Transcript What is Wi-Fi?

Wi-Fi
Wireless Communications
Sheldon Lou
www.powerpointpresentationon.blogspot.com
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 interoperable.
US Frequency Bands
Band
UHF ISM
S-Band
S-Band ISM
C-Band
C-Band satellite downlink
C-Band Radar (weather)
C-Band ISM
C-Band satellite uplink
X-Band
X-Band Radar (police/weather)
Frequency range
902-928 MHz
2-4 GHz
2.4-2.5 GHz
4-8 GHz
3.7-4.2 GHz
5.25-5.925 GHz
5.725-5.875 GHz
5.925-6.425 GHz
8-12 GHz
8.5-10.55 GHz
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
Notes
(1997)
(1999)
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.
Wireless LAN Networks
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
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.
WLAN Architecture--Mesh
 Mesh: Every client in
the network also acts
as an access or relay
point, creating a “selfhealing” and (in
theory) infinitely
extensible network.
 Not yet in widespread
use, unlikely to be in
homes.
WLAN Architecture—Infrastructure Mode
To Wired Network
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.
Comparison of Two Structures
Expansion
Flexibility
Control
Routing
Coverage
Reliability
Infrastructure
X
Ad hoc
X
X
X
X
X
Extended Service Area
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.
 Two BSSs coverage areas can largely overlap to
increase the capacity for a particular area. If so, the two
access points will use different channels, as we will
explain later.
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.
Antennas, continued
 Antennas come in all
shapes and styles:
Omni-directional:
 Vertical Whip
 Ceiling mount
Directional:
 Yagi (“Pringles can”)
 Wall mounted panel
 Parabolic dish
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 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
communication
 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
(CSMA/CA).
How does CSMA/CA (Carrier Sensing
Multiple Access with Collision Avoidance)
Work? (p. 189, Example 4.18)
RTS/CTS (Request-to-send/clear-to-send)
 Use Request-to-send/clear-to-send (RTS/CTS)
mechanism (p. 191-192, Fig. 4.17 & p. 462, Fig. 11.14)
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.
 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
users.
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.
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.
 The coverage areas of two access points (Basic Service
Sets, BSS) may overlap to increase capacity. For
example, up to 8 users can use VoIP simultaneously
through one access point. With two overlapping APs, 16
users can talk simultaneously. But the two APs have to
use non-overlapping channels.
Modulation
Gaussian frequency shift keying (GFSK) is
used (Sec. 3.6.1, p. 97).
Wi-Fi network services
Distribution and integration
Association, re-association, and
disassociation
Authentication and deauthentication
Providing privacy
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 the
same access point.
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 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.
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 the
registration information to determine which
access point to use for any mobile station.
Reassociation
 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.
Disassociation
 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
disassociating.
Authetication/deauthentication
 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 association.
WiMax
What is WiMax?
 WiMax is a radio technology that promises to deliver
two-way Internet access at speeds of up to 75 Mbps
at long range.
 Its backers claim that WiMax can transmit data up to
30 miles between broadcast towers and can blanket
areas more than a mile in radius with bandwidth that
exceeds current DSL and cable broadband
capabilities.
 So, some believe that it could slash the cost of
bringing broadband to remote areas.
WiMax (Cont’d)
 WiMax, short for Worldwide Interoperability for
Microwave Access, is the latest of the wireless "last
mile" broadband technologies.
 ISP see WiMax as a means of connecting rural or
remote areas with broadband service, something
that would be technically, physically or economically
difficult to do by burying wire for DSL or cable
connections.
 Laying wires is especially difficult in hilly areas like
Susquehanna.
Benefits over Satellite
 In rural areas, the real competition to WiMax would
be satellite data services.
 The benefit that WiMax offers over satellite is that
satellite offers limited uplink bandwidth (upload data
rates are not as high as download data rates).
 Further, satellite suffers with high latency.
WiMax (Cont’d)
 In congested cities, WiMax products could shift
traffic to help relieve heavy demand on broadband
networks.
 WiMax will work with other shorter-range wireless
standards, including Wi-Fi, which has taken off as
an easy way to provide Internet access throughout a
home or business.
 Eventually, advocates hope to see the standard
evolve into a mobile wireless Internet service similar
to cellular data technologies. It may not ever be as
wide-area as cellular but will offer higher data rates.
WiMax Protocols
 The protocols that govern WiMax have been
standardized. They are collectively referred to as
802.16.
 Like Wi-Fi = 802.11, WiMax = 802.16.
 Overall vision for 802.16 is that carriers (e.g., ISP)
would set up base stations connected to a public
(wired) network. This is like cellular.
 Each base station would support hundreds of fixed
subscriber stations. Fixed means that subscriber
stations do not move. Plans to expand the standard
to include mobile stations is in the working.
More on WiMax
 Base stations will use the 802.16 protocols to
dynamically allocated uplink/downlink bandwidth to
subscriber stations based on their demand.
 802.16 has been developed for several frequency
bands (various licensed frequencies in 10-66 GHz,
also licensed and unlicensed frequencies in 2-11
GHz).
 In the unlicensed bands, 802.16 can be used as a
backhaul for wi-fi systems or a longer-range
alternative, i.e., replacing hotspots with hotzones.
Some Technical Specs on WiMax
 The radio technology is based on OFDM.
 802.16 standards incorporate use of adaptive
antenna arrays, which can be used to create
dynamic beams in desired directions.
 Standards offer option for a mesh mode network
topology.
Mesh Networking in WiMax
 When a subscriber unit is not in line of sight with the
base station (does not have a good signal strength),
then it may be able to make a peer-to-peer
connection to a neighbor, i.e., hop to a neighbor’s
subscriber unit.
 The neighbor’s unit may be in line of sight with the
base station, in which case this neighbor would
serve as a relay station (a repeater).
 If the neighbor’s unit is not in line-of-sight then
another hop can be made.
Mesh Mode
Residential
Trunk
Trunk
(Wired)
Network
Business
WiMax Costs
 Analysts estimate that subscriber stations for home
access will initially cost up to $300.
 Base stations will cost as little as $5,000 but will
reach $100,000, depending on their range. Each
base station may be able to support up to 60 T1
class subscriber lines.
 In some cases, consumers would lease subscriber
stations from carriers the way they do with cable settop boxes as part of their service plans.
WiMax Predictions
 Base stations will be able to connect to other base
stations within a range of up to 30 miles with data
transfer speeds of up to 75 megabits per second.
 Subscriber stations, the set-top box-like devices, will
connect to base stations with ranges of up to three
miles and transfer speeds of up to 15 megabits per
second.
 WiMax T1 class lines may cost 10% of wired T1
costs.
 Products will start being available 2005-2006.
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WiFi
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Thanks