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.
The IEEE (Eye-triple-E, Institute of Electrical and Electronics
Engineers Inc.) is a non-profit, technical professional association of
more than 360,000 individual members in approximately 175
countries. The Wireless Ethernet Compatibility Alliance started the
Wi-Fi--wireless fidelity--certification program to ensure that
equipment claiming 802.11 compliance was genuinely
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
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.
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.
Ad Hoc Structure
Mobile stations communicate to each
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.
Mesh: Every client in
the network also acts
as an access or relay
point, creating a “self-
healing” and (in
Not yet in widespread
use, unlikely to be in
WLAN Architecture—Infrastructure Mode
To Wired 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
Comparison of Two Structures
Infrastructure Ad hoc
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
Antennas come in all
shapes and styles:
Yagi (“Pringles can”)
Wall mounted panel
How Can Several Users Communicate
As we have discussed, 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. (3G and 4G cell phones are
somewhat different, as we will explain later.)
For data exchange, many users can share one channel.
A user sends information when no one else is sending.
New technologies try to accommodate both voice and
data transmissions, as we will discuss in this course.
Share one channel in data
In data communication, data are grouped into packets/frames.
Each packet/frame contains a number of bits of information.
Devices (phones, computers, etc.) don’t communicate
simultaneously. It’s like they are sharing one single cable (the air in
this case), only one person can use it at one time.
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
How does CSMA/CA (Carrier Sensing
Multiple Access with Collision Avoidance)
Work? (p. 189, Example 4.18)
Request-to-send/clear-to-send (RTS/CTS) mechanism
(p. 191-192, Fig. 4.17 & p. 462, Fig. 11.14)
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
Other terminals go to the virtual carrier-sensing mode
(NAV signal on), therefore the source terminal sends its
packet with no contention.
After completion of the transmission, the destination
station sends an ACK, opening contention for other
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
We discuss two methods specified in
802.11, FHSS and DSSS.
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.
Use Barker code (Example 3.16, p. 116).
Complementary code keying (CCK)
Used to increase the data rate to 11 Mbps
Example 17, p. 119
Sec. 11.3.4, p. 457
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), see p.
454, Fig. 11.5.
Three APs can coexist without any hop collision, that
results in a threefold increase in the capacity of the cell.
Hop rate = 2.5 hops per second.
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 (see Fig.
11.6, P. 455).
APs located close to each other can choose
different channels to mitigate interference.
Gaussian frequency shift keying (GFSK) is
used (Sec. 3.6.1, p. 97).
Wi-Fi network services
Distribution and integration
Association, re-association, and
Authentication and deauthentication
This service is used by mobile stations in an
infrastructure network every time they send data.
Once a frame has been accepted by an access
point, it uses the distribution service to deliver
the frame to its destination. Any communication
that uses an access point travels through the
distribution service, including communications
between two mobile stations associated with the
same access point.
Integration is a service provided by the
distribution system; it allows the
connection of the distribution system to a
non-IEEE 802.11 network. The integration
function is specific to the distribution
system used and therefore is not specified
by 802.11, except in terms of the services
it must offer.
Delivery of frames to mobile stations is
made possible because mobile stations
register, or associate, with access points.
The distribution system can then use the
registration information to determine which
access point to use for any mobile station.
When a mobile station moves between basic
service areas within a single extended service
area, it must evaluate signal strength and
perhaps switch the access point with which it is
associated. Reassociations are initiated by
mobile stations when signal conditions indicate
that a different association would be beneficial;
they are never initiated by the access point. After
the reassociation is complete, the distribution
system updates its location records to reflect the
reachability of the mobile station through a
different access point.
To terminate an existing association, stations
may use the disassociation service. When
stations invoke the disassociation service, any
mobility data stored in the distribution system is
removed. Once disassociation is complete, it is
as if the station is no longer attached to the
network. Disassociation is a polite task to do
during the station shutdown process. The MAC
is, however, designed to accommodate stations
that leave the network without formally
Physical security is a major component of a wired LAN
security solution. Wired network’s equipment can be
locked inside offices. Wireless networks cannot offer the
same level of physical security, however, and therefore
must depend on additional authentication routines to
ensure that users accessing the network are authorized
to do so. Authentication is a necessary prerequisite to
association because only authenticated users are
authorized to use the network. (In practice, though, many
access points are configured for "open-system" mode
and will authenticate any station.)
Deauthentication terminates an authenticated
relationship. Because authentication is needed before
network use is authorized, a side effect of
deauthentication is termination of any current