Basic Wireless
Network 1
Chapter 5
Basic Wireless
Network 1
Wireless Networks
•Wireless Technology overview
•The IEEE 802.11 WLAN Standards
Basic Wireless
Network 1
Wireless ?
• A wireless LAN or WLAN is a wireless local area network
that uses radio waves as its carrier.
• The last link with the users is wireless, to give a network
connection to all users in a building or campus.
• The backbone network usually uses cables
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Why Wireless LANs (WLANs)
• Mobility (portability) and Flexibility
• Places where there is no cabling infrastructure / Hard to wire
areas
• Reduced cost of wireless systems
• Improved flexibility of wireless systems
• Cost
– Relatively low cost of deployment
– Continual drop in price for WLAN equipment
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Common Topologies
The wireless LAN connects to a wired LAN
• There is a need of an access point that bridges wireless LAN traffic into the
wired LAN.
• The access point (AP) can also act as a repeater for wireless nodes,
effectively doubling the maximum possible distance between nodes.
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What is 802.11?
• A family of wireless LAN (WLAN) specifications developed by a working
group at the Institute of Electrical and Electronic Engineers (IEEE)
• Defines standard for WLANs using the following four technologies
• Frequency Hopping Spread Spectrum (FHSS)
• Direct Sequence Spread Spectrum (DSSS)
• Infrared (IR)
• Orthogonal Frequency Division Multiplexing (OFDM)
• Versions: 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac
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ISM Frequency Bands
ISM (Industrial, Scientific and Medical) frequency bands:
• 900 MHz band (902 … 928 MHz)
• 2.4 GHz band (2.4 … 2.4835 GHz)
• 5.8 GHz band (5.725 … 5.850 GHz)
Anyone is allowed to use radio equipment for transmitting in these bands (provided
specific transmission power limits are not exceeded) without obtaining a license.
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WLAN Standards
Wireless
LAN
2.4 GHz
5 GHz
802.11
802.11b
802.11g
HiSWANa
802.11a
HiperLAN2
(2 Mbps)
(11 Mbps)
(22-54 Mbps)
(54 Mbps)
(54 Mbps)
(54 Mbps)
HomeRF 2.0
Bluetooth
HomeRF 1.0
(10 Mbps)
(1 Mbps)
(2 Mbps)
802.11e
802.11f
802.11h
802.11i
(QoS)
(IAPP)
(TPC-DFS)
(Security)
802.11 Protocols under development
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WLAN Standards
•
Several WLAN standards, e.g.:
– IEEE 802.11b offering 11 Mbit/s at 2.4 GHz
– The same radio spectrum is used by Bluetooth
• A short-range technology to set-up wireless personal area networks
with gross data rates less than 1 Mbit/s
– IEEE 802.11a, operating at 5 GHz and offering gross data rates of 54
Mbit/s
– IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz.
– IEEE 802.11n up and coming standard up to 300 Mbit/s (two spatial
streams; 600 Mbit/s with 4 spatial streams)
– …
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IEEE 802 wireless network technology
options
Network definition
IEEE standard
Known as
Wireless personal area network
(WPAN)
IEEE 802.15.1
Bluetooth
Low-rate WPAN (LR-WPAN)
IEEE 802.15.4
ZigBee
IEEE 802.11
WiFi
IEEE 802.16
WiMAX
Wireless local area network (WLAN)
Wireless metroplitan area network
(WMAN)
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Characteristics of selected
wireless link standards
Data rate (Mbps)
200
54
5-11
802.11n
802.11a,g
802.11b
4
1
802.11a,g point-to-point
data
802.16 (WiMAX)
UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
3G cellular
enhanced
802.15
.384
3G
UMTS/WCDMA, CDMA2000
.056
2G
IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Mid-range
outdoor
Long-range
outdoor
200m – 4 Km
5Km – 20 Km
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WLAN : 802.11b -Expand
The first 802.11 standard deployed.
Supports 1, 2, 5.5 and 11 Mbps data rates in the 2.4 GHz
ISM (Industrial-Scientific-Medical) band
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WLAN : 802.11a -Expand
Operates in the 5 GHz UNII (Unlicensed National
Information Infrastructure) band
Incompatible with devices operating in 2.4GHz
Supports Data rates up to 54 Mbps (6,9,12,18,24,36,48,54
Mbps).
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WLAN : 802.11g -Expand
Supports data rates as high as 54 Mbps on the 2.4 GHz
band
Provides backward compatibility with 802.11b equipment
Supports Data rates up to 54 Mbps (6,9,12,18,24,36,48,54
Mbps).
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WLAN : 802.11n -Expand
Supports data rates as high as 144 Mbps on the 2.4 GHz
band (802.11g/n) and 300 Mbps on 5 GHz band (802.11a/n)
for 2X2 stream
The maximum bandwidth per spatial stream in 802.11a/n is
150 Mbps, which means that an 802.11a/n AP outfitted with
three transmit and three receive antennas can deliver
maximum theoretical throughput of 450 Mbps
Supports up to 4 stream MIMO
Provides backward compatibility with 802.11a/b/g equipment
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WLAN : 802.11ac -Expand
Supports data rates as high as 1.3 Gbps (also called gigabit
wireless)
Operates only in 5 GHz band
Supports 80/160 MHz channel
Supports up to 8 stream MIMO
The maximum bandwidth in 802.11ac is 433 Mbps per spatial
stream, and the maximum number of spatial streams supported
are eight. So the theoretical maximum throughput on an 802.11ac
network will eventually be several times that of Gigabit Ethernet.
First-generation devices (wave 1), however, are limited to using
either two or three transmit and receive antennas to deliver a
theoretical throughput maximum of 866 Mbps or 1.3 Gbps
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WLAN : MIMO
802.11 a/b/g: Supports 54 Mbps on single 20 MHz stream
802.11 n: Two stream 144 Mbps for 20MHz 2.4GHz band
(802.11g/n) and 300 Mbps for 40 MHz 5GHz band (802.11a/n) or
Three stream 450 Mbps for 40 MHz 5GHz band
802.11 ac: Three stream 1.3 Gbps for 80 MHz 5 Ghz band
Basic Wireless
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WLAN components
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Wireless Lan NIC
• hardware installed in
computing device that
enables it to
communicate on a
network.
Basic Wireless
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Access Points (AP)
• Allows stations to associate with it
• Supports Distributed Coordination Function (DCF)
and Point Coordination Function (PCF)
• Provides management features
– Join/Associate with BSS
– Time synchronisation (beaconing)
– Power management
• all traffic flows through APs
• Supports roaming
Access Points (AP)
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Wireless Router
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WIFI Certification(1)
• Wi-Fi certification is provided by the Wi-Fi
Alliance (http://www.wi-fi.org), a global,
nonprofit, industry trade association devoted to
promoting the growth and acceptance of
WLANs.
• The Wi-Fi Alliance is an association of vendors
whose objective is to improve the
interoperability of products that are based on
the 802.11 standard by certifying vendors for
conformance to industry norms and adherence
to standards.
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WIFI Certification(2)
• Certification includes all
three IEEE 802.11 RF
technologies, as well as
early adoption of pending
IEEE drafts, such as
802.11n, and the WPA and
WPA2 security standards
based on IEEE 802.11i
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WIFI Certification (3)
• The roles of these three organizations can be
summarized as follows:
• ITU-R regulates allocation of RF bands.
• IEEE specifies how RF is modulated to carry
information.
• Wi-Fi ensures that vendors make devices that are
interoperable.
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IEEE 802.11 terminology
System Architecture of an
infrastructure network
802.11 LAN
802.x LAN
Basic Service Set
– group of stations using same radio
frequency
Access
STA1
(BSS)
Point (AP)
– station integrated into the wireless LAN
and the distribution system
BSS1
Portal
Access
Point
– terminal with access mechanisms to
wireless medium and radio contact to
access point
Distribution System
Access
Point
ESS
Station (STA)
Distribution System (DS)
– interconnection network to form one
logical network
BSS2
Extended
STA2
STA3
802.11 LAN
Service Set (EES)
– based on several BSS
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IEEE 802.11 BSS
•
IEEE 802.11 allows the building of ad hoc networks between stations, thus forming one
or more BSSs.
– In this case, a BSS comprises a group of stations using the same radio frequency.
– Several BSSs can either be formed via the distance between the BSSs or by using
different carrier frequencies.
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IEEE 802.11 BSS
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IEEE 802.11 Extended Service Set
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Divided into overlapping channels.
For e.g. the 2.4000–2.4835 GHz band is divided into 13 channels each of width 22 MHz but spaced only 5 MHz
apart, with channel 1 centred on 2.412 GHz and 13 on 2.472 GHz
Availability of channels is regulated by country (e.g. Japan adds a 14th channel 12 MHz above channel 13). 3
channels are non overlapping
Given the separation between channels 1, 6, and 11, the signal on any channel should be sufficiently
attenuated to minimally interfere with a transmitter on any other channel.
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802.11 MAC (Multiple Access Control)
Multiple access control: Different nodes must gain
access to the shared multiple (for instance a wireless
channel) otherwise there will be collisions.
Access methods:
FDMA
Assigning channels in frequency domain
TDMA
Assigning time slots in time domain
CDMA
Assigning code sequences in code domain
CSMA
Assigning transmission opportunities in time
domain on a statistical basis
:
LLC
MAC
PHY
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Wireless LAN cannot implement
CSMA/CD for 3 reasons
•Station must be able to send and receive
data at the same time.
•Collision may not be detected because of the
hidden terminal problem.
•Distance between stations in wireless LANs
can be great. Signal fading could prevent a
station at one end from hearing a collision at
other end.
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IEEE 802.11: CSMA/CA
Before sending a frame, source senses the medium by
checking the energy level at the carrier frequency.
Backoff until the channel is idle.
After the channel is found idle, the station waits for a period of
time called the Distributed interframe space (DIFS); then the
station sends a control frame called request to send (RTS).
After receiving RTS, the destination waits for a period called
Short interframe space (SIFS), the destination station sends a
control frame, called Clear to Send (CTS) to source. This
control frame indicates that the destination station is ready to
receive data.
Source sends data after waiting for SIFS
Destination sends ACK after waiting for SIFS.
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IEEE 802.11: CSMA/CA
Slot Time
SIFS
DIFS
Operating Frequency
Maximum Data Rate
50 µsec
28 µsec
SIFS + (2 * Slot Time)
2.4 GHz
2 Mbps
CSMA/CA Parameter
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IEEE 802.11: CSMA/CA
Design Wireless
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