit flow for multimedia applications
 Delivering such data at a steady rate is essential
 because variations in delay known as jitter can disrupt reception
(cause pops or clicks in audio/make video freeze for a short time)
 Isochronous network is designed to accept and send data at a fixed
rate, R
 Network interface is such that data must be handed to the
network for transmission at exactly R bits per second
 For example, an isochronous mechanism designed to transfer voice
operates at a rate of 64,000 bits per second
 A sender must generate digitized audio continuously
 A receiver must be able to accept and play the stream

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TRANSMISSION MODE
 The term transmission mode defines the direction of the flow
of information between two communication devices i .e ,
 It tells the direction of signal flow between the two devices.
1. SIMPLEX TRANSMISSION MODE.
2. HALF DUPLEX TRANSMISSION MODE
3. FULL DUPLEX TRANSMISSION MODE

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SIMPLEX MODE
 In simplex mode transmission information sent in only one
direction.
 Device connected in simplex mode is either sent only or
received only that is one device can only send, other device
can only receive.
 Communication is unidirectional.

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HALF DUPLEX
 In half duplex transmission data can be sent in both the
directions, but only in one direction at a time.
 Both the connected device can transmit and receive but not
simultaneously.
 When one device is sending the other can only receive and
vice-versa.

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FULL DUPLEX
 In full duplex transmission, data can be sent in both the
directions simultaneously.
 Both the connected devices can transmit and receive at the
same time.
 Therefore it represents truly bi-directional system.
 In full duplex mode, signals going in either
 Direction share the full capacity of link.

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Unicast, Broadcast and Multicast
 Unicast: Unicast is the term used to describe communication
where a piece of information is sent from one point to another
point. In this case there is just one sender, and one receiver
 Broadcast: Broadcast is the term used to describe
communication where a piece of information is sent from one
point to all other points. In this case there is just one sender,
but the information is sent to all connected receivers.
 Multicast: Multicast is the term used to describe
communication where a piece of information is sent from one
or more points to a set of other points. In this case there is may
be one or more senders, and the information is distributed to a
set of receivers (theer may be no receivers, or any other
number of receivers).

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Types of network
 Computer networks can be characterized by their size as well
as their purpose.
 The size of a network can be expressed by the geographic area
they occupy and the number of computers that are part of the
network.
 Networks can cover anything from a handful of devices
within a single room to millions of devices spread across
the entire globe.
 Some of the different networks based on size are:
 Personal area network, or PAN
 Local area network, or LAN
 Metropolitan area network, or MAN
 Wide area network, or WAN

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Personal Area Network
 A personal area network, or PAN, is a computer network
organized around an individual person within a single
building.
 This could be inside a small office or residence.
 A typical PAN would include one or more computers,
telephones, peripheral devices, video game consoles and
other personal entertainment devices.
 This type of network provides great flexibility. For example, it
allows you to:
 Send a document to the printer in the office upstairs while
you are sitting on the couch with your laptop.
 Upload the photo from your cell phone to desktop
computer.
 Watch movies from an online streaming service to your TV.

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Local Area Network
 A local area network, or LAN, consists of a computer network at a
single site, typically an individual building.
 A LAN is very useful for sharing resources, such as data storage and
printers. LANs can be built with relatively inexpensive hardware,
such as hubs, network adapters and Ethernet cables.
 The smallest LAN may only use two computers, while larger LANs
can accommodate thousands of computers.
 A LAN typically relies mostly on wired connections for
increased speed and security, but wireless connections can also
be part of a LAN.
 High speed and relatively low cost are the defining characteristics
of LANs.
 LANs are typically used for single sites where people need to
share resources among themselves but not with the rest of the
outside world.
 If a local area network, or LAN, is entirely wireless, it is referred
to as a wireless local area network, or WLAN.

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MAN
Metropolitan Area Network
 A metropolitan area network, or MAN, consists of a
computer network across an entire city, college campus or
small region.
 A MAN is larger than a LAN, which is typically limited to a
single building or site.
 Depending on the configuration, this type of network can
cover an area from several miles to tens of miles.
 A MAN is often used to connect several LANs together to
form a bigger network.

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WAN
 A wide area network, or WAN, occupies a very large area,
such as an entire country or the entire world.
 A WAN can contain multiple smaller networks, such as LANs
or MANs.
 The Internet is the best-known example of a public WAN
 A WAN is a geographically-dispersed collection of LANs.
 A network device called a router connects LANs to a WAN.
 In IP networking, the router maintains both a LAN address and
a WAN address.

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Comparison of LAN MAN and WAN
Local Area Network Metropolitan Area Network Wide Area Network
1.LAN is a group of computers
and other network devices
which are connected together.
MAN is a larger network of
computers and other network
devices which are connected
together usually spans several
buildings or large
geographical area.
WAN is a group of computers and
other network devices which are
connected together which is not
restricted to a geographical
location. Internet is WAN
All the devices that are part of
LAN are within a building or
multiple building.
All the devices that are part of
MAN are span across
buildings or small town.
All the devices that are part of
WAN have no geographical
boundaries.
LAN network has very high
speed mainly due to proximity
of computer and network
devices.
MAN network has lower
speed compared to LAN.
WAN speed varies based on
geographical location of the
servers. WAN connects several
LANs
LAN connection speeds can be
10Mbps/ 100Mbps/ 1000Mpbs
MAN connection speeds can
be 10Mbps or 100Mbps.
WAN connection speeds can be
10Mbps or 100Mbps.
LAN uses Guided Media MAN uses Guided Media or
Unguided media.
WAN mainly uses Guided Media or
Unguided media. Its long distance
communications, which may or
may not be provided by public
packet network.

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Other Types of Area Networks
 While LAN and WAN are by far the most popular network
types mentioned, you may also commonly see references to
these others:
 Wireless Local Area Network - a LAN based on Wi-Fi
wireless network technology
 Campus Area Network - a network spanning multiple LANs
but smaller than a MAN, such as on a university or local
business campus.
 Storage Area Network - connects servers to data storage
devices through a technology like Fibre Channel.
 System Area Network - links high-performance computers
with high-speed connections in a cluster configuration. Also
known as Cluster Area Network.

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Topology in Network
Network topologies are categorized into the
following basic types:
 bus
 ring
 star
 tree
 mesh

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Topology in Network
 Various configurations, called topologies, have been used to administer
LANs
 Ring topology A configuration that connects all nodes in a closed loop on
which messages travel in one direction.
 Star topology A configuration that centers around one node to which all
others are connected and through which all messages are sent. A star
network features a central connection point called a "hub node" that may be
a network hub , switch or router .
 Bus topology All nodes are connected to a single communication line that
carries messages in both directions. A single cable, the backbone functions
as a shared communication medium that devices attach or tap into with an
interface connector.
 Tree Topology Tree topologies integrate multiple star topologies together
onto a bus. In its simplest form, only hub devices connect directly to the tree
bus, and each hub functions as the root of a tree of devices.
 Mesh Topology Mesh topologies involve the concept of routes. Unlike each
of the previous topologies, messages sent on a mesh network can take any
of several possible paths from source to destination.

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Tree
Mesh

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Wireless LAN: Bluetooth
Bluetooth
 A short-range communication system intended to replace the
cables connecting devices
 Key Features: Robustness, low power, and low cost
 Provides a way to connect and exchange information between
devices such as mobile phones, PCs, printers, such as mobile
phones, PCs, printers, GPS receivers, digital cameras, and
video game consoles
 Speed of upto 1 Mbps

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Wireless LAN: Wi-Fi
 Typically called wireless LAN or Wireless Ethernet
 More powerful than Bluetooth and used in corporate settings
to connect PCs to PCs, corporate settings to connect PCs to
PCs, printers, servers, etc
 Supported by most PC OS, game consoles, smartphones,
printers, etc
 A Wi-Fi enabled device can connect to the Internet when
within range of a wireless network connected to the Internet
 Interconnected access points popularly called hotspots
 Hotspot - can comprise an area as small as a single room with
wireless-opaque walls or as large as many square miles
covered by overlapping access points.

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Internetworking devices
Increasing power and complexity
 Hubs
 Bridges
 Switches
 Routers
 Computers within a LAN are often connected using a hub
 LAN to LAN connections are often performed with a bridge.
 Segments of a LAN are usually connected using a switch.
 LAN to WAN connections are usually performed with a
router.

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Hubs
A hub interconnects two or more workstations into a
local area network.
When a workstation transmits to a hub, the hub
immediately resends the data frame to all connecting
links.
Hubs expand one Ethernet connection into many. For
example, a four-port hub connects up to four machines

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Hubs

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Bridge
 A bridge connects networks and forwards
frames from one network to another.
BRIDGE
A B
C D
E F
G H
PORTS

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Selective Forwarding
 If A sends a frame to E - the frame must
be forwarded by the bridge.
 If A sends a frame to B - there is no reason
to forward the frame.
BRIDGE
A B
C D
E F
G H

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Bridge interconnection
Identical LANs
dissimilar LANs

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Bridges vs Routers
Bridge: A bridge is a
device that connects two
segments of the same
network. The two
networks being
connected can be alike or
dissimilar.
Bridges are protocol-
independent. They
simply forward packets
without analyzing and
re-routing messages.
Router: A router is a device that
connects two distinct networks.
Routers are similar to bridges,
but provide additional
functionality, such as the
ability to filter messages and
forward them to different
places based on various
criteria.
The Internet uses routers
extensively to forward packets
from one host to another.

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Switches
 A switch is a combination of a hub and a bridge.
 It can interconnect two or more workstations, but like a bridge,
it observes traffic flow and learns.
 When a frame arrives at a switch, the switch examines the
destination address and forwards the frame out the one
necessary connection.
 Major role: isolating traffic patterns and providing multiple
access. This design is usually done by the network manager.
 Switches are easy to install and have components that are hot-
swappable.

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Routers
 The device that connects a LAN to a WAN or a WAN to a
WAN (the INTERNET! – uses IP addresses).
 A router accepts an outgoing packet, removes any LAN
headers and trailers, and encapsulates the necessary WAN
headers and trailers.
 Because a router has to make wide area network routing
decisions, the router has to dig down into the network layer of
the packet to retrieve the network destination address.
 Thus, routers are often called “layer 3 devices”. They operate
at the third layer, or OSI network layer, of the packet.
 Routers often incorporate firewall functions.
 An example of a router’s operation is shown on the next slide.