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Wi-Fi
Wireless Communications
Sheldon Lou
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What is Wi-Fi?
The standard for wireless local area networks(WLANs). It’s like a common language that allthe devices use to communicate to each other.If you have a standard, people can make allsorts 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 ElectronicsEngineers Inc.) is a non-profit, technical professional association of
more than 360,000 individual members in approximately 175countries. The Wireless Ethernet Compatibility Alliance started theWi-Fi--wireless fidelity--certification program to ensure that
equipment claiming 802.11 compliance was genuinely interoperable.
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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
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Wi-Fi Standards
Standard Speed Freq band Notes
802.11 2 Mbps 2.4 GHz (1997)
802.11a 54 Mbps 5 GHz (1999)
802.11b 11 Mbps 2.4 GHz
802.11g 54 Mbps 2.4 GHz
802.11n 600 Mbps 2.4/5 GHz
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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 mostfamiliar 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 licenseto set up and operate a wireless network.
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U-NII (Unlicensed National
Information Infrastructure) Bands
802.11n can operate at the 5G U-NII bands:
U-NII Low (U-NII-1): 5.15-5.25 GHz.
U-NII Mid (U-NII-2): 5.25-5.35 GHz.
U-NII Worldwide: 5.47-5.725 GHz.
U-NII Upper (U-NII-3): 5.725 to 5.825GHz.
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U-NII bands
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Wireless LAN Networks
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WLAN Architecture— Ad Hoc Mode
Ad-Hoc mode: Peer-to-peer setup whereclients can connect to each other directly.
Generally not used for business networks.
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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 itwhen the meeting ends.
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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 widespreaduse, unlikely to be in
homes.
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WLAN Architecture—Infrastructure Mode
To Wired Network
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Infrastructure network
There is an Access Point (AP), which becomes thehub of a “star topology.”
Any communication has to go through AP. If aMobile Station (MS), like a computer, a PDA, or a
phone, wants to communicate with another MS, itneeds to send the information to AP first, then APsends 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.
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Comparison of Two Structures
Infrastructure Ad hoc
Expansion X
Flexibility XControl X
Routing X
Coverage XReliability X
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Extended Service Area
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Roaming
In an extended service area, a mobile station (MS) canroam from one BSS (Basic Service Set) to another.
Roughly speaking, the MS keeps checking the beaconsignal 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.
Two BSSs coverage areas can largely overlap toincrease the capacity for a particular area. If so, the twoaccess points will use different channels, as we willexplain later.
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Antennas
All WLAN equipment comes with a built-inomni-directional antenna, but some select
products will let you attach secondary
antennas that will significantly boostrange.
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Antennas, continued
Antennas come in all
shapes and styles:
Omni-directional:
Vertical WhipCeiling mount
Directional:
Yagi (“Pringles can”)
Wall mounted panel
Parabolic dish
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How Can Several Users Communicate
Simultaneously?
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 phonecalls, 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.
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Share one channel in data
communication
In data communication, data are grouped into packets/frames. Eachpacket/frame contains a number of bits of information.
Devices (phones, computers, etc.) don’t communicatesimultaneously. It’s like they are sharing one single cable (the air inthis 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 freecan an MS sends packets.
If some station is sending or receiving signal, the MS that intends tosend will generate a random waiting time and wait for its turn. If several MSs are all waiting for their turns, since their waiting timesare randomly generated and thus not equal, they will not start
sending simultaneously. Thus collision (two or more MSs sendingsignals simultaneously) is avoided.
It’s called Carrier Sensing Multiple Access with Collision Avoidance(CSMA/CA).
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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 packetidentifying the source address, destination address, andthe length of the data to be sent.
The destination station responds with CTS packet.
The source terminal receives the CTS and sends thedata.
Other terminals go to the virtual carrier-sensing mode(NAV signal on), therefore the source terminal sends itspacket with no contention.
After completion of the transmission, the destinationstation sends an ACK, opening contention for other users.
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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.
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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 thisensures 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.
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Frequency Hopping in 802.11
The frequency can hop over 78 hopping channels eachseparated 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, thatresults in a threefold increase in the capacity of the cell.
Hop rate = 2.5 hops per second.
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Frequency bands for DSSS
FHSS uses 1 MHz bandwidth (narrowband), but thecenter frequency hops over 76 MHz. DSSS uses a chiprate 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.
The coverage areas of two access points (Basic ServiceSets, BSS) may overlap to increase capacity. For
example, up to 8 users can use VoIP simultaneouslythrough one access point. With two overlapping APs, 16users can talk simultaneously. But the two APs have touse non-overlapping channels.
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Modulation
Gaussian frequency shift keying (GFSK) is
used.
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Wi-Fi network services
Distribution and integration
Association, re-association, and
disassociation
Authentication and deauthentication
Providing privacy
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Distribution
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 thesame access point.
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Integration
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 integrationfunction is specific to the distribution
system used and therefore is not specified
by 802.11, except in terms of the servicesit must offer.
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Association
Delivery of frames to mobile stations is
made possible because mobile stations
register, or associate, with access points.
The distribution system can then use theregistration information to determine which
access point to use for any mobile station.
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Reassociation
When a mobile station moves between basicservice areas within a single extended servicearea, it must evaluate signal strength andperhaps switch the access point with which it is
associated. Reassociations are initiated bymobile stations when signal conditions indicatethat 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 thereachability of the mobile station through adifferent access point.
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Disassociation
To terminate an existing association, stationsmay use the disassociation service. Whenstations invoke the disassociation service, anymobility data stored in the distribution system is
removed. Once disassociation is complete, it isas if the station is no longer attached to thenetwork. Disassociation is a polite task to doduring the station shutdown process. The MAC
is, however, designed to accommodate stationsthat leave the network without formallydisassociating.
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Authetication/deauthentication
Physical security is a major component of a wired LANsecurity solution. Wired network’s equipment can belocked inside offices. Wireless networks cannot offer thesame level of physical security, however, and thereforemust depend on additional authentication routines to
ensure that users accessing the network are authorizedto do so. Authentication is a necessary prerequisite toassociation because only authenticated users areauthorized to use the network. (In practice, though, manyaccess points are configured for "open-system" modeand will authenticate any station.)
Deauthentication terminates an authenticatedrelationship. Because authentication is needed beforenetwork use is authorized, a side effect of deauthentication is termination of any currentassociation.
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