Wi-Fi, also known as IEEE 802.11, is a set of standards that allow wireless devices to communicate. It operates in unlicensed frequency bands like 2.4 GHz and 5 GHz. There are different Wi-Fi standards that support varying speeds like 802.11b at 11 Mbps and 802.11a at 54 Mbps. Wi-Fi networks can be configured in different modes like ad-hoc for device-to-device or infrastructure which uses an access point. Access points allow multiple devices to connect and share a wireless signal. Wi-Fi uses techniques like spread spectrum, carrier sensing, and packetization to allow many devices to communicate simultaneously over the same wireless channel.

1. Wi-Fi
Presented By:
Manish Srivastava

2. What is Wi-Fi?
The standard for wireless local area networks
(WLANs). It’s like a common language that all
the devices use to communicate to each other.
If you have a standard, people can make all
sorts of devices that can work with each other.
It’s actually IEEE 802.11, a family of standards.

3. US Frequency Bands
Band Frequency range
UHF ISM 902-928 MHz
S-Band 2-4 GHz
S-Band ISM 2.4-2.5 GHz
C-Band 4-8 GHz
C-Band satellite downlink 3.7-4.2 GHz
C-Band Radar (weather) 5.25-5.925 GHz
C-Band ISM 5.725-5.875 GHz
C-Band satellite uplink 5.925-6.425 GHz
X-Band 8-12 GHz
X-Band Radar (police/weather) 8.5-10.55 GHz

4. Wi-Fi Standards
Standard Speed Freq band
802.11 2 Mbps 2.4 GHz
802.11a 54 Mbps 5 GHz
802.11b 11 Mbps 2.4 GHz
802.11g 54 Mbps 2.4 GHz

5. ISM Band
ISM stands for industrial, scientific, and medical.
ISM bands are set aside for equipment that is
related to industrial or scientific processes or is
used by medical equipment. Perhaps the most
familiar ISM-band device is the microwave oven,
which operates in the 2.4-GHz ISM band. The
ISM bands are license-free, provided that
devices are low-power. You don't need a license
to set up and operate a wireless network.

6. Wireless LAN Networks


7. Wi-Fi network services
Distribution and integration
Association, re-association, and
disassociation
Authentication and deauthentication
Providing privacy

8. WLAN Architecture—Ad Hoc Mode
 Ad-Hoc mode: Peer-to-peer setup where
clients can connect to each other directly.
Generally not used for business networks.

9. Ad Hoc Structure
Mobile stations communicate to each
other directly.
It’s set up for a special purpose and for a
short period of time. For example, the
participants of a meeting in a conference
room may create an ad hoc network at the
beginning of the meeting and dissolve it
when the meeting ends.

10. WLAN Architecture--Mesh
 Mesh: Every client in
the network also acts
as an access or relay
point, creating a “self-
healing” and (in
theory) infinitely
extensible network.
 Not yet in widespread
use, unlikely to be in
homes.

11. WLAN Architecture—Infrastructure Mode
To Wired Network

12. Infrastructure network
 There is an Access Point (AP), which becomes the
hub of a “star topology.”
 Any communication has to go through AP. If a
Mobile Station (MS), like a computer, a PDA, or a
phone, wants to communicate with another MS, it
needs to send the information to AP first, then AP
sends it to the destination MS
 Multiple APs can be connected together and handle
a large number of clients.
 Used by the majority of WLANs in homes and
businesses.

13. Comparison of Two Structures
Infrastructure Ad hoc
Expansion X
Flexibility X
Control X
Routing X
Coverage X
Reliability X

14. Extended Service Area


15. Roaming
 In an extended service area, a mobile station (MS) can
roam from one BSS (Basic Service Set) to another.
 Roughly speaking, the MS keeps checking the beacon
signal sent by each AP and select the strongest one and
connect to that AP.
 If the BSSs overlap, the connection will not be
interrupted when an MS moves from one set to another.
If not, the service will be interrupted.

16. Antennas
All WLAN equipment comes with a built-in
omni-directional antenna, but some select
products will let you attach secondary
antennas that will significantly boost
range.

17. Antennas, continued
Antennas come in all
shapes and styles:
 Omni-directional:
Vertical Whip
Ceiling mount
 Directional:
Yagi (“Pringles can”)
Wall mounted panel
Parabolic dish

18. How Can Several Users Communicate
Simultaneously?
There is a difference between a network
designed for voice conversation and one for
data exchange.
 For voice conversations, like telephone and cell phone
calls, each person has a dedicated channel during the
entire conversation.
 For data exchange, many users can share one channel.
A user sends information when no one else is sending.

19. Share one channel in data
communication
 In data communication, data are grouped into packets/frames.
Each packet/frame contains a number of bits of information.
Before an MS (mobile station) sends its packets, it checks to see if
someone else is sending information. Only when the medium is free
can an MS sends packets.
 If some station is sending or receiving signal, the MS that intends to
send will generate a random waiting time and wait for its turn. If
several MSs are all waiting for their turns, since their waiting times
are randomly generated and thus not equal, they will not start
sending simultaneously. Thus collision (two or more MSs sending
signals simultaneously) is avoided.
 It’s called Carrier Sensing Multiple Access with Collision Avoidance
(CSMA/CA).

20. RTS/CTS (Request-to-send/clear-to-
send)
 Use Request-to-send/clear-to-send (RTS/CTS)
mechanism to avoid collision when two MSs cannot hear
each other (blocked by a wall …).
 A terminal ready for transmission sends an RTS packet
identifying the source address, destination address, and
the length of the data to be sent.
 The destination station responds with CTS packet.
 The source terminal receives the CTS and sends the
data.
 After completion of the transmission, the destination
station sends an ACK, opening contention for other

21. Spread spectrum in 802.11
It is a requirement imposed by the
regulatory authorities for devices in ISM
band in order to reduce interference.
There is also limitations on transmitted
power.
We discuss two methods specified in
802.11, FHSS and DSSS.

22. DSSS in 802.11
Used by 802.11b
Symbol transmission rate = 1Mbps
Multipath spread of up to 1/1 Mbps = 1 µs does
not cause ISI. For indoor applications this
ensures that the system does not suffer from ISI.
Chip rate = 11 Mcps
Resolution is on the order of 1/11 Mcps = 90 ns.

23. Frequency Hopping in 802.11
 The frequency can hop over 78 hopping channels each
separated by 1 MHz. The first channel, Channel 0,
starts at 2.402 GHz. Channel 1 is at 2.403 GHz,
Channel 2, 2.404 GHz, and so on up to Channel 77 at
2.479 GHz (US, Canada, and Europe standards).
 These frequencies are divided into three patterns of 26
hops each corresponding channel numbers (0, 3, 6, 9,
…, 75), (1, 4, 7, 10, …, 76), (2, 5, 8, 11, …, 77)
Hop rate = 2.5 hops per second.

24. Frequency bands for DSSS
 FHSS uses 1 MHz bandwidth (narrowband), but the
center frequency hops over 76 MHz. DSSS uses a chip
rate of 11 Mcps which occupies around 26 MHz of
bandwidth (wideband).
 The ISM band at 2.4 GHz is divided into 11 overlapping
channels spaced by 5 MHz
 APs located close to each other can choose different
channels to mitigate interference.
 The coverage areas of two access points (Basic Service
Sets, BSS) may overlap to increase capacity.

25. Thank you.