Digital network lecturer8

DIT
Dar es Salaam institute of Technology (DIT)
ETU 08102
Digital Networks
Ally, J
jumannea@gmail.com

Wireless Local Area Wireless (WLAN)
DIT

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1980’s ~
1990’s ~
LAN (Local Area Network)
Local Area Network (Wired)
Remote Access to Main Computer
Printers, Data Storage, etc. are shared
Internet Usage (e-mail, WWW, etc.)

WiFi
 Almost all wireless LAN now are 802.11
based
 Competing technologies, e.g. HiperLAN can’t
compete on volume and cost
 802.11 is also known as WiFi = Wireless
Fidelity
 Fidelity = Compatibility between wireless
equipment from different manufacturers
 WiFi Alliance is a non-profit organization that
does the compatibility testing (WiFi.org)
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What is WLAN
 A Wireless Local Area Network (WLAN)
network design uses the idea of an access
point and clients.
 The access point or router is the equivalent of
a base station in a cellular network.
 It acts as a bridge between the wired
backbone and the wireless client traffic.
 Examples of client devices include desktops,
laptops, smart phones, game consoles and
printers.
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WLAN Architecture
 Stations is all components that can connect into a
wireless medium in a network.
 Basic Service Set (BSS) is a set of all stations that can
communicate with each other.
 Extended Service Set (ESS) is a set of connected BSSs.
Access points in an ESS are connected by
distribution system
 Distribution System (DS) connects access points in an
extended service set.
DS can be wired or wireless. Current wireless
distribution systems are mostly based on MESH
protocols.
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Types of Wireless LANs
 WLAN can be configured into different
architectures, depending requirements on the
system.
 These physical architecture include the
following:
 Ad Hoc
 Infrastructure
 Mesh
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DIT
Wireless Local Area Networks (WLANs)
 WLANs connect “local” computers (100m range)
 Breaks data into packets
 Channel access is shared (random access)
 Backbone Internet provides best-effort service
01011011
Internet
Access
Point
0101 1011

DIT
WLAN Network Architecture
Basic Service Set (BSS): a set of stations which communicate
with one another
Ad hoc network
• Only direct communication
possible
• No relay function
Infrastructure Mode
• Stations communicate with AP
• AP provides connection to wired network
(e.g. Ethernet)
• Stations not allowed to communicate directly
• Some similarities with cellular
Source: AirTight Networks

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WLAN Network Architecture (2)
Extended Service Set(ESS) : a set of BSSs interconnected by a
distribution system (DS)
Local Area Network (e.g. Ethernet)
Future: Meshed Wireless Backhaul
Source: AirTight Networks

WLAN Bands and Channels
 WLAN operate in the
unlicensed ISM radio bands
around 2.4 GHz and 5 GHz.
 The 2.4 GHz band is used
by 802.11b, 802.11g, and
802.11n.
 It consists of 14 channels
ranging from 2.412 GHz to
2.484 GHz.
 Not all channels can be
used in all regions, and
local restrictions govern
which channels are
available.
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WLAN Bands and Channels (2)
 By contrast, 802.11a, 802.11n, and 802.11ac
amendments all utilize the 5 GHz band ranging from
5.15 GHz to 5.875 GHz.
 The radio bands available for WLAN in 5 GHz are a mix
of ISM and Unlicensed National Information
Infrastructure (U-NII) bands.
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802.11 Channels and Channel Wide
 Starting at the center frequency of 2.412 MHz,
IEEE 802.11 gives every 5 MHz a new channel
number.
 The channel wide is 20 MHz and 40 MHz for
channel bounding (introduced in the 802.11n
amendment).
 If there are different/parallel WLAN setups, it is
crucial not to run them in the same frequency
range.
 Separation of at least one channel is
mandatory to avoid co-channel interference.
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802.11 Channel Separation
 802.11 channel separation 802.11 Overlapping Channels
 802.11 Adjacent Channels
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Bandwidth Configuration
 The original 802.11 specification allowed for channel
bandwidths of 20 MHz.
 Subsequent revisions expanded channel bandwidths
to 40 MHz, 80 MHz, and 160 MHz.
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Bandwidth Configuration for 802.11n and
802.11ac
 802.11n allows the use of 20 MHz and 40 MHz modes.
 802.11ac allows the use of 20 MHz, 40 MHz, 80 MHz,
80+80 MHz, and 160 MHz configurations.
 The 802.11ac amendment is available only in the 5 GHz
band, and not in the 2.4 GHz band.
 802.11ac offers both contiguous and noncontiguous 160
MHz bandwidth configurations.
 The noncontiguous 160 MHz transmission is typically
referred to as 80+80 mode.
 In the contiguous 160 MHz transmission, the signal is
structured such that two 80 MHz transmissions are placed
side-by-side to occupy 160 MHz of contiguous spectrum.
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Bandwith Allocation for 802.11n & 802.11ac
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Key WLAN Enabling Technologies
 The 802.11 specifications use a range of key
technologies that enable them to robustly achieve
high data rates with reasonable spectrum
utilization.
 For example, DSSS offers greater resilience to
interference in the 2.4 GHz spectrum, which is also
utilized by other wireless technologies.
 The 802.11a standard was one of the first
commercial wireless standards to utilize OFDM.
 The 802.11n amendment was also one of the first
commercial standards allowing for up to 4x4 MIMO.
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Direct Spread Sequence Spectrum (DSSS)
 The IEEE 802.11 committee adopted the use of DSSS
technology to protect against interference.
 In DSSS transmissions, the modulated signal is multiplied
by a pseudorandom spreading code and the receiver is
only able to demodulate the signal by applying the same
spreading code in its tracking of the signal.
 An 11-bit Barker sequence was used as spreading code
for 1 Mb/s and 2 Mb/s data rates.
 The 5.5 Mb/s and 11 Mb/s data rates use complementary
code keying (CCK), which uses an 8-bit spreading code.
 Today, the 802.11b standard is referred to as DSSS
HR/DSSS. HR refers to high rate, which corresponds to
the 5.5 Mb/s and 11 Mb/s data rates in 802.11b.
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Orthogonal Frequency Division Multiplexing
(OFDM)
 OFDM was first adopted as part of the 802.11a specification,
and then 802.11g specification.
 All major revisions of the 802.11 standard, including
802.11n, and 802.11ac are based on OFDM technology.
 For 802.11a/g, 802.11n, and 802.11ac, the OFDM
transmission uses a constant symbol rate (constant
subcarrier spacing) for all bandwidth configurations.
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Multiple Input Multiple Output (MIMO)
 MIMO technology, involves multiple antennas at the
transmitter and receiver, is an important technology
featured in the WLAN standard.
 MIMO improve the reliability and the throughput of
communication over wireless channels.
 MIMO was first adopted in WLAN as part of the 802.11n
standard, and later expanded in 802.11ac.
 802.11n supports antenna configurations ranging from
single input, single output (SISO) to 4x4 MIMO, with
802.11ac extending the capability to 8x8 MIMO.
 MIMO deployments, such as 4x4 or 8x8, are typically used
in access points.
 Due to complexity most end devices uses single antenna.
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Benefits of using MIMO
 MIMO in WLAN is increasing data rates through the use of
multiple spatial streams (known as spatial multiplexing).
 MIMO technology increase the diversity or redundancy of
the transmission through spatial diversity.
 For example, if a SISO system were able to achieve a data
rate of 100 Mb/s, an 8x8 MIMO system with 8 spatial
streams could achieve a maximum data rate of 800 Mb/s.
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Theoretical Data Rate Calculation
 Wireless LAN provides high data rates by using a
combination of higher order modulation schemes, higher
bandwidths, and MIMO technology.
 Determining the maximum data rate is a straightforward
calculation,
Max Data Rate = (Data Carriers x Spatial Streams x Bits per
symbol x Code Rate) / Symbol Duration
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802.11a/g Data Rate Calculations
 802.11a and 802.11g use nearly identical signal
structures, and each standard uses the same number of
data carriers, symbol rates, and code rates.
 Parameters Affecting 802.11a/g Data Rate
 Maximum Data Rate = (48 x 1 x 6 x ¾) / 4µs
= 54 Mbps
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802.11n Data Rate Calculation
 802.11n adds a larger number of subcarriers through its
40 MHz bandwidth high throughput (HT) option.
 By implementing 4x4 MIMO, 802.11n expands the
number of spatial streams from 1 (SISO) to 4.
 Parameters Affecting 802.11n Data Rate
 Maximum Data Rate = (108 x 4 x 6 x 5/6) / 3.6µs
= 600 Mbps
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802.11ac Data Rate Calculation
 802.1ac adds a larger number of subcarriers through the
160 MHz bandwidth very high throughput (VHT) option.
 Added spatial streams, use of the 256-QAM modulation
scheme, and a higher code rate also contribute to
maximum data throughput.
 Parameters Affecting 802.11ac Data Rate
 Maximum Data Rate = (512 x 8 x 8 x 5/6) / 3.6µs
= 6.933 Gbps
DIT

Digital network lecturer8

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