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7/29/2019 89088468 Wireless Communications and Networks Ppt
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Cellular Wireless Networks
ICS 620
Fall 2003Week #9
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Cellular Network Organization
Use multiple low-power transmitters (100 W or
less)
Areas divided into cells
Each served by its own antenna Served by base station consisting of transmitter,
receiver, and control unit
Band of frequencies allocated
Cells set up such that antennas of all neighbors are
equidistant (hexagonal pattern)
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Cellular Spectrum
A band
10 MHz
333 channels
30khz
B band
10 MHz
333 channels
30khz
825 835 845
870 880 890
824 846.5 849
869A band
10 MHz
333 channels
30khz
B band
10 MHz
333 channels
30khz
Phone Transmit
Base Transmit
891.5 894
Aband
Aband
Aband
Aband
Bband
Bband
1 MHz33 chan
1.5 MHz50 chan
2.5 MHz83 chan
1 MHz
33 chan
1.5 MHz
50 chan
2.5 MHz
83 chan
20 MHz Guard
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Frequency Reuse
Adjacent cells assigned different frequencies to
avoid interference or crosstalk
Objective is to reuse frequency in nearby cells
10 to 50 frequencies assigned to each cell Transmission power controlled to limit power at that
frequency escaping to adjacent cells
The issue is to determine how many cells must
intervene between two cells using the same frequency
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Approaches to Cope with
Increasing Capacity
Adding new channels
Frequency borrowingfrequencies are taken fromadjacent cells by congested cells
Cell splittingcells in areas of high usage can besplit into smaller cells
Cell sectoringcells are divided into a number ofwedge-shaped sectors, each with their own set of
channels Microcellsantennas move to buildings, hills,
and lamp posts
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Cellular System Overview
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Cellular Systems Terms
Base Station (BS)includes an antenna, acontroller, and a number of receivers
Mobile telecommunications switching office(MTSO)connects calls between mobile units
Two types of channels available between mobileunit and BS
Control channelsused to exchange informationhaving to do with setting up and maintaining calls
Traffic channelscarry voice or data connectionbetween users
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Steps in an MTSO Controlled Call
between Mobile Users
Mobile unit initialization
Mobile-originated call
Paging Call accepted
Ongoing call
Handoff
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Additional Functions in an MTSO
Controlled Call
Call blocking
Call termination
Call drop Calls to/from fixed and remote mobile
subscriber
bil di i
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Mobile Radio Propagation
Effects
Signal strength
Must be strong enough between base station and mobile
unit to maintain signal quality at the receiver
Must not be so strong as to create too much co-channelinterference with channels in another cell using the
same frequency band
Fading
Signal propagation effects may disrupt the signal andcause errors
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Handoff Performance Metrics
Cell blocking probabilityprobability of a newcall being blocked
Call dropping probabilityprobability that a callis terminated due to a handoff
Call completion probabilityprobability that anadmitted call is not dropped before it terminates
Probability of unsuccessful handoffprobability
that a handoff is executed while the receptionconditions are inadequate
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Handoff Performance Metrics
Handoff blocking probabilityprobability that ahandoff cannot be successfully completed
Handoff probabilityprobability that a handoffoccurs before call termination
Rate of handoffnumber of handoffs per unittime
Interruption durationduration of time during ahandoff in which a mobile is not connected toeither base station
Handoff delaydistance the mobile moves fromthe point at which the handoff should occur to the
point at which it does occur
H d ff S i U d
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Handoff Strategies Used to
Determine Instant of Handoff
Relative signal strength
Relative signal strength with threshold
Relative signal strength with hysteresis Relative signal strength with hysteresis and
threshold
Prediction techniques
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Power Control
Design issues making it desirable to include
dynamic power control in a cellular system
Received power must be sufficiently above the
background noise for effective communication Desirable to minimize power in the transmitted signal
from the mobile
Reduce co-channel interference, alleviate health concerns, save
battery power
In SS systems using CDMA, its desirable to equalize
the received power level from all mobile units at the BS
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Types of Power Control
Open-loop power control
Depends solely on mobile unit
No feedback from BS
Not as accurate as closed-loop, but can react quicker tofluctuations in signal strength
Closed-loop power control
Adjusts signal strength in reverse channel based onmetric of performance
BS makes power adjustment decision andcommunicates to mobile on control channel
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Traffic Engineering
Ideally, available channels would equalnumber of subscribers active at one time
In practice, not feasible to have capacity
handle all possible load ForNsimultaneous user capacity andL
subscribers
LNblocking system
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First-Generation Analog
Advanced Mobile Phone Service (AMPS)
In North America, two 25-MHz bands allocated
to AMPS
One for transmission from base to mobile unit
One for transmission from mobile unit to base
Each band split in two to encourage
competition
Frequency reuse exploited
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Frequency Division MultipleAccess
Definition - FDMA is a multiple accessmethod in which users are assigned
specific frequency bands. The user hassole right of using the frequency bandfor the entire call duration. (Qualcomm, 1997)
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FDMA
Frequency Division Multiple Access
F
requency
Time
Chan A
Chan B
Chan C
Chan D
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AMPS Operation
Subscriber initiates call by keying in phonenumber and presses send key
MTSO verifies number and authorizes user
MTSO issues message to users cell phoneindicating send and receive traffic channels
MTSO sends ringing signal to called party
Party answers; MTSO establishes circuit and
initiates billing information Either party hangs up; MTSO releases circuit,
frees channels, completes billing
Diff B t Fi t d
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Differences Between First and
Second Generation Systems
Digital traffic channelsfirst-generation systemsare almost purely analog; second-generationsystems are digital
Encryptionall second generation systemsprovide encryption to prevent eavesdropping
Error detection and correctionsecond-generationdigital traffic allows for detection and correction,giving clear voice reception
Channel accesssecond-generation systems allowchannels to be dynamically shared by a number ofusers
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Time Division Multiple Access
Definition - TDMA is an assignedfrequency band shared among a fewusers. However, each user is allowed to
transmit in predetermined time slots.Hence, channelization of users in the sameband is achieved through separation in
time.(Qualcomm, 1997)
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TDMA
Time Division Multiple Access
F
requency
Time
Chan A
Chan B
Mobile Wireless TDMA Design
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Mobile Wireless TDMA Design
Considerations
Number of logical channels (number of time slotsin TDMA frame): 8
Maximum cell radius (R): 35 km
Frequency: region around 900 MHz Maximum vehicle speed (V
m):250 km/hr
Maximum coding delay: approx. 20 ms
Maximum delay spread (m): 10 s
Bandwidth: Not to exceed 200 kHz (25 kHz perchannel)
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GSM Network Architecture
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Mobile Station
Mobile station communicates across Um interface(air interface) with base station transceiver insame cell as mobile unit
Mobile equipment (ME)physical terminal, suchas a telephone or PCS
ME includes radio transceiver, digital signal processorsand subscriber identity module (SIM)
GSM subscriber units are generic until SIM isinserted
SIMs roam, not necessarily the subscriber devices
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Base Station Subsystem (BSS)
BSS consists of base station controller andone or more base transceiver stations (BTS)
Each BTS defines a single cell
Includes radio antenna, radio transceiver and alink to a base station controller (BSC)
BSC reserves radio frequencies, manageshandoff of mobile unit from one cell toanother within BSS, and controls paging
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Network Subsystem (NS)
NS provides link between cellular network and
public switched telecommunications networks
Controls handoffs between cells in different BSSs
Authenticates users and validates accounts Enables worldwide roaming of mobile users
Central element of NS is the mobile switching
center (MSC)
Mobile Switching Center (MSC)
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Mobile Switching Center (MSC)
Databases
Home location register (HLR) databasestoresinformation about each subscriber that belongs toit
Visitor location register (VLR) database
maintains information about subscribers currentlyphysically in the region
Authentication center database (AuC)used forauthentication activities, holds encryption keys
Equipment identity register database (EIR)keeps track of the type of equipment that exists atthe mobile station
TDMA Format Time Slot
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TDMA FormatTime Slot
Fields
Trail bitsallow synchronization of transmissionsfrom mobile units
Encrypted bitsencrypted data
Stealing bit - indicates whether block containsdata or is "stolen"
Training sequenceused to adapt parameters ofreceiver to the current path propagationcharacteristics
Strongest signal selected in case of multipathpropagation
Guard bitsused to avoid overlapping with otherbursts
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GSM Speech Signal Processing
GSM Signaling Protocol
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GSM Signaling Protocol
Architecture
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Functions Provided by Protocols
Protocols above the link layer of the GSMsignaling protocol architecture providespecific functions:
Radio resource management Mobility management
Connection management
Mobile application part (MAP)
BTS management
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Code Division Multiple Access
Definition - CDMA is a method in which usersoccupy the same time and frequencyallocations, and are channelized by unique
assigned codes. The signals are separated atthe receiver by using a correlator that acceptsonly signal energy from the desired channel.
Undesired signals contribute only to noise.(Qualcomm, 1997)
CDMA
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CDMA
Code Division Multiple Access
Code
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Capacity
CDMA has the ability to deliver 10 to 20times the capacity as FDMA for the
same bandwidth. CDMA also has acapacity advantage over TDMA by 5 to7 times.
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Advantages of CDMA Cellular
Frequency diversityfrequency-dependenttransmission impairments have less effect onsignal
Multipath resistancechipping codes used forCDMA exhibit low cross correlation and lowautocorrelation
Privacyprivacy is inherent since spreadspectrum is obtained by use of noise-like signals
Graceful degradationsystem only graduallydegrades as more users access the system
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Drawbacks of CDMA Cellular
Self-jammingarriving transmissions frommultiple users not aligned on chip boundariesunless users are perfectly synchronized
Near-far problemsignals closer to the receiverare received with less attenuation than signalsfarther away
Soft handoffrequires that the mobile acquiresthe new cell before it relinquishes the old; this ismore complex than hard handoff used in FDMAand TDMA schemes
Mobile Wireless CDMA Design
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Mobile Wireless CDMA Design
Considerations
RAKE receiverwhen multiple versions of asignal arrive more than one chip interval apart,RAKE receiver attempts to recover signals frommultiple paths and combine them
This method achieves better performance than simplyrecovering dominant signal and treating remainingsignals as noise
Soft Handoffmobile station temporarily
connected to more than one base stationsimultaneously
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Principle of RAKE Receiver
Types of Channels Supported by
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Types of Channels Supported by
Forward Link
Pilot (channel 0) - allows the mobile unit toacquire timing information, provides phasereference and provides means for signal strengthcomparison
Synchronization (channel 32) - used by mobilestation to obtain identification information aboutcellular system
Paging (channels 1 to 7) - contain messages forone or more mobile stations
Traffic (channels 8 to 31 and 33 to 63)theforward channel supports 55 traffic channels
Forward Traffic Channel
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Forward Traffic Channel
Processing Steps
Speech is encoded at a rate of 8550 bps
Additional bits added for error detection
Data transmitted in 2-ms blocks with forward
error correction provided by a convolutionalencoder
Data interleaved in blocks to reduce effects of
errors
Data bits are scrambled, serving as a privacy mask
Forward Traffic Channel
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Forward Traffic Channel
Processing Steps (cont.)
Power control information inserted into traffic
channel
DS-SS function spreads the 19.2 kbps to a rate of
1.2288 Mbps using one row of 64 x 64 Walshmatrix
Digital bit stream modulated onto the carrier using
QPSK modulation scheme
ITUs View of Third-Generation
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ITU s View of Third-Generation
Capabilities
Voice quality comparable to the public switchedtelephone network
144 kbps data rate available to users in high-speedmotor vehicles over large areas
384 kbps available to pedestrians standing ormoving slowly over small areas
Support for 2.048 Mbps for office use
Symmetrical / asymmetrical data transmissionrates
Support for both packet switched and circuitswitched data services
ITUs View of Third-Generation
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ITU s View of Third Generation
Capabilities
An adaptive interface to the Internet to reflect
efficiently the common asymmetry between
inbound and outbound traffic
More efficient use of the available spectrum ingeneral
Support for a wide variety of mobile equipment
Flexibility to allow the introduction of new
services and technologies
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Alternative Interfaces
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CDMA Design Considerations
Bandwidthlimit channel usage to 5 MHz
Chip ratedepends on desired data rate, need for
error control, and bandwidth limitations; 3 Mcps
or more is reasonable Multirateadvantage is that the system can
flexibly support multiple simultaneous
applications from a given user and can efficiently
use available capacity by only providing thecapacity required for each service
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Paging & SMS
Evolution of Paging
Tone Boy, early 1960s
Tone-Voice, late 1960s
Digital Pagers, 1970s
Numeric Paging Systems
Alpha/Numeric Paging Systems
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Paging
Larger coverage area in each site
Signal, Numeric, Alpha-numeric
Marketed by coverage area.
Features--Web messaging, modemmessaging
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Paging
Current Applications
Fax Forwarding
E-Mail Forwarding
Voice Mail Notification
Automated Problem Notification
Two-way Paging
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Wireless Local Loop
Wired technologies responding to need forreliable, high-speed access by residential,
business, and government subscribers
ISDN, xDSL, cable modems
Increasing interest shown in competing wirelesstechnologies for subscriber access
Wireless local loop (WLL)
Narrowbandoffers a replacement for existingtelephony services
Broadbandprovides high-speed two-way voice anddata service
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WLL Configuration
Advantages of WLL over Wired
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Advantages of WLL over WiredApproach
Costwireless systems are less expensive due to
cost of cable installation thats avoided
Installation timeWLL systems can be installed
in a small fraction of the time required for a newwired system
Selective installationradio units installed for
subscribers who want service at a given time
With a wired system, cable is laid out in anticipation of
serving every subscriber in a given area
Propagation Considerations for
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Propagation Considerations forWLL
Most high-speed WLL schemes use millimeter
wave frequencies (10 GHz to about 300 GHz)
There are wide unused frequency bands available above
25 GHz
At these high frequencies, wide channel bandwidths
can be used, providing high data rates
Small size transceivers and adaptive antenna arrays can
be used
Propagation Considerations for
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Propagation Considerations forWLL
Millimeter wave systems have someundesirable propagation characteristics
Free space loss increases with the square of the
frequency; losses are much higher in millimeterwave range
Above 10 GHz, attenuation effects due torainfall and atmospheric or gaseous absorption
are large Multipath losses can be quite high
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Fresnel Zone
How much space around direct path betweentransmitter and receiver should be clear ofobstacles?
Objects within a series of concentric circles around the
line of sight between transceivers haveconstructive/destructive effects on communication
For point along the direct path, radius of firstFresnel zone:
S= distance from transmitter
D = distance from receiver
DS
SDR
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Atmospheric Absorption
Radio waves at frequencies above 10 GHz
are subject to molecular absorption
Peak of water vapor absorption at 22 GHz
Peak of oxygen absorption near 60 GHz
Favorable windows for communication:
From 28 GHz to 42 GHz
From 75 GHz to 95 GHz
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Effect of Rain
Attenuation due to rain Presence of raindrops can severely degrade the
reliability and performance of communication links
The effect of rain depends on drop shape, drop size,
rain rate, and frequency Estimated attenuation due to rain:
A = attenuation (dB/km)
R = rain rate (mm/hr)
a and b depend on drop sizes and frequency
baRA
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Effects of Vegetation
Trees near subscriber sites can lead to multipathfading
Multipath effects from the tree canopy are
diffraction and scattering Measurements in orchards found considerable
attenuation values when the foliage is within 60%
of the first Fresnel zone
Multipath effects highly variable due to wind
Multipoint Distribution Service
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u t po t st but o Se v ce(MDS)
Multichannel multipoint distribution service(MMDS)
Also referred to as wireless cable
Used mainly by residential subscribers and small
businesses
Local multipoint distribution service (LMDS)
Appeals to larger companies with greater bandwidth
demands
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Advantages of MMDS
MMDS signals have larger wavelengths andcan travel farther without losing significantpower
Equipment at lower frequencies is lessexpensive
MMDS signals don't get blocked as easilyby objects and are less susceptible to rainabsorption
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Advantages of LMDS
Relatively high data rates
Capable of providing video, telephony, and
data
Relatively low cost in comparison with
cable alternatives
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802.16 Standards Development
Use wireless links with microwave or millimeterwave radios
Use licensed spectrum
Are metropolitan in scale Provide public network service to fee-paying
customers
Use point-to-multipoint architecture with
stationary rooftop or tower-mounted antennas
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802.16 Standards Development
Provide efficient transport of heterogeneous trafficsupporting quality of service (QoS)
Use wireless links with microwave or millimeter
wave radios Are capable of broadband transmissions (>2
Mbps)
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Protocol Architecture
Physical and transmission layer functions: Encoding/decoding of signals
Preamble generation/removal
Bit transmission/reception
Medium access control layer functions: On transmission, assemble data into a frame with
address and error detection fields
On reception, disassemble frame, and perform address
recognition and error detection Govern access to the wireless transmission medium
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Protocol Architecture
Convergence layer functions: Encapsulate PDU framing of upper layers into
native 802.16 MAC/PHY frames
Map upper layers addresses into 802.16addresses
Translate upper layer QoS parameters intonative 802.16 MAC format
Adapt time dependencies of upper layer trafficinto equivalent MAC service
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IEEE 802.16.1 Services
Digital audio/video multicast
Digital telephony
ATM
Internet protocol
Bridged LAN
Back-haul Frame relay
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IEEE 802.16.3 Services
Voice transport
Data transport
Bridged LAN
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IEEE 802.16.1 Frame Format
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IEEE 802.16.1 Frame Format
Header - protocol control information Downlink headerused by the base station
Uplink headerused by the subscriber to convey
bandwidth management needs to base station
Bandwidth request headerused by subscriber to
request additional bandwidth
Payloadeither higher-level data or a MAC
control message CRCerror-detecting code
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MAC Management Messages
Uplink and downlink channel descriptor
Uplink and downlink access definition
Ranging request and response
Registration request, response and acknowledge Privacy key management request and response
Dynamic service addition request, response and
acknowledge
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MAC Management Messages
Dynamic service change request, response,and acknowledge
Dynamic service deletion request and
response Multicast polling assignment request and
response
Downlink data grant type request
ARQ acknowledgment
Physical LayerUpstream
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y y pTransmission
Uses a DAMA-TDMA technique
Error correction uses Reed-Solomon code
Modulation scheme based on QPSK
Physical LayerDownstream
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y yTransmission
Continuous downstream mode For continuous transmission stream (audio, video)
Simple TDM scheme is used for channel access
Duplexing technique is frequency division duplex
(FDD) Burst downstream mode
Targets burst transmission stream (IP-based traffic)
DAMA-TDMA scheme is used for channel access
Duplexing techniques are FDD with adaptivemodulation, frequency shift division duplexing(FSDD), time division duplexing (TDD)
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Wireless LAN Technology
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Wireless LAN Applications
LAN Extension
Cross-building interconnect
Nomadic Access
Ad hoc networking
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LAN Extension
Wireless LAN linked into a wired LAN onsame premises
Wired LAN
Backbone
Support servers and stationary workstations
Wireless LAN
Stations in large open areas
Manufacturing plants, stock exchange trading floors,and warehouses
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Multiple-cell Wireless LAN
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Cross-Building Interconnect
Connect LANs in nearby buildings
Wired or wireless LANs
Point-to-point wireless link is used
Devices connected are typically bridges or
routers
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Nomadic Access
Wireless link between LAN hub and mobiledata terminal equipped with antenna
Laptop computer or notepad computer
Uses: Transfer data from portable computer to office
server
Extended environment such as campus
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Ad Hoc Networking
Temporary peer-to-peer network set up tomeet immediate need
Example:
Group of employees with laptops convene for ameeting; employees link computers in a
temporary network for duration of meeting
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Wireless LAN Requirements
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
Infrared (IR) LANs
Spread spectrum LANs
Narrowband microwave
Strengths of Infrared Over
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Microwave Radio
Spectrum for infrared virtually unlimited Possibility of high data rates
Infrared spectrum unregulated
Equipment inexpensive and simple
Reflected by light-colored objects
Ceiling reflection for entire room coverage
Doesnt penetrate walls
More easily secured against eavesdropping Less interference between different rooms
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Drawbacks of Infrared Medium
Indoor environments experience infraredbackground radiation
Sunlight and indoor lighting
Ambient radiation appears as noise in aninfrared receiver
Transmitters of higher power required
Limited by concerns of eye safety and excessive
power consumption Limits range
IR Data Transmissionh i
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Techniques
Directed Beam Infrared
Ominidirectional
Diffused
i d f d
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Directed Beam Infrared
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
Cross-building interconnect between bridges or
routers
O i idi i l
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Ominidirectional
Single base station within line of sight of allother stations on LAN
Station typically mounted on ceiling
Base station acts as a multiport repeater
Ceiling transmitter broadcasts signal received
by IR transceivers
IR transceivers transmit with directional beamaimed at ceiling base unit
Diff d
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Diffused
All IR transmitters focused and aimed at apoint on diffusely reflecting ceiling
IR radiation strikes ceiling
Reradiated omnidirectionally
Picked up by all receivers
Spread Spectrum LANC fi i
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Configuration
Multiple-cell arrangement (Figure 13.2)
Within a cell, either peer-to-peer or hub
Peer-to-peer topology
No hub
Access controlled with MAC algorithm
CSMA
Appropriate for ad hoc LANs
Spread Spectrum LANC fi i
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Configuration
Hub topology 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:
Transmit to / receive from hub only Broadcast using omnidirectional antenna
N b d Mi LAN
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Narrowband Microwave LANs
Use of a microwave radio frequency bandfor signal transmission
Relatively narrow bandwidth
Licensed
Unlicensed
Li d N b d RF
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Licensed Narrowband RF
Licensed within specific geographic areas toavoid potential interference
Motorola - 600 licenses in 18-GHz range
Covers all metropolitan areas
Can assure that independent LANs in nearby
locations dont interfere
Encrypted transmissions prevent eavesdropping
U li d N b d RF
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Unlicensed Narrowband RF
RadioLAN introduced narrowband wirelessLAN in 1995
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