WiFi

1
WiFi

• WiFi is the wireless way to handle networking.
• Wi-fi is a wireless technology that uses
• radio frequency to transmit data through
• the air
• It is also known as 802.11 networking.
• The big advantage of WiFi is its simplicity.

2
Walkie-Talkie Network

• If you want to understand wireless networking at
  its simplest level, think about a pair of
  walkie-talkie.
• These are small radios that can transmit and
  receive radio signals.
• The only problem would be that the data rate
  would be very slow.
• Another problem the walkie-talkies could not be
  used to connect to the internet.

3
WiFis Radio Technology

• The radios used in WiFi are not so different from
  the radios used in walkie-talkies.
• They have the ability to transmit and receive.
• They have the ability to convert 1s and 0s into
  radio waves and then back into 1s and 0s.
• There are major differences, of course.

4
WiFis Radio Technology (Contd)

• WiFi radios that work with the 802.11b and
  802.11g standards transmit at 2.4 GHz, while
  those that comply with the 802.11a standard
  transmit at 5 GHz. The higher frequency allows
  higher data rates.
• The radios used for WiFi have the ability to
  change frequencies,i.e frequency hop between
  channels.

5
Wireless Networking Standards

• WiFi refers to the protocols that allow wireless
  networking.
• These protocols are codified in standards.
• Standards are mutually agreed upon rules adopted
  by the industry on how the wireless networks
  operate.
• There are several standards that enable wireless
  local area networks (WLANs).

6
802.11b, 802.11a, and 802.11g

• 802.11b was first to reach the marketplace.802.11b
  transmits at 2.4 GHz and go up to 11 Mbps.
• 802.11a was next. It operates at 5 GHz and can
  handle up to 54 Mbps.
• 802.11g is a mix of both worlds. It operates at
  2.4Ghz but it has the 54 megabits per second
  speed of 802.11a.

7
802.11b

• Any 802.11b signal occupies approximately 30 MHz.
  
• Thus, 802.11b signal overlaps with several
  adjacent channel frequencies.
• Any given area can therefore support at most 3
  access points (operating on different channels)
  at once.

8
802.11b (Contd)
Neighboring APs use different channels to reduce
interference. Reuse cluster size is equal to
3.
1
Access Point
2
3
9
802.11b (Contd)
2 Mbps
5.5 Mbps
11 Mbps
10
802.11a

• 802.11a specification operates at radio
  frequencies between 5.15 and 5.825 GHz, i.e.
  802.11a utilizes 300 MHz bandwidth in Unlicensed
  National Information Infrastructure (U-NII) band.
• The FCC has divided total 300 MHz in this band
  into three distinct 100 MHz bands low, middle,
  and high, each with different legal maximum power.

Band Channel High band 5.725-5.825 GHz
9-12 Middle band 5.25-5.35 GHz 5-8 Low
band 5.15-5.25 GHz 1-4
11
802.11a (Contd)

• The low and middle bands have a total of 200 MHz
  of frequency.
• This 200 MHz supports 8 non-overlapping channels.

12
802.11a (Contd)
Neighboring APs use different channels to reduce
interference. Reuse cluster size is equal to
8.
1
Access Point
7
2
3
6
4
5
8
13
802.11a (Contd)
802.11a
802.11b
2 Mbps
12 Mbps
5.5 Mbps
24 Mbps
36 Mbps
48 Mbps
11 Mbps
54 Mbps
14
802.11g

• 802.11g operates over 3 non-overlapping channels.
• 802.11g operates in 2.4 GHz band but it delivers
  data rates from 6 Mbps to 54 Mbps.

15
802.11g

• Once again, 802.11gs "backward compatibility"
  with 802.11b.
• So both 11a and 11g offer the same data rates.
  Which is better?

16
Comparing 11a and 11g (Contd)

• 802.11a operates in underused 5 GHz band 802.11g
  operates in heavily used 2.4 GHz band.
• Higher number of channels in 11a allows more
  flexibility in avoiding interference.
• 802.11a has range 150-300 ft in practical
  scenarios. 11g has range comparable to 11b
  (approximately 1000 ft).
• 11a range is smaller than 11b and 11g. This is
  because 11a operates at a much higher frequency
  band.

17
Comparing 11a and 11g (Contd)

• Because of its smaller range, 11a requires more
  Access Points to a region, thereby increasing
  cost. It is the most expensive of the three
  options.
• 802.11b is the cheapest and most popular WLAN
  option.
• 802.11g is more expensive than 11b but cheaper
  than 11a.

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What does a typical 802.11 Packet look like?

• Typical 802.11 packet
• Preamble
• PLCP (Physical Layer Convergence Procedure)
• Data
• CRC is the cyclic redundancy check

Preamble
PLCP Header
Data
CRC
19
How are Multiple Transmitters Supported?

• In 802.11 systems, only one user is allowed to
  communicate with a receiver at a time (cannot use
  another frequency channel support a second or
  third additional user).
• The way the one user is selected depends on the
  carrier sense multiple access with collision
  avoidance (CSMA/CA) random access method.

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CSMA

• To help illustrate the operation of CSMA, we will
  use an analogy of a dinner table conversation.
• Lets represent our wireless medium as a dinner
  table, and let several people engaged in polite
  conversation at the table represent the wireless
  nodes.

21
CSMA (Contd)

• The term multiple access covers what we already
  discussed above When one wireless device
  transmits, all other devices using the wireless
  medium hear the transmission.

22
2 Way Handshake

• Node with packet to send monitors channel.
• If channel idle for specified time interval
  called DIFS, then node transmits.
• If channel busy, then
• node continues to monitor until channel idle for
  DIFS.
• At this point, terminal backs-off for random time
  (collision avoidance) and attempts transmitting
  after waiting this random amount of time.

23
2 Way Handshake (Contd)

• First way of the 2 way handshake was for the
  transmitter to send its information packet to the
  destination node, after following the collision
  avoidance method described above.
• If the packet reaches the destination without
  problems, the destination sends a short packet
  over the wireless medium acknowledging the
  correct reception.
• This packet is typically called an ACK packet.
  ACK is the second way of the 2 way handshake.

24
4 Way Handshake

• Listen before you talk
• If medium is busy, node backs-off for a random
  amount of time after waiting DIFS, just as
  before.
• But now, instead of packet, sends a short
  message Ready to Send (RTS). This message is
  basically attempting to inform others that I
  have something to send.

25
4 Way Handshake (Contd)
Access Point
Laptop
RTS
CTS
Data
ACK
26
Adding WiFi to Your Computer

• One of the best things about WiFi is how simple
  it is.
• 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.

27
Adding WiFi to an Older Computer

• 802.11a, 802.11b or 802.11g network card.
• For a laptop, this card will normally be a PCMCIA
  card
• For a desktop machine, a PCI card
• Install the card

28
Adding WiFi to an Older Computer

• Install the drivers for the card
• Find an 802.11 hotspot.
• Access the hotspot.

29
Locating Hotspots

• There are many WiFi hotspots now available in
  public places like restaurants, hotels, libraries
  and airports. For example, Starbucks.
• You can also create your own hotspot in your
  home, as we will see in a little bit.
• One way to find a hotspot is to go on-line.

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Finding Hotspots O-Line
31
Connecting to a Hotspot

• There are actually two steps to making a
  connection.
• The first is to have your notebook "talk" to the
  hotspot
• On the newest machines, an 802.11 card will
  automatically connect with an 802.11 hotspot and
  a network connection will be established.

32
Connecting to a Hotspot (Contd)

• On older machines you often have to go through a
  simple 3-step process to connect to a hotspot.

33
Connecting to a Hotspot (Contd)

• Access the software for the 802.11 card --
  normally there is an icon for the card down in
  the system tray at the bottom right of the
  screen.
• Click the "Search button" in the software. The
  card will search for all of the available
  hotspots in the area and show you a list.
• Double-click on one of the hotspots to connect to
  it.

34
Connecting to a Hotspot (Contd)

• On older machines you often have to go through a
  simple 3-step process to connect to a hotspot.
• Find what is the SSID of the hotspot
• All the search feature (in newer equipment) is
  doing is grabbing these two pieces of information
  from the radio signals generated by the hotspot
  and displaying them for you.

35
Connecting to a Hotspot (Contd)

• On most notebook models, you will see some sort
  of signal icon on the bottom right hand corner of
  your screen or a lit indicator on the notebook
  itself, which will give you feedback for "On" and
  signal strength (a red screen means your radio is
  Off a green screen indicates it is On).

OFF
ON
36
Connecting to a Hotspot (Contd)

• You can also see the quality of the signal by
  clicking on the radio icon (may vary by system)
  

37
Connecting to a Hotspot (Contd)

• Sign up with a wireless Internet service provider
  and configure your notebook according to their
  instructions.
• If you don't subscribe to a service, chances are
  you will need to use your credit card to pay for
  access every time you want to connect.
• After this, you will end up at the log-on page of
  the wireless provider (or, in some cases, the
  wireless location).

38
Connecting to a Hotspot (Contd)

• Enter your user name and password if you are
  already a customer.
• Once you successfully log on, you should see the
  following icon in your tool bar, indicating the
  connection has been made

39
WiFi Security

• WiFi hotspots can be open or secure.
• If a hotspot is open, then anyone with a WiFi
  card can access the hotspot.
• If it is secure, then the user needs to know a
  WEP key to connect.
• WEP stands for Wired Equivalent Privacy

40
WiFi Security (Contd)

• WEP is an encryption system for the data that
  802.11 sends through the air.
• Specifically, it is the process of encoding bit
  stream in such a way that only the person (or
  computer) with the key (a digital sequence) can
  decode it.

41
Setting up a Hotspot at Home

• If you already have several computers hooked
  together on an Ethernet network and want to add a
  wireless hotspot to the mix, you can purchase a
  Wireless Access Point and plug it into the
  Ethernet network.

Wireless Access Point
42
Setup 1
43
WiFi Range

• In a typical home, hotspot will provide coverage
  for about 100 feet (30.5 meters) in all
  directions, although walls and floors do cut down
  on the range.

44
One Type of Amplifier
Or a directional antenna can be used to
give better range in a particular direction.
45
Another Way to Amplify WiFi Signals
A WiFi repeater is installed to extend coverage.
Wireless Access Point
46
Configuring a Hotspot

• Most wireless access points come with default
  values built-in.
• Once you plug them in, they start working with
  these default values.
• However, you may want to change things.
• You normally get to set three things on your
  access point.

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Things to Configure in a Hotspot

• The SSID -- Service Set IDentifier is a sequence
  of charactersthat uniquely names a WLAN.
• The channel the radio link used by access
  point/router to communicate to wireless devices.
• The WEP key -- The default is to disable WEP.

48
Applications

• Common applications for Wi-Fi include Internet
  and VoIP phone access, gaming, and network
  connectivity for consumer electronics such as
  televisions, DVD players, and digital cameras

49
Advantages of Wi-Fi
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Disadvantages

• Planning Depending on the goal
• Security Greater exposure to risks
• Access
• Compromising Data
• Denial of Service
• Speed Slower than cable
• Range Affected by various medium

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Difference between Bluetooth and Wi-Fi
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