Networks and Networking
Background
EMC 165: Computer & Network Engineering
Spring 2005
Profs. Chuah & Kishore
Last Time/Today
• Last time, we motivated study of computer &
network engineering using some case studies.
• Today, we introduce
– Several types of networks we will discuss in
greater detail during the semester; and
– Important concepts and terminology related to
study of modern networks.
What is a Communication Network?
• A collection of devices that can exchange
information between each other.
• Devices can be computers, phones, other
appliances, sensors, etc.
• Information can be all kinds of data: computer
files, voice, video stream, etc.
• Information is exchanged over wires (copper,
optical fibers) or wirelessly using radio signals
that are transmitted and received by antennas,
or both.
What is a Communication Network?
(Cont’d)
• To fully connect all devices in a network,
additional equipment may be needed.
• These additional devices (e.g., access points,
routers) form interconnections (wired and/or
wireless) over which information can traverse as
it moves from a source device to a destination
device.
• The path a data stream takes from source to
destination is called a route.
Network Examples
1. Internet
• One of the greatest things about the Internet is
that nobody really owns it.
• It is a global collection of networks, both big and
small.
• These networks connect together in many
different ways to form the single entity that we
know as the Internet. In fact, the very name
comes from this idea of interconnected
networks.
The Internet Concept
The Internet Concept (Cont’d)
Internet: Network of Networks
• Every computer that is connected to the Internet
is part of a network, even the one in your home.
• For example, you may use a modem and dial a
local number to connect to an Internet Service
Provider (ISP).
• At school/work, you may be part of a local area
network (LAN), but you most likely still connect
to the Internet using an ISP that your
school/company has contracted with.
Internet: Network of Networks
(Cont’d)
• When you connect to your ISP, you become part
of their network.
• The ISP may then connect to a larger network
and become part of their network.
• The Internet is simply a network of networks.
Connecting Network of Networks
• The amazing thing here is that there is no
overall controlling network.
• Instead, there are several high-level networks
connecting to each other through Network
Access Points or NAPs.
• All the networks that make up the Internet rely
on NAPs, backbones and routers to talk to each
other.
Internet: Clients & Servers
• Internet servers make the Internet possible. All
of the machines on the Internet are either
servers or clients.
• The machines that provide services to other
machines are servers.
• The machines that are used to connect to those
services are clients.
• There are Web servers, e-mail servers, FTP
servers and so on serving the needs of Internet
users all over the world.
Internet: Clients & Servers (Cont’d)
• When you connect to www.cnn.com to read a
page, you are a user sitting at a client's
machine.
• You are accessing the cnn Web server. The
server machine finds the page you requested
and sends it to you.
• Clients locate servers using addresses called
Internet Protocol (IP) addresses.
• We will cover these and more details in the
upcoming weeks.
2. Cellular Networks
• Cellular telephony is one of the fastest growing
technologies on the planet.
• It has been around roughly 15-20 years now.
Presently, we are starting to see the third
generation of the cellular phones coming to the
market.
• New phones allow users to do much more than
hold phone conversations.
Beyond Voice
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•
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Store contact information
Make task/to-do lists
Keep track of appointments
Calculator
Send/receive email
Send/receive pictures
Send/receive video clips
Get information from the internet
Play games
Integrate with other devices (PDA’s, MP3 Players,
etc.)
Cells, Bases, and Mobiles
Base station: local radio tower to which a mobile sends its signals.
Mobile: User handset.
Sends/receives all signals
from base station.
Normal
Telephone
System
Wired connection
Cell: coverage area of base station
Multiple Cells
• Over a region, say a city, a cellular service
provider will typically install multiple base
stations, leading to multiple interconnected cells.
• As mobiles move from one cell to another, the
phone call is handed from one base station to
the next.
Multiple Cells (Cont’d)
• A base station provides coverage
(communication capabilities) to users on mobile
phones within its coverage area.
• Users outside the coverage area
receive/transmit signals with too low power for
reliable communications.
• Users within the coverage area transmit and
receive signals from the base station.
• The base station itself is connected to other
base stations and to the wired telephone
network.
How it all works
• In the upcoming weeks, we will review how cell
phones operate, learn about cellular network
components, the differences in the generations
of cellular technology, etc.
• In the process, we will learn how wireless
signals are generated, how they move through
the air, and how receivers can extract these
signals from the air.
3. Wireless Local Area Networks
(WLANs)
• A Wireless local area network is a network of
computers that communicate with each other
without wires within a 100 feet or so.
• Most popular WLAN is WiFi; it is also known as
802.11 networking.
More on Wi-Fi
• You can connect computers anywhere in your
home or office without the need for wires. The
computers connect to the network using radio
signals, and computers can be up to 100 feet or
so apart.
• In order to form such networks, the computers
have to have special equipment.
• This typically comes in the form of a Wi-Fi card,
which are collection of computer chips that
enable Wi-Fi connections.
Wi-Fi Components
• Many new laptops already come with a WiFi
card built in -- in many cases you don't have to
do anything to start using WiFi.
• It is also easy to add a WiFi card to an older
laptop or a desktop PC.
• Once a Wi-Fi card is
available, you just have to
access a hotspot.
Wi-Fi Components (Cont’d)
• Hotspot: a connection point for a WiFi network.
It is a small box (called an access point) that is
hardwired into the Internet. The box contains an
802.11 radio that can simultaneously talk to up
to 100 or so 802.11 cards.
• There are many WiFi hotspots now available in
public places like restaurants, hotels, libraries
and airports. For example, Starbucks.
• The number of hotspots in the world is growing
daily.
• You can also create a hotspot at home.
Wi-Fi Hotspots
• One way to find a hotspot is to go on-line.
• For example, the following websites:
Wi-Fi: A Network in the Network of
Networks
Internet
Wi-Fi Access Point/Router
WLAN
Wi-Fi Ready Devices
More on Wi-Fi
• In the next few weeks, we will see how Wi-Fi
works, how you can set one up at home/work,
how Wi-Fi networks can be extended to cover
larger areas, etc.
• We will also see how existing Wi-Fi technology
is being used to create ad-hoc, self-configuring
networks (more on this later).
4. Ad-Hoc/Sensor Networks
• Thus far, all the wireless networks we have
mentioned have a centralized architecture: user
terminals communicate strictly with a base
station or an access point.
• Currently, there is a lot of research work being
conducted in ad-hoc wireless networks, where
terminals can communicate directly with each
other. (It is not necessary that terminals talk with
access points first.)
Wi-Fi Example
• For example, Wi-Fi cards can operate in both
centralized (or infrastructure) mode as well as
ad-hoc mode.
• Peer-to-peer communication is enabled.
Ad-Hoc Mode
• Ad Hoc connections can be used to share
information directly between devices. This mode
is also useful for establishing a network where
wireless infrastructure does not exist.
• Some uses,
– Synchronize data between devices.
– Retrieve multimedia files from one device and
“play” them on another device.
– Print from a computer to a printer without
wires.
• Ad Hoc wireless networking has very important
application in sensor networks.
Sensors
• Most basic definition: sensors are devices that
are able to detect changes in a physical quantity
or event.
• Another definition: sensors are electronic or
electromechanical devices that are used to
detect and supply either physical or
environmental information.
More on Sensors
For example, sensors can measure:
Air Temperature
Barometric Pressure
Distance
Evaporation
Fuel
Moisture & Temp
Leaf Wetness
Position (GPS)
Precipitation
Relative Humidity
Soil Heat Flux
Soil Moisture
Solar Radiation
Soil/Water Temperature
Snow Depth
Wind Speed & Direction
Water Quality
pH Conductivity
Structural VibrationsBioToxins
Etc.
Sensor Networks
• Sensors there have important applications in
transportation, healthcare, manufacturing,
environmental monitoring, homeland security,…
• In most of these applications, a large number of
sensors are installed in a region over which
pertinent data has to be collected.
• Sensors have to report their measured data
back to a central unit, which processes and
monitors data collection and may make control
decisions, e.g., alert users of high/low measured
data.
Sensor Network (Cont’d)
The communication required to forward collected
data to a central processor occurs over a peerto-peer network, i.e., an ad-hoc network.
Data
Collection
Data
Collection
Wireless
Links
Radio
Antenna
Sensor
Wired Connections
Sensor Networks (Cont’d)
• This semester, we will discuss several
existing/emerging sensor networks, design of
ad-hoc networks, etc.
• In the process, we will also review the important
research areas in this emerging networking
topic.
5. Satellite Systems: GPS
• Several key communication networks rely on
satellite links.
• An important satellite system is Global
Positioning System (GPS).
• GPS is primarily a navigation tool, giving users
with GPS terminals accurate measurements of
location, velocity, etc. These terminals obtain
the measurements by receiving signals from
satellites located several thousand miles in the
space.
GPS Components
• Three basic
components:
– User terminal (gives
navigation
measurements).
– 24 Satellites (used as
artificial/intelligent stars
to determine location of
user terminals)
– 5 Ground stations
(collectively control and
monitor the satellites).
More to Come on GPS
• During the semester, we will learn how GPS
works, how user terminal uses satellites to give
highly accurate navigation data, etc.
• In the process, we will discuss satellite systems
in general, how satellites are launched into
space, learn about different types of satellites,
the components required in communication
satellites, etc.
5. Home Networks
A Network connecting devices within a home,
e.g., computers, printers, TV, DVD players,
music servers, (even) refrigerators, microwave
ovens, etc.
Gateway
Between 1 &3
Broadband Connection
(e.g., DSL, Cable,
Wireless, etc.)
Home Network
Example of Present Home Networks
More on Home Networking
• In the future, more and more devices can be
connected on the home network.
• We will discuss these possibilities and
demonstrate how sensor networks may
converge with present day home networks to
give smart homes.
Important Terminology & Concepts
Packet and Circuit Switched
Networks
• Most of the networks we have talked about thus
far are packet-switched networks.
• To understand this concept, we must compare
these types of networks to circuit-switched
networks.
• To under both packet and circuit switched
networks, we must begin with comparing analog
to digital.
Analog Versus Digital
• The data or information we can communicate
over a network can be qualified as either analog
or digital.
• Imagine plotting the information as a function of
time.
– Analog signals can take on a continuous range of
values (e.g., voice).
– Digital signals can only take on one of a possible set
of values (e.g., text message).
Analog to Digital Conversion
• Most modern networks carry digital information.
• Thus, analog signals, like voice, have to be
converted from analog to digital.
• We now overview this process.
Analog Speech Signal
• Microphone outputs an analog electrical signal.
• For example, here is a graph showing the
analog wave created by saying the word "hello“.
A to D Conversion
• In digital systems, this analog signal is converted
to a sequence of 0’s and 1’s.
• One way this can be done is to sample the
analog signal in fixed time intervals:
Sampling instances
Quantization
• This sampling process is called quantization.
Quantization
• The quantized values that make up the digital signal (say
{0, 0.1, 0.2, …, 1}) are then converted to binary format,
i.e, into bits.
Bits
• Binary signals can only take on one of two
values: 0 or 1, commonly known as bits.
• To understand bits, we first look at digits.
• A digit is a single place that can hold numerical
values in the range of 0 to 9.
• Digits are normally combined together to make
larger numbers (called decimal numbers).
Bits (Cont’d)
• For example, 6,957 has four digits. 7 is hold the
1s place, 5 is holding the 10s, 9 is holding the
100’s place, and 6 is holding the 1000’s place.
• Each digit is placeholder for the next higher
power of ten.
• Each place can have 10 different values; this
implies that digits use a base-10 number
system.
• Bits use a base-2 (binary) number system.
Bits (Cont’d)
• Reason computers and communication systems
use binary number system is that electronic
components that perform binary operations are
cheap.
• Bit stands for Binary digIT. A binary digit is a
sequence of 0’s and 1’s that represent numbers.
1110101001010101010101010100110111110101000101010010101010
More on Bits
0=0
1=1
2 = 10
3 = 11
4 = 100
5 = 101
6 = 110
7 = 111
8 = 1000
9 = 1001
10 = 1010
11 = 1011
12 = 1100
13 = 1101
14 = 1110
15 = 1111
16 = 10000
17 = 10001
18 = 10010
19 = 10011
20 = 10100
There are simple rules that have been developed to
convert a digit to a bit.
Some Decimal Number Rules
• To understand binary numbers, begin by
recalling elementary school math. When we first
learned about numbers, we were taught that, in
the decimal system, things are organized into
columns:
H|T|O
1|9|3
• Here, "H" is the hundreds column, "T" is the tens
column, and "O" is the ones column. So the
number "193" is 1-hundreds plus 9-tens plus 3ones.
Decimal Number Rules (Cont’d)
• Years later, we learned that the ones column
meant 100, the tens column meant 101, the
hundreds column 102 and so on, such that
102|101|100
1| 9|3
• Thus, the number 193 is really
(1*102)+(9*101)+(3*100) = 1*100+9*10+3*1.
Binary Number Rules
• The binary system works under the exact same
principles as the decimal system, only it
operates in base 2 rather than base 10. In other
words, instead of columns being 102|101|100
they are 22|21|20
• For example, the binary sequence 11, is
1*21+1*20 = 3.
• While the binary sequence 10010 is
(1*24)+(0*23)+(0*22)+(1*21)+(0*20) =
(1*16)+(0*8)+(0*4)+(1*2)+(0*1) = 18.
Bytes
• Bits are rarely seen alone in computers.
• They are typically grouped together into 8 bit
collections, called bytes.
• With 8 bits in a byte, a byte can represent values
ranging from 0 to 255:
0 = 00000000
1 = 00000001
2 = 00000010
.
.
.
254 = 11111110
255 = 11111111
How is Quantized Digital Signal
converted to Bits?
• After quantization, a digital signal takes on one
of a discrete set of values {y1,y2,….yN}.
• Each possible discrete value can be represented
in binary format.
• Whenever the digital signal takes on particular
value yi, the value yi is converted to binary
format, i.e., a sequence of 0’s and 1’s.
• Result is a binary (digital) signal.
Packets
• In communication networks, bits are transported
in groups called packets.
• Packets can vary in length (depending on the
type of network), e.g., from hundreds of bits to
thousands of bits.
• To understand this, consider the process of
sending an email message.
Email Example
• An email message is written using characters on
a keyboard.
• These characters only take on one of a set of a
possible values (are thus inherently digital).
• There is a standard method for converting text
messages to bits, called ASCII conversion.
Brief Aside: ASCII
• American Standard Code for Information
Interchange (ASCII) is a standard method for
converting text characters (including !@#$%....)
into binary format.
• Standard method means all systems, software
packages, operating systems, etc. use same
conversion process.
• Standard method is preferred because once
converted to binary format, message can be
understood by all software applications.
Email Example (Cont’d)
• Once the message has been converted to bits,
the bits are groups into packets.
• These packets are then sent over the wired or
wireless connection to the router, which is
connected to the internet.
• The packets then move through the internet to
the destination.
• At the destination, the packets are reassembled
into a stream of bits and then reconverted from
bits to a text message.
Packet and Circuit Switched
Networks
• In packet switched networks, each packet is
independently routed from the source terminal to
the destination terminal (e.g, Internet). As a
result, packets from the same data stream (e.g.,
email message) can take different paths through
the network.
• In circuit-switched networks, each packet in a bit
stream takes the same exact path from source
to destination.
Circuit-Switched
Network connections (wired or wireless)
Fixed Route
Network Routers
Circuit-Switched
• In circuit switched networks, a path from source to
destination is fixed by reserving network resources
from source to destination for the duration of the
message transmission.
• As a result, packets arrive in order at the
destination.
• This is different from packet-switched networks.
Packet-Switched
Route for Packets 1 & 2
Route for Packet 3
Route for Packet 4
Packet-Switched
• In packet-switched networks, no route
reservations are made. This way each link can
support more than one message stream (implies
more information can be packed into each link).
• Independent routing for each packet implies that
data traffic can be distributed more evenly over
all network devices.
• Result: packets can arrive out of order at
destination, thereby requiring reassembly.
Review of Today’s Lecture
• We introduced some relevant communication
networks, which we will review in greater detail
during the semester.
• We learned some important networking
concepts that characterize modern computer
and communication networks.
Some References for Today’s
Lecture
• http://www.math.grin.edu/~rebelsky/Courses/152
/97F/Readings/student-binary.html
• http://www.techonline.com/community/ed_resour
ce/feature_article/4153
• http://www.howstuffworks.com