Wireless LANs (IEEE802.1)
Guevara Noubir
Northeastern University
noubir@ccs.neu.edu
Slides partially from “Mobile Communications” by J. Schiller Chapter 7.
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Wireless LAN Applications
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LAN Extension
Cross-building interconnect
Nomadic Access
Ad hoc networking
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LAN Extension
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Wireless LAN linked into a wired LAN on
same premises
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Wired LAN
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Backbone
Support servers and stationary workstations
Wireless LAN
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Stations in large open areas
Manufacturing plants, stock exchange trading floors, and
warehouses
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Multiple-cell Wireless LAN
Cross-Building Interconnect
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Connect LANs in nearby buildings
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Wired or wireless LANs
Point-to-point wireless link is used
Devices connected are typically bridges or
routers
Nomadic Access
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Wireless link between LAN hub and mobile
data terminal equipped with antenna
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Laptop computer or notepad computer
Uses:
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Transfer data from portable computer to office
server
Extended environment such as campus
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Ad Hoc Networking
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Temporary peer-to-peer network set up to meet
immediate need
Example:
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Group of employees with laptops convene for a
meeting; employees link computers in a temporary
network for duration of meeting
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Wireless LAN Requirements
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Throughput
Number of nodes
Connection to backbone LAN
Service area
Battery power consumption
Transmission robustness and security
Collocated network operation
License-free operation
Handoff/roaming
Dynamic configuration
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Wireless LAN Categories
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Infrared (IR) LANs
Spread spectrum LANs
Narrowband microwave
Strengths of Infrared Over
Microwave Radio
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Spectrum for infrared virtually unlimited
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Infrared spectrum unregulated
Equipment inexpensive and simple
Reflected by light-colored objects
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Possibility of high data rates
Ceiling reflection for entire room coverage
Doesn’t penetrate walls
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More easily secured against eavesdropping
Less interference between different rooms
Drawbacks of Infrared Medium
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Indoor environments experience infrared
background radiation
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Sunlight and indoor lighting
Ambient radiation appears as noise in an infrared
receiver
Transmitters of higher power required
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Limited by concerns of eye safety and excessive power
consumption
Limits range
IR Data Transmission
Techniques
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Directed Beam Infrared
Ominidirectional
Diffused
&
Directed Beam Infrared
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Used to create point-to-point links
Range depends on emitted power and degree of
focusing
Focused IR data link can have range of
kilometers
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Cross-building interconnect between bridges or
routers
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Ominidirectional
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Single base station within line of sight of all
other stations on LAN
Station typically mounted on ceiling
Base station acts as a multiport repeater
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Ceiling transmitter broadcasts signal received by IR
transceivers
IR transceivers transmit with directional beam
aimed at ceiling base unit
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Diffused
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All IR transmitters focused and aimed at a point
on diffusely reflecting ceiling
IR radiation strikes ceiling
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Reradiated omnidirectionally
Picked up by all receivers
Spread Spectrum LAN
Configuration
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Multiple-cell arrangement
Within a cell, either peer-to-peer or hub
Peer-to-peer topology
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No hub
Access controlled with MAC algorithm
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CSMA
Appropriate for ad hoc LANs
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Spread Spectrum LAN
Configuration
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Hub topology
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Mounted on the ceiling and connected to backbone
May control access
May act as multiport repeater
Automatic handoff of mobile stations
Stations in cell either:
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Transmit to / receive from hub only
Broadcast using omnidirectional antenna
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Narrowband Microwave LANs
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Use of a microwave radio frequency band for
signal transmission
Relatively narrow bandwidth
Licensed
Unlicensed
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Licensed Narrowband RF
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Licensed within specific geographic areas to
avoid potential interference
Motorola - 600 licenses in 18-GHz range
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Covers all metropolitan areas
Can assure that independent LANs in nearby
locations don’t interfere
Encrypted transmissions prevent eavesdropping
Unlicensed Narrowband RF
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RadioLAN introduced narrowband wireless
LAN in 1995
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Uses unlicensed ISM spectrum
Used at low power (0.5 watts or less)
Operates at 10 Mbps in the 5.8-GHz band
Range = 50 m to 100 m
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Approach
Idea
SDMA
segment space into
cells/sectors
Terminals
only one terminal can
be active in one
cell/one sector
Signal
separation
cell structure, directed
antennas
TDMA
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
all terminals are
active for short
periods of time on
the same frequency
synchronization in
the time domain
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FDMA
segment the
frequency band into
disjoint sub-bands
simple, established,
robust
inflexible, antennas
Disadvantages typically fixed
inflexible,
frequencies are a
scarce resource
Comment
only in combination
with TDMA, FDMA or
CDMA useful
digital, flexible
guard space
needed (multipath
propagation),
synchronization
difficult
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
CDMA
spread the spectrum
using orthogonal codes
every terminal has its all terminals can be active
own frequency,
at the same place at the
uninterrupted
same moment,
uninterrupted
filtering in the
code plus special
frequency domain
receivers
Advantages very simple, increases established, fully
capacity per km²
23
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
flexible, less frequency
planning needed, soft
handover
complex receivers, needs
more complicated power
control for senders
Basis for 3rd generation
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Protocol
Throughput
Pure-ALOHA
S = Ge-2G
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ACDABA-F
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AC-ABAGA---BAGD
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infrastructure
network
AP
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wired network
AP: Access Point
AP
ad-hoc network
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802.x LAN
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TCP
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802.11 MAC
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IEEE 802.11a and IEEE 802.11b
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IEEE 802.11a
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Makes use of 5-GHz band
Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
Uses orthogonal frequency division multiplexing (OFDM)
Subcarrier modulated using BPSK, QPSK, 16-QAM or 64QAM
IEEE 802.11b
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Provides data rates of 5.5 and 11 Mbps
Complementary code keying (CCK) modulation scheme
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